CA2033235A1 - Method and apparatus for injection molding ceramic valves - Google Patents
Method and apparatus for injection molding ceramic valvesInfo
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- CA2033235A1 CA2033235A1 CA 2033235 CA2033235A CA2033235A1 CA 2033235 A1 CA2033235 A1 CA 2033235A1 CA 2033235 CA2033235 CA 2033235 CA 2033235 A CA2033235 A CA 2033235A CA 2033235 A1 CA2033235 A1 CA 2033235A1
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Abstract
Abstract of the Disclosure An apparatus and method for injection molding a unitary ceramic part such as an engine valve contemplates a mold having a die cavity therein shaped like an engine valve. The die cavity is formed with a head portion of the valve at a first portion of the cavity and a stem portion of the valve at a second portion of the cavity.
Molding material is injected slowly into the die cavity by an injection molding apparatus connected to the mold. As the molding material is slowly injected into the cavity, the material moves up through the die cavity forcing air out of a vented portion on the mold until the die cavity is filled. A temperature control plate, connected to the mold adjacent the first portion of the die cavity, allows a cooling fluid to circulate therein in order to permit controlled cooling of the part from the head portion of the valve down to the stem portion. After the valve has cooled and solidified it is removed.
Molding material is injected slowly into the die cavity by an injection molding apparatus connected to the mold. As the molding material is slowly injected into the cavity, the material moves up through the die cavity forcing air out of a vented portion on the mold until the die cavity is filled. A temperature control plate, connected to the mold adjacent the first portion of the die cavity, allows a cooling fluid to circulate therein in order to permit controlled cooling of the part from the head portion of the valve down to the stem portion. After the valve has cooled and solidified it is removed.
Description
88-152 -~- 2~3~3~
METHOD AND APPARATUS FOR INJECTION MOLDING
CERAMIC VALVES
Backqround of the Invention The present invention gellerally relates to a method and app~ratus for fabricating ceramic parts and, more particularly, to a method and apparatus for fabricating ceramic engine valves through the use of a low pressure injection molding technique.
It is well known that engine valves have been made from special steels. Valves formed of special steels, however, have exhibited shortcomings with regard to resistance to corrosion, heat and wear.
In view of the disadvantages of steel engine va1ves, valves madP from ceramic materials have been proposed for use in engines. Ceramic materials have been proposed as a material for engine valves because such materials are light weight and exhibit good resistance to heat, corrosion and wear. U.S. Patent No. 4,359,022 discloses a method or making a valve havin~ a stem portion and a top or head portion from a ceramic material. The stem portion is made from a silicon nitride powder mixture which is sintered by hot pressing in an atmosphere of nitrogen gas. The top portion is molded from a silicon nitride powder mi~ture and subsequently joined integrally with the stem portion by hot pressing.
U.S. Patent No. 4,696,777 discloses a method for making a valve having a head portion and a stem portion.
The head portion is first molded from a ceramic powder material and subsequently dried. A joining surface on the head portion is next contacted with a mass of ceramic powder material which forms the stem portion of the ,, , . : ~
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valve. The mass of ceramic material from which the stem portion is formed is held adjacent the head portion within a rubber mold. Once assembled, the head portion and the stem portion are subjected to static pressure for obtaining an integrally molded valve. The valve is subsequently fired to complete the process.
While both of the above noted methods teach forming valves from ceramic materials, a simpler method is desired whereby a valve or other ceramic part may be formed as a unitary element rather than as two separate elements which must subsequently be joined together.
Methods for forming ceramic parts by employing an injection molding technique have also been employed in the prior art. Such methods, however, have not been fully successful because the parts, after being injected into a mold, have tended to shrink away from the walls of the mold as they have cooled, resulting in parts not being formed to near-net-shape. Further, the parts have developed shrink voids or cracks internally as they have cooled.
Accordingly, there is a need for an improved method and apparatus for forming a unitary part, such as an engine valve, by an injection molding technique to a near-net-shape while preventing shrink voids from forming 25 therein. `~
Summar~ of the Invention The present invention sets forth an improved method and apparatus for fabricating ceramic parts, such as engine valves, from a ceramic-containing molding material by using a low pressure injection molding technique. A ceramic-containing molding material is first 2~33235 88-15~ -3-melted and subjected to a vacuum prior to being injected into the base of a die cavity contained within a heated mold. As the molding material is injected, the material moves up through the die cavity forcing air out of a vented portion of the mold at the top of the die cavity.
The part is then cooled under pressure through the use of a water cooled top plate which provides slow controlled cooling of the part from the head portion of the valve to the stem portion. This controlled cooling, while maintaining the ceramic molding material under pressure, results in a valve being formed to a near-net-shape and prevents shrink voids from forming in the valve. The method and apparatus of the present invention are also useful in the fabrication of other ceramic pa~ts using low pressure injection molding.
In accordance with one aspact o~ the present invention, a method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein is provided and includes the steps of:
injecting ceramic molding material into the die caYity having first and second portions at the first location on the mold until the die cavity is filled to form the ceramic part; venting gases from the die cavity as the ceramic material is injected; cooling the ceramic molding material for a time sufficient to solidify the ceramic molding material, the cooling step including the step of accelerating the cooling of the mold at the second location thereon to accelerate the cooling of the ceramic material from the first portion of the die cavity to the second portion thereof while maintaining pressure on the ceramic material in the die cavity through the mold at the first location thereon for a time sufficient to ~ ' 21~3~3~
substantially solidify the ceramic material; and removing the solidified ceramic part from the mold.
The ceramic part preferably is an automobile engine valve and the die cavity is formed with the head portion of the valve at the first portion of the cavity while the stem portion of the valve is formed at the second portion of the cavity. Further, the gas~s vented from the die cavity as the ceramic material is injected are preferably vented adjacent to the first portion of the die cavit~.
The step of accelerating the cooling of the mold is preferably performed by circulating a cooling fluid, such as water, through a temperature control plate on the mold. The temperature control plate is preferably located at the second location on the mold. The second location on the mold is located adjacent the first portion of the die cavity where the head of the valve is formed.
The method further comprises the steps of heating the mold at the first location thereon and heating the mold at the second location thereon prior to injecting the ceramic molding material into the die cavity. These heating steps serve to enhance the flow of the ceramic molding material as it is injected into the die cavity.
During the step of accelerating the cooling of 25 the mold the pressure is maintained on the ceramic ~ -material in the die cavity through the mold at the first location thereon by way of further molding material. As a result, a portion of the further molding material is allowed to enter into the die cavity as the molding material in the die cavity cools.
The step of heating the mold at the first location thereon may continue during the step of - 2~3323~
88-152 _5_ accelerating the cooling of the mold at the second location thereon in order to enhance the flow of the portion of further molding material as it enters into the die cavity as the molding material in the die cavity cools.
The step of cooling the ceramic material further includes the step of discontinuing the heating of the mold at the first location thereon after the ceramic material has substantially solidified in order that the ceramic material may completely solidify.
In accordance with another aspect of the present invention, a method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein is provided and comprises the steps ~f: inj~cting ceramic molding material into the die cavity having first and second portions at the first location on the mold until the die cavity is filled to form the ceramic part: venting gases from the die cavity as the ceramic material is injected; cooling said ceramic molding material for a time sufficient to solidify said molding material, said cooling step including the step of accelerating the cooling of the ceramic material from the first portion of the die cavity to the second portion thereof while maintaining pressure on the ceramic material in the die cavity through the mold at the first location thereon for a time sufficient to substantially solidify the ceramic material; and removing the solidified ceramic ~ !:
part from the mold.
Thè step of accelerating the cooling of the .
ceramic material from the first portion of the die cavity 30 to the second portion thereof is preferably performed by ~ ;
cooling the mold at the second location on the mold. The second location on the mold is adjacent the first portion : ' 2~3'~3~
of the die cavity where the head of the valve is formed.
In accordance with a further aspect of the present invention, a molding apparatus for forming a ceramic part is provided and comprises: a mold having a first location, a second location and a die cavity therein, the die cavity having a first portion and a second portion; means for injecting a ceramic molding material into the die cavity at the first location on the mold; means located on the mold for venting gases from the 10 die cavity as the ceramic material is injected into the -die cavity; and means for controlling the temperature of the mold at the second location on the mold, the temperature controlling means cooling the mold at the second location on the mold to accelerate the cooling of ~
the ceramic material from the first portion of the die cavity to the second portion thereof to substantially solidify the ceramic material.
The die cavity iS preferably shaped li~e an automobile engine valve. The die cavity iS formed with a head portion of the valve at the first portion of the die cavity and a stem portion of the valve at the second portion of the die cavity.
The venting means preferably comprises at least one venting washer. The venting means is preferably located adjacent the first portion of the die cavity.
The temperature controlling means preferably comprises means for circulating a fluid through the mold at the second location on the mold. The circulating means comprises a plate having ducts therein through which the fluid is circulated. The second location on the mold is preferably adjacent the first portion the die cavity where the head of the valve is formed.
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The temperature controlling means further comprises means for changing the temperature of the fluid ` circulating through the mold so that a heated fluid is permitted to circulate through the mold prior to injecting the molding material into the die cavity and a cooling fluid is permitted to circulate through the mold after the molding material has been injected into the die cavity.
The molding apparatus further comprises means for heating the mold at the first location thereon for enhancing the flow of the molding material as it is injected into the die cavity.
Accordingly, it is an object of this invention to provide a method and apparatus for making a part, such as an engine valve, from a ceramic material whereby the part may be formed as a unitary element. It is also an object of this invention to provide a method and apparatus for molding a high quality ceramic engine valve to a near-net-shape while preventing shrink voids from forming therein. Other obiects and advantages will be apparent 20 from the following description, the accompanying drawings ` `, and the appended claims.
Brief Description of the Drawinas Fig. 1 is a schematic side view of a conventional injection molding apparatus having a mold of this invention attached thereto;
Fig. 2 is an exploded perspective view of the mold of Fig. l;
Fig. 3 is a schematic side view of the mold of Fig. l; and Fig. 4 is a perspective view of a valve made by the method and apparatus of this invention.
-g 2~3~3c3 Detailed Description of the Preferred Embodiment The mold of the present invention, generally designated by the reference numeral 10, is shown in Fig. 1 attached to a conventional injection molding machine 20.
5 The conventional injection molding machine 20 may comprise any known low pressure injection molding machine capable of injecting a ceramic-containing molding material into a mold. For purposes of illustration only, a commercially available low pressure air injection molding machine 20 is 10 shown and comprises a frame 22 supporting a heated mixing pot 24 which holds molding material 23 to be injected into the mold 10. The mi~ing pot 24 has located on an upper portion thereof a cover 26 which is capable of clamping onto the mixing pot 24 in an air-tight manner. The cover 15 26 has a stirring blade 28 connected thereto which e~tends ~;
down into the mixing pot 24. The stirring blade 28 has a motor (not shown) drivenly connected thereto for ratating the stirring blade 28 within the mixing pot 24. The stirring blade 28 serves to mix the molding material 23 20 within the mixing pot 24 in order to allow air bubbles within the molding material 23 to escape therefrom. Also, by mixing the molding material 23 within the mixing pot 24, the molding material's ability to subsequently flow into the mold 10 is enhanced.
A connecting line 32, attached to cover 26, permits the mi~ing pot 24 to communicate with a low pressure air source 31 and a vacuum pump 34. The low pressure air source 31 may comprise either a portable compressor or an in-house low pressure air system. The vacuum pump 34, as shown in Fig. 1, is driven by a motor 35 and coupled thereto by a belt 37.
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88-152 _9_ Connecting line 32 is connected at an end away from the mixing pot 24 to a control valve 30, such as an electronically controlled solenoid valve, which controls communication with the mixing pot 24 between the low s pressure air source 31 and the vacuum pump 34. The control valve 30 is connected to the vacuum pump 34 by a vacuum connecting line 36 and is connected to the low pressure air source 31 by a low pressure air connecting line 18.
The valve 30 is controlled either to permit low pressure air to enter into the mixing pot 24 for injecting the molding material 23 into the mold 10 or to allow the vacuum pump 34 to exhaust the air contained within the -mi~ing pot 24 to help withdraw air bubbles from the molding material 23.
The mold 10 communicates with the mixing pot 24 through an injection line 38 and a control valve 40, such as an electro-pneumatic control valve. The control valve 40 is installea in injection line 38 to control the movement of the molding material 23 into the mold lO. The injection line 38 comprises a plurality of injection tubes 41 connected to one another by couplings 39. The control ~ .
valYe 40, when closed, serves to prevent air from entering into the injection line 38 by way of the mold 10 while the vacuum pump 34 is exhausting air from the mixing pot 24.
25 As will be discussed in more detail below, when the ~ -control valve 40 is closed it may also serve to permit the molding material 23 to build-up pressure at the valve 40 while low pressure air is allowed to enter into the mixing pot 24 by the control valve 30. After the molding `30 material 23 is allowed to build-up pressure at the valve `:~
40, a surge of molding material 23 will enter into the ~;
mold 10 when the valve 40 is opened.
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The mold 10 may be secured onto a heated platen 52 which is connected to the injection line 38. Two threaded members 78, only one of which is shown in Fig. 1, are located on opposite sides of the platen 52. The two members 78 threadedly engage within a support beam 79 in order to adjustably support the beam 79. The support beam 79 carries a piston-cylinder unit 77 thereon which is capable of pressing down onto the mold 10 in order to clamp the mold 10 onto the platen 52.
The mold 10, as best sho~n in Figs. 2 and 3, comprises a main body portion 42 having a die cavity 44 therein which is shaped like an engine valve. The die caYity 44 is formed with a head section of the valve at a first head portion 46 of the cavity 44 and a stem section of the valve at a second stem portion 48 of the cavity 44. The mold 10 is heated at a first location 50 thereon by way of the heated platen 52 upon which it is secured.
The mold 10 is also heated at a second location 58 thereon by way of a temperature control plate 54. By heating the 2~ mold 10, the molding material 23 is permlttea to more easily flow into the die cavity 44, thereby insuring that the entire die cavity 44 becomes completely filled with the molding material 23.
The temperature control plate 54 has ducts 56 therein and is connected to the mold 10 at the second location 58 thereon. The temperature control plate 54 is fastened onto the mold 10 by suitable means such as a plurality of fastener bolts 60 which extend through openings 62 in the control plate 54 and threadedly engage into openings 64 in the main body portion 42~ Bolts 60 also extend through washers 70, which are located between the second location 58 on the mold and the control plate 2~33~3~
54. Washers 70 separate the main body portion 42 from control plate 54 and permit a vented gap 68 to be formed in the mold 10 fo-r venting gases therefrom when the molding material is injected into the mold 10.
A fluid, such as water, :is circulated through the ducts 56 in order to permit controlled heating or cooling of the mold 10 at its second location 58. The fluid may be circulated through the ducts 56 by a commercially available fluid source 80 having a temperature control mechanism associated therewith for controlling and changing the temperature of the fluid. The fluid source 80 is connected to opposite ends of each of the ducts 56 by a plurality of fluid lines 66.
The motor associated with the stirring blade 28 and the control valves 30 and 40 on injection moldin~
machine 20 may be controlled manually by an operator by ~`
way of switches 71 located on a control panel 73.
Alternatively, a processor (not shown) may be employed to provide automatic control of the motor associated with stirring blade 28 and the control valves 30 and 40.
The method employed by the present invention for molding a valve 75 having stem section 74 and a head section 76, as shown in Fig.4, from a ceramic-containing molding material will now be explained. The ;
ceramic-containing molding material 23 may comprise, for e~ample, a composition including a silicon carbide ceramic powder and a number of conventional additives such as wax, zinc stearate, boron and carbon black. The zinc stearate is typically added to enhance the flow of the molding 30 material 23 as it is injected into the die cavity 44. The :
wax is added to act as a binder for the composition. When the part is made from a silicon carbide composition, it ' . ' ,:, 2~3323~
must go through a partial binder removal step and a sintering step after being removed from the mold 10. The boron and carbon black are added as sintering aids for the sintering step.
Alternatively, the molding material 23 may comprise a composition including a silicon nitride ceramic powder, wax, iron oxide and zinc stearate. The zinc stearate functions to enhance the flow of the molding material 23 as it is injected into the die cavity 44 while the wax acts as a binder for the composition. When the part is made from a silicon nitride composition, it must go through a binder burnout step and a nitriding step after being removed from the mold 10. The iron oxide is conventionally added as an aid for the nitriding step.
The ceramic-containing molding material 23 is first placed within the heated mixing pot 24 of the injection molding machine 20 and the cover 26 is locked thereto in an air-tight manner. The control valve 30 is then operated to permit the vacuum pump 34 to e~haust air from the mixing pot 24 as the molding material is heated and melted. While the vacuum pump 34 is extracting air from the mixing pot 24, the stirring blade 28 is actuated to stir the molding material 23 in order to permit any air bubbles contained in the molding material 23 to escape therefrom and be e2hausted from the mixing pot 24 by the vacuum pump 34. Also, by stirring the molding material 23, its ability to subsequently flow into the mold 10 is enhanced.
After the molding material 23 is substantially free of all air bubbles, the stirring blade 28 is stopped and the valve 30 is subsequently operated to close off the mixing pot 24 from the vacuum source 34. The rotatlon of the stirring blade 28 is stopped before the vacuum source 2~33~3~
is closed off from the mixing pot 24 in order to prevent any air from being mixed into the molding material 23.
The valve 30 is then operated to permit low pressure air .:
to enter into the mixing pot 24 through the line 32. The low pressure air which enters into the mixing pot 24 applies pressure to the molding material 23, thereby forcing the molding material 23 out of the mi~ing pot 24, through line 38 and into mold 10.
The molding material 23 must be able to flow in a slow, but steady manner into the mold 10 in order to completely force all of the air within the die cavity 44 out of the vented gap 68. Not all molding materials, however, are capable of flowing easily into the mold 10.
Thus, in order to aid the flow of such a molding material into the mold 10, the control valve 40 may be left in a closed position for a short time as the low pressure air is initially applied to the mixing pot 24 in order to permit the molding material 23 to build-up pressure at the ~ .
valve 40. After a pressure build-up occurs, the control valve 40 can then be opened resulting in a surge of molding material 23 flowing into the die cavity 44. If ~he molding material is capable of flowing easily into the mold 10, due to its inherent physical properties, then a pressure build-up at the valve 40 is not required. ~ .
The mold 10 is initially heated at:its first and second locations 50 and 58, respectively, prior to .
injecting the ceramic molding material 23 into the die :
cavity 44. Heating the mold 10 serves to enhance the ~low of the ceramic molding material 23 as it is injected into ~ ~.
the die cavity 44. The mold 10 is heated at its first location 50 by way of the heated platen 52 and is heated at its second location 58 by way of heated water ~3233 circulating through ducts 56 within plate 54. After the molding material 23 has been injected into the die cavity 44, the mold 10 remains heated only at its first location 50. As will be discussed in more detail below, at this point in the process, the mold 10 is cooled at its second location 58.
As the molding material 23 is injected into the die cavity 44, the material 23 moves slowly up through the die cavity 44 forcins the air therein out of the vented lo gap 68 of the mold lo. The moldins material is permitted to flow into the die cavity 44 until both the stem portion 48 and the head portion 46 have been filled with the molding material 23. After the die cavity 44 has been filled with the molding material 23, pressure is maintained on the molding material 23 until the part has substantially cooled and solidified. Since the mold 10 remains heated at its first location 50 and pressure is maintained on the molding material ~3 until the part has substantially cooled, extra molding material is allowed to move into the die cavity 44 as the in;ected material within the die cavity 44 begins to cool and contract therein. This insures that the part is molded to a near-net-shape, free of any shrink voids therein.
As mentioned above, once the molding material 23 has been injected into the die cavity 44, the part is allowed to cool until it has solidified. The cooling of the part may be accelerated by circulating the fluid associated with the fluid source 80, now cooled by the source 80, through the ducts 56 until the part has at least substantially solidified. The rate of cooling of the part may be controlled by adjusting the temperature ; `
control mechanism associated with the fluid source 80 to ~3~23~
change the temperature of the fluid traveling through the cooling ducts 56. Since the temperature control plate 54 is located at the second location 58 on the mold, controlled cooling of the part proceeds from the head portion 46 of the die cavity 44 down to the stem portion 48 until the entire part has at least substantially solidified. After the molding material 23 has substantially solidified, the mold 10 may then be removed from the heated platen 52 and allowed to stand until the molding material 23 has completely solidified.
Alternatively, the heated platen 52 may be turned off and the mold may be left attached to the platen 52, with or without cooling fluid circulating through ducts 56, until the part has completely solidified. ~fter the part has completely solidified, the ceramic part is then removed from the mold 10.
After the part has been removed from the mold 10, i~ must proceed through various ~inal steps before it is completed. For example, if the moldi~g material 23 comprises a silicon carbide composition, as discussed above, the part will proceed through a partial binder removal step, a sintering step and a surface finishing step~ Alternatively, if the molding material 23 comprises a silicon nitride composition, the part will proceed through a binder burnout step, a nitriding step and a surface finishing step.
Having thus described the method and apparatus for making an engine valve of this invention in detail and ~ ' by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention. For example, although the molded article is disclosed as being an engine valve, it will be appreciated that any other ' : ` '-, :.:' ` `. J~ ,~, . - ' `": , ,: , ` .. . .
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article having a shape and size different from that of an engine valve may be molded by the method and apparatus of this invention. Also, while the molding material was described as comprising either a silicon carbide composition or a silicon nitride composition, it will be appreciated that the moldin~ material may comprise any other molding composition capable of being molding in the manner set forth herein.
: : , . , i ;: ,, , . ., . ,.. ,:,:; .,, . , ., .,,:,, . ~. : :. : .
METHOD AND APPARATUS FOR INJECTION MOLDING
CERAMIC VALVES
Backqround of the Invention The present invention gellerally relates to a method and app~ratus for fabricating ceramic parts and, more particularly, to a method and apparatus for fabricating ceramic engine valves through the use of a low pressure injection molding technique.
It is well known that engine valves have been made from special steels. Valves formed of special steels, however, have exhibited shortcomings with regard to resistance to corrosion, heat and wear.
In view of the disadvantages of steel engine va1ves, valves madP from ceramic materials have been proposed for use in engines. Ceramic materials have been proposed as a material for engine valves because such materials are light weight and exhibit good resistance to heat, corrosion and wear. U.S. Patent No. 4,359,022 discloses a method or making a valve havin~ a stem portion and a top or head portion from a ceramic material. The stem portion is made from a silicon nitride powder mixture which is sintered by hot pressing in an atmosphere of nitrogen gas. The top portion is molded from a silicon nitride powder mi~ture and subsequently joined integrally with the stem portion by hot pressing.
U.S. Patent No. 4,696,777 discloses a method for making a valve having a head portion and a stem portion.
The head portion is first molded from a ceramic powder material and subsequently dried. A joining surface on the head portion is next contacted with a mass of ceramic powder material which forms the stem portion of the ,, , . : ~
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valve. The mass of ceramic material from which the stem portion is formed is held adjacent the head portion within a rubber mold. Once assembled, the head portion and the stem portion are subjected to static pressure for obtaining an integrally molded valve. The valve is subsequently fired to complete the process.
While both of the above noted methods teach forming valves from ceramic materials, a simpler method is desired whereby a valve or other ceramic part may be formed as a unitary element rather than as two separate elements which must subsequently be joined together.
Methods for forming ceramic parts by employing an injection molding technique have also been employed in the prior art. Such methods, however, have not been fully successful because the parts, after being injected into a mold, have tended to shrink away from the walls of the mold as they have cooled, resulting in parts not being formed to near-net-shape. Further, the parts have developed shrink voids or cracks internally as they have cooled.
Accordingly, there is a need for an improved method and apparatus for forming a unitary part, such as an engine valve, by an injection molding technique to a near-net-shape while preventing shrink voids from forming 25 therein. `~
Summar~ of the Invention The present invention sets forth an improved method and apparatus for fabricating ceramic parts, such as engine valves, from a ceramic-containing molding material by using a low pressure injection molding technique. A ceramic-containing molding material is first 2~33235 88-15~ -3-melted and subjected to a vacuum prior to being injected into the base of a die cavity contained within a heated mold. As the molding material is injected, the material moves up through the die cavity forcing air out of a vented portion of the mold at the top of the die cavity.
The part is then cooled under pressure through the use of a water cooled top plate which provides slow controlled cooling of the part from the head portion of the valve to the stem portion. This controlled cooling, while maintaining the ceramic molding material under pressure, results in a valve being formed to a near-net-shape and prevents shrink voids from forming in the valve. The method and apparatus of the present invention are also useful in the fabrication of other ceramic pa~ts using low pressure injection molding.
In accordance with one aspact o~ the present invention, a method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein is provided and includes the steps of:
injecting ceramic molding material into the die caYity having first and second portions at the first location on the mold until the die cavity is filled to form the ceramic part; venting gases from the die cavity as the ceramic material is injected; cooling the ceramic molding material for a time sufficient to solidify the ceramic molding material, the cooling step including the step of accelerating the cooling of the mold at the second location thereon to accelerate the cooling of the ceramic material from the first portion of the die cavity to the second portion thereof while maintaining pressure on the ceramic material in the die cavity through the mold at the first location thereon for a time sufficient to ~ ' 21~3~3~
substantially solidify the ceramic material; and removing the solidified ceramic part from the mold.
The ceramic part preferably is an automobile engine valve and the die cavity is formed with the head portion of the valve at the first portion of the cavity while the stem portion of the valve is formed at the second portion of the cavity. Further, the gas~s vented from the die cavity as the ceramic material is injected are preferably vented adjacent to the first portion of the die cavit~.
The step of accelerating the cooling of the mold is preferably performed by circulating a cooling fluid, such as water, through a temperature control plate on the mold. The temperature control plate is preferably located at the second location on the mold. The second location on the mold is located adjacent the first portion of the die cavity where the head of the valve is formed.
The method further comprises the steps of heating the mold at the first location thereon and heating the mold at the second location thereon prior to injecting the ceramic molding material into the die cavity. These heating steps serve to enhance the flow of the ceramic molding material as it is injected into the die cavity.
During the step of accelerating the cooling of 25 the mold the pressure is maintained on the ceramic ~ -material in the die cavity through the mold at the first location thereon by way of further molding material. As a result, a portion of the further molding material is allowed to enter into the die cavity as the molding material in the die cavity cools.
The step of heating the mold at the first location thereon may continue during the step of - 2~3323~
88-152 _5_ accelerating the cooling of the mold at the second location thereon in order to enhance the flow of the portion of further molding material as it enters into the die cavity as the molding material in the die cavity cools.
The step of cooling the ceramic material further includes the step of discontinuing the heating of the mold at the first location thereon after the ceramic material has substantially solidified in order that the ceramic material may completely solidify.
In accordance with another aspect of the present invention, a method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein is provided and comprises the steps ~f: inj~cting ceramic molding material into the die cavity having first and second portions at the first location on the mold until the die cavity is filled to form the ceramic part: venting gases from the die cavity as the ceramic material is injected; cooling said ceramic molding material for a time sufficient to solidify said molding material, said cooling step including the step of accelerating the cooling of the ceramic material from the first portion of the die cavity to the second portion thereof while maintaining pressure on the ceramic material in the die cavity through the mold at the first location thereon for a time sufficient to substantially solidify the ceramic material; and removing the solidified ceramic ~ !:
part from the mold.
Thè step of accelerating the cooling of the .
ceramic material from the first portion of the die cavity 30 to the second portion thereof is preferably performed by ~ ;
cooling the mold at the second location on the mold. The second location on the mold is adjacent the first portion : ' 2~3'~3~
of the die cavity where the head of the valve is formed.
In accordance with a further aspect of the present invention, a molding apparatus for forming a ceramic part is provided and comprises: a mold having a first location, a second location and a die cavity therein, the die cavity having a first portion and a second portion; means for injecting a ceramic molding material into the die cavity at the first location on the mold; means located on the mold for venting gases from the 10 die cavity as the ceramic material is injected into the -die cavity; and means for controlling the temperature of the mold at the second location on the mold, the temperature controlling means cooling the mold at the second location on the mold to accelerate the cooling of ~
the ceramic material from the first portion of the die cavity to the second portion thereof to substantially solidify the ceramic material.
The die cavity iS preferably shaped li~e an automobile engine valve. The die cavity iS formed with a head portion of the valve at the first portion of the die cavity and a stem portion of the valve at the second portion of the die cavity.
The venting means preferably comprises at least one venting washer. The venting means is preferably located adjacent the first portion of the die cavity.
The temperature controlling means preferably comprises means for circulating a fluid through the mold at the second location on the mold. The circulating means comprises a plate having ducts therein through which the fluid is circulated. The second location on the mold is preferably adjacent the first portion the die cavity where the head of the valve is formed.
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The temperature controlling means further comprises means for changing the temperature of the fluid ` circulating through the mold so that a heated fluid is permitted to circulate through the mold prior to injecting the molding material into the die cavity and a cooling fluid is permitted to circulate through the mold after the molding material has been injected into the die cavity.
The molding apparatus further comprises means for heating the mold at the first location thereon for enhancing the flow of the molding material as it is injected into the die cavity.
Accordingly, it is an object of this invention to provide a method and apparatus for making a part, such as an engine valve, from a ceramic material whereby the part may be formed as a unitary element. It is also an object of this invention to provide a method and apparatus for molding a high quality ceramic engine valve to a near-net-shape while preventing shrink voids from forming therein. Other obiects and advantages will be apparent 20 from the following description, the accompanying drawings ` `, and the appended claims.
Brief Description of the Drawinas Fig. 1 is a schematic side view of a conventional injection molding apparatus having a mold of this invention attached thereto;
Fig. 2 is an exploded perspective view of the mold of Fig. l;
Fig. 3 is a schematic side view of the mold of Fig. l; and Fig. 4 is a perspective view of a valve made by the method and apparatus of this invention.
-g 2~3~3c3 Detailed Description of the Preferred Embodiment The mold of the present invention, generally designated by the reference numeral 10, is shown in Fig. 1 attached to a conventional injection molding machine 20.
5 The conventional injection molding machine 20 may comprise any known low pressure injection molding machine capable of injecting a ceramic-containing molding material into a mold. For purposes of illustration only, a commercially available low pressure air injection molding machine 20 is 10 shown and comprises a frame 22 supporting a heated mixing pot 24 which holds molding material 23 to be injected into the mold 10. The mi~ing pot 24 has located on an upper portion thereof a cover 26 which is capable of clamping onto the mixing pot 24 in an air-tight manner. The cover 15 26 has a stirring blade 28 connected thereto which e~tends ~;
down into the mixing pot 24. The stirring blade 28 has a motor (not shown) drivenly connected thereto for ratating the stirring blade 28 within the mixing pot 24. The stirring blade 28 serves to mix the molding material 23 20 within the mixing pot 24 in order to allow air bubbles within the molding material 23 to escape therefrom. Also, by mixing the molding material 23 within the mixing pot 24, the molding material's ability to subsequently flow into the mold 10 is enhanced.
A connecting line 32, attached to cover 26, permits the mi~ing pot 24 to communicate with a low pressure air source 31 and a vacuum pump 34. The low pressure air source 31 may comprise either a portable compressor or an in-house low pressure air system. The vacuum pump 34, as shown in Fig. 1, is driven by a motor 35 and coupled thereto by a belt 37.
332~
88-152 _9_ Connecting line 32 is connected at an end away from the mixing pot 24 to a control valve 30, such as an electronically controlled solenoid valve, which controls communication with the mixing pot 24 between the low s pressure air source 31 and the vacuum pump 34. The control valve 30 is connected to the vacuum pump 34 by a vacuum connecting line 36 and is connected to the low pressure air source 31 by a low pressure air connecting line 18.
The valve 30 is controlled either to permit low pressure air to enter into the mixing pot 24 for injecting the molding material 23 into the mold 10 or to allow the vacuum pump 34 to exhaust the air contained within the -mi~ing pot 24 to help withdraw air bubbles from the molding material 23.
The mold 10 communicates with the mixing pot 24 through an injection line 38 and a control valve 40, such as an electro-pneumatic control valve. The control valve 40 is installea in injection line 38 to control the movement of the molding material 23 into the mold lO. The injection line 38 comprises a plurality of injection tubes 41 connected to one another by couplings 39. The control ~ .
valYe 40, when closed, serves to prevent air from entering into the injection line 38 by way of the mold 10 while the vacuum pump 34 is exhausting air from the mixing pot 24.
25 As will be discussed in more detail below, when the ~ -control valve 40 is closed it may also serve to permit the molding material 23 to build-up pressure at the valve 40 while low pressure air is allowed to enter into the mixing pot 24 by the control valve 30. After the molding `30 material 23 is allowed to build-up pressure at the valve `:~
40, a surge of molding material 23 will enter into the ~;
mold 10 when the valve 40 is opened.
-` 2~33~3~
The mold 10 may be secured onto a heated platen 52 which is connected to the injection line 38. Two threaded members 78, only one of which is shown in Fig. 1, are located on opposite sides of the platen 52. The two members 78 threadedly engage within a support beam 79 in order to adjustably support the beam 79. The support beam 79 carries a piston-cylinder unit 77 thereon which is capable of pressing down onto the mold 10 in order to clamp the mold 10 onto the platen 52.
The mold 10, as best sho~n in Figs. 2 and 3, comprises a main body portion 42 having a die cavity 44 therein which is shaped like an engine valve. The die caYity 44 is formed with a head section of the valve at a first head portion 46 of the cavity 44 and a stem section of the valve at a second stem portion 48 of the cavity 44. The mold 10 is heated at a first location 50 thereon by way of the heated platen 52 upon which it is secured.
The mold 10 is also heated at a second location 58 thereon by way of a temperature control plate 54. By heating the 2~ mold 10, the molding material 23 is permlttea to more easily flow into the die cavity 44, thereby insuring that the entire die cavity 44 becomes completely filled with the molding material 23.
The temperature control plate 54 has ducts 56 therein and is connected to the mold 10 at the second location 58 thereon. The temperature control plate 54 is fastened onto the mold 10 by suitable means such as a plurality of fastener bolts 60 which extend through openings 62 in the control plate 54 and threadedly engage into openings 64 in the main body portion 42~ Bolts 60 also extend through washers 70, which are located between the second location 58 on the mold and the control plate 2~33~3~
54. Washers 70 separate the main body portion 42 from control plate 54 and permit a vented gap 68 to be formed in the mold 10 fo-r venting gases therefrom when the molding material is injected into the mold 10.
A fluid, such as water, :is circulated through the ducts 56 in order to permit controlled heating or cooling of the mold 10 at its second location 58. The fluid may be circulated through the ducts 56 by a commercially available fluid source 80 having a temperature control mechanism associated therewith for controlling and changing the temperature of the fluid. The fluid source 80 is connected to opposite ends of each of the ducts 56 by a plurality of fluid lines 66.
The motor associated with the stirring blade 28 and the control valves 30 and 40 on injection moldin~
machine 20 may be controlled manually by an operator by ~`
way of switches 71 located on a control panel 73.
Alternatively, a processor (not shown) may be employed to provide automatic control of the motor associated with stirring blade 28 and the control valves 30 and 40.
The method employed by the present invention for molding a valve 75 having stem section 74 and a head section 76, as shown in Fig.4, from a ceramic-containing molding material will now be explained. The ;
ceramic-containing molding material 23 may comprise, for e~ample, a composition including a silicon carbide ceramic powder and a number of conventional additives such as wax, zinc stearate, boron and carbon black. The zinc stearate is typically added to enhance the flow of the molding 30 material 23 as it is injected into the die cavity 44. The :
wax is added to act as a binder for the composition. When the part is made from a silicon carbide composition, it ' . ' ,:, 2~3323~
must go through a partial binder removal step and a sintering step after being removed from the mold 10. The boron and carbon black are added as sintering aids for the sintering step.
Alternatively, the molding material 23 may comprise a composition including a silicon nitride ceramic powder, wax, iron oxide and zinc stearate. The zinc stearate functions to enhance the flow of the molding material 23 as it is injected into the die cavity 44 while the wax acts as a binder for the composition. When the part is made from a silicon nitride composition, it must go through a binder burnout step and a nitriding step after being removed from the mold 10. The iron oxide is conventionally added as an aid for the nitriding step.
The ceramic-containing molding material 23 is first placed within the heated mixing pot 24 of the injection molding machine 20 and the cover 26 is locked thereto in an air-tight manner. The control valve 30 is then operated to permit the vacuum pump 34 to e~haust air from the mixing pot 24 as the molding material is heated and melted. While the vacuum pump 34 is extracting air from the mixing pot 24, the stirring blade 28 is actuated to stir the molding material 23 in order to permit any air bubbles contained in the molding material 23 to escape therefrom and be e2hausted from the mixing pot 24 by the vacuum pump 34. Also, by stirring the molding material 23, its ability to subsequently flow into the mold 10 is enhanced.
After the molding material 23 is substantially free of all air bubbles, the stirring blade 28 is stopped and the valve 30 is subsequently operated to close off the mixing pot 24 from the vacuum source 34. The rotatlon of the stirring blade 28 is stopped before the vacuum source 2~33~3~
is closed off from the mixing pot 24 in order to prevent any air from being mixed into the molding material 23.
The valve 30 is then operated to permit low pressure air .:
to enter into the mixing pot 24 through the line 32. The low pressure air which enters into the mixing pot 24 applies pressure to the molding material 23, thereby forcing the molding material 23 out of the mi~ing pot 24, through line 38 and into mold 10.
The molding material 23 must be able to flow in a slow, but steady manner into the mold 10 in order to completely force all of the air within the die cavity 44 out of the vented gap 68. Not all molding materials, however, are capable of flowing easily into the mold 10.
Thus, in order to aid the flow of such a molding material into the mold 10, the control valve 40 may be left in a closed position for a short time as the low pressure air is initially applied to the mixing pot 24 in order to permit the molding material 23 to build-up pressure at the ~ .
valve 40. After a pressure build-up occurs, the control valve 40 can then be opened resulting in a surge of molding material 23 flowing into the die cavity 44. If ~he molding material is capable of flowing easily into the mold 10, due to its inherent physical properties, then a pressure build-up at the valve 40 is not required. ~ .
The mold 10 is initially heated at:its first and second locations 50 and 58, respectively, prior to .
injecting the ceramic molding material 23 into the die :
cavity 44. Heating the mold 10 serves to enhance the ~low of the ceramic molding material 23 as it is injected into ~ ~.
the die cavity 44. The mold 10 is heated at its first location 50 by way of the heated platen 52 and is heated at its second location 58 by way of heated water ~3233 circulating through ducts 56 within plate 54. After the molding material 23 has been injected into the die cavity 44, the mold 10 remains heated only at its first location 50. As will be discussed in more detail below, at this point in the process, the mold 10 is cooled at its second location 58.
As the molding material 23 is injected into the die cavity 44, the material 23 moves slowly up through the die cavity 44 forcins the air therein out of the vented lo gap 68 of the mold lo. The moldins material is permitted to flow into the die cavity 44 until both the stem portion 48 and the head portion 46 have been filled with the molding material 23. After the die cavity 44 has been filled with the molding material 23, pressure is maintained on the molding material 23 until the part has substantially cooled and solidified. Since the mold 10 remains heated at its first location 50 and pressure is maintained on the molding material ~3 until the part has substantially cooled, extra molding material is allowed to move into the die cavity 44 as the in;ected material within the die cavity 44 begins to cool and contract therein. This insures that the part is molded to a near-net-shape, free of any shrink voids therein.
As mentioned above, once the molding material 23 has been injected into the die cavity 44, the part is allowed to cool until it has solidified. The cooling of the part may be accelerated by circulating the fluid associated with the fluid source 80, now cooled by the source 80, through the ducts 56 until the part has at least substantially solidified. The rate of cooling of the part may be controlled by adjusting the temperature ; `
control mechanism associated with the fluid source 80 to ~3~23~
change the temperature of the fluid traveling through the cooling ducts 56. Since the temperature control plate 54 is located at the second location 58 on the mold, controlled cooling of the part proceeds from the head portion 46 of the die cavity 44 down to the stem portion 48 until the entire part has at least substantially solidified. After the molding material 23 has substantially solidified, the mold 10 may then be removed from the heated platen 52 and allowed to stand until the molding material 23 has completely solidified.
Alternatively, the heated platen 52 may be turned off and the mold may be left attached to the platen 52, with or without cooling fluid circulating through ducts 56, until the part has completely solidified. ~fter the part has completely solidified, the ceramic part is then removed from the mold 10.
After the part has been removed from the mold 10, i~ must proceed through various ~inal steps before it is completed. For example, if the moldi~g material 23 comprises a silicon carbide composition, as discussed above, the part will proceed through a partial binder removal step, a sintering step and a surface finishing step~ Alternatively, if the molding material 23 comprises a silicon nitride composition, the part will proceed through a binder burnout step, a nitriding step and a surface finishing step.
Having thus described the method and apparatus for making an engine valve of this invention in detail and ~ ' by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention. For example, although the molded article is disclosed as being an engine valve, it will be appreciated that any other ' : ` '-, :.:' ` `. J~ ,~, . - ' `": , ,: , ` .. . .
~332~
article having a shape and size different from that of an engine valve may be molded by the method and apparatus of this invention. Also, while the molding material was described as comprising either a silicon carbide composition or a silicon nitride composition, it will be appreciated that the moldin~ material may comprise any other molding composition capable of being molding in the manner set forth herein.
: : , . , i ;: ,, , . ., . ,.. ,:,:; .,, . , ., .,,:,, . ~. : :. : .
Claims (24)
1. A method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein, comprising the steps of:
injecting ceramic molding material into said die cavity having first and second portions at said first location on said mold until said die cavity is filled to form said ceramic part;
venting gases from said die cavity as said ceramic material is injected;
cooling said ceramic molding material for a time sufficient to solidify said ceramic molding material, said cooling step including the step of accelerating the cooling of said mold at said second location thereon to accelerate the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof while maintaining pressure on said ceramic material in said die cavity through said mold at said first location thereon for a time sufficient to substantially solidify said ceramic material; and removing the solidified ceramic part from said mold.
injecting ceramic molding material into said die cavity having first and second portions at said first location on said mold until said die cavity is filled to form said ceramic part;
venting gases from said die cavity as said ceramic material is injected;
cooling said ceramic molding material for a time sufficient to solidify said ceramic molding material, said cooling step including the step of accelerating the cooling of said mold at said second location thereon to accelerate the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof while maintaining pressure on said ceramic material in said die cavity through said mold at said first location thereon for a time sufficient to substantially solidify said ceramic material; and removing the solidified ceramic part from said mold.
2. The method of claim 1, wherein said ceramic part is an automobile valve and said die cavity is formed with the head portion of said valve at said first portion of said cavity while the stem portion of said valve is formed at said second portion of said cavity.
3. The method of claim 1, wherein said gases are vented adjacent said first portion of said die cavity.
4. The method of claim 1, wherein said second location on said mold is adjacent said first portion of said die cavity.
5. The method of claim 1, wherein said step of accelerating the cooling of said mold comprises circulating a cooling fluid through a temperature control plate on said mold.
6. The method of claim 5, wherein said cooling fluid comprises water.
7. The method of claim 1, wherein said step of accelerating the cooling of said mold comprises circulating a cooling fluid through said mold at said second location.
8. The method of claim 7, wherein said cooling fluid comprises water.
9. The method of claim 1 further comprising the steps of heating said mold at said first location thereon and heating said mold at said second location thereon prior to injecting said ceramic molding material into said die cavity in order to enhance the flow of said ceramic molding material as it is injected into said die cavity.
10. The method of claim 9, wherein during said step of accelerating the cooling of said mold said pressure is maintained on said ceramic material in said die cavity through said mold at said first location thereon by further molding material, thereby resulting in a portion of said further molding material entering into said die cavity as said Molding material in said die cavity cools.
11. The method of claim 10, wherein said step of heating said mold at said first location thereon continues during said step of accelerating the cooling of said mold at said second location thereon in order to enhance the flow of said portion of said further molding material as it enters into said die cavity.
12. The method of claim 11 wherein said step of cooling said ceramic material further includes the step discontinuing the heating of said mold at said first location thereon after said ceramic material has substantially solidified in order that said ceramic material may completely solidify.
13. A method of injection molding a ceramic part in a mold having a first location, a second location and a die cavity therein, comprising the steps of:
injecting ceramic molding material into said die cavity having first and second portions at said first location on said mold until said die cavity is filled to form said ceramic part;
venting gases from said die cavity as said ceramic material is injected;
cooling said ceramic molding material for a time sufficient to solidify said ceramic molding material, said cooling step including the step of accelerating the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof while maintaining pressure on said ceramic material in said die cavity through said mold at said first location thereon for a time sufficient to substantially solidify said ceramic material; and removing the solidified ceramic part from said mold.
injecting ceramic molding material into said die cavity having first and second portions at said first location on said mold until said die cavity is filled to form said ceramic part;
venting gases from said die cavity as said ceramic material is injected;
cooling said ceramic molding material for a time sufficient to solidify said ceramic molding material, said cooling step including the step of accelerating the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof while maintaining pressure on said ceramic material in said die cavity through said mold at said first location thereon for a time sufficient to substantially solidify said ceramic material; and removing the solidified ceramic part from said mold.
14. The method of claim 13, wherein said accelerating step is performed by cooling said mold at said second location on said mold.
15. The method of claim 14, wherein said second location on said mold is adjacent said first portion of said die cavity.
16. A molding apparatus for forming a ceramic part, comprising:
a mold having a first location, a second location and a die cavity therein, said die cavity having a first portion and a second portion;
means for injecting a ceramic molding material into said die cavity at said first location on said mold;
means located on said mold for venting gases from said die cavity as said ceramic material is injected into said die cavity; and means for controlling the temperature of said mold at said second location on said mold, said temperature controlling means cooling said mold at said second location on said mold to accelerate the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof to substantially solidify said ceramic material.
a mold having a first location, a second location and a die cavity therein, said die cavity having a first portion and a second portion;
means for injecting a ceramic molding material into said die cavity at said first location on said mold;
means located on said mold for venting gases from said die cavity as said ceramic material is injected into said die cavity; and means for controlling the temperature of said mold at said second location on said mold, said temperature controlling means cooling said mold at said second location on said mold to accelerate the cooling of said ceramic material from said first portion of said die cavity to said second portion thereof to substantially solidify said ceramic material.
17. The molding apparatus as claimed in claim 16, wherein said die cavity is shaped like an automobile engine valve, said die cavity is formed with a head portion of said valve at said first portion of said die cavity and a stem portion of said valve at said second portion of said die cavity.
18. The molding apparatus as claimed in claim 16, wherein said venting means comprises at least one venting washer.
19. The molding apparatus as claimed in claim 16, wherein said venting means is located adjacent said first portion of said die cavity.
20. The molding apparatus as claimed in claim 16, wherein said second location on said mold is adjacent said first portion of said die cavity.
21. The molding apparatus as claimed in claim 16, wherein said temperature controlling means comprises means for circulating a fluid through said mold at said second location on said mold.
22. The molding apparatus as claimed in claim 21, wherein said circulating means comprises a plate having ducts therein through which said fluid is circulated.
23. The molding apparatus as claimed in claim 21, wherein said temperature controlling means further comprises means for changing the temperature of said fluid circulating through said mold so that a heated fluid is permitted to circulate through said mold prior to injecting said molding material into said die cavity and a cooling fluid is permitted to circulate through said mold after said molding material has been injected into said die cavity.
24. The molding apparatus as claimed in claim 16 further comprising means for heating said mold at said first location thereon for enhancing the flow of said molding material as it is injected into said die cavity.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US46611190A | 1990-01-16 | 1990-01-16 | |
| US466,111 | 1990-01-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2033235A1 true CA2033235A1 (en) | 1991-07-17 |
Family
ID=23850518
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2033235 Abandoned CA2033235A1 (en) | 1990-01-16 | 1990-12-27 | Method and apparatus for injection molding ceramic valves |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2033235A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2488842A (en) * | 2011-03-11 | 2012-09-12 | Osspray Ltd | Ceramic discharge tip for a dental abrasion device |
-
1990
- 1990-12-27 CA CA 2033235 patent/CA2033235A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2488842A (en) * | 2011-03-11 | 2012-09-12 | Osspray Ltd | Ceramic discharge tip for a dental abrasion device |
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