JP2000167861A - Method for gas pressure injection molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To completely prevent a shrink by filling a molten resin in a mold so as to form a block in which no gas is intruded in a portion having a fear of leaking pressurized gas. SOLUTION: A molten resin is injection filled in a cavity 4 in the state that a mold 1 is closed. Thereafter, pressurized gas is supplied to an introducing passage 8 provided in the mold 1. The gas is press injected from a cavity surface 4b side into the cavity 4 through a gap between a gas injecting pin 6 and a moving side mold 3. The gas is press injected between a rear surface of a molding except a block surrounded by an auxiliary rib and the surface 4b corresponding thereto to urge a surface of the molding to its corresponding cavity surface 4a. The gas is arrived at a root of the rib while urging the molding to the surface 4a. Thus, the gas arriving at the root of the rib cannot exceed the rib to prevent leakage of the gas and to realize prevention of a sink.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method, and more particularly, to a gas pressure injection molding method involving pressurized gas injection between a resin injected into a cavity and a cavity surface. .

[0002]

2. Description of the Related Art Generally, in injection molding, when molding a molded product having a thick portion protruding on the back surface (non-design surface) side, the resin shrinks due to cooling, so that it is possible to cope with the thick portion on the back surface side. It is widely known that a recess called a sink is formed on the surface (design surface) side of a molded product to be formed.

Conventionally, the most common method for preventing sinking is to increase the injection pressure, extend the injection time, and cool the resin in the cavity to some extent while applying the supply pressure of the molten resin (resin pressurization method). Are known. However, shrinkage prevention by the resin pressing method is disclosed in
As shown in Japanese Patent No. 75247, the molding conditions are different depending on the thickness of the molded product and the like, so that the molding operation becomes complicated and sufficient sink prevention cannot be achieved unless a high resin pressure is applied. There is a problem that it causes burrs and the work load for removing the burrs increases. In addition, when an excessive resin pressure is applied, there is a problem in dimensional accuracy such that warpage occurs in a molded product. Furthermore, in the resin pressurization method, pressure is easily transmitted to a thick portion near the gate, but pressure is not sufficiently applied to a thick portion away from the gate portion, and depending on the position of the thick portion, the pressure is not completely increased. For example, the sink cannot be eliminated.

In Japanese Patent Application Laid-Open No. 50-75247, after injecting a molten resin into a cavity, one side of a molded product is pushed up with a valve body, and one side of the molded product and one side of the molded product are molded. A pressurized gas injection molding method is proposed in which a cavity is formed between the core and a pressurized gas, and a pressurized gas is press-fitted into the cavity and the other surface of the molded product is pressed against the corresponding cavity surface. .

In this gas pressure injection molding method, a pressurized gas is injected instead of applying a resin pressure in the resin pressurization method, and the occurrence of sink marks is prevented by the pressurized gas. However, in this method, there is no disclosure as to prevention of leakage of pressurized gas from a mold dividing surface, an ejector pin, a slide core, or the like when a complicated mold generally used in injection molding is used.

Japanese Unexamined Patent Publication No. Hei 4-501090 discloses that a molten resin having a volume smaller than the volume of a cavity, specifically, a molten resin having a volume of 90 to 95% of the volume of a cavity is injected and then left in the cavity. A pressurized gas injection molding method for pressurizing a pressurized gas into an empty space is described. In this gas pressure injection molding method, a method is described in which a pressurized gas is press-fitted into a predetermined position surrounded by ribs or the like, but a method for preventing gas leakage when a practical mold is used is described. There is no effective description.

Further, in the specification of W093 / 14918, in a gas pressure injection molding method, in order to increase the pressure efficiency of a pressurized gas, a sealed structure for preventing gas from leaking from a parting surface of a mold is disclosed. A method is disclosed which uses a mold in which a weir having a tapered shape such as a triangle is provided in a cavity, in addition to a mold. In this case, a method is described in which the ejector pin is sealed with an O-ring, and further the O-ring is sealed up to the mold parting surface.
This method requires a lot of trouble in manufacturing the mold, and when a mold for producing a molded product having an opening is used, a pressurized gas is prevented from flowing into a mold cavity surface which forms the surface of the molded product. Is not described at all.

Further, in the specification of WO96-02379, a mold having a sealing structure in which a sealing material is applied to a parting surface or the like of a mold is used. 30 to 90% of the difference between the volume of the molten resin and the volume of the resin that shrinks when cooled to room temperature
After filling an excess resin), a pressurized gas is injected between the molten resin mass and the mold, and the molten resin mass is pressed against the inner surface of the mold cavity to form the mold. ing.

However, in this method, it is necessary to provide a sealed chamber capable of gas sealing up to around the protruding pin, for example, which causes a problem that the load on the apparatus is large, and that air or gas in the mold is removed from the inside of the mold cavity. However, there is a problem that it is necessary to fill a relatively large amount of excess resin or to fill the resin at a high pressure in order to drive the resin. In addition, JP-A-10-146857 and JP-A-10-146873 disclose:
A method is disclosed in which a mold is filled with a molten resin, a closed section for pressurized gas injection is formed by using a molded product side wall and an auxiliary rib, and then gas is pressed into the closed section. However, in this method, there is no detailed description on a method for a molded article having an opening and the like, and specific measures for a mold having a complicated shape are not sufficient.

That is, in the conventional gas pressure injection molding method, there is no simple and reliable method for preventing leakage of the pressurized gas.

[0011]

However, in recent years,
Demand for large molded products such as housings of OA equipment and home electric appliances, and furthermore, automobile parts and the like is increasing, and demand for thinner molded products for cost reduction of products is also increasing. In the case of a thin and large molded product, a reinforcing portion generally called a rib or a boss is usually provided on the back surface in order to maintain strength. The thicker the ribs and bosses are, the higher the reinforcing effect is, and a flow assisting effect that allows the resin to be easily filled in the mold is obtained.

However, as a method for preventing the sink of the thick portion as described above, a pressurized gas is injected between the molten resin filled in the mold and the mold to press the molten resin onto the design surface of the mold. When using pressurized gas injection molding, it is necessary to prevent the gas pressurized into the mold from leaking out of the mold as much as possible.However, there is a gap in the general mold core that connects with the atmosphere through which gas can pass. It often exists.

The present invention has been made in view of such a conventional problem, and has as its object to completely prevent sinking by a gas pressure injection molding method.

[0014]

According to the present invention, in order to achieve the above object, a mold cavity is filled with a molten resin and a pressurized gas is injected between the filled resin and the mold. Then, in injection molding in which the molded product design surface is pressed against the corresponding cavity surface, the mold is melted in the mold so that a section where gas does not enter into the area where the pressurized gas may leak is formed. An object of the present invention is to provide a gas pressure injection molding method characterized by filling a resin.

The object to be molded according to the present invention is particularly effective for a molded article having a thick portion protruding toward the rear surface, which is liable to cause sink marks on the surface. The thick portion includes, for example, a locally protruding thick portion such as a rib or a boss, as well as a region in which the thickness is varied over a wide range over a fixed portion. Further, the present invention is effective for a mold on the side into which gas is injected, that is, a mold having a portion that communicates with the atmosphere through a gap formed in a mold cavity surface that forms a back surface side with respect to a molded product design surface. It is. It is also effective for a mold having a shape in which a part of the mold core on the side into which gas is injected is in contact with the mold cavity forming the design surface of the molded product. Here, the mold core portion is a portion that forms a mold convex portion opposite to a cavity that forms a molded product design surface, and is a portion that generally forms a mold concave portion that forms a boss or a rib. The gap formed in the mold core portion described here has a clearance through which gas can pass though a molten resin cannot normally enter, and a slide core, a dividing surface of the mold core, a inserting piece,
Depending on the presence of a protruding pin, etc., on the mold structure or
It is a gap inevitably formed in manufacturing. Further, the slide core here is a part (one part) that forms the core, and is a part that moves (so-called slides) in a direction different from the cavity and the core that are divided when the mold is opened and closed. There is a gap through which the pressurized gas can pass through the contact surface between the pressurized gas and the mold core as the main mold, and the pressurized gas may leak through the mold mating surface (parting). Also, the division surface of the mold core is a mating surface formed when each member is combined when the mold core is composed of a plurality of members, and the pressurized gas also passes through this mating surface. There is a gap that can be formed, and the pressurized gas may leak through this gap. In addition, the insert piece has a structure in which part of the mold core is incorporated into the main mold of the mold core, and there is a gap between the main mold and the insert piece that allows the passage of pressurized gas. However, when the insert is in contact with the mold mating surface (parting), the mold is connected to the atmosphere through the parting surface, or a bolt and a mold for fixing the insert to the main mold are used. There is a case where the pressurized gas leaks due to conduction with the atmosphere through a gap or the like. In addition, a gap through which the pressurized gas can pass usually exists between the protrusion pin and the mold, and the pressurized gas may leak through this gap.

In the case where a molded article having an opening is produced, a part of a mold core portion is formed in a convex shape, and the convex portion comes into contact with a mold cavity forming a design surface of the molded article. Use a mold. In this case, the gas injected from the mold core side may leak from the convex part of the mold core to the mold cavity surface side forming the molded product design surface. In the present invention, a closed section is formed by the rib or the rib and the side wall with the molten resin filled in the mold so as to form a section excluding a portion where the pressurized gas may leak. Here, the rib forming the closed section is placed at a position in contact with the bottom surface, and a rib having a height equal to or more than the bottom wall thickness is continuously formed to surround a portion where the pressurized gas may leak, The closed section is formed such that the rib and the side face are continuous. Here, when the closed section is formed by the rib and the side wall, it is desirable that the rib be located near the upper part of the side wall at a position where they are in contact with each other. Here, the vicinity of the upper part is the height of the side surface of 9
It means that the rib exists up to a height of 0% or more, and more preferably, the rib has a shape forming a closed section up to the height of the side wall.

The press-fitting of the pressurized gas in the present invention may be performed by a pin that forms a slit that is opened in a slit shape that does not allow the molten resin to enter at the time of injection but allows the pressurized gas to pass therethrough.
A sintered body, a valve body having a poppet mechanism, or the like can be used. The resin that can be used in the present invention is not particularly limited as long as it is generally called a thermoplastic resin. For example, rubber-reinforced styrene resins such as polystyrene, high impact polystyrene, and medium impact polystyrene, styrene-acrylonitrile copolymer (SAN resin), acrylonitrile-butyl acrylate rubber-
Styrene copolymer (AAS resin), modified polyphenylene ether resin (m-PPE), acrylonitrile-ethylenepropyl rubber-styrene copolymer (AES), acrylonitrile-polyethylene chloride-styrene copolymer (ACS), ABS resin (for example, , Acrylonitrile-
Styrene resins such as butadiene-styrene copolymer, acrylonitrile-butadiene-styrene-alpha-methylstyrene copolymer, acrylonitrile-methyl methacrylate-butadiene-styrene copolymer), acrylic resins such as polymethyl methacrylate (PMMA),
Olefins such as low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP); vinyl chlorides such as polyvinyl chloride and polyvinylidene chloride; ethylene vinyl chloride vinyl acetate copolymer; ethylene vinyl chloride Vinyl chloride copolymer resin such as copolymer, polyethylene terephthalate (PETP, PE
T), polybutylene terephthalate (PBTP, PB
T) and other polyester resins, polycarbonate (P
C), polycarbonate resins such as modified polycarbonate, and polyamide resins such as polyamide 66, polyamide 6, and polyamide 46. Polyoxymethylene copolymer,
Polyacetal (POM) resin such as polyoxymethylene homopolymer, other engineering resin, super-engineering resin such as polyethersulfone (PES), polyetherimide (PEI), thermoplastic polyimide (TPI), polyetherketone (PE)
K), polyether ether ketone (PEEK), polyphenylene sulfide (PSU), etc., cellulose derivatives such as cellulose acetate (CA), cellulose acetate butyrate (CAB), ethyl cellulose (EC), liquid crystal polymer, liquid crystal aromatic polyester And the like.

Also, thermoplastic polyurethane elastomer (TPU), thermoplastic styrene butadiene elastomer (TSBC), thermoplastic polyolefin elastomer (TPO), thermoplastic polyester elastomer (TP
EE), thermoplastic vinyl chloride elastomer (TPV)
C), and a thermoplastic elastomer such as a thermoplastic polyamide elastomer (TPAE) can also be used. In the present invention, one or more blends of the above-described thermoplastic resins may be used, or a filler and / or an additive may be contained.

Hereinafter, further description will be made with reference to the drawings. FIG. 1 is a sectional view showing an example of a mold 1 used in the present invention. As shown in the figure, the mold 1 is composed of a fixed mold 2 and a movable mold 3, and a cavity 4 is formed between the mold 2 and the molten resin at the time of molding. FIG.
Fig. 1 shows a molded product 15 formed by the mold 1;
The plan view is a perspective view of the opening. Molded product 15
Is a box-shaped member having two side walls 16a to 16d around the periphery, two thick ribs 12 and a thick boss 13 provided across the base portion 18 on the bottom surface, and a back surface (non-design Surface), and the outside is the surface (design surface). Further, a portion surrounded by the auxiliary rib 14 forms a section 19 where the pressurized gas does not enter. 6a indicates a gas injection port formed by a gas injection pin 6 described later.

On the other hand, on the side of the cavity surface 4b corresponding to the back surface of the molded product 15, an ejector pin 5 connected to the atmosphere is embedded. A gas injection pin 6 is provided on the cavity surface 4b side. This gas injection pin 6
Is embedded in the movable mold 3 with its tip facing the cavity 4 from the cavity surface 4b, and is provided with a pressurized gas source (not shown) via a valve (not shown).
The pressurized gas sent from the gas introduction path 8 connected to the mold 4 is supplied to the cavity 4 through a gap left between the movable mold 3 and the moving mold 3. In the figure, reference numeral 7 denotes an O for preventing the pressurized gas from escaping from the joint of the mold components.
It is a ring. Reference numeral 10 in the figure denotes a slide core for forming an opening in a molded product. In the figure, reference numeral 11 denotes an insert piece in which a part of the mold core is manufactured as a separate part.
And 11a shows a dividing line by a mating surface of the main mold and the nest, and is generally designed to have a clearance through which the molten resin cannot enter, but there is a possibility that a pressurized gas may pass through. FIG. 12a shows a cavity portion for forming a thick rib 12, and 13a shows a cavity for forming a thick boss. In the figure, reference numeral 14 denotes an auxiliary rib provided to prevent leakage of pressurized gas injected between the mold core surface and the molten resin, and 14a denotes a mold cavity for forming the rib. Is shown.

FIG. 3 is a schematic view of a molded article having an opening at the bottom of the molded article. FIG. 3 (a) is a plan view and FIG. 3 (b).
Is a perspective view. FIG. 4 is a schematic view of a mold for molding FIG. Further, the molding method of the present invention will be described with reference to FIGS. First, a molten resin is injected into the cavity 4 with the mold 1 closed. After the injection and filling of the molten resin, a pressurized gas is supplied from a pressurized gas source (not shown) to a gas introduction path 8 provided in the mold 1. The pressurized gas may be, for example, air or carbon dioxide gas, but is preferably an inert gas such as nitrogen. The type of gas used is preferably selected depending on the pressure of the pressurized gas, molding material, molding conditions, and the like. The pressure of the pressurized gas varies depending on the type of resin used, the shape of the molded product, the size of the molded product, and the like, but is usually 10 to 250 kgf / cm ² .

The pressurized gas supplied to the gas introduction passage 8 passes through a gap between the gas injection pin 6 and the movable mold 3 and is pressed into the cavity 4 from the cavity surface 4b side. This pressurized gas is applied to the molded product 1 other than the section surrounded by the auxiliary rib 14.
5 and the corresponding cavity surface 4b are pressed in, thereby pressing the surface of the molded product 15 against the corresponding cavity surface 4a. The pressing by the pressurized gas suppresses the occurrence of sink marks on the surface of the molded product 15 on the cavity surface 4a side, improves the transferability on the cavity surface 4a side, and reduces the appearance defect due to sink marks, uneven gloss, and the like. Problems are also reduced. Further, the releasability when the molded product 15 is taken out from the mold 1 is also improved.

In order for the pressure of the pressurized gas introduced into the cavity 4 to effectively press the surface of the molded product 15 against the cavity surface 4a, the cavity 4
It is necessary to prevent the pressurized gas injected into the mold from leaking out of the mold 1 and the mold cavity surface 4a. FIG.
FIG. 3 is a schematic view of a state near an ejector pin surrounded by an auxiliary rib 14 when a pressurized gas is press-fitted into a cavity 4 after filling a mold 1 with a molten resin.

The pressurized gas injected from the gap around the gas injection pin 6 reaches the base of the auxiliary rib 14 while pressing the molded product 15 against the cavity surface 4a. Here, the pressurized gas reaching the base of each rib 14 presses the rib 14 in the direction shown by the arrow, and the pressurized gas cannot cross the rib, so that the pressurized gas and the molten resin in the portion where the ejector pin is located Since it does not penetrate into the mold cavity, leakage of the pressurized gas from the gap with the ejector focus mold can be prevented. In general, the higher the pressure of the pressurized gas, the easier the leakage occurs. However, as shown in FIG. 5, the higher the pressure of the pressurized gas, the stronger the ribs 14 are pressed against the cavity surface, and the higher the pressure of the pressurized gas, the more the ribs 14 get over the ribs. Will be blocked.
Further, at this time, the surface side corresponding to the position of the thick rib 12 where sink is likely to occur in normal molding is prevented from being sink by pressing with the pressurized gas which is press-fitted.

In the present invention, the width of the thick portion 12 is set to w,
When the thickness at the periphery of 2 is t, w ≧ (3/5)
This is effective for a molded product 15 having a thick portion 12 such that t is satisfied. That is, the molded article 15 having such a thick portion 12 is difficult to prevent sink in normal injection molding, but according to the present invention, it can be surely solved.

In the present invention, the rib and the side wall are substantially T
It is preferable that the auxiliary rib is set to have the same height as the side wall at the portion 21 where the character shapes are connected (FIG. 2B). In the present invention, “equal or higher” means
The height of the auxiliary rib means preferably 90% or more of the height of the side wall. More preferably, the height of the auxiliary rib is designed to be the same as that of the side wall. FIG. 2B shows this most preferred embodiment. When the mold shown in FIG. 4 is used to produce a molded product having an opening as shown in FIG. 3, the gas press-fitted between the mold and the molten resin is supplied by the auxiliary ribs 14.
If there is no, as shown by the arrow, it enters between the mold cavity surface forming the surface of the molded product and the resin, and as a result, causes sink on the surface of the molded product with respect to the thick portion 12. This phenomenon can also be prevented by forming the gas press-in exclusion region by the auxiliary rib 14.

Here, the thickness of the auxiliary rib 14 for forming the gas injection exclusion area 19 is not particularly limited as long as it is less than the thickness of the thick part 12, but it is particularly for reasons such as strength reinforcement. If it is not necessary to increase the thickness of the base portion 18, it is preferable that the thickness be equal to or less than half the thickness t1 of the base portion 18, which is designed by ordinary injection molding. The design of the height of the auxiliary rib 14 can be changed by molding conditions, etc.
Above, preferably 5 mm or more, more preferably 7 mm or more. Also, the auxiliary ribs may be tapered (angled) to facilitate release, as used in normal injection molded product design.

In the present invention, the amount of resin to be injected is not particularly limited, and can be used for any of short shots, just shots, and overpacks. In order to ensure the prevention of leakage, it is preferable to fill the mold with an excessive amount of resin and sufficiently fill the auxiliary rib 14 with molten resin.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION

[0030]

Example 1 A molded article having a box shape and a main part thickness of 2.0 mm as shown in FIG. 6 was formed. The injection of the pressurized gas and the gas seal are the same as those described in FIG.
The gas injection position G was set between each side wall and each rib a, b.
The thickness of each rib shown by a and b is 4.0 mm.

The molding material is high impact polystyrene (HIPS), ABS resin, modified PPE resin (m-PP
E) was used, and a molding was molded by immediately injecting a pressurized gas after injecting an excessive amount (103% by weight) of the molding material relative to the cavity volume. The respective molding conditions are shown below. In addition, the ratio of the filled molten resin, in each mold, resin used, molding conditions,
A molded product was molded by just molding to just fill the cavity, and the weight of the molded product was calculated by setting the weight of the molded product to 100% by weight and measuring the weight of the molded product filled with an excessive amount of molten resin. (A) Material used: HIPS ・ Cylinder temperature: 200 ° C. ・ Injection pressure: 100 kg / cm ² (gauge pressure) ・ Resin holding pressure: 20 kg / cm ² (gauge pressure) ・ Pressurized gas pressure: 100 kg / cm ² ( (Gauge pressure) Gas injection time: 3 seconds Gas holding time: 20 seconds (B) Material used: ABS ・ Cylinder temperature: 230 ° C. ・ Injection pressure: 100 kg / cm ² (gauge pressure) ・ Resin holding pressure: 20 kg / cm ² (Gauge pressure) Pressure of pressurized gas: 100 kg / cm ² (Gauge pressure) Gas injection time: 3 seconds Gas holding time: 20 seconds (C) Material used: m-PPE ・ Cylinder temperature: 260 ° C. ・ Injection pressure : 100kg / cm ² (gauge pressure) resin dwell pressure: 20kg / cm ² (gauge pressure) of - pressurized gas pressure: 100kg / cm ² (gauge pressure) during the gas injection : 3 sec Gas retention time:; 20 seconds the molded article appearance is determined by the naked eye, yet thick rib 12
And the sink of the design surface on the opposite side were measured. Gas pressure gas 1
After press-fitting at 00 kg / cm ² for 3 seconds, the press-fitting valve was closed and the pressurized gas was held in the mold for 20 seconds. At this time, the residual pressure of the pressurized gas in the mold (the pressure value after holding for 20 seconds) ) Was measured. Table 1 shows the measurement results.

[0032]

Comparative Example 1 Example 1 except that the auxiliary rib 14 was not formed.
Gas pressure injection molding was performed using the same resin as in Example 1 and using the same resin and the same method as in Example 1. The appearance of the molded product was visually judged, and the sink of the design surface opposite to the thick rib 12 was measured. Table 1 shows the measurement results.

[0033]

[Table 1]

From the results shown in Table 1, the molded article according to the present invention is:
The molded article had good appearance and almost no sink mark. In the case of a complex mold that is difficult with conventional pressurized gas injection molding or requires a lot of time for the mold structure, pressurized gas injection molding was difficult, but the present invention makes it very simple. , The industrial use range has been greatly expanded.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a mold used in the present invention.

FIG. 2 is a view showing a molded product obtained by the mold of FIG. 1;

FIG. 3 is an example of a molded product according to the present invention.

FIG. 4 is a view showing a mold for producing the molded product of FIG. 3;

FIG. 5 is an explanatory view of a pressurized gas leakage preventing action according to the present invention.

FIG. 6 is an example of a molded article according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 mold 2 fixed mold 3 moving mold 4 cavity 4a, 4b cavity surface 5 ejector pin 6 gas injection pin 7 O-ring 8 gas introduction path 9 resin gate 10 slide core 11 insert piece 11a insert piece and mold Dividing surface 12 Thick rib 12a Cavity forming thick rib 13 Thick boss 13a Cavity forming thick boss 14 Auxiliary rib 14a Cavity forming auxiliary rib 15 Molded product 16 Side wall 17 Opening formed by slide core Reference Signs List 18 base part 19 gas injection exclusion area 20 opening part located in base part 21 joint part between auxiliary rib and side wall

Claims (2)

[Claims] 1. A mold resin is filled in a mold cavity,
In a gas pressure injection molding method in which a pressurized gas is injected between a filled resin and a mold and a molded product design surface is pressed against a corresponding cavity surface, the pressurized gas may leak. A gas pressure injection molding method, characterized in that a mold is filled with a molten resin so as to form a section in which gas does not enter a portion. 2. The gas pressure injection molding method according to claim 1, wherein the section in which gas does not enter is formed by a resin rib filled in a mold.