CA1090076A - Method and apparatus for injection foam molding - Google Patents

Abstract

ABSTRACT
A method and apparatus for the manufacture by foam molding of plastic products by first injecting into a mold solid plastic resin and then injecting a foamed plastic resin of the same basic composition as the solid plastic resin. The process and apparatus consisting of a single extruder and single injection chamber wherein the solid and foamed plastic resin are extruded and stored until shot into the mold by hydraulic pressure. Controls and provided for the correct amount of each type of resin to be extruded and stored ad the process can be adapted so that the sprue portion of the product will be solid resin and by a simple addition, coloring may be added so that the basic resin, method or apparatus will not require changes.

Description

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BACKGROUND
Field of Invention This invention relates to the method and apparatus for the manufacture o~ structural foam articles having a predetermined skin thickness and a foam interior from the same basic resin compound.
Description of Prior Art The present method of manufacturing structural foamed articles consist chiefly of two type of methods and 1~ apparatus. In one method as set out in the Angel patent 3,268,636 a method is described wherein a hlowing agent is mixed with a plastic resin and then extruded into an accumulator until the proper amount is collected and kept at a temperature and pressure to prevent foaming. A valve is then transferred which now connects the accumulator to a mold wherein the plastic resin is forced into the mold where it will now foam and expand forming a cell-like structure.
The external skin of the article is chilled faster than the inside and therefore will form a continuous surface without holes or blemishes. However, with this method it is very difficult to control the skin thickness since the whole method relies on the mixing of the foaming agent, the foaming action of the plastic mixture, the temperature and pressure of the accumulator and mold s~ructure, and its temperature gradient during cooling.

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In the other method of injection molding foam products described in the Garner patent 3,59~,290, two extruders are used wherein each contains a different type plastic resin. Here one extruder will inject a solid plastic of one type material into the mold, then a valve is transferred and a second type plastic, which will foam, is extruded into the mold to form the internal structure of the article. The mold halves are then opened a controlled amount and the foamed internal structure will fully develop.
Here the skin thickness can be controlled because a fixed amount of that particular plastic was available within the mold from the first extruder and the foam will continue to expand giving the desired cell structure as the mold is allowed to open. Both quantities of each type plastic extruded is controlled by the timing of the extruders during the injection process so that the skin thickness and ultimate density of the article is controlled on a repeatable basis.
Although not a problem but rather an expense in this second method is the use of two extruders for the skin and core of the article. My invention overcomes this drawback by needing only one extruder.

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10900~6 In the former example the skin thickness was uncontrolled because when the accumulator was connected to the mold, the plastic flow was virtually uncontrolled and there~ore the skin thickness variation and the ultimate core cell structure is a function of many variables which can change from shot to shot.
Here my invention establishes a fixed quantity o solid material which will fix the minimum wall thickness as the foamed plastic is injected into the mold.
_MMARY
Broadly speaking the present invention may be defined as providing a process for manuacturing resinous components having a dense non-cellular outer surface and a cellular inner structure comprising the steps of: extruding a foamable resinous material into an injection chamber wherein partial volume expan-; sion occurs; extruding next a non-cellular resinous material into the injection chamber, the non-cellular resinous material being of the same polymer composition as the firstly extruded material; discontinuing the extrusion o the resinous materials;
establishing communication by a valve means between the injection chamber and mold means; injecting first the lastly extruded non-cellular resinous material into the mold means at a controlled rate; injecting next the partially foamed resincus material.
rapidly into the mold; and terminating communication between the injection chamber and the mold means.
The above method may be carried out in an injection foam molding apparatus comprisiny: an extruder mean~ with operatively associated drive means; an injection chamber; pipe means connecting the extruder means to the injection chamber; a blowing agent source connected to the extruder means wherein ,.
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1090(r,~i blowing agent is metered into the plasticized resin melt stream;
means controlling the injection chamber wherein the plasticized resin foams and expands to a predetermined volume in the injection chamber; a manifold means with passages connecting ~he injection chamker to nozzle means wherein said nozzle means is affixedly held to the manifold and in communication with mol,d means; the nozzle means having a shut-off valve disposed therein; means operatively associated with the injection chamber for rapidly forcing the foamed plasticized resin from the injection chamber to the mold means; and ~he predetermined volume of the injection chamber is a function of the volume of the passages from the blowing agent source to the nozzle and the volume of the mold means.
Thus, this invention utilizes a single extruder and auxiliary mixer wherein a foaming agent is added by a nitrogen pump. The outlet of the mixer is connected to an injection chamker and from there to the mold. By operating the extruder with the nitrogen pump turned on, a steady stream of foamed plasticized material can be extruded into the injection ch,~mber.
After a predetermined volume is reached a switch will turn off the nitrogen pump and the unit will now extrude a solid plasticized material until a second switch is tripped which will stop the extruder~ The shut-off valve in the nozzle will now open and the extruded plasticized material in the injection chamber will be shot into the mold. 'I'he first portion to enter the mold cavity will be the solid material and form the outer skin of the article to be produced then the second portion of the accumulated material will enter the mold forming the cell-like internal structure of the article. The shut-off valve will now seal the nozzle and the cycle will be repeated.
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The cycle of the apparatus can be operated such that a solid material is at both the beginning and end of the shot. This will produce a so]id plastic sprue and allow a smooth finish at the break point of the sprue and article leaving a solid material in the nozzle and passages available for the next article to be produced.
A static mixer following the extruder can be added as well as a liquid coloring pump so that the color of the plastic can be changed without completely changing the material in the hopper. This object will make color changes to the finished product more convenient and less costly.
Depending on the size of the part to be manufactured, additional sprues or runners can be added so that the material can be injected into the mold cavit~ simultaneously through more than one opening. Also the density of the part ; produced is a function of the volume or size of the pipes and bores in the system and an ob~ect of this invention is to consider these volumes as related to other elements in the system.
A further object is to allow the extruder to be shut off auring periods of inactiv~ity thus conserving power and energy as compared to most extruders used in this art - which must be operated continuously. During the shot cycle of such continuouly operated extruders, the material extruded is dumped overboard and discarded, or accumulated, reground, and then mixed in with virgin plastic. This procedure of dumping material is not necessary in my inYention.
In the Drawings:
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Figure 1 is an overall partial section of the bm:
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extruder, mixer, piping, injection chamber and mold sections;
Figure 2 is a view showing solid plastic resin displacing injection chamber piston;
Figure 3 is a view showing foamed plastic resin displacing injection chamber piston;
Figure 4 is a view showing additional solid plastic resin displacing injection chamber piston;
Figure S is a view showing solid material injected into mold from injection chamber;
Figure 6 is a view showing the foamed material injected into mold from injection chamber;
Figure 7 is a view showing first extruded solid plastic resin injected into mold; and Figure 8 is a view showing the no~zle shut-off and completion of the cycle.
D~SCRIPTION
The overall apparatus of my invention can be viewed in Figure 1 which shows a hopper:12 and an extruder 10 driven by a motor means 15 with the outlet of the extr.uder lQ
connected by pipe 14 to a static mixer 16. Near the outlet of the extruder 10 is a pump 18 having a fluid blowing agent .
source such as nitrogen connected to it. Some other type of blowing agents such as anot~er gas or a liquid cou~d be used in lieu of the nitrogen. Also connected near the outlet of the extruder 10 is a pump 20 having a source of fluid color additive available as well. The check valves 19 and 21 are associated with the blowing agent pump 18 and color additiYe bm:
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pump 20 respectively. The flites on the extruder 10 are modified and fittings 11 and 22 are adapted to accept the inputs from the blowing agent and color additive pumps so these fluids en-ter the center of the melt stream. A plurality of fittings may be radially inserted for both the blowing agent and color additive to better distribute the fluids into the melt stream. The blowing agent source may also be in a plurality of input locations in front of and behind the location shown in Figure 1 depending on the type of plastic and the size of the part to be produced. These locations would be in the front half of the extruder since a more rearward location could cause leakage of the blowing agent, if a gas, out of khe hopper The mixer 16 contains a static element 17 which mixes the plasticized resin with either the fluid blowing agent or the color additive or with them both.
The outlet of the mixer 16 is affixed to manifold 24 which contains a ball 25 and pin 26 at the inlet to form check valve 27, as disclosed in U.S. Patent 3,806,291, into manifold bore 28. An injection cylinder 30 is shown housed in conjunction with;the manifold 24 for convenience and could be a separate element connected by piping or tubing to the manifold. A bore 31 forms the inside diameter of the injection cylinder 30 having a conic surface 32 communicating from this bore to diameter 34 and thence to manifold bore 28. Closely fitting inside injection cylinder bore 31 is piston 35. The piston 35 is connected with piston rod 38 to plastic injection hydraulic cylinder 39. The volume bounded by conic surface 32, bore 31 and piston 35 form injection 3 n chamber 47.

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Mold halves 45 and 46 are shown in Figure 1 in a closed position held in place by the moving plate 50 and stationary plate 52 respectively of a typical clamp section of an injection molding machine.
The mold cavity 54 is connected to the sprue 55 to sprue bushing 57 through bore 58 and to nozzle 59 through nozzle bore 60. The sprue bushing 57 is affixedly held to the mold half 46 by any convenient means and the nozzle 59 is likewise affixedly held to manifold 24, Nozzle shut-off valve 63 is a hydraulically operated cylinder 65 with rod end 64 closely fitted into nozzle bore 60. Communicating with manifold bore 28 is cross bore 61 in nozzle 59 which intersects with nozzle bore 60 so that a continuous path from the plastic injection chamber 47 is made with the cavity 54 of the mold. A seal 62 in the nozzle 59 prevents leakage of plasticized material especially during the shot cycle.
Four limit switches 66, 68, 70 and 72 are located at the piston rod end of plastic injection chamber 47 and are affixedly held to the injection cylinder body 33 by rods 74 and 75. These switches are adjustable so that the correct amount of solid and foamed material can be injected for a ; given shot. These switches are connected to other electrical circuit elements such as relays or contactors so that proper signals and voltages can be delivered to operate properly in ; the correct timing sequence in the system. These switches are shown activated by the piston 35 but could be actuated ; by any convenient means signaling the various positions of the piston in the body of the injection chamber, For example bm:

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a linear transducer could develop an elec-trical vol-tage proportional to distance and hence to volume which could be used in the same manner as the switches just shown.
The injection chamber hydraulic cylinder 39 is piped into the hydraulic circuitry and branches into three lines.
One line is connected to a pilot operated check valve 79 and then to a pressure relief valve 80 which dumps to tank. The other two branches go to directional valve 78 and pilot operated check valve 76.
The hydraulic pump 41 is connected to a pressure relief valve 42 to tank and through a check valve 83 to a variable orifice 77 which connects to directional valve 78. A
directional control valve 85 is connected to relief valve 42 and to tank to relieve pump 41 pressure after accumulator is charged. Another branch from pump 41 is connected to directional valve 43 through check valve 82 to pilot operated - -check valve 76. In parallel connections between the check valves 82 and 76 are a pressure switch 44 and a hydraulic accumulator 40. The pressure switch 44 may be connected to other electrical circuit elements for control purposes. The accumulator although appearing as a single element in the hydraulic system could have a plurality of hydraulic accumulators connected in parallel.
The sizes of the bores in the manifold 24 and injection chamber 47 as well as the size of the piping and bores from the nitrogen fit-ting 11 in the extruder to the mixer 16 and manifold 24 will be commensurate with the size of the foamed part to be produced and the ultimate densi-ty of the part. The part size itself is of course determined by bm:

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the mold cavity 54 and a g.iven shot size of the injection chamber 47, determined by either the full injection chamber volume or that set by the switches (66, 68, 70, and 72) associated with the injection chamber to control the function of the extruder 10 and nitrogen pump 20, will determine the final density of the part to be produced when considered in conjunction with the pipes and bores sizes between the various element in the system.
These bores and pipes or passages which connect from the fittings in the extruder to the injection chamber and from the injection chamber to the mold could represent a significant amount of the total volume of the injection chamber or in contrast a small portion of it. By way of example~
if the total pipe volume as described is only 5% of the injection chamber volume, and assuming the plastic when unfoamed has a density of 1.00 and when fully foamed would have a density of 0.50, then the final minimum part density would equal 0.525 if maximum injection chamber volume is used. Obviously by injecting more solid material the part density can be increased up to 1.00 which would be for a full solid part and no foam. If the total pipe volume is 20% of the injection chamber size, with the same assumption as above, then the final minimum part density would equal 0.60 if maximum injector chamber volume is used. Still further if the pipe volume is 20% and only half of the injection chamber volume as controlled by the switches is used, the density would only go up to 0.70 since the pipe volume is now equivalent to ~0% of the available injection chamber volume.
The following table will show the pipe and bore volumes in - _ g_ bm:

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-` ~O9~U76 terms of percentage of the injection chamber volume versus the part density using 1.00 as the solid part density, 0.50 as the fully foamed part density and using full volume of the injection chamber as the base assuming only solid material in the pipes and bores. The general range of thermoplastic densities are from 0.91 to 1.50 and the actual density desired for any given thermoplastic can be calculated from the table by multiplying the actual density of the thermoplastic by the density shown since this chart is set-up based on a thermoplastic density of 1.00.

PIPE VOLUME PART DENSITY
_ 1% .505 5% .S25 10% .55 15% .575 20% .60 25% .625 30% .65 40% .70 650% 75 .
70% .85 ~0% . 90 90% .95 100% 1.00 Again it becomes very obvious that the total volume of the pipes and bores between the elements must be small in order to properly control the density of the part and the practical range of the volume of bores and pipe would be from nil, since it is impossible to have zero, to about ~0%. The 30 preferred and best range of pipe and bore volume would be from 5% to 25%. Here again, the number of nozzles, and size of the part will determine if the volume in the nozzles, pipes and bores become prohibitive from an economic and practical poi.nt which would require then that the injection chamber be changed .- to allow a reduced density of the part.

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~ , "' 0~)76 Although only one nozzle is shown it is understood that a plurality of nozzles could be used.
Mode of Operation After an appropriate warm-up time so that heater bands associated with the extxuder 10 and injection cylinder 30 would be up to temperature and other circuit elements have stablized, the unit and system would be ready to operate.
Hydraulic pump 41 would be turned on and directional valve 43 energized to port fluid into the accumulator 40. When this accumulator is fully charged, pressure switch 4~ will de-activate directional valve 43 and the pressure will be trapped. Simultaneously directional valve 85 is energized venting pressure relief valve 42 and fluid would now be dumped at a lower pressure over relief valve 42 to tank.
Pilot operated check valve 79 would now be opened and the relief valve 80 would be set to a variable pressure compatible with the polymer being plasticized to provide back pressure to injection cylinder piston 35, Extruder 10 would be turned on and would start to extrude plasticized resin into pipe 14 and through optional mixer 16 opening the check valve 27 at the end of the mixer and allowing the solid resin to flow through manifold bore 28, bore 34 and into injection chamber 47 pushing back piston 35 allowing the solid material to flow into the chamber bore 31 as shown in Figure 2. When the correct amount of solid material 23 is accumulated in the injection chamber 47 both limit switches 66 and 68 will be activated which will turn on pump 18; that will begin pumping a metered amount of blowing agent, in this case nitrogen gas, through check valve 19 and into the end of the extruder 10 bm:
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where it will be forced into the melt and carried up stream through pipe 14 into the auxiliary mixer 16 where a static element 17 inside mixes the resin and blowing agent together.
The mixture of resin and blowing agent shown in Figure 3 now continue to force the piston 35 back, filling the injection chamber 47 and at the same time the mixture 36 partially expands in the chamber with the amount of mixture 36 expansion controlled by the setting of relief valve 80, until the correct amount for the part to be molded is accumulated.
At which time limit switch 70 is activated which now shuts off pump 18 and stops the metering of the blowing agent. Check valve 19 will prevent plasticized resin from coming into the pump 18 while the extruder continues to operate.
The extruder 10 continues extruding solid material into the injection chamber 47 until limit switch 72 is activated which signals that a sufficient amount of solid material 49 has been accumulated as seen in Figure 4 and this also signals to shut off the extruder so that no further resin will be extruded or injected.
Referring now to Figure 1, the pilot operated check valve 79 will now be closed so that no fluid can flow over relief valve 80 and directional control valve 78 will be energized. With pump 41 still operating, fluid will flow through variable orifice 77 into the injection hydraulic cylinder 39 causing it to ease the piston 35 forward and the melt will close check valve 27 which will prevent any flow of resin back to the extruder. The nozzle shut-off valve 63 will open and allow the free flow of resin from the injection chamber 47 through diameter 34 intersecting with manifold bm:

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9a~076 bore 28 in line with cross bore 61 in nozzle 59 connecting to nozzle bore 60 through sprue bushing bore 58 into mold cavity 54 through sprue 55 in mold half 46, best shown in Figure 5. The rod 64 of cylinder 65 will stop clear of bore 61 but in front of the seal 62 in nozzle to help prevent leakage of resin. Now with the piston 35 moving forward at a controlled velocity because of variable orifice 77, plasticized solid resin 49 will now be forced into the mold cavity 54 formed by mold halves 45 and 46. A short time delay can occur to allow the outer skin of the plasticized resin 49 to firm up, then pilot operated check valve 76 will be energized and the stored fluid in the hydraulic accumulator 40 will surge into injection hydraulic cylinder 39 causing piston 35 to shoot or rapidly force the foamed mixture 36 into the mold as shown in Figure 6 causing it to flow into the center of the previous solid melt pushing it into the far reaches of the mold and causing the part molded now to have a laminar outer surface structure with a finite thickness, depending on the amount of solid plastic, and a foamed or cellular inner structure. If the process were stopped at this time, the part, after it comes out of the mold and was degated or had the sprue removed, would have a cellular structure at the sprue point that would be difficult to cover or would require some secondary manufacturing operation. To prevent this, the solid material 23 which was extruded first into the injec-tion chamber 47 will now be forced into the mold as shown in Figure 7 which will fill the bores and passages with a solid resin material which will cause the gate or sprue to be solid. When this operation is complete, limit switch 66 bm:

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,_ will signal the completion of the shot and nozzle shut-off valve 63 will operate to shut off nozzle as shown in Figure 8 and complete the cycle. While the molded part is cooling, the extruder 10 is now turned on and the plasticizing cycle will start again. Also the accumulator ~0 will be recharged with hydraulic fluid by operating the directional control valve 43 and that portion of the cycle will start again. It is obvious that the various limit switches will be connected to other electrical circuit element so that the proper sequencing and time delays will be met in compliance with the par-t to be produced.
Pump 20 is shown in Figure 1 connected to a color additive source which will allow coloring to be added, for example to the whole part, or the skin of the part could be one color and the core of the part could be another color.
This would operate in conjunction with check valve 21 which would open to allow the flow of additive from pump 20 into the end of the extruder 10 and, when the pump is shut off, would prevent plasticized melt from flowing back up into the pump itself.
In summary, a single action extruder capable of plasticizing resin is piped to an injection chamber. Near the end of the extruder is connected a foaming agent source to input the foaming agent into the center of the melt stream in front of a mixer which will thoroughly distribute the foaming agent into the melt. By controlling the sequencing of the extruder and the foaming agent pump, the foamed resin is sandwiched in the injection chamber with solid resin on each side. A unique fact is that the foamed resin is allowed bm:
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to expand in the injection chamber prior to it being shot into the mold which reduces the need for holding the melt at a much higher pressure, as practiced in the art, prior to it being shot into the mold. Thus by only operating the extruder when necessary and adding a foaming agent at the appropriate time in the cycle, the solid and foamed plasticized resin can be extruded from a single extruder into the injection -~
chamber and then shot into the mold.
It is apparent that changes may be made to the process and structure described without departing from the scope of the invention as sought to be defined in the following claims.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An injection foam molding apparatus comprising:
a. an extruder means with operatively associated drive means;
b. an injection chamber;
c. pipe means connecting said extruder means to said injection chamber;
d. a blowing agent source connected to said extruder means wherein blowing agent is metered into the plasticized resin melt stream;
e. means controlling said injection chamber wherein said plasticized resin foams and expands to a predetermined volume in said injection chamber;
f. a manifold means with passages connecting said injection chamber to nozzle means wherein said nozzle means is affixedly held to said manifold and in communication with mold means;
g. said nozzle means having a shut-off valve disposed therein;
h. means operatively associated with said injection chamber for rapidly forcing said foamed plasticized resin from said injection chamber to said mold means; and i. said predetermined volume of said injection chamber is a function of the volume of the passages from said blowing agent source to said nozzle and the volume of said mold means.

2. The apparatus of claim 1 wherein a switch means controls the volume of said plasticized resin injected into said injection chamber.

3. The apparatus of claim 1 wherein a control means regulates the speed of plasticized resin flow into said mold means from said injection chamber.

4. The apparatus of claim 1 wherein a mixing chamber is disposed in communication between said extruder means and said injection chamber.

5. The apparatus of claim 1 wherein a valve means is disposed in communication between said extruder means and said injection chamber.

6. The apparatus of claim 1 wherein said volume of the passages from said blowing agent source to said nozzle means is from 5% to 25% of the total volume of said injection chamber.

7. The apparatus of claim 1 wherein said volume of the passages from said blowing agent source to said nozzle means is from 1% to 40% of the total volume of said injection chamber.

8. The apparatus of claim 2 wherein a control means regulates the speed of plasticized resin flow into said mold means from said injection chamber.

9. The apparatus of claim 8 wherein the said control means is a variable orifice.

10. The apparatus of claim 4 wherein a valve means is disposed in communication between said extruder means and said injection chamber.

11. The apparatus of claim 9 wherein a valve means is disposed in communication between said extruder means and said injection chamber.

12. The apparatus of claim 1 wherein said extruder means and said blowing agent source have means to operate intermittantly.

13. The apparatus of claim 5 wherein said blowing agent source has means to operate intermittantly.

14. The apparatus of claim 13 wherein said extruder means has means to operate intermittantly.

15. The apparatus of claim 10 wherein said extruder means and said blowing agent source have means to operate intermittantly.

16. A process for manufacturing resinous components having a dense non-cellular outer surface and a cellular inner structure comprising the steps of:
a. extruding a foamable resinous material into an injection chamber wherein partial volume expansion occurs;
b. extruding next a non-cellular resinous material into said injection chamber, said non-cellular resinous material being of the same polymer composition as said firstly extruded material;
c. discontinuing the extrusion of the resinous material;
d. establishing communication by a valve means between said injection chamber and mold means;
e. injecting first said lastly extruded non-cellular resinous material into said mold means at a controlled rate;
f. injecting next said partially foamed resinous material rapidly into said mold; and g. terminating communication between said injection chamber and said mold means.

17. The process of claim 16 wherein a color additive pump disposes a color additive into the resinous material and said resinous material and said additive are mixed in a mixing chamber.

18. A process for manufacturing resinous components having a dense non-cellular outer surface and a cellular inner structure comprising the steps of:

a. first extruding a non-cellular resinous material into an injection chamber;
b. extruding a foamable resinous material into said injection chamber wherein partial volume expansion occurs, said foamable material being of the same polymer composition as said firstly extruded material;
c. extruding next a second amount of non-cellular resinous material into said injection chamber said second non-cellular material being of the same polymer composition as said firstly extruded material;
d. discontinuing the extrusion of the resinous materials;
e. establishing communication by a valve means between said injection chamber and mold means;
f. injecting said lastly extruded resinous material into said mold means at a controlled rate;
g. injecting said partially foamed resinous material and said firstly extruded non-cellular material rapidly into said mold means; and h. terminating communication between said injection chamber and said mold means.

19. The process of claim 16 wherein a delay occurs after said first injection step.

20. The process of claim 18 wherein a delay occurs after said injection of said lastly extruded resinous material.