JPH07117084A - Method and mold for injection molding of thermoplastic resin - Google Patents

Abstract

PURPOSE:To prevent the generation of a sink on the surface becoming the appearance surface of an injection molded product. CONSTITUTION:A fixed mold and a movable mold are opposed to each other to form a mold cavity and the molding surface of the mold cavity corresponding to the non-appearance surface of a molded product is coated with a heat insulating membrane whose contact angle is larger than that of a molding resin material to collectively generate the volumetric shrinkage caused by the cooling of a molten resin on the side of the coating layer to prevent the generation of a sink on the opposed appearance surface.

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 a mold for injection molding which prevent sink marks from being generated on the surface of a molded product obtained by injection molding of a thermoplastic resin. It is possible to provide a product having an excellent surface appearance in a housing of a machine, a cover or the like.

[0002]

2. Description of the Related Art A method for producing a thermoplastic resin molded product (hereinafter abbreviated as a molded product) by injection molding has advantages such as high productivity and easy molding of a complicated shape. Widely used for daily sundries. In the case of a molded product that has ribs, bosses, etc., or thick-walled parts that are formed for the purpose of increasing the mechanical strength of the molded product, etc. It often occurs concentrated on a part. In general, the roots of ribs, bosses, etc. are thicker than other parts, so sink marks are particularly likely to occur. Moreover, since the outer surface is often a flat surface, the skin layer itself contracts in the surface direction and is easily peeled off from the cavity surface. Since it is difficult to peel from the skin, the skin layer on the external surface is more likely to be drawn into the negative pressure inside, and sink marks are more likely to occur on the external surface.

The depth of the sink mark increases as the rib thickness and the boss diameter increase, and also increases as the resin temperature increases. In other words, the larger the shrinking volume and the larger the temperature difference until solidification, the more the amount of shrinkage accumulates.
The sink mark grows. The occurrence of such sink marks on the appearance surface of the molded product significantly impairs the commercial value of the molded product, and is therefore one of the most deficient defects to be avoided.

In order to prevent such sink marks, various methods have been conventionally proposed. For example, Japanese Patent Publication 61-
53208, JP-A-63-268611,
JP-A-64-63122 and the like introduce a method of using high-pressure gas as an auxiliary. According to these methods, surface defects such as air traps and intense flow marks are likely to occur in the molded product. Further, there are serious problems such as dangers associated with the use of high-pressure gas, difficulty of gas handling, and excessive investment in control equipment. In addition, JP-A-60-1
Japanese Patent No. 51015 discloses a method in which molten resin is injected and filled into a slightly opened mold cavity, and then the mold is clamped, and the molten resin is pressure-compressed to offset the volume contraction of the resin, thereby preventing sink marks in the entire molded product. However, in JP-A-60-92813, after injection molding, only the thick portion is put into a molten state and compressed in accordance with a sink to cancel the volume contraction of the resin.
A method for preventing sink marks in the thick portion has been proposed. Although it is possible to eliminate sink marks by these methods, it is necessary to have a heating device and a compression device, and the cost is high,
It has the drawback of limiting the compression direction and constraining the product design.

On the other hand, in Japanese Unexamined Patent Publication No. 56-167410, the back surface of the place where the sink mark is generated is locally heated by a heater to a temperature not lower than the mold temperature, and the cooling rate at that place is made slower than the other places. The method to obtain a molded product without sink marks is
Japanese Patent Publication No. 6-9126 discloses that a through hole of a porous member is provided on the back surface of a portion of a molded article where sink marks are likely to occur, and compressed air is introduced from the through hole to press the back surface of the molded article. A method for obtaining a molded product without sink marks on the surface is introduced. These methods have the same idea as the present invention in that the sink mark is not generated on the surface which is the appearance of the molded product, and it is generated on the back surface thereof to prevent the defect of the molded product, but the former is the same. Since the mold temperature needs to be locally raised to a high temperature or cooled, it has the drawback of lengthening the molding cycle. The latter has a through hole of a porous member in the mold and further supplies compressed air. There is a problem that the cost is high because a special mold structure and ancillary equipment are required.

[0006]

Generally, the molten resin introduced into the mold cavity is cooled from the moment of contact with the metal mold surface forming the mold cavity to form a surface skin layer,
It gradually solidifies while growing the skin layer toward the inside. At this time, a negative pressure is generated inside the resin layer due to the rapid volume contraction that occurs at the glass transition temperature, and acts as a force to draw the surface skin layer inside. It is said that at the moment when this force exceeds the self-shape retaining property of the skin layer, the skin layer peels from the cavity contact surface and is drawn inward, which is a significant cause of sink marks. The present invention solves various problems of the above-mentioned conventional methods by reversely utilizing such a phenomenon that is a significant cause of sink marks. Therefore, the object of the present invention is to
In particular, it is an object of the present invention to provide an injection molding method that can easily and economically eliminate the occurrence of sink marks on the external surface of a molded product that has ribs, bosses, etc., and thick parts that are likely to occur during injection molding. To do.

[0007]

SUMMARY OF THE INVENTION The inventors of the present invention have found that a mold cavity surface corresponding to a non-appearance surface of a molded product is covered with a heat insulating material having a higher contact angle with water than a molding resin material. The skin layer formed in contact with the heat insulating material surface of the molten resin injected into the cavity is delayed in skin layer formation as compared with the skin layer in contact with the opposite mold cavity surface, and at the same time, the peelability is enhanced and The present inventors have completed the present invention by finding that shrinkage is likely to be induced and generated on the coated surface side in the form of a sink, and sink can be prevented from occurring on the opposing external surface. That is, according to the present invention, a part or the whole of the inner surface of the mold on the fixed side or the movable side corresponding to the non-appearance surface of the molded product in the mold cavity for injection molding is brought into contact with water rather than the filled thermoplastic resin. With a heat insulating thin film with a high angle formed, a predetermined amount of molten thermoplastic resin is injected and filled into the mold cavity and cooled, so that the volumetric shrinkage caused by the solidification shrinkage of the molten thermoplastic resin is not applied to the external surface. It is an injection molding method of a thermoplastic resin that is concentrated on one side to prevent sink marks from being formed on the opposite appearance surface. In particular, the heat insulating thin film has a contact angle with water of 100 degrees or more and a thin film thickness of 0.5 μm.
It is the injection molding method of the thermoplastic resin characterized by being in the range of up to 1000 μm. Further, in the method of injection molding a thermoplastic resin, the heat insulating thin film is made of a fluororesin. Furthermore, in the mold cavity for injection molding, the contact angle to water is 100 degrees or more and the thin film has a thickness of 0 on a part or the whole of the inner surface of the fixed side or the movable side corresponding to the external surface of the molded product. It is a mold for injection molding of a thermoplastic resin, characterized in that a heat insulating thin film made of a fluorine-based resin having a range of 0.5 μm to 1000 μm is formed.

The present invention will be described in detail below. Looking first at the phenomenon of sink marks, when the molten resin fills the mold cavity and solidifies, the resin material reaches its glass transition point at the time when it is most involved in sink mark generation. It's time. The abrupt volume contraction due to the glass transition occurs at an extremely early stage of the cooling and solidifying process and is an instantaneous event in terms of time. At this time, a negative pressure is generated inside the resin layer due to the rapid volume contraction caused by the glass transition, and acts as a force to draw the skin layer inside. It is said that at the moment when this force exceeds the self-shape retaining property of the skin layer, the skin layer peels from the cavity contact surface and is drawn inward, which is a significant cause of sink marks. Therefore, the sink mark phenomenon occurs in the skin layer on the side where the growth of the skin layer is delayed and the solidification is insufficient and the self-shape retention is poor.

However, it cannot be said that the side having a slower growth of the skin layer and insufficient solidification and poor self-shape retention does not always come off first. For example, where there are ribs, bosses, etc. where sink marks are most likely to occur, sink marks occur without exception on the exterior surface composed of a simple surface close to a flat surface, and on the back surface where ribs, bosses, etc. are installed. Is known not to occur. The back surface on which ribs, bosses, etc. are installed has a complicated shape, which deteriorates cooling efficiency and delays the growth of the skin layer, resulting in poor self-shape retention. This phenomenon cannot be fully explained by the above-mentioned conventional theory of generation, because the self-shape retention property of is quickly secured. In the cooling step, the surface layer is cooled, so that the portion that undergoes a rapid volume contraction when the molten resin undergoes glass transition moves from the surface layer toward the inside, and the stress due to the volume contraction occurs in the internal molten resin layer. Although it is transmitted and accumulated as a negative pressure inside the molten resin layer, stress in the surface direction is also generated on the surface portion solidified to form the skin layer due to the subsequent heat shrinkage. That is, the sink mark phenomenon is considered to be determined by the force relationship between the force of the negative pressure generated inside the resin layer due to the volume contraction to draw the skin layer inside and the adhesive force between the mold cavity surface and the skin layer. The latter effect is greater.

Generally, in a thick wall portion having a simple shape, there is no significant difference in the adhesive force between the fixed type and the movable type surface steel material forming the cavity with the resin, so that the growth of the skin layer and the resulting self-shape retention property. The surface on which the sink mark occurs depends on the degree of. Therefore, in the present invention, the mold cavity surface corresponding to the appearance surface of the molded product is left as the conventional metal surface,
A resin skin layer formed by coating a part or the whole of the mold surface of the mold cavity corresponding to the non-appearance surface of the facing molded product with a heat-insulating thin film having a higher contact angle with water than the molding resin material. And the peelability between the coated surface and
The volume shrinkage generated during the cooling process of the molten resin is absorbed in this portion. In other words, the surface of the molded product becomes easy to move by peeling the surface of the molded product from the mold cavity surface by the action of the high peeling property between the coating surface having a high contact angle and the resin skin layer, and the deformation amount due to shrinkage is drawn in to cause sink marks. , Which reduces the sink marks on the opposite appearance surface. This occurs on the non-appearance side because the amount of shrinkage when the molded resin undergoes a phase change from melt to solid is constant with respect to the mold cavity volume after the gate, which is the resin inlet, is closed. It is possible to prevent the occurrence of sink marks on the surface, which is the appearance surface of the molded product, because the amount of the sink marks disappeared from the opposite mold non-coated surface.

The mold used in the present invention has a contact angle with water of 100 ° or more, preferably 100, on a part or the whole of the surface of the mold cavity corresponding to the non-appearance surface of the molded product.
It is characterized by being formed of a heat insulating thin film having a thickness of 0.5 μm to 1000 μm at a temperature of up to 110 degrees. Since the mold-coated surface having such a contact angle with water is larger than the contact angle with water of a general thermoplastic resin of 70 to 90 degrees, it exhibits repulsion to the molding resin material and the interface is easily peeled off.

When the thermoplastic resin is injection-molded according to the present invention, the contact angle of water on the mold-covering surface of the mold cavity corresponding to the non-appearance surface of the molded product is 100 degrees or more. Sinks are concentrated on the mold-covered surface, and almost no sinks are generated on the opposing appearance surface. When the contact angle of the heat insulating thin film is less than 100 degrees, the peeling property from the molding resin material is small, and there is no difference in the peeling property between the coated mold surface and the uncoated mold surface, and the effect is not exhibited. Further, in the present invention, the heat insulating thin film having a contact angle with water of 100 degrees or more should be formed only on the non-appearance surface. It is not preferable because it occurs on the side. From such a principle, it is possible to prevent the occurrence of sink marks by forming a heat insulating thin film such as a polyimide resin having a contact angle to water of 100 degrees or less, preferably 70 to 90 degrees, which is close to that of the molding resin, on the opposite appearance surface sides. It is possible to do so.

Therefore, as a material capable of forming a heat insulating thin film satisfying the above-mentioned conditions and having heat resistance capable of withstanding the injection molding temperature of a general thermoplastic resin, polytetrafluoroethylene [(-CF2-CF2- ) N], tetrafluoroethylene-hexafluoropropylene copolymer [-CF2-CF2-) n-(-CF2-C (CF3
-) Fm], polychlorotrifluoroethylene [(-C
F2 -CClF-) n], tetrafluoroethylene-purple oroalkyl vinyl ether copolymer [(-CF
2--CF2--n-(-CF2-C (ORf) F-)
m], ethylene-tetrafluoroethylene copolymer [(-CH2-CH2-CF2-CF2-) n], polyvinylidene fluoride [(-CH2-CF2-) n],
Ethylene-chlorotrifluoroethylene copolymer [-C
H2-CH2-CF2--CClF-) n], polyvinyl fluoride [-CH2-CHF-) n] and the like are desirable, and these fluorine-based resins may be used alone or in a blend. Also, it may be used in combination with other inorganic materials.

As a method for coating the surface of the mold cavity with a fluorine resin alone or in a blend, an aqueous dispersion of the above fluorine resin or a solvent diluent is applied by spraying or brushing, and then heat treatment is performed. It can be carried out to form a coating film. As a method of using a fluorine-based resin in combination with another inorganic material, nickel / phosphorus, chromium, ceramics, etc. are formed on the surface of the mold so as to be porous, and then the above-mentioned fluorine-based resin is used for the porous coating. After impregnating the quality coating, heat treatment can be performed to form a composite heat insulating thin film of a fluororesin / inorganic material. As a method of forming the above-mentioned inorganic porous film on the metal surface, for example, electroless plating method for nickel and phosphorus, and electroplating method for chromium,
For ceramics, methods such as a thermal spraying method, a plasma jet method and an ion plating method can be applied.
The thickness of the fluororesin coating film and the fluororesin / inorganic composite coating is in the range of 0.5 μm to 1000 μm, more preferably 1 to 500 μm. If it is thinner than 0.5 μm, the thin film strength cannot withstand injection molding for a long time, and if it is thicker than 1000 μm, the heat insulating property makes the molding cycle longer and it is difficult to adjust the product size and the mold cavity size. May be. As the resin exhibiting the effect of preventing sink mark by the injection molding method of the present invention, polystyrene, rubber-modified polystyrene, AS resin,
Most of the thermoplastic resins such as ABS resin, polypropylene, polyethylene, vinyl chloride resin, polycarbonate resin, etc., or resins mixed with a filler reinforcing material are available.

[0015]

As described above, in the present invention, by coating the mold surface of the mold cavity corresponding to the non-appearance surface of the molded product with the heat insulating thin film having a contact angle higher than that of the molding resin material,
With the skin layer formation delayed, the skin layer is selectively peeled off preferentially, and the sink phenomenon caused by the volume shrinkage of the resin during cooling is selectively generated at the skin layer peeling portion on the cavity coating surface side. , It is possible to prevent the occurrence of sink marks on the surface which is the appearance surface of the molded product corresponding to the surface not covered with the cavity. That is, it is possible to eliminate the sink mark on the appearance surface by only making the mold cavity surface corresponding to the non-appearance surface of the molded product high peelability, not the mold cavity surface corresponding to the appearance surface of the molded product. Corresponding cavities can be textured, plated, machined, etc., as with conventional metal molds. Further, since the heat insulating coating is thin, the amount of heat shield is small and the molding cycle is hardly affected. As described above, the effect of the present invention is to easily solve the sink mark defect on the external surface of the molded product, which has been difficult to solve conventionally.

[0016]

EXAMPLES Examples will be described below with reference to the drawings. The contact angle of the heat insulating coating material with water is 23 ° C. at room temperature using an automatic contact angle meter CA-Z type (manufactured by Kyowa Interface Science Co., Ltd.).
It was measured in.

Example 1 In this example, as shown in FIG. 1, 60 × 150 m
4 mm in thickness and 5 in height on the back surface of the flat plate portion 2 having a thickness of m and a thickness of 2 mm.
Molded article 1 having ribs 3, 4, and 5 having a length of 90 mm and a length of 90 mm
Was molded. A gate 6 for injecting a molten thermoplastic resin is formed on the side surface of the flat plate 1. The ribs 3, 4, 5 are significantly thicker than the plate thickness of the flat plate 1. Due to this thickness, the surface side 3a of the molded product 1 corresponding to the positions of the ribs 3, 4, 5 when using a normal injection molding method,
Sinking occurs remarkably in the parts 4a and 5a. As a mold commonly used in the present embodiment, as shown in FIG. 2, the front side 7a of the molded product 1 corresponds to the fixed side cavity 7, and the back side 8a of the molded product 1 corresponds to the movable side cavity 8. Then, the movable side cavity was made into a nest type like the movable side nest 9, and the nest was removed when performing various coating processes. Molten resin is sprue 13, runner 1
4, the gate 6 is filled and the cavity 12 is filled. Pre-hardened steel (made by Daido Steel: NAK80) was used as the steel material of the mold.

After cleaning the surface of the movable cavity corresponding to the non-appearance rear surface of the molded product with acetone, roughening treatment with sandblasting and further cleaning with acetone,
Polytetrafluoroethylene-based composite enamel coating (manufactured by Daikin Industries, Ltd., Polyflon TFE / TC-710)
0) was coated using an air spray gun and heat-treated to form a film having a thickness of 40 μm. The contact angle of the obtained film with water was 104 degrees. The mold is attached to an injection molding machine with a clamping force of 75 tons and the mold temperature is 50 ° C.
And a polystyrene resin having a resin temperature of 210 ° C. (Estyrene H-65 manufactured by Nippon Steel Chemical Co., Ltd .;
The contact angle with water was 77 degrees. The melt was injection-filled with a filling time of 1.5 seconds, cooled for 20 seconds, the mold was opened, and the molded product was taken out. When the surface condition of the obtained molded product 1 was examined, almost no sink marks were seen on the appearance side of the molded product on the mold non-covered surface side at the positions 3a, 4a, 5a where the sink marks were originally likely to occur. On the other hand, sink marks were seen on the side surfaces and the periphery of the ribs 3, 4, and 5 on the back surface of the molded product on the mold coating side.

Example 2 After the surface of the cavity on the movable mold side corresponding to the non-appearance surface of the molded product of the mold used in Example 1 was roughened by sandblasting, an alumina / titanium oxide ceramic was prepared by gas spraying. Is sprayed to form a film having a thickness of 100 μm, and after rough grinding, polytetrafluoroethylene fine particle water dispersion (Polyflon TFE dispersion manufactured by Daikin Industries, Ltd.) is impregnated and heat-treated to form a polytetrafluoroethylene-impregnated ceramic layer. Formed. The contact angle of water of the obtained film with water was 106 degrees. The above mold was attached to an injection molding machine having a mold clamping force of 75 tons, the mold temperature was maintained at 60 ° C., and the resin temperature used in Example 1 was 21 in the mold cavity.
A polystyrene resin melt at 0 ° C. was injection-filled with a filling time of 1.5 seconds, cooled for 20 seconds, the mold was opened, and the molded product was taken out. When the surface condition of the obtained molded product 1 was examined, almost no sink marks were found on the surface of the molded product on the mold-uncoated surface side at positions 3a, 4a, and 5a where the sink marks were originally likely to occur. On the other hand, sink marks were seen on the side surfaces and the periphery of the ribs 3, 4, and 5 on the back surface of the molded product on the mold coating side.

Comparative Example 1 A molded product obtained by molding the mold used in Example 1 under the same conditions as in Example 1 using the surface of the movable cavity of the mold without polishing to form a film without polishing. On the surface of No. 1, large sink marks were generated at positions 3a, 4a and 5a.

Comparative Example 2 The movable cavity of the mold used in Example 1 had a thickness of 200 mm.
A 0 μm polytetrafluoroethylene sheet was screwed to form a coating layer. The contact angle of the polytetrafluoroethylene sheet with water was 105 degrees. The above mold was attached to an injection molding machine having a mold clamping force of 75 tons, the mold temperature was maintained at 60 ° C., and the mold cavity was filled with the polystyrene resin melt having a resin temperature of 210 ° C. used in Example 1 for 1.5 seconds. After injection and filling, and cooling for 40 seconds, the mold was opened and the molded product was taken out. When the surface condition of the obtained molded product 1 was examined, almost no sink marks were found on the surface of the molded product on the mold non-covered surface side at positions 3a, 4a and 5a where the sink marks were originally likely to occur. The rib is deformed.

[0022]

According to the injection molding method of the present invention, the mold surface of the mold cavity corresponding to the non-appearance surface of the molded product is coated with a heat insulating thin film having a contact angle higher than that of the molding resin material to cool the molten resin. The volume shrinkage caused by this is concentratedly generated on the side of the heat insulating coating layer to reduce the occurrence of sink marks on the opposing appearance surface, and to produce a molded product with less appearance defect marks on the appearance surface. Then, these effects are molded by using a mold covered with a heat-insulating thin film that specifies the contact angle and film thickness of the thin film of the mold cavity, which is the non-appearance surface, without the need for special auxiliary equipment. It's just realized. Further, there are great economic effects such that the injection molding method of the present invention can be carried out without departing from general molding conditions and molding environment.

[Brief description of drawings]

FIG. 1 is a perspective view of a molded product 1.

FIG. 2 is a vertical cross-sectional view of a mold.

[Explanation of symbols]

 1 Molded Product 2 Flat Plates 3, 4, 5 Back Side Rib 6 Gate 7 Fixed Side Cavity 8 Movable Side Cavity 9 Movable Side Nest 10 Fixed Mold 11 Movable Mold 12 Cavities 3a, 4a, 5a Positions where sink marks are likely to occur

Claims (4)

[Claims] 1. A contact with water, rather than a thermoplastic resin to be filled, on a part or all of the inner surface of the mold on the fixed side or the movable side corresponding to the non-appearance surface of the molded product in the mold cavity for injection molding. With a heat insulating thin film with a high angle formed, a predetermined amount of molten thermoplastic resin is injected and filled into the mold cavity and cooled, so that the volumetric shrinkage caused by the solidification shrinkage of the molten thermoplastic resin is not applied to the external surface. Injection molding method of thermoplastic resin that is concentrated on one side to prevent sink marks on the opposing appearance surface. 2. The heat insulating thin film has a contact angle of 1 with water.
The thickness of the thin film is not less than 00 degrees and is 0.5 μm to 1000 μm.
The thermoplastic resin injection molding method according to claim 1, wherein 3. The thermoplastic resin injection molding method according to claim 1, wherein the heat insulating thin film is made of a fluororesin. 4. A mold cavity for injection molding, wherein a part or the whole of the inner surface of the fixed side or the movable side corresponding to the external surface of the molded product has a contact angle to water of 100 degrees or more and a thin film. An injection molding die of a thermoplastic resin, characterized in that a heat-insulating thin film made of a fluororesin having a thickness of 0.5 μm to 1000 μm is formed.