CA1298047C

CA1298047C - Method of injection-molding foamed products

Info

Publication number: CA1298047C
Application number: CA000547125A
Authority: CA
Inventors: Hideo Kikuchi; Mitsuo Wada; Yasuo Nishimura; Yoshio Kinoshita; Osamu Endo; Mitsuaki Okada; Makoto Shimada
Original assignee: Kobayashi KK; Shigeru Kogyo KK
Current assignee: Kobayashi KK; Shigeru Kogyo KK
Priority date: 1986-09-19
Filing date: 1987-09-17
Publication date: 1992-03-31
Anticipated expiration: 2009-03-31
Also published as: GB8721563D0; DE3730611A1; GB2197244A; GB2197244B; AU7821087A; KR890004853A; AU601483B2

Abstract

ABSTRACT OF THE DISCLOSURE
A method of injection-molding foamed products.
Raw material consisting of a foaming type plastisol is heated within a cylinder of an injection molding machine so that the raw material is gelatinized. The gelatinized raw material is injected into a cavity of a mold through a nozzle of the injection molding machine.
During the injection, the raw material is foamed at the nozzle. The raw material in the foamed state is filled in the cavity of the mold and is cooled and solidified therewithin.

Description

lZ98Q~7 l TITLE OF THE INVENTION
METHOD OF INJECTION-MOLDING FOAMED PRODUCTS

BACKGROUND OF THE INVENTION
The present invention relates to a method of injection-molding foamed products.
In the conventional injection molding of foamed products, a raw material such as, for e~ample, powder or pellets of vinyl chloride resin is heated, melted and foamed within a cylinder of an injection molding machine. The foamed resin is injected from a nozzle into a cavity of a mold to form the foamed products.
However, it is not easy for the above-described technique to control the foaming, making it difficult to obtain desirable foamed products. In addition, it is difficult to always fill a constant amount of foaming raw material into the cavity of the mold, thereby resulting in a short-shot. Moreove~, the conventional method requires a special injection molding machine, resulting in increase in the cost of equipment.
Another method is also known in which molten resin is filled in a mold and is subsequently heated and foamed therewithi~n. In this case, additional cycles are necessary for heating and cooling the mold, so that the forming time is lengthened. Moreover, of the resin having entered the cavity, a portion close to the mold -cavity surface is brought to a higher temperature level lZ980~7 1 and is foamed vigorously. Therefore, foaming marks remain on the surface of the foamed product manufactured, and the product surface is not smooth in roughness. Thus, the product is poor in appearance and S Eeeling or touch, and inferior in wear resistance and the like.

OBJECT AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an injection molding method by which it is made easy to control foaming to thereby enable the foaming to be brought to a desirable condition or state, and it is ensured that a constant amount of raw material can be injected into a mold.
It is another object of the invention to provide an injection molding method by which it is made possible to smooth product surfaces.
It is still another object of the invention to provide an injection moldlng method in which a universal or general-purpose injection molding machine can be utilized to enable the cost of equipment to be reduced.
According to the invention, there is provided a method of injection-molding foamed products, comprising the steps of:
heating a raw material consisting of a foaming type plastisol within a cylinder of an injection molding machine to gelatinize the raw material;
injecting the gelatinized raw material into a 1 cavity of a mold through a nozzle of the injection molding machine, the raw material being foamed at the nozzle during the injection, and the raw material in the foamed state being filled in the cavity of the mold; and cooling and solidifying the foamed raw material within the cavity of the mold.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic cross-sectional view showing an injection molding machine and a mold employed in carrying-out of a method according to an embodiment of the invention, the machine and the mold being in a position prior to injection;
Fig. 2 is a view similar to Fig. 1, but showing the injection molding machine and the mold in a position after injection;
Fig. 3 is an enlarged cross-sectional view showing a nozzle employed in the injection molding machine illustrated in Fig. 1;
Fig. 4 is an enlarged front elevational view showing a stationary mold half of the mold illustrated in Fig. 1;
Fig. 5 is an enlarged front elevational view showing a movable mold half of the mold illustrated in Fig. 1;
Fig. 6 is an enlarged cross-sectional view showing the mold illustrated in Fig. 1, which is in an open position;

1~8~4~
1 Fig. 7 is a cross-sectional view showing a foamed product formed by the use of the mold illustrated in Fig. 6;
Fig. 8 is a front elevational view showing the foamed product illustrated in Fig. 7;
Fig. 9 is a fragmental enlarged cross-sectional view showing a skin layer and a cushion layer of the product illustrated in Fig. B; and Fig. 10 is a fragmental cross-sectional view showing a nozzle employed in a method according to another embodiment of the invention.

DETAILED DESCRIPTION
The invention will now be described in detail, by way of e~ample, with reference to the accompanying drawings, Referring first to Figs. 1 and 2, an injection molding machine 10 comprises a cylinder 11, a screw 12 rotatably arranged within the cylinder 11, and a hopper 13 mounted to the cylinder 11 adjacent a rear end thereof. Four sets of band heaters 14a, 14b, 14c and 14d are disposed about the cylinder 11, and are adapted to be controlled independently of each other. A nozzle 15 is mounted to the forward end of the cylinder 11.
The cylinder 11 is formed therein with a passage 16 whose forward end portion 16a is tapered gently.
As best shown in Fig. 3, the nozzle lS is of a 1298~

1 funnel-shape having an axially extending passage 17.
The passage 17 is comprised of a first passage section 17a contiguous to a forward end 16a of the aforesaid passage 16 and tapered at a gradient higher than the latter, and a second passage section 17b which is small in diameter and long in length and which is contiguous to the ~irst passage section 17a. The second passage section 17b in the illustrated embodiment is tapered gently, but may have a uniform cross-sectional area over the entire length. The second passage section 17b has a mean diameter of approximately 2 mm, for example, and is extremely small in diameter as compared with the diameter (approximately 60 mm) o~ the passage 16 in the cylinder 11. The second passage section 17b has its length on the order of about 3 to 7 cm.
The injection molding machine 10 is of universal or general-purpose type which is used to inject usual molten resins. Only the nozzle 15 is made specially, and has its length about three times the usual nozzle.
A mold 20 is a generally used one and comprises a stationary platen 21 and a movable platen 22 which is moved by a moving mechanism, not shown, toward and away from the~ stationary platen 21. A stationary mold half 23 and a movable mold half 24 are mounted to the stationary and movable platens 21 and 22, respectively.
As shown in detail in Figs. 4 through 6, the i298047 l confronting surfaces of the respective stationary and movable mold halves 23 and 24 are formed with rectangular recesses 25 and 26, respec,ively. When the stationary and movable mold halves 23 and 24 are closed, t:he recesses 25 and 26 cooperate with each other to define a mold cavity 27 as shown in Figs. 1 and 2. The recess 25 in the stationary mold half 23 has a forming bottom surface 25a for forming a surface of a foamed product 40 subsequently to be described. The forming surface 25a is formed with a grain pattern as illustrated in Fig. 4.
As shown in Figs. 1 and 2, the stationary mold half 23 is formed with a sprue 28 in communication with the second passage section 17b of the nozzle 15, a gate 29 contiguous to the cavity 27, and a runner 30 connecting the sprue 28 and the gate 29 to each other.
As shown in Fig. 6, the movable mold half 24 is formed therethrough with a plurality of pin bores 31 having their respective one ends which open at a forming bottom surface 26a of the recess 26, adjacent the peripheral edge thereof. Ejector pins 32 are received in the pin bores 31, respectively.
An injection molding apparatus constructed as described above operates in the following manner, to injection-mold the foamed product 40 shown in Fig. 7.
At the outset, as shown in Fig. 6, when the mold 20 is in an open position, an insert or a plate-like core 41 is mounted to the forming surface 26a of 1 the recess 26 in the movable mold half 24, by means of pressure sensitive adhesive double coated tapes or the like. The core 41 is preferably formed of a gas-permeable material such as, for example, a woody fiber board. The pin bores 31 are closed by the core 41, and a slight gap 45 is formed between the peripheral edge 41a of the core 41 and the peripheral edge of the recess 26, i.e., the peripheral edge of the cavity 27.
After attachment of the core 41, the movable platen 22 is moved toward the stationary platen 21 to bring the movable mold half 24 into engagement with the stationary mold half 23, thereby closing the mold 20.
On the other hand, a raw material designated by the reference numeral 50 in Figs. 1 and 2 is fed from the hopper 13 into the cylinder 11. Used as the raw material 50 is a foaming type plastisol having the expansion ratio of about 3 or less, for example, a plastisol containing vinyl chloride resin paste, plasticizer, foaming agent and fine or minute hollow glass beads. The screw 12 is rotated to feed the raw material toward the nozzle 15 and is retracted during this feeding stroke. The raw material is heated by the band heaters 14a to 14d and is gelatinized in the course of passing through the passage 16 of the cylinder 11.
The band heaters 14a to 14d are so controlled that temperature signals fed-back respectively from temperature sensors l9a to l9d embedded in the wall of the cylinder 11 are brought respectively to setting \

lZg8~
1 levels, that is, in such a manner that the te~perature of the raw material rises g~adually toward the nozzle 15. At the forward end 16a of the passage 16 closest to lhe nozzle 15, the raw material is heated up to a temperature level slightly lower than the foaming temperature.
When the gelatinized raw material is fully filled in a space in the cylinder 11 in front of the screw 12, the screw 12 is advanced to inject the raw material by an amount corresponding to the advance stroke. As described previously, the raw material is heated within the passage 16 of the cylinder 11, is gelatinized, and reaches the temperature level close to the foaming temperature at a location just before the nozzle 15. Friction heat is generated when the raw material is caused to pass through the first passage section 17a in the nozzle 15 which has the cross-sectional passageway area reduced abruptly, and through the second passage section 17b which is extremely narrow. Due to the friction heat, the raw material is brought to a temperature level equal to or above the foaming temperature at the second passage section 17b.
As a result, the raw material is foamed primarily within the second passage section 17b of the nozzle lS. Foams generated are small in size because the raw material is compressed under high pressure.
As described above, since the foaming takes place at the nozzle 15, it is easy to control the \

i2~8C~7`

1 foaming, and it is possible to always supply a constant amount of raw material from the injection molding machine 10 into the mold 20.
Since the plastisol is used as a starting raw material, the plastisol is maintained high in fluidity even if it is gelatinized. Moreover, since the fine glass beads are contained in the raw material, the raw material is further increased in fluidity. Thus, it is ensured that the raw material foamed at the passage 17 in the nozzle 15 is smoothly filled in the cavity 27 at high speed and at low injection pressure, without staying at the sprue 28, the runner 30 and the like.
The filled raw material is cooled and solidified to form a pad 42 shown in Fig. 7. As a result, there is obtained the foamed product 40 as illustrated in Fig. 7.
It is not easy to accurately grasp the foaming phenomenon, but it is supposed that the foaming raw material is reduced in pressure within the cavity 27 - and, therefore, the foams grow.
In the foamed product 40, the core 41 and the pad 42 disposed on one side of the core 41 are united with each other in a superposed fashion. Within the pad 42, as shown in Fig. 9, the foams are gradually reduced in size toward the surface 44a. In other words, the pad 42 is formed so as to have a cushion layer 43 in which the foams are relatively large in size and the expansion ratio is high, and a skin layer 44 contiguous to the cushion layer 43. In the skin layer 44, the foams are 1~38047 1 relatively small in size and the expansion ratio is low, or no foams are formed. The surface 44a of the skin layer 44 is free from foaming marks, and the grain pattern is applied to the surface 44a of the skin layer 44, as shown in Fig. 8, by transfer from the forming surface 25a. It is guessed that the skin layer 44 low in expansion ratio and the smooth surface 44a are obtained due to the facts that a portion of the raw material close to the cavity surface of the mold 20 is early cooled and is restrained from being foamed, and that the raw material is high in fluidity.
Moreover, because of the high fluidity, it is ensured that the raw material is filled at low injection pressure in the narrow gap 45 shown in Fig. 6, so that it is possible to form a fine end or terminal portion 42a. Accordingly, no flashes or the like appear on the peripheral edge of the foamed product 40 as viewed from the front side, thereby making it unnecessary to effect a terminal treatment or trimming.
Furthermore, because of the low injection pressure, the core 41 is prevented from being deformed, even if the core 41 is relatively low in strength.
During the injection, a part of gas generated due to the foaming at the nozzle 15 early enters the mold 20. ~owever, the gas is penetrated into the core 41 made of a woody fiber board, and is partially discharged to the outside through the pin bores 31 and grooves, not shown, formed in the parting surface of the ~2~

1 mold 20. Accordingly, good or superior filling of the raw material into the cavity 27 is not prevented by the gas. Further, since generation of foams large in size within the raw material is restrained by the glass beads, it is ensured that the skin layer 44 is obtained which has the fine surface 44a.
After the above-described injection molding, the movable mold half 24 is moved away from the stationary mold half 23 to open the mold 20.
Subsequently, the ejector pins 32 are actuated to separate the foamed product 40 from the movable mold half 24.
The terminal portion 42a may be formed so as to extend to the rear side of the core 41, as indicated by the reference numeral 42' in Fig. 7. In this case, a gap is required to be formed also between the rear side of the core 41 and the forming surface 26a of the movable mold half 24.
The core, before the injection of raw material, may be retained in position on the movable mold half by vacuum attracting means. Further, the core may be a resin-formed article or a sheet metal, in addition to the woody fiber board.
A fastener or fasteners for attachment of any component parts may be set on the stationary mold half, for example. In this case, the fastener or fasteners is or are united with a foamed product by injection of the foaming raw material.

i298~

1 The invention is applicable to interior accessories for automobiles and the like.
EXPERI MENT
A foaming type plastisol used as a raw material was composed of 100 weight part of a paste of PVC (polyvinyl chlorideJ resulting from emulsion polymerization, 85 weight part of DUP (diundecyl phthalate) as plasticizer, 6 weight part of Sn stabilizer, 5 weight part of Ba-Zn stabilizer, 2 weight part of ADCA (azodicarbonamide) as foaming agent, 2 weight part of Mg-Al lubricant, and 10 weight part of hollow glass beads. The hollow glass beads were very fine or minute, and were on the order of 100 micron meters.
Of the composition of the aforesaid plastisol, the components except for the glass beads were first mixed with each other by a kneader and, subsequently, vacuum degassing took place to form a mixture having its initial viscosity of 1500 cps. The glass beads were added to the mixture to obtain a plastisol whose viscosity is 3000 cps.
As a base material, a woody fiber board impregnated with phenol resin and having a thickness of 3 mm was set in the mold 20. The above plastisol was fed from the hopper 13.
Sections A to D of the cylinder 11 were controlled in temperature to 140 degrees C, 150 degrees C, 160 degrees C and 180 degrees C, respectively. The 1~98047 1 resin was injected at a low injection pressure of 60 to 200 Kg/cm2. The mold 20 was maintained at temperature on the order of about 20 to 30 degrees C, and the clamping force was 50 tons.
The resin reached the foaming temperature of 190 degrees C (temperature at which resin is foamed for one second) in the course of passing through the nozzle 15, and was foamed. The resin was elevated in temperature to 210 degrees C at the outlet of the nozzle 15.
In the formed article 40, the pad 42 was of a double structure having the cushion layer 43 of 1.5 mm thick and the skin layer 44 of 0.5 mm thick. The surface 44a of the skin layer 44 was good in appearance and feeling or touch. The terminal portion 42a of 0.5 mm thick was also finely formed at the peripheral edge 41a of the core 41. The surface 44a of the skin layer 44 was completely free from pin holes, irregular color, jetting, flow marks, sharkskin and weld lines.
Moreover, wales and two-stage foaming phenomenon did not occur which frequently appear in foamed products.
Physical properties of the pad 42 were such that the specific gravity was 0.85, the tension strength was 73 Kg, the elongation percentage was 225%, the tear strength was 33.6 Kg/cm, and the mean expansion ratio including the skin layer 44 and the cushion layer 43 was 1.8.
Fig. 10 shows an arrangement of a cylinder 111 ~2~8~'7 1 and a nozzle 115, employed in carrying-out of a method according to another embodiment of the invention. The cylinder 111 is formed therethrough with a passage 116 having a uniform cross-sectional area over the entire length. The nozzle 115 is screwed into the forward end of the passage 116. The nozzle 115 is formed so as to have its length longer than that of a universal or general-purpose nozzle, and is formed with a first passage 117a, a second passage 117b and a third passage 117c. The passage 117a is in particular long and is tapered gently as compared with that of the general-purpose nozzle. The second passage 117b is short and uniform in cross-sectional area over the entire length.
The third passage 117 is tapered gently. In case of the lS nozzle 115, friction heat is generated during passing of raw material elevated to a temperature level close to the foaming temperature, through the first passage 117a gradually decreasing in cross-sectional area, and through the narrow second passage 117b. As a result, primarily in the second and third passages 117b and 117c, the raw material is elevated to a temperature level higher than the foaming temperature so that foaming takes place. In the embodiment illustrated in Fig. 10, a band heater 114 is disposed also about the nozzle 115. The band heater 114 is in response to a signal fed-back from a temperature sensor 119 embedded in the nozzle 115, to maintain the nozzle 115 at a temperature level slightly lower than the foaming 4'7 1 temperature of the raw material. Other feature and arrangement of the embodiment illustrated in Fig. 10 are substantially similar to those of the embodiment shown in Fig. 1. Thus, in Fig. 10, parts or components like or similar to those shown in Fig. 1 are designated by the same or like reference numerals, and the description of such like or similar parts and components will therefore be omitted.

Claims

1. A method of injection-molding foamed products, comprising the steps of:
heating a raw material consisting of a foaming type plastisol within a cylinder of an injection molding machine to gelatinize the raw material;
injecting the gelatinized raw material into a cavity of a mold through a nozzle of the injection molding machine, the raw material being foamed at the nozzle during the injection, and the raw material in the foamed state being filled in the cavity of the mold; and cooling and solidifying the foamed material within the cavity of the mold.

2. A method as defined in claim 1, wherein said gelatinized raw material is brought to a temperature level higher than a foaming temperature due to friction heat during passing through the nozzle of the injection molding machine, and is thereby foamed.

3. A method as defined in claim 1, wherein, with an insert being beforehand housed within the cavity of the mold, said raw material in the foamed state is injected into the cavity and is cooled and solidified so that a resin pad is formed in united relation to said insert and in a superposed fashion.

4. A method as defined in claim 3, wherein said insert is formed of a gas permeable material.

5. A method as defined in claim 4, wherein said insert is made of a woody fiber board.

6. A method as defined in claim 3, wherein said insert is in the form of a plate, and is so disposed as to define a narrow gap between a peripheral edge of said insert and a peripheral edge of said cavity, the raw material being filled in said gap to form a terminal portion of said pad.

7. A method as defined in claim 1, wherein as said foaming type plastisol, a mixture is used which contains a vinyl chloride resin paste, a plasticizer and a foaming agent.

8. A method as defined in claim 7, wherein fine hollow glass beads are added to said mixture.