CA2350991A1 - Method and apparatus for injection molding - Google Patents
Method and apparatus for injection molding Download PDFInfo
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- CA2350991A1 CA2350991A1 CA 2350991 CA2350991A CA2350991A1 CA 2350991 A1 CA2350991 A1 CA 2350991A1 CA 2350991 CA2350991 CA 2350991 CA 2350991 A CA2350991 A CA 2350991A CA 2350991 A1 CA2350991 A1 CA 2350991A1
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Abstract
An injection molding method and device, and a controller for sequentially opening and closing valves in the injection molding device. The methods and devices are effective to reduce the clamping force needed to clamp multiple cavity molds.
Description
METHOD AND APPARATUS FOR INJECTION MOLDING
BACKGROUND
The present invention relates to a method and apparatus for injection molding, and in particular, to a method and apparatus for molding injection molded parts.
Conventionally, a variety of methods have been utilized for injection molding in various fields. Of these methods, a molding metal mold of a runnerless (hot runner) system has been widely used.
There are a variety of molds in such a hot runner system. For example, there is a valve gate system in which it is possible to move a needle pin forward and backward relative to a gate so as to open and close the gate mechanically.
In a valve gate system, it is possible to prevent a gate portion from causing problems such as gate stringiness and to enlarge the gate diameter so that the flow of a melted resin into a mold is smooth. Generally, the opening and closing control of the gate in the valve gate system is carried out such that an air cylinder or a hydraulic cylinder type actuator is included in the metal mold, and such an actuator is controlled in synchronism with a molding process. This synchronization is realized generally by a process of opening a valve in response to a mold clamping signal, starting a timer, and closing a gate when the timer counts the lapse of the predetermined time.
In another example, a valve is opened in response to a mold clamping completion signal from an injection molding machine, and a gate is closed in response to a completion signal of a holding pressure.
A conventional molding process uses two platens, a movable platen and a stationary platen. A hydraulic cylinder applies a certain force to push a movable platen against a stationary platen. Molding members within or attached to the platens form a molding cavity. The force is maintained on the stationary platen or die plate while a molten material is injected into the molding cavity. The molten material is injected into the cavity with an extrusion screw until the pressure inside the cavity reaches a predetermined molding pressure. After injecting the molten material, the molten material is allowed to cool and solidify, the force is then released, and the plates are separated and the process begins anew.
Some injection molding machines use a mold with only one cavity, thereby allowing for the production of one molded object per cycle. Total cycle time is the sum of the fill time and the cool down time. The cool down time is generally substantially longer than the fill time. For example, a typical fill time is about 5 seconds, whereas a typical cooling time is about 30 seconds, for a total of about 35 seconds for the production of one molded article.
To reduce process time per molded article, some injection molding machines utilize molds with a plurality of cavities for forming a plurality of molded articles. The molten material fills into each of the cavities simultaneously.
While this may double the fill time, for example, to about 8 seconds, the cooling time remains fixed at about 30 seconds. The total time of this process is about 38 seconds for the production of two molded articles. Thus, using multiple cavities increases the efficiency almost two-fold.
A problem with the multiple cavity method, however, is that the mold clamping force must also be doubled since the article molding area is doubled.
As a result, a larger injection molding machine must be used to apply the extra force needed to hold the molding platens together. A larger injection molding machine costs more, takes up more floor space, and requires more power. Therefore, using multiple cavity molds with the conventional method sacrifices cost for greater time efficiency.
BRIEF SUMMARY
Taking the foregoing conventional problems into consideration, an object of the present invention is to provide an injection molding method that does not require increasing the mold clamping force for molding articles in a multiple cavity mold, thereby providing the simultaneous benefits of both time and cost savings in the same process.
BACKGROUND
The present invention relates to a method and apparatus for injection molding, and in particular, to a method and apparatus for molding injection molded parts.
Conventionally, a variety of methods have been utilized for injection molding in various fields. Of these methods, a molding metal mold of a runnerless (hot runner) system has been widely used.
There are a variety of molds in such a hot runner system. For example, there is a valve gate system in which it is possible to move a needle pin forward and backward relative to a gate so as to open and close the gate mechanically.
In a valve gate system, it is possible to prevent a gate portion from causing problems such as gate stringiness and to enlarge the gate diameter so that the flow of a melted resin into a mold is smooth. Generally, the opening and closing control of the gate in the valve gate system is carried out such that an air cylinder or a hydraulic cylinder type actuator is included in the metal mold, and such an actuator is controlled in synchronism with a molding process. This synchronization is realized generally by a process of opening a valve in response to a mold clamping signal, starting a timer, and closing a gate when the timer counts the lapse of the predetermined time.
In another example, a valve is opened in response to a mold clamping completion signal from an injection molding machine, and a gate is closed in response to a completion signal of a holding pressure.
A conventional molding process uses two platens, a movable platen and a stationary platen. A hydraulic cylinder applies a certain force to push a movable platen against a stationary platen. Molding members within or attached to the platens form a molding cavity. The force is maintained on the stationary platen or die plate while a molten material is injected into the molding cavity. The molten material is injected into the cavity with an extrusion screw until the pressure inside the cavity reaches a predetermined molding pressure. After injecting the molten material, the molten material is allowed to cool and solidify, the force is then released, and the plates are separated and the process begins anew.
Some injection molding machines use a mold with only one cavity, thereby allowing for the production of one molded object per cycle. Total cycle time is the sum of the fill time and the cool down time. The cool down time is generally substantially longer than the fill time. For example, a typical fill time is about 5 seconds, whereas a typical cooling time is about 30 seconds, for a total of about 35 seconds for the production of one molded article.
To reduce process time per molded article, some injection molding machines utilize molds with a plurality of cavities for forming a plurality of molded articles. The molten material fills into each of the cavities simultaneously.
While this may double the fill time, for example, to about 8 seconds, the cooling time remains fixed at about 30 seconds. The total time of this process is about 38 seconds for the production of two molded articles. Thus, using multiple cavities increases the efficiency almost two-fold.
A problem with the multiple cavity method, however, is that the mold clamping force must also be doubled since the article molding area is doubled.
As a result, a larger injection molding machine must be used to apply the extra force needed to hold the molding platens together. A larger injection molding machine costs more, takes up more floor space, and requires more power. Therefore, using multiple cavity molds with the conventional method sacrifices cost for greater time efficiency.
BRIEF SUMMARY
Taking the foregoing conventional problems into consideration, an object of the present invention is to provide an injection molding method that does not require increasing the mold clamping force for molding articles in a multiple cavity mold, thereby providing the simultaneous benefits of both time and cost savings in the same process.
In one aspect, the invention is a method for sequentially injecting a molten material comprising clamping a stationary platen and a movable platen at a clamping force to define at least two cavities, opening a first valve gate to inject a molten material into a first cavity, closing the first valve gate when the pressure inside the first cavity reaches a set-point pressure, opening a second valve gate to inject the molten material into a second cavity, and closing the second valve gate when pressure inside the second cavity reaches a set-point pressure.
In a second aspect, the invention is an injection molding apparatus comprising a mold having at least two mold cavities, a molten material inlet system in communication with said at least two mold cavities, at least two valve gates in said molten material inlet, wherein each of said at least two valve gates are associated with one of said mold cavities; and a controller adapted to sequentially open and close said valve gates.
In a third aspect, the invention is a controller for use with a injection 1 S molding device having a mold with at least two cavities, the controller comprises means for opening a first valve gate associated with a first mold cavity to initiate a flow of molten material into the first mold cavity, means for closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure, means for opening a second valve gate associated with a second cavity to initiate a flow of molten material into a second mold cavity, and means for closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, the invention being defined only by the claims following this detailed description.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein:
FIG. 1 is a schematic view of an injection molding apparatus suitable for S the method of injection molding a molten material, provided by the present invention.
FIG. 2 is a schematic view of a part of the injection molding apparatus showing a state immediately after clamping the mold, and a state in which the introduction of the molten material is initiated, in the method of injection molding, provided by the present invention.
FIG. 3 is a flowchart illustration of the sequential injection molding process of the present invention.
FIG. 4 schematically shows a change in injection velocity with time for a conventional injection molding method and a sequential injection molding method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, the injection molding apparatus suitable for use in the method of injection-molding a thermoplastic or thermoset resin, provided by the present invention, will be outlined below with reference to FIG. 1. The injection molding apparatus includes an injection cylinder 12 having a resin-feeding screw 10 inside, a stationary platen 40, a movable platen 44, an inlet 26, valve gates 50, tie bars 34, a clamping hydraulic cylinder 30 and a hydraulic piston 32. The movable platen 44 is actuated with the hydraulic piston 32 in the hydraulic cylinder 30 to move in parallel on the tie bars 34.
A mold is formed by a stationary mold member 36 and a movable mold member 46. The stationary mold member 36 is attached to the stationary platen 40, and the movable mold member 46 is attached to the movable platen 44.
The platens 40, 44, the tie bars 34, and the cylinder 30 and piston 32 define a clamping system for applying a clamping pressure to the mold members 36, 46.
The movable platen 44 is moved towards the stationary platen 40 until the -S-movable mold member 46 is engaged with the stationary mold member 36, and the mold is clamped to form cavities 22, 24. This clamped position is illustrated in FIG. 2.
After the mold has been clamped, the clamping force is controlled with the clamping hydraulic cylinder 30. The clamping force may also be controlled by toggle or an electric machine. The molten material flows into the cavities 22, via an inlet 26. The valve gates 50, 52 face cavities 22, 24 and, in a preferred embodiment, there is a valve gate associated with each cavity. The valve gates 50, 52 open and close the inlet 26 to the cavities 22, 24. Suitable valve gates 50, 52 are any valves known in the injection molding art. Particularly preferred valves include valve gates. After the molten material cools and hardens, the clamping force is released and the movable platen 44 is moved away from the stationary platen 40, in order to release the molded product.
For the exemplary two-cavity mold shown in FIG. 2, the sequential 1 S injection molding method begins with clamping the mold with at a mold clamping force. The controller 60 then closes valve gate 52 and opens valve gate 50.
Molten material fills cavity 22. A pressure transducer P2 is associated with valve gate 50, and another pressure transducer P1 is associated with the cavity 22 itself.
Similarly, a pressure transducer P4 is associated with valve gate 52 and another pressure transducer P3 is associated with cavity 24. In an alternate embodiment the pressure transducers P1', P3' associated with the cavity are located near an opposite wall from the inlet. Pressure transducers P1', P3' are useful to monitor the pressure inside the cavity near the end of the fill. Lt will be understood that it is not necessary to have a pressure transducer at both the valve gate and the cavity.
One of ordinary skill in the art will understand that it is only necessary to have a pressure transducer associated with either each valve gate, each cavity, or a combination of both. The pressure transducers 66 transmit their respective readings to the controller 60, which uses either or both pressure readings to determine when to close valve gate 50 and open valve gate 52. The controller closes valve gate 50 and opens valve gate 52 when the pressure inside cavity reaches a set-point pressure. Molten material then fills into cavity 24, and pressure transducers P4, P3 associated with valve gate 52 and cavity 24, respectfully, transmit information to the controller. When the pressure inside cavity 24 reaches the set-point pressure, the controller closes valve gate 52 and the molten material is allowed to cool and solidify.
S FIG. 3 is a flowchart illustration of the sequential injection molding process of the present invention. It will be understood that each step of the flowchart illustration can be implemented by computer program instructions or can be done manually. These computer program instructions may be loaded onto a computer or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart step. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart step. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart step.
It will be understood that each step of the flowchart illustration can be implemented by special purpose hardware-based computer systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions, or can be done manually.
An injection molding machine utilizing a sequential injection molding process has a plurality of cavities formed by the movable mold member 46 and the stationary mold member 36. For an injection molding machine with m cavities, where n equals 1 to m, the process begins with step 100 by closing the clamp with a mold clamping force calculated by the equation:
mold clamping force = (molding pressure)/(surface area of cavity n) The molding pressure is predetermined and is calculated based upon the type of S molding material and the desired characteristics of the molded article. In step 110, a first valve gate is opened which faces a first cavity. 'The first cavity is then injected with molten material using a resin-feeding screw at a predetermined injection velocity in step 120. The injection velocity may be changed or may be kept constant as the cavity becomes filled with molten material. The time it takes to fill the cavity to the set-point pressure depends on the size of the cavity and the injection velocity. In a preferred embodiment, the injection velocity is varied and it takes about one second to about ten seconds to fill the cavity to the set-point pressure. In step 130, the pressure inside the cavity is measured and compared to the set-point molding pressure. The process goes back to step 120 if the pressure 1 S inside the cavity is less than the set-point molding pressure. The first valve gate is closed once the pressure inside the cavity reaches the set-point molding pressure in step 140. The process goes back to step 110 and repeats for n cavities. After all of the cavities are full at the set-point pressure, the molten material inside the cavities is allowed to cool and solidify in step 150. The cooling process takes about 20 seconds to about 40 seconds, depending upon the size of the molded article and the type and temperature of the molded material. After cooling, the mold clamping force is released and the clamp is opened in step 160. The sequential injection molding process ends with step 170, when the molded articles are ejected from the molding cavities.
The mold clamping force required is reduced significantly in a sequential injection molding process for a multiple cavity mold. 'This is because the area to be pressurized does not increase when there are multiple cavities. For a mold with multiple cavities, the area to be pressurized remains constant and equals the area of one cavity since each cavity in the mold is pressurized and closed sequentially.
Therefore, the mold clamping force required in a two-cavity mold is reduced to almost half by using the sequential injection molding method compared to a _g_ conventional method. The mold clamping force required in a three-cavity mold the force required is reduced by over fifty percent compared to the force required in the conventional method. This significant reduction in mold clamping force allows for a reduction in the press size, which in turn allows for dramatic cost savings in terms of production cost per molded article.
FIG. 4 shows how the injection velocity varies during the step of filling a cavity for a standard injection molding process compared to a sequential injection molding process in a two-cavity mold. The injection velocity is controlled by the machine set-point of the resin-feeding screw 10. In a standard injection molding process the cavities are filled with molten material simultaneously and in the sequential method the cavities are filled sequentially. Both processes may be carried out with more than two cavities. The sequential molding process, however, has at least two cavities.
In a standard injection molding process, the injection pressure is set at the set-point molding pressure. The injection velocity of the molten material is at a filling flow rate when the valve gate is first opened. As illustrated in FIG.
4, the injection velocity is kept at filling flow rate until the cavities are almost full. The injection velocity is then gradually tapered down from the filling flow rate so that the molten material can fill up the entire cavity. Once the pressure inside the cavity reaches the set-point molding pressure, the injection velocity is brought down to zero. Decreasing the injection velocity ensures that the molten material is uniform inside the cavities, thereby yielding a higher quality molded article.
In the sequential injection molding process, the injection pressure is set at the set-point molding pressure. The injection velocity of the molten material is at a filling flow rate when a valve gate is opened. The injection velocity is kept at the filling flow rate until a cavity is almost full and then gradually tapered down until the pressure inside a cavity reaches the set-point molding pressure. The difference in the sequential method compared to the standard method, is that the injection velocity is increased again to the filling flow rate when the second valve gate is opened. This adds approximately 0.5 seconds to about 4 seconds to the total fill-time for the process. In a preferred embodiment, the ramp up of the injection velocity to the filling flow rate is rapid so that the total process time does not increase significantly.
It is contemplated that numerous modifications may be made to the injection molding method and apparatus of the present invention without departing from the spirit and scope of the invention as defined in the claims. For example, while the exemplary embodiment shown in the drawings has two mold cavities, those skilled in the art will appreciate that the same sequential steps can be used to control the flow of molten material into molds having more than two cavities.
In addition, for molds having more than two cavities, there may be a valve gate associated with each cavity, with each valve gate opened and closed sequentially.
Alternately, for molds having more than two cavities, there may be fewer valve gates than cavities, as long as there are at least two cavities. In this embodiment, at least one of the valve gates would control the inlet to at least two cavities.
Accordingly, while the present invention has been described herein in relation to 1 S several embodiments, the foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, arrangements, variations, or modifications and equivalent arrangements.
Rather, the present invention is limited only by the claims appended hereto and the equivalents thereof.
In a second aspect, the invention is an injection molding apparatus comprising a mold having at least two mold cavities, a molten material inlet system in communication with said at least two mold cavities, at least two valve gates in said molten material inlet, wherein each of said at least two valve gates are associated with one of said mold cavities; and a controller adapted to sequentially open and close said valve gates.
In a third aspect, the invention is a controller for use with a injection 1 S molding device having a mold with at least two cavities, the controller comprises means for opening a first valve gate associated with a first mold cavity to initiate a flow of molten material into the first mold cavity, means for closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure, means for opening a second valve gate associated with a second cavity to initiate a flow of molten material into a second mold cavity, and means for closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, the invention being defined only by the claims following this detailed description.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein:
FIG. 1 is a schematic view of an injection molding apparatus suitable for S the method of injection molding a molten material, provided by the present invention.
FIG. 2 is a schematic view of a part of the injection molding apparatus showing a state immediately after clamping the mold, and a state in which the introduction of the molten material is initiated, in the method of injection molding, provided by the present invention.
FIG. 3 is a flowchart illustration of the sequential injection molding process of the present invention.
FIG. 4 schematically shows a change in injection velocity with time for a conventional injection molding method and a sequential injection molding method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, the injection molding apparatus suitable for use in the method of injection-molding a thermoplastic or thermoset resin, provided by the present invention, will be outlined below with reference to FIG. 1. The injection molding apparatus includes an injection cylinder 12 having a resin-feeding screw 10 inside, a stationary platen 40, a movable platen 44, an inlet 26, valve gates 50, tie bars 34, a clamping hydraulic cylinder 30 and a hydraulic piston 32. The movable platen 44 is actuated with the hydraulic piston 32 in the hydraulic cylinder 30 to move in parallel on the tie bars 34.
A mold is formed by a stationary mold member 36 and a movable mold member 46. The stationary mold member 36 is attached to the stationary platen 40, and the movable mold member 46 is attached to the movable platen 44.
The platens 40, 44, the tie bars 34, and the cylinder 30 and piston 32 define a clamping system for applying a clamping pressure to the mold members 36, 46.
The movable platen 44 is moved towards the stationary platen 40 until the -S-movable mold member 46 is engaged with the stationary mold member 36, and the mold is clamped to form cavities 22, 24. This clamped position is illustrated in FIG. 2.
After the mold has been clamped, the clamping force is controlled with the clamping hydraulic cylinder 30. The clamping force may also be controlled by toggle or an electric machine. The molten material flows into the cavities 22, via an inlet 26. The valve gates 50, 52 face cavities 22, 24 and, in a preferred embodiment, there is a valve gate associated with each cavity. The valve gates 50, 52 open and close the inlet 26 to the cavities 22, 24. Suitable valve gates 50, 52 are any valves known in the injection molding art. Particularly preferred valves include valve gates. After the molten material cools and hardens, the clamping force is released and the movable platen 44 is moved away from the stationary platen 40, in order to release the molded product.
For the exemplary two-cavity mold shown in FIG. 2, the sequential 1 S injection molding method begins with clamping the mold with at a mold clamping force. The controller 60 then closes valve gate 52 and opens valve gate 50.
Molten material fills cavity 22. A pressure transducer P2 is associated with valve gate 50, and another pressure transducer P1 is associated with the cavity 22 itself.
Similarly, a pressure transducer P4 is associated with valve gate 52 and another pressure transducer P3 is associated with cavity 24. In an alternate embodiment the pressure transducers P1', P3' associated with the cavity are located near an opposite wall from the inlet. Pressure transducers P1', P3' are useful to monitor the pressure inside the cavity near the end of the fill. Lt will be understood that it is not necessary to have a pressure transducer at both the valve gate and the cavity.
One of ordinary skill in the art will understand that it is only necessary to have a pressure transducer associated with either each valve gate, each cavity, or a combination of both. The pressure transducers 66 transmit their respective readings to the controller 60, which uses either or both pressure readings to determine when to close valve gate 50 and open valve gate 52. The controller closes valve gate 50 and opens valve gate 52 when the pressure inside cavity reaches a set-point pressure. Molten material then fills into cavity 24, and pressure transducers P4, P3 associated with valve gate 52 and cavity 24, respectfully, transmit information to the controller. When the pressure inside cavity 24 reaches the set-point pressure, the controller closes valve gate 52 and the molten material is allowed to cool and solidify.
S FIG. 3 is a flowchart illustration of the sequential injection molding process of the present invention. It will be understood that each step of the flowchart illustration can be implemented by computer program instructions or can be done manually. These computer program instructions may be loaded onto a computer or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart step. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart step. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart step.
It will be understood that each step of the flowchart illustration can be implemented by special purpose hardware-based computer systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions, or can be done manually.
An injection molding machine utilizing a sequential injection molding process has a plurality of cavities formed by the movable mold member 46 and the stationary mold member 36. For an injection molding machine with m cavities, where n equals 1 to m, the process begins with step 100 by closing the clamp with a mold clamping force calculated by the equation:
mold clamping force = (molding pressure)/(surface area of cavity n) The molding pressure is predetermined and is calculated based upon the type of S molding material and the desired characteristics of the molded article. In step 110, a first valve gate is opened which faces a first cavity. 'The first cavity is then injected with molten material using a resin-feeding screw at a predetermined injection velocity in step 120. The injection velocity may be changed or may be kept constant as the cavity becomes filled with molten material. The time it takes to fill the cavity to the set-point pressure depends on the size of the cavity and the injection velocity. In a preferred embodiment, the injection velocity is varied and it takes about one second to about ten seconds to fill the cavity to the set-point pressure. In step 130, the pressure inside the cavity is measured and compared to the set-point molding pressure. The process goes back to step 120 if the pressure 1 S inside the cavity is less than the set-point molding pressure. The first valve gate is closed once the pressure inside the cavity reaches the set-point molding pressure in step 140. The process goes back to step 110 and repeats for n cavities. After all of the cavities are full at the set-point pressure, the molten material inside the cavities is allowed to cool and solidify in step 150. The cooling process takes about 20 seconds to about 40 seconds, depending upon the size of the molded article and the type and temperature of the molded material. After cooling, the mold clamping force is released and the clamp is opened in step 160. The sequential injection molding process ends with step 170, when the molded articles are ejected from the molding cavities.
The mold clamping force required is reduced significantly in a sequential injection molding process for a multiple cavity mold. 'This is because the area to be pressurized does not increase when there are multiple cavities. For a mold with multiple cavities, the area to be pressurized remains constant and equals the area of one cavity since each cavity in the mold is pressurized and closed sequentially.
Therefore, the mold clamping force required in a two-cavity mold is reduced to almost half by using the sequential injection molding method compared to a _g_ conventional method. The mold clamping force required in a three-cavity mold the force required is reduced by over fifty percent compared to the force required in the conventional method. This significant reduction in mold clamping force allows for a reduction in the press size, which in turn allows for dramatic cost savings in terms of production cost per molded article.
FIG. 4 shows how the injection velocity varies during the step of filling a cavity for a standard injection molding process compared to a sequential injection molding process in a two-cavity mold. The injection velocity is controlled by the machine set-point of the resin-feeding screw 10. In a standard injection molding process the cavities are filled with molten material simultaneously and in the sequential method the cavities are filled sequentially. Both processes may be carried out with more than two cavities. The sequential molding process, however, has at least two cavities.
In a standard injection molding process, the injection pressure is set at the set-point molding pressure. The injection velocity of the molten material is at a filling flow rate when the valve gate is first opened. As illustrated in FIG.
4, the injection velocity is kept at filling flow rate until the cavities are almost full. The injection velocity is then gradually tapered down from the filling flow rate so that the molten material can fill up the entire cavity. Once the pressure inside the cavity reaches the set-point molding pressure, the injection velocity is brought down to zero. Decreasing the injection velocity ensures that the molten material is uniform inside the cavities, thereby yielding a higher quality molded article.
In the sequential injection molding process, the injection pressure is set at the set-point molding pressure. The injection velocity of the molten material is at a filling flow rate when a valve gate is opened. The injection velocity is kept at the filling flow rate until a cavity is almost full and then gradually tapered down until the pressure inside a cavity reaches the set-point molding pressure. The difference in the sequential method compared to the standard method, is that the injection velocity is increased again to the filling flow rate when the second valve gate is opened. This adds approximately 0.5 seconds to about 4 seconds to the total fill-time for the process. In a preferred embodiment, the ramp up of the injection velocity to the filling flow rate is rapid so that the total process time does not increase significantly.
It is contemplated that numerous modifications may be made to the injection molding method and apparatus of the present invention without departing from the spirit and scope of the invention as defined in the claims. For example, while the exemplary embodiment shown in the drawings has two mold cavities, those skilled in the art will appreciate that the same sequential steps can be used to control the flow of molten material into molds having more than two cavities.
In addition, for molds having more than two cavities, there may be a valve gate associated with each cavity, with each valve gate opened and closed sequentially.
Alternately, for molds having more than two cavities, there may be fewer valve gates than cavities, as long as there are at least two cavities. In this embodiment, at least one of the valve gates would control the inlet to at least two cavities.
Accordingly, while the present invention has been described herein in relation to 1 S several embodiments, the foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, arrangements, variations, or modifications and equivalent arrangements.
Rather, the present invention is limited only by the claims appended hereto and the equivalents thereof.
Claims (20)
1. A method for sequentially injecting a molten material comprising:
clamping a stationary platen and a movable platen at a clamping force to define at least two mold cavities;
opening a first valve gate to inject a molten material into a first mold cavity;
closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure;
opening a second valve gate to inject the molten material into a second mold cavity; and closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
clamping a stationary platen and a movable platen at a clamping force to define at least two mold cavities;
opening a first valve gate to inject a molten material into a first mold cavity;
closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure;
opening a second valve gate to inject the molten material into a second mold cavity; and closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
2. The method of claim 1 wherein after closing the second valve gate, said molten material is held in said mold cavities at said clamping pressure until said material cools and solidifies into molded articles.
3. The method of claim 1, wherein a hydraulic cylinder, toggle, or electric machine is used to clamp the stationary platen and the movable platen at said clamping force.
4. The method of claim 1, wherein a valve gate faces each of said mold cavities.
5. The method of claim 4, wherein the valve gate are controlled independently of each other by a control unit.
6. The method of claim 1, wherein the pressure at the valve gate is measured by pressure transducers associated with said valve gates.
7. The method of claim 1, wherein the pressure inside the mold cavities is measured by pressure transducers associated with said mold cavities.
8. The method of claim 5, wherein the control unit opens and closes the valve gate based on signals transmitted by said pressure transducers to the control unit.
9. The method of claim 1, wherein the flow rate of injection of said molten material is reduced from a filling flow rate when the first valve gate is closed and then increased to the filling flow rate after the second valve gate is opened.
10. An injector molding apparatus comprising:
a mold having at least two mold cavities;
an molten material inlet system in communication with said at least two mold cavities;
at least two valves in said molten material inlet, wherein each of said at least two valves are associated with one of said mold cavities; and a controller adapted to sequentially open and close said valves.
a mold having at least two mold cavities;
an molten material inlet system in communication with said at least two mold cavities;
at least two valves in said molten material inlet, wherein each of said at least two valves are associated with one of said mold cavities; and a controller adapted to sequentially open and close said valves.
11. The apparatus of claim 10, wherein the number of valves and the number of mold cavities are equal.
12. The apparatus of claim 10, further comprising a stationary platen and a movable platen adapted to apply a clamping pressure to said mold having said at least two mold cavities.
13. The apparatus of claim 10, wherein said molten material inlet comprises at least two channels, each of said channels associated with one of said mold cavities, each of said channels in communication with and branching from a common inlet.
14. The apparatus of claim 13, wherein each of said channels includes one of said valve gates.
15. The apparatus of claim 10, wherein said valves comprise valve gates.
16. The apparatus of claim 10, further comprising a clamping system, said clamping system applying a clamping pressure to said mold.
17. The apparatus of claim 16, wherein said clamping pressure is lower than the pressure that would be required by a non-sequential controller.
18. A controller for use with an injection molding device having a mold with at least two cavities, the controller comprising:
means for opening a first valve gate associated with a first mold cavity to initiate a flow of molten material into the first mold cavity;
means for closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure;
means for opening a second valve gate associated with a second mold to initiate a flow of molten material into a second mold cavity; and means for closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
means for opening a first valve gate associated with a first mold cavity to initiate a flow of molten material into the first mold cavity;
means for closing the first valve gate when the pressure inside the first mold cavity reaches a set-point pressure;
means for opening a second valve gate associated with a second mold to initiate a flow of molten material into a second mold cavity; and means for closing the second valve gate when pressure inside the second mold cavity reaches a set-point pressure.
19. The controller of claim 18, wherein said first valve gate closing means is responsive to a pressure transducer associated with said first valve gate, and said second valve gate closing means is responsive to a pressure transducer associated with said second valve gate.
20. The controller of claim 18, wherein said first valve gate closing means is responsive to a pressure transducer associated with said first mold cavity, and said second valve gate closing means is responsive to a pressure transducer associated with said second mold cavity.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US59568100A | 2000-06-16 | 2000-06-16 | |
| US09/595,681 | 2000-06-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2350991A1 true CA2350991A1 (en) | 2001-12-16 |
Family
ID=24384227
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2350991 Abandoned CA2350991A1 (en) | 2000-06-16 | 2001-06-18 | Method and apparatus for injection molding |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2350991A1 (en) |
| MX (1) | MXPA01006245A (en) |
-
2001
- 2001-06-18 MX MXPA01006245 patent/MXPA01006245A/en not_active Application Discontinuation
- 2001-06-18 CA CA 2350991 patent/CA2350991A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| MXPA01006245A (en) | 2003-05-19 |
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