WO1996007527A1 - Low-pressure injection molding method - Google Patents

Abstract

A method of low-pressure injection molding a synthetic resin by using a metal mold obtained by coating a wall surface which constitutes a cavity with a layer of a heat resisting resin of a thickness of more than 0.1 mm and not more than 0.5 mm, in which method injection including low-speed injection with a flow velocity in a cavity of a synthetic resin mold of not higher than 100 mm/sec is carried out. According to the low-pressure injection molding method of the present invention, an injection molded product having an improved glossiness, substantially free from sink marks and hesitation marks and having excellent eternal appearance and a high stress crack resistance is obtained.

Description

 Description Low-pressure injection molding <Technical field>

 The present invention relates to a low-pressure injection molding method for synthetic resin.

 More specifically, the present invention relates to a synthetic resin using a metal mold obtained by coating a wall constituting a cavity of a metal mold body with a heat-resistant resin layer having a specific thickness range. The present invention relates to a low-pressure injection molding method for a synthetic resin in which injection including low-speed injection in a mold cavity is performed.

 Background technology>

 As a synthetic resin injection molding method, a low-pressure injection molding method with low injection pressure or mold clamping force has been widely introduced and put into practical use.

 As a low-pressure injection molding method, gas assist is introduced in PI astics T echno 1 ogy, Aril, 44 (1994) or USP 4, 101, 617, etc. G There is an injection molding method. This molding method involves injecting a synthetic resin into the mold cavity and then injecting a gas body, making use of the fact that the pressure loss of the gas body is small to even the end of the synthetic resin flow. This is a method of transmitting the injection pressure to the mold and performing low-pressure injection molding.

In EP 0,599,099 A1, the present inventors have proposed an oligomer assist injection molding method and a liquid. The assist injection molding method is a method of similarly performing low-pressure injection molding using an oligomer or a liquid.

 The Sandwich injection molding method, which has many introductions, including BP 1, 156, 217, injects the first synthetic resin first, and then the second synthetic resin. In this method, a structure having a sandwich structure is formed while satisfying the mold cavity. When a synthetic resin containing a foaming agent is used as the second synthetic resin, low pressure injection molding can be performed by the foaming force of the inner core resin.

 In addition, the mold clamping plate is slightly retracted to inject the synthetic resin in a state where the mold cavity is deepened (thickened), and then the mold clamping plate is advanced to move the mold cavity to a predetermined depth. Injection compression molding, in which the resin thickness is reduced to a small value and the entire mold cavity is filled with synthetic resin at low pressure, has recently become widely used.

 The Hettinger low-pressure injection molding method, which performs injection at extremely low speeds with extremely low injection pressure, is introduced in Synthetic Resins, Vol. 37, N0.9, page 34 (1991), etc. It is used well depending on the shape of the molded product.

These low-pressure injection molding methods have the advantage that the filling pressure of the synthetic resin can be reduced as compared with general injection molding methods, but the flow velocity of the synthetic resin in the mold cavity is slower. Disadvantage. As described above, if the flow velocity of the synthetic resin in the mold cavity becomes slow, the reproducibility of the mold surface on the surface of the injection molded product is poor, the surface becomes rough, and the molded product has a high resistance to dust. The track cracking properties (hereinafter abbreviated as "SCR") have also deteriorated, and improvements are required.

 In the conventional injection molding method, the molding conditions are variously changed to improve the reproducibility of the mold surface and the solvent resistance. Among the various molding conditions, the most influential is the mold temperature. It is effective to raise the mold temperature to near the softening temperature of the synthetic resin. However, when the mold temperature is increased, the cooling time required for cooling and solidifying the plasticized resin increases, and the molding efficiency decreases. Therefore, there is a need for a method that does not increase the mold temperature, improves the reproducibility of the mold surface, or that does not increase the required cooling time even if the mold temperature is increased. ing. As an example of the latter, there is a method in which holes for heating and cooling are attached to a mold, and heating and cooling of the mold are repeated by alternately flowing a heat medium and a coolant. However, this method has the disadvantages that it consumes a large amount of heat and requires a long cooling time.

 In conventional injection molding, the inventors of the present invention described a method for improving the reproducibility of the mold surface by coating the mold wall surface forming the mold cavity with a substance having low thermal conductivity. It is proposed in WO publication 93Z0696980.

 However, in general, there are few reports on the study of the interrelationship between the thickness of the heat insulating eyebrows covering the mold wall surface, various molding conditions, and the surface condition of the injection molded product.

<Disclosure of invention> Generally, gas assist injection molding, liquid assist injection In the low-pressure injection molding method used in the present invention, such as a molding method, an oligomer assist injection molding method, an injection compression molding method, a Sandwich injection molding method, a Hettinginger low-pressure injection molding method, etc. The flow velocity of the synthetic resin in the mold cavity is slow, and often the flow velocity of the resin changes rapidly during the flow. Since the flow speed of the resin is slow, the reproducibility of the mold surface is poor, and the flow speed changes suddenly.When the flow speed changes suddenly, the appearance of the resin, which is generally called a hedging mark, is poor. A roaming mark occurs. Furthermore, if the flow velocity in the synthetic resin mold cavity is reduced, the solidified eyebrows become thicker, and the SCR (stress resistance) generally deteriorates. Therefore, it has been required to improve these appearance defects and SCR. The present invention has been made for the purpose of responding to this problem.

 The present inventors have studied the relationship between low-speed flow of resin, low injection pressure, and a mold covered with an insulated eyebrow in the low-pressure injection molding method. The present invention was completed by studying the relationship with SCR and the like.

That is, the present invention provides a mold obtained by coating a wall of a metal mold body, which constitutes a cavity, with a heat-resistant resin layer having a thickness of more than 0.1 mm and not more than 0.5 mm. The present invention relates to a low-pressure injection molding method for a synthetic resin in which a flow including a low-speed injection at a flow rate of 10 O mm Z seconds or less is performed using a synthetic resin mold cavity. Further, the present invention relates to the synthetic resin, wherein the synthetic resin contains a resin reinforced material, wherein the molded article after molding the resin has substantially no protrusion of the resin reinforced material on the surface thereof. It relates to the molding method.

 The present invention also relates to the above-mentioned molding method, wherein the synthetic resin containing the resin-reinforced material is a rubber-reinforced synthetic resin.

 The present invention also relates to the above-mentioned molding method, wherein the rubber-reinforced synthetic resin is a rubber-reinforced polystyrene resin.

 Furthermore, the present invention relates to the above-mentioned molding method, wherein the thickness of the heat-resistant resin eyebrow is in the range of 0.15 to 0.45 mm.

 Further, in the present invention, the low-pressure injection molding method comprises the steps of: a gas-assist injection molding method, a liquid-assist injection molding method, an oligomer-assist injection molding method, an injection compression molding method, and a core resin. The present invention relates to the above-mentioned molding method comprising at least one of the group consisting of a sandwich injection molding method made of foamed resin and a Hettinga low-pressure injection molding method.

 <Brief description of drawings>

 FIGS. 1A, 1B and 1C show the low pressure injection molding method in which the present invention is carried out (gas assist injection molding method, oligomer assist injection molding method, liquid assist injection molding method). FIG.

 Figure 2 shows the relationship between temperature and viscosity for various synthetic resins and various fluids.

3A and 3B show another molding example of the gas assist injection molding method. FIGS. 4A and 4B are views for explaining an injection compression molding method as a low-pressure injection molding method in which the present invention is carried out.

 Figures 5, 6, 7 and 8 show the temperatures near the mold wall surface when the temperature of the mold body was 50 and the temperature of the rubber-reinforced polystyrene was 240 ° C. Indicates the distribution change (calculated value).

 Figure 9 shows the flow pattern in a synthetic resin mold cavity.

 Figure 10 shows the shear heating (calculated value) of a synthetic resin flowing through a mold cavity.

 Figures 11A and 11B show that the use of a heat-insulating layer-coated mold improves SCR (stress crack resistance) even at low speed injection molding.

 Fig. 12 shows the time-dependent change of the resin pressure applied to the mold wall surface during injection molding of synthetic resin.

 FIG. 13 shows a method for measuring SCR.

 FIGS. 14A and 14B show the relationship between the gloss of the molded product and the flow velocity in the mold cavity of the synthetic resin and the thickness of the polyimide as the heat-resistant resin layer, respectively.

 FIGS. 15A and 15B show schematic views of cross sections near the surface of the injection molded article.

Explanation of reference numerals

 1 Mold body

 Type 2 key bit

3 Synthetic resin 4 Where the flow velocity changes

5 Gas body

6 Molded product

 7 Molding 156

8 gate

 9 Resin flow end

 1 0 Thick part

 1 1 Gas channel

 1 2 Mold

1 3 solidified eyebrows

 1 4 Flow tip

 1 5 Flow toward the wall

 16 Flow rate of synthetic resin

 1 7 Center of mold cavity

1 8 Speed distribution curve

 1 9 Shear rate distribution curve

 2 0 Solidified layer or surface layer

 2 1 Inner core

 2 2 Concave side of molded product

2 3 Convex side of molded product

 2 4 Switch resin to gas

2 5 Test piece

 2 6 Where bending stress is applied

 2 7 Load

2 8 Rubber particles <Best mode for carrying out the invention>

 The synthetic resin used in the present invention is a thermoplastic resin that can be used for general injection molding, for example, a polyolefin such as polyethylene polypropylene, Polystyrene, styrene-acrylonitrile copolymer, styrene-based resin such as rubber-reinforced polystyrene, polyamide, polyester, polycarbonate G, methacrylic resin, and vinyl chloride resin. Preferably, the synthetic resin contains 1 to 60% of a resin reinforcement. Examples of the resin-reinforced material include various types of rubber, glass fibers, various types of fibers such as carbon fibers, and inorganic powders such as talc, calcium carbonate, and kaolin. A synthetic resin that can be used favorably is a rubber-reinforced synthetic resin, and among them, a rubber-reinforced polystyrene resin is particularly preferably used.

 The rubber-reinforced polystyrene resin used in the present invention refers to rubber-reinforced polystyrene, ABS resin, AAS resin, MBS resin, and the like in which a rubber phase is distributed in an island shape in a resin phase.

Rubber-reinforced polystyrene is a resin in which a rubber phase such as polybutadiene or SBR is dispersed in an island shape in a resin phase of a polymer mainly composed of styrene. ABS resin is a resin in which rubber phases such as polybutadiene and SBR are dispersed in the form of islands in a resin phase of a copolymer mainly composed of styrene and acrylonitrile. . AAS resin is an ATA resin in a resin phase of a copolymer mainly composed of styrene and acrylonitrile. Is a resin in which the rubber phase of rubber is dispersed in the form of islands.The MBS resin has islands in the resin phase consisting of a copolymer mainly composed of styrene and methyl methacrylate. It is a resin dispersed in a shape.

 Furthermore, blends mainly composed of these resins can also be used in the present invention. For example, a blend with a rubber reinforced polystyrene resin blended with poly (vinylene ether) can be used favorably. The injection molded products of these resins molded by the present invention have an extremely good balance between performance and economy, and are suitable as light electric appliances, housing for electronic appliances, various daily necessities, various industrial parts, and the like. Used for The mold body of the mold specified in the present invention is made of iron or iron-based steel material, aluminum, or aluminum-based alloy, zinc alloy, base metal. It means a metal mold generally used for molding synthetic resins such as lithium-copper alloy. In particular, molds made of steel can be used successfully. 0

The heat-resistant resin used as the heat-insulating layer in the present invention has a glass transition temperature of 140 ° C or more, preferably 160 ° C or more, and / or a melting point of 230 ° C or more. Preferably, it is a heat-resistant resin of 250 ° C or higher. The heat conductivity of the heat-resistant resin of the present invention is generally 0.0001 to 0.02 cal / cm'sec * ° C, which is much smaller than that of metal. Further, the heat-resistant resin has a toughness of elongation at break of 5% or more, preferably 10% or more, and more preferably 15% or more. I like it. The tensile elongation at break is measured according to ASTMD 638, and the tensile speed at the time of measurement is 5 mm / min.The heat-resistant resin that can be favorably used as a heat insulating layer in the present invention is used in the main chain. It is a heat-resistant polymer having an aromatic ring, and examples thereof include various amorphous heat-resistant polymers and various polyimides that are soluble in organic solvents.

 Examples of the noncrystalline heat-resistant polymer include polysulfone, poly-tersulfone, polyarylsulfone, polyarylate, and polyolefin Xylene ether. .

 There are various types of polyimides, and linear high molecular weight polyimides and partially crosslinked polyimides can be used favorably. In general, linear high molecular weight polyimide has a large tensile elongation at break, is tough, and has excellent durability, and can be used particularly well. A linear high molecular weight polyimide having a repeating unit of the polymer shown in Table 1 can be used favorably.

In the present invention, the closer the coefficient of thermal expansion between the heat-resistant resin layer and the mold body, the better. Generally, in synthetic resin injection molding, heating and cooling are performed repeatedly during molding, and stress is generated between the heat-resistant resin and the mold body if the coefficients of thermal expansion differ greatly. Also, prior to injection molding, when the mold body is covered with a heat-resistant resin eyebrow, stress is generated similarly by heating and cooling. When this stress exceeds a certain value, the heat-resistant resin layer is separated from the mold body. Table 2 shows examples of the thermal expansion coefficient of the low thermal expansion polyimide. Various low thermal expansion polyimides can be used successfully. In the table, Bifix and Free can freely shrink the film when the polyimide precursor is imidized to form a polyimide phenol. The fixed force (Free) has the meaning that the polymer chain is fixed in a rectangular frame, the shrinkage that occurs during imidization is suppressed, and the polymer chain is oriented in-plane by the stress (Bifix). . After the polyimide precursor solution is applied to the mold body, the thermal expansion coefficient of the polyimide formed by heating has an intermediate value between Free and Bifix.

(Hereinafter the margin)

table 1

 Repeating unit for polymerization break

R: R, and R:

表 2

Table 2

The thermal expansion coefficient of the low thermal expansion Poryi Mi de [10- ⁵ K- "]

In general, injection molding has economic value where a molded article having a complicated shape can be obtained in one molding. In order to coat the complex mold surface with a heat-resistant resin and firmly adhere to the surface, a heat-resistant polymer solution or a solution of a heat-resistant polymer precursor is applied and then heated. It is most preferred to form a heat resistant polymer by heat treatment. Therefore, it is preferable that the heat-resistant polymer or the heat-resistant polymer precursor of the present invention can be dissolved in a solvent.

 As another example of the heat-resistant resin of the present invention, an epoxy resin, silicone resin, or melamine resin to which flexibility is imparted can also be used favorably. Particularly, a modified epoxy resin imparted with flexibility, for example, a modified epoxy resin modified with a tough resin can be used favorably.

 The adhesion between the heat-resistant resin layer as the heat insulating layer of the present invention and the mold body needs to be large, and is 0.5 kg Z 10 mm or more at room temperature, preferably 0.5 mm. 8 kg 10 mm width or more, more preferably 1 kg Z 10 mm width or more. Here, the above-mentioned adhesion force is expressed as a separation force when the closely adhered heat insulating layer is cut to a width of 10 mm and the cut width is pulled at a speed of 2 Om mZ in a direction perpendicular to the bonding surface. This separation varies considerably depending on the measurement location and the number of measurements, but it is important that the minimum value is large, and it is preferable that the separation be large and stable. . The adhesion in the present invention is the minimum value of the adhesion of the main part of the mold.

As a further embodiment of the present invention, heat resistance as a heat insulating layer In order to further improve the smoothness of the surface of the thin layer of resin eyebrows, to further improve the abrasion resistance of the surface, or to improve the releasability, the thickness of the heat-resistant resin layer is approximately Another material thinner than 1/5, preferably thinner than around 1Z10, can be coated on the heat-resistant resin layer if necessary.For example, synthetic resin sheets and molds A paint generally used as a hard coat, which is used for improving scratch resistance, can be applied to the surface of the glass. An example of this is a thermosetting silicone-based hardcoat, especially a hard-coated silicone-based hardcoat with an epoxy compound. Can be used well and is preferred for the present invention. In addition, it is possible to satisfactorily apply a fluororesin / silicone polymer in order to improve the releasability. If necessary, a thin metal layer can be coated on the heat-resistant resin layer as a heat insulating layer. For example, chrome plating or nickel plating can be coated on the heat insulating layer. In the case of providing a thin metal eyebrow, it is preferable that the thickness of the metal layer be 110 or less of the thickness of the heat-resistant resin layer.

The thickness of the heat-resistant resin layer as the heat insulating layer is appropriately selected within a range of more than 0.1 mm and 0.5 mm or less. Preferably it is 0.15 mm or more and 0.45 mm or less. In the low-pressure injection molding of the present invention, if the thickness of the heat insulating layer is 0.1 mm or less, the effect as the heat insulating layer cannot be sufficiently exerted. In other words, the invention relates to the flow velocity in a synthetic resin mold cavity. Temperature, the thickness of the heat-resistant resin layer as the heat insulating layer, and the reproducibility of the mold surface reproducibility of the molded product.The required thickness of the heat insulating layer was determined by the synthetic resin. There is a close relationship with the flow velocity in the mold cavity of the synthetic resin.If the flow velocity in the mold cavity of the synthetic resin is 10 Om mZ seconds or less, the thickness of the heat insulating layer exceeding 0.1 mm is required. Become.

 On the other hand, in general, in injection molding, the relationship between the mold temperature (T d) and the cooling time (0) required for the mold is theoretically expressed by the following equation.

^{θ = - {Ό 2/2} π a) - 1 n [{π /) {(T x - T d) / (T c one T d)}]

 Θ Cooling time (sec)

 D Maximum wall thickness of molded product (cm)

 T c Cylinder temperature (:. C)

 T x Curing temperature of molded product (° C)

 Thermal diffusivity of resin

 T d Mold temperature (T)

 The cooling time (0) is proportional to the maximum wall thickness (D) of the part and is a function of the (T x — T d) value. Covering the mold body with a heat-insulating layer acts in the same direction as increasing the thickness of the molded product and shortening the cooling time.

The thermal diffusivity of the heat-resistant resin used as the heat-insulating eyebrow and the synthetic resin injected are almost the same, and increasing the thickness of the heat-resistant resin layer as the heat-insulating layer by 0.5 mm requires the above formula. Equivalent to increasing the maximum thickness (D) of a molded product by 0.5 mm . Therefore, increasing the thickness of the heat-resistant resin layer as a heat-insulating layer to more than 0.5 mm is not economically favorable because the molding cycle time is extremely large.

 The surface of the mold body is covered with a heat-insulating layer made of a heat-resistant resin. When the injected heated resin comes into contact with the heat-insulating layer surface, the surface of the mold (heat-insulating layer) receives heat from the resin and heats up. The lower the thermal conductivity of the heat insulating eyebrows and the thicker the heat insulating layer, the higher the mold surface temperature. In the present invention, since molding is performed by low-speed injection, the mold surface temperature is softened for at least 0.4 seconds after the injected synthetic resin comes into contact with the cooled mold surface. It is preferable that the temperature is higher than the temperature. If, for example, there is no insulating eyebrow on the mold surface, the mold surface temperature will be almost the same as the mold body temperature after 0.1 seconds. As in the present invention, by covering the mold body with a heat insulating layer having a thickness of 0.1 to 0.5 mm, the mold surface is kept at a temperature equal to or higher than the softening temperature for 0.4 seconds. be able to.

 Changes in the mold surface temperature during injection molding can be calculated from the respective temperatures, specific heat, mature conductivity, density, latent heat of crystallization, etc. of the synthetic resin, the mold body, and the heat-resistant resin eyebrows as the heat insulating layer. You. For example, using ADINA and ADINAT (software Xa developed at the Massachusetts Institute of Technology) can be calculated by transient heat conduction analysis using the nonlinear finite element method.

The softening temperature of the resin described here means that the synthetic resin It is the temperature at which it can be deformed. For amorphous resin, the vicat softening temperature (ASTMD 1525), for the hard crystalline resin, the heat deformation temperature (ASTMD 648 load 18.6 kg / cm ² ), In the case of crystalline resins, these are the heat deformation temperatures (ASTMD 648, load 4.6 kg Z cm ² ). Examples of the hard crystalline resin include polyoxymethylene, Nylon 6, and Nylon 66. Examples of the soft crystalline resin include various polyethylene and polyproylene. Pyrene and the like.

The low-pressure injection molding method described in the present invention is an injection molding method in which the injection pressure of a synthetic resin and a liquid or liquid injected into a mold cavity is low. In general, the injection molding method is performed at a high injection pressure of about 100 kg / cm ² , whereas the low pressure injection molding method of the present invention uses a low pressure injection molding method including a low pressure injection of less than half of the injection pressure. It is.

 The low-pressure injection molding method of the present invention includes a gas-assist injection molding method, a liquid-assist injection molding method, an oligomer-assist injection molding method, an injection compression molding method, and a sand in which the core resin is a foamed resin. It is selected from at least one of the group consisting of the injection molding method and the Hettinger low-pressure injection molding method.

In the low-pressure injection molding method of the present invention, the synthetic resin is molded by injection molding including low-speed injection in which the flow velocity in the mold cavity is 100 mmsec or less. This low-speed injection includes various cases such as when the injection speed of the synthetic resin temporarily becomes low, when the flow stops momentarily, and when the injection speed is low overall. Species cases may be included. The injection speed can be calculated from both the flow distance of the synthetic resin in the mold cavity and the value obtained by measuring the flow time of the synthetic resin based on the advancement time of the injection screw. . When a synthetic resin is injected by a gas assist injection molding method and then a gas body is injected to satisfy the mold cavity, the synthetic resin is generally injected at a high pressure, and then the gas body is generally synthesized. Injection and molding are performed with less than half the pressure of resin, but the speed of resin flow after switching to low pressure gas is greatly reduced. When a high-pressure synthetic resin is switched to a low-pressure gas, a flow mark with a bad appearance is generated. This is commonly referred to as a hedging mark. The resin flows at a low speed beyond the hedging mark, and the reproducibility of the mold surface generally deteriorates.

 Hereinafter, the present invention will be described with reference to the drawings.

 In FIGS. 1A, 1B and 1C, the synthetic resin 3 is injected into the mold cavity 2 formed by the mold body 1 (FIG. 1A), and then the gas body 5 is injected into the mold cavity 2. 2

I B), then open the mold 1 and remove the molded product 6 (Fig.

I C ) . Although the synthetic resin 3 is injected at a high injection pressure, the injection pressure of the gas body 5 is significantly lower than the injection pressure of the synthetic resin 3 and less than half of the injection pressure of the synthetic resin. Therefore, generally, the flow velocity of the injected synthetic resin in the mold cavity changes in conjunction with the switching of injection from synthetic resin to gas. When this flow velocity changes4, the surface of the molded product is generally referred to as a roughness mark. Bad flow marks with bad appearance remain. Also, the reproducibility of the mold surface is poor in the area between the hedging mark and the flow end. In addition, since the resin is filled at a low flow rate, the SCR of the molded product also deteriorates.

 The present invention is a molding method which solves these problems, that is, a heat insulating layer having a thickness of more than 0.1 mm and not more than 0.5 mm is formed on the wall surface forming the mold cavity of the metal mold body. Synthetic resin that is injected using a mold obtained by coating the heat-resistant resin layer as described above, including a low-speed injection with a flow velocity in the mold cavity of the synthetic resin of 10 Om mZ seconds or less. According to this method, there is substantially no protrusion of the resin-reinforced material on the surface of the molded product, the reproducibility of the mold surface is excellent, and an injection molded product with improved SCR can be obtained. This has the effect of making it possible.

Similar results can be obtained by changing the gas body of gas assist injection molding to another fluid. FIG. 2 is a diagram showing the relationship between temperature and viscosity of various synthetic resins and various fluids shown in Table 3. Various fluids can be used in the same manner as long as the viscosity at the time of injection is not more than 1 Z50 of the viscosity of the synthetic resin injected first. The use of a liquid instead of a gas as the fluid is a liquid-assist injection molding method, and the use of a low-molecular-weight polymer is the oligomer-cast injection molding method. In this liquid assist injection molding method and the oligomer assist injection molding method, the same results as described with reference to FIGS. 1A, 18 and 1 are obtained. Three

S polymer

 Rubber reinforced polystyrene Asahi Kasei polystyrene HIPS492

HIPS # 492

 ■■ ίιΒ 丄 菜, Μ: ヽ tftu ヽ

 (tolL W)

 Polypropylene Asahi Kasei Polypro M1500

 PP M1500

 .Π, feb ヽ * fj ヽ High-density polystyrene Suntech-ichi HD J300

PE J300 Low molecular weight polystyrene Hymer SB—150

PS SB150

 Ligang + Ϊ¾ naju, U (JU pani / ki 1 shigoshu W) St) low molecular weight polystyrene hymer ST— 120

PS ST120

 Average molecular weight (10, 000) (manufactured by Niyo Todani ®_ £ Co., Ltd.) Low molecular weight high density polyethylene high wax 800 ワ ッ ク ス

 HD-PE800P

 Hira, JTT; HO, (h) D J / EB1L W) S) Low molecular weight low density polyethylene High wax 720P

 LD-PE720P

 , Τΰ / Κι¾ · /, ULU 开 ^ bffllL child W) SH) low molecular weight low density polyethylene high wax NL800

LD-PENL800

 (Viscosity method: average molecular S6, 400) (Mitsui Petrochemical Co., Ltd.) Low molecular weight polypropylene biscol 550-P

PP 550P

 (Vapor osmotic pressure method: Average molecular weight S4,000) (Sanyo Chemical Industries, Ltd.) Polyethylene glycol Polyethylene glycol 6000

PEG 6000

(Weight average molecular weight 7,500) (Wako Pure Chemical Industries, Ltd.) Figures 3A and 3B show another molded article formed by gas assist injection molding. In FIGS. 3A and 3B, the molded article 7 has a thick portion 10 formed from the gate 8 to the resin flowing end 9. When a synthetic resin is injected from the gate 8 and then a gas body is injected, the gas body proceeds selectively in a thick portion, and a hollow part generally called a gas channel 11 is formed. The pressure of the injected gas is evenly transmitted to the resin flowing end 9 by the gas pressure of the gas channel 11. Figure 3B shows the cross section A-A 'of the molded product in Figure 3A. Even with such molded products, the same problems of mold surface reproducibility and SCR as shown in Figs.1A, 1B and 1C occur.However, in the present invention, an appropriately selected heat resistance A good molded product can be obtained by using a mold covered with a conductive resin eyebrow.

4A and 4B show an injection compression molding method which is one of the low pressure injection molding methods described in the present invention. In FIGS. 4A and 4B, the mold clamping plate of the injection molding machine was slightly retracted, so that the moving side of the mold 1 was retracted, and the mold cavity 2 was deepened (thickened). In this state, the synthetic resin 3 is injected (FIG. 4A), and then the mold clamping plate is advanced, whereby the moving side of the mold 1 is advanced to reduce the mold cavity to a predetermined depth (thickness). ) To fill the mold cavity with synthetic resin at low pressure (Fig. 4B). Conventionally, in general, when synthetic resin is injected and then the depth (wall thickness) of the mold cavity begins to decrease, the flow velocity of the synthetic resin in the mold cavity usually changes. Poor appearance Flow marks occur. In addition, when the depth (wall thickness) of the mold cavity is reduced, when the mold cavity is filled with a low pressure, the flow velocity of the synthetic resin in the mold cavity is low, and therefore, that portion is not sufficient. The reproducibility of the mold surface of the molded product becomes poor, and the glossiness becomes poor. In addition, the SCR of the molded article also deteriorates due to the slow flow rate of the synthetic resin in the mold cavity. In contrast, in the present invention, the appearance defect and the SCR are improved by using a mold covered with a heat-resistant resin layer as a selected heat-insulating layer.

 Below, the reproducibility of the mold surface and S C

The reason why R is extremely bad and the reason why it is improved by using the mold covered with the specific heat-resistant resin eyebrow of the present invention will be described with reference to FIGS. 5 to 12. FIG.

5, 6, 7 and 8 show that the temperature of the mold body is 50. C shows the change (calculated value) of the temperature distribution of the synthetic resin layer or Z near the mold wall surface and the heat-resistant resin layer when the rubber-reinforced polystyrene was injection-molded at 240 ° C. <The numerical value of each curve in the figure indicates the time (seconds) since the heated synthetic resin came into contact with the cooled mold wall. The heated synthetic resin comes into contact with the mold wall surface and cools down rapidly, while the mold surface receives heat from the heated synthetic resin and rises in temperature _(as shown in the figure, the mold surface is insulated). When coated with a heat-resistant resin layer (Polyimide) as a layer (Figs. 6 and 7), the temperature rise on the heat-resistant resin eyebrow surface that comes into contact with the synthetic resin increases, and the temperature decreases Will also be smaller. When the mold surface is covered with the heat-resistant resin eyebrows, the shorter the time after the synthetic resin comes into contact with the mold wall, the higher the mold surface temperature will be. The same effect can be obtained as when the mold temperature is greatly increased.

 Figure 8 shows how the temperature of the polyimide surface of the thermal insulation layer changes with time since the resin came into contact with the surface. 0.4 seconds after the resin comes into contact with the mold surface, the mold must be coated to a thickness exceeding 0.1 mm with polyimide to keep the mold surface above the curing temperature of the synthetic resin. . Heat resistance The thickness of the resin layer varies depending on the mold temperature and the softening temperature of the synthetic resin. Generally, the mold temperature is around 50 ° C and the softening temperature of the synthetic resin that is practically used is 10 ° C. The temperature is around 0 ° C or higher, which determines the thickness of the heat-resistant resin as the heat insulating eyebrows.

As in the present invention, in a low-pressure injection molding method in which the flow velocity of a synthetic resin in a mold cavity is generally low, the synthetic resin reproduces the mold surface on the contact surface after the synthetic resin comes into contact with the mold wall surface. It takes time for the injection pressure to be applied to a certain degree, and the die surface temperature when the injection pressure is applied is thought to decrease. When molding by a molding method in which the flow rate of the resin changes rapidly during injection molding, the appearance tends to change suddenly due to the change in the flow rate of the resin. In the present invention, a change in the appearance of the resin is suppressed by setting the thickness of the heat insulating member to a thickness exceeding 0.1 mm. In addition, for at least 0.4 seconds after the synthetic resin contacts the mold surface, The appearance is improved by injection molding under the condition that the surface temperature is equal to or higher than the softening temperature of the synthetic resin.

 FIG. 9 shows the flow pattern of the synthetic resin in the mold cavity, and FIG. 10 shows that shear heat is generated by the flow of the resin.

 In Fig. 9, when the plasticized thermoplastic resin is injected into the mold cavity composed of the cooled mold 1 and 2, the injected synthetic resin contacts the cooled mold wall and solidifies immediately. Form eyebrows 13. Then, the synthetic resin that is drawn and injected advances in the solidified layer 13, reaches the flow front (F 10 w Front) 14, and then flows toward the mold wall 15, so-called Fountain F 1 ow. As shown in the velocity distribution curve 18, the flow velocity 16 of the synthetic resin is the highest at the center 17 of the mold cavity, and becomes slower as it approaches the solidified layer 13. The shear force shown in the shear rate distribution curve 19 is generated in the flowing synthetic resin, and heat is generated in proportion to the shear force. This calorific value is extremely large when the injection speed is high, and is generated at a position in contact with the solidified layer, and functions to prevent the solidified layer from increasing. Figure 10 shows how much heat is generated depending on the injection speed. Fig. 10 shows the temperature distribution of the synthetic resin immediately after filling the mold by calculating the calorific value under the following conditions while changing the flow speed of the synthetic resin.

 Mold: steel

Die key size: 60 x 29 x 2 mm (width x length x thickness) Synthetic resin Asahi Kasei Polystyrene 4 9 2

 Synthetic resin temperature 240 ° C

 Mold temperature 50 ° C

 Injection filling time 0.1 seconds, 0.4 seconds, 1.0 seconds Software used for calculation: MOLDFLOW

Temperature distribution in the resin cross section immediately after resin filling at three points: 25 mm (A), 15 mm (B) and 2665 mm (C) from the MOLDFL OW / FLOW gate made by PTYL td. It is shown. Generally, when the flow rate of a synthetic resin is high, the calorific value is extremely large, and the heat is generated along the solidified layer, so that the growth of the solidified layer is suppressed and the resin is filled in a state where the solidified layer is thin. Is done. That is, in the conventional injection molding of a synthetic resin, the solidified eyebrows are suppressed from increasing due to self-heating, and molding is performed while securing a flow path. On the other hand, in the low-speed injection as in the present invention, the heat generation is small, and the solidified layer is thickened during the flow by the low-speed flow, and the resin is filled in a state where the solidified eyebrows are thick. In low-speed injection molding, injection molding is performed under extremely strict conditions in view of flow in this sense. It is estimated that thickening of the solidified layer in this way would worsen the SCR of the force-formed product.

Next, the fact that the present invention improves the SCR will be described with reference to FIGS. 11A and 11B. The low-speed injection thickens the solidified layer and deteriorates the SCR. In the present invention, a mold covered with a heat-resistant resin layer as a heat insulating layer is used to improve this. To use.

In Figures 11A and 11B, when synthetic resin is injection-molded in a cooled metal mold, the injected molten resin comes into contact with the cooled mold wall, and the solidified layer immediately forms on the surface. 0 is formed, and the solidified layer 20 becomes a heat insulating layer, and the inner core 21 is gradually cooled and contracted. In a sufficiently cooled molded product, compressive stress remains in the surface layer 20 due to shrinkage of the inner core 21.The residual compressive stress depends on the ratio of the thickness C of the inner core 21 to the thickness S of the surface layer 20, CZS. It is proportional (Fig. 11 A 0 I). When injection molding is performed using a mold covered with heat-resistant resin eyebrows that are heat insulating layers, the solidified layer 20 that is formed immediately after injection is molded using a normal mold that is not covered with the heat insulating layer. It becomes thinner than. The residual compressive stress of a sufficiently cooled molded article increases in proportion to C '/ S' (1 in Fig. 11.8), and the residual compressive stress is caused by the heat-resistant resin layer as a heat insulating layer. Larger than when molding with an uncoated mold. When bending stress is applied to this injection-molded product, the compressive stress on the concave side 22 of the molded product further increases, and conversely, the convex side 23 of the molded product loses the compressive stress, and a tensile stress is applied (Fig. 11A Π), (Fig. 11B Π). In the case of Fig. 11B, where the residual compressive stress of the surface layer is large, the tensile stress of the surface layer when bending is reduced. _(In general, the compressive strength of synthetic resin is much larger than the tensile strength, so synthetic resin When a bending stress is applied to the molded product, the molded product is ruptured from the convex side of the molded product. Difficult to read. As described above, a molded article molded with a mold covered with a heat-resistant resin layer as a heat insulating layer has a high resistance to soil rupture when stress is applied. It is presumed that this is the reason why the SCR of the injection molded article obtained according to the present invention becomes strong.

 Next, the low flow rate of the synthetic resin in the mold cavity in low pressure injection molding will be described with reference to FIG. 12. Figure 12 shows the change over time in the resin pressure applied to the mold wall during injection molding as a model difference between conventional injection molding and gas assist injection molding. In Fig. 12, the pressure curves near the gate are denoted by A and a, the pressure curves near the flow end are denoted by Cc, and the pressure curves near the middle of the flow are denoted by B and b. In the case of conventional injection molding, the pressure applied to the vicinity of the gate rises in the curve of A and A 'and becomes the maximum in the full shot, and the pressure applied to the middle part of the resin flows increases in the curve of B and increases. The pressure at the end of the flow reaches its maximum in the short shot, and the pressure applied near the flow end increases in the curve C, reaches the maximum in the full shot, and decreases with the dwell time. The maximum value of each of A, B, and C is larger as the position in the molded article is closer to the gate, and the final applied pressure differs greatly depending on the position.

On the other hand, in the case of gas assist injection molding, the resin is first injected at a high pressure, the injection of the resin is stopped during the injection, and a gas body with a pressure significantly lower than the resin injection pressure is injected. Satisfies the mold cavity. In this gas assist injection molding, the pressure applied near the gate first rises along the curve A, and When the resin is switched to a gas inside, the resin pressure at 24 becomes almost the same as that of the gas, and then changes at a constant pressure as shown by the a curve. After the gas is injected, the pressure in the middle of the resin flow rises as indicated by the curve b, becomes almost the same as the gas pressure at full shot, and is maintained at the pressure. The pressure of the resin near the flow end is shown by curve c, which is almost the same as the gas pressure at the time of the full shot, and is almost maintained at the time of the dwell. Gas assist injection molding is characterized in that the pressure near the gate, in the middle of the flow, and at the end of the flow are almost the same at full shot. As a result, gas assist injection molding is widely used because the residual stress in the molded product becomes uniform and the dimensional accuracy of the molded product is improved. However, in gas assist injection molding, as shown by curves b and c, the rise of resin pressure is slow, and the flow velocity in this part in the mold cavity is slow, and the molded product in this part It also has the drawback that the appearance and SCR are poor. The present invention aims at improving the above-mentioned drawbacks by coating the surface of a mold body with a heat-resistant resin.

 According to the low pressure injection molding method of the present invention, the glossiness, the sink mark and the roughness mark are improved, and an injection molded product excellent in appearance and SCR of the molded product can be obtained.

(Hereinafter the margin) 【Example】

 Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

 The following molds, materials, and measurement methods are used.

 (1) Die main body 1: Made of steel (S55C), the mold cavity has a side gate of 2 mm (thickness) X 100 mm x 100 mm. The mold surface is mirror-finished and chrome plated.

(2) Die body 2: Made of steel (S55C), mold cavity is 3 mm (thickness) X 70 mm x 378 mm, rectangular shape, 5 mm x 5 mm The size of the rib is in the longitudinal direction and has a side gate at the corner. The mold surface is mirror-like and chrome-plated.

(3) Polyimide precursor and cured polyimide: linear high molecular weight precursor solution “Trenice # 300” (manufactured by Toray Industries, Inc.) . The performance of the cured polyimide was 300 ° C at Tg force, 0.005 ca1 / cm-sec at thermal conductivity, and the elongation at break was 60%. is there.

(4) Polyimide coating mold: Apply a polyimide precursor solution to each mold body, heat to 160 ° C to make partial imidization, and then apply the coating. The heating at 160 ° C. was repeated 2 to 8 times, and finally, heating was performed to 290 ° C. to make 100% imidization, and the mold surface was coated with polyimide. The surface is polished to a mirror surface, and the thickness of the polyimide is 0.025 mm, 0 mm. Make a mold covering mold of 0.5 mm, 0.075 mm, 0.1 mm, 0.15 mm, 0.16 mm, 0.175 mm.

 (5) Modified epoxy resin: “SEMEDYN EP — 07” (manufactured by SEMEDYN Corporation). After curing, the thermal conductivity is 0.

0 0 0 5 c a 1 / c m · s e c ·. C, elongation at break is 12%, and pencil hardness is H.

 (6) Modified epoxy resin-coated mold: A modified epoxy resin is applied to mold body 1, and heated to 50 ° C and then to 100 ° C to be cured. Polish the surface to make it mirror-like.

 (7) Synthetic resin for injection molding:

 HIPS492: Rubber-reinforced polystyrene "Asahi Kasei Polystyrene 492" manufactured by Asahi Kasei Corporation

 HIPS 495: Rubber reinforced polystyrene "Asahi Kasei Polystyrene 495" manufactured by Asahi Kasei Corporation

 PMMA: Metal acrylate resin "Delut 80N" manufactured by Asahi Kasei Kogyo Co., Ltd.

 (8) Gloss measurement method: According to JIS K7105 (reflection angle: 60 degrees).

(9) SCR measurement method: Use the cantilever method shown in Fig.13. In Fig. 13, the solvent is brought into contact with a test piece 25 (2 x 200 x 100 mm size) where a bending stress is applied, and a crack is applied under a load of 27. Watch the time when break occurs and breaks.

Example 1 Injection molding is performed with PMMA using the mold body 1 and a 0.175 mm thick polyimide coated mold of the mold body 1. The molding conditions were as follows: the temperature of the injection cylinder was 260 ° C, the mold temperature was 60 ° C, and the injection pressure was changed to reduce the average flow velocity in the mold cavity of the synthetic resin to 13 mm ns. Molded by injection and high-speed injection of 23 Om mZ seconds. The SCR of the injection-molded product was measured by the break time, and is shown in Table 4.

 Molded products using polyimide coated molds have improved SCR, especially at low speed injection.

(Hereinafter the margin)

Table 4

Low-speed injection: 13 mmZ seconds flow velocity in synthetic resin mold cavity High-speed injection: 〃 230 mmZ seconds Polyimide coated S mold: 0.175 mm coated polyimide

Example 2

 HIPS 495 is injection-molded by using the mold body 1 and a polyimide mold having various thicknesses of the mold body 1. The molding conditions were a cylinder temperature of 220 ° C and a mold temperature of 35 ° C.The injection speed was changed by changing the injection pressure, and the thickness of the polyimide film was also changed. The gloss of the molded article was measured and is shown in Figs. 14A and 14B.

 The gloss of the molded article varies greatly depending on the flow velocity in the mold cavity of the synthetic resin.To obtain high gloss at low injection speeds with a flow velocity of 10 Omm ns or less The thickness of the polyimide must be more than 0.1 mm, and preferably 0.15 mm or more.

Example 3

 The conventional high-speed injection molding, the conventional low-speed injection molding, and the mold body 2 and the mold body 2 covered with 0.16 mm thick polyimide are used. Injection molding is performed by four types of molding methods: gas assist injection molding and oligomer assist injection molding. HIPS492 is used as the synthetic resin, and the chemical foaming agent azodicarbonamide is blended with the PSST120 shown in Table 3 and Figure 2 as the oligomer. . The molding is performed at a cylinder temperature of 230 ° C and a mold temperature of 40 ° C.

Injection molding was performed using each molding method and each mold combination.The gloss of each molded product, the presence or absence of sink marks on the back of the rib, and the presence or absence of heaviness marks were measured.The results are shown in Table 5. Show. Table 5 In the case of B, C, and D, which include low-speed injection, the gloss at the flow end when injection molding is performed with a chrome metal mold is significantly worse. On the other hand, when the molding is performed using a polyimide coating mold, the gloss at the flow end is good even at low-speed injection.

 A schematic diagram of a cross-section near the surface layer at the flow end of a molded product that was injection-molded using a polymide-coated mold and a Cr-mesh mold (mold body). These are shown in Figures 15A and 15B, respectively. These figures are based on transmission electron micrographs (TEM) (magnification: 500,000). When molded with a polyimide-coated mold, the surface of the molded article 29 has substantially no protrusion of the rubber particles 28 of the resin-reinforced material, and is excellent in mold surface reproducibility.

 Low-pressure injection molding methods such as gas-assist injection molding method, liquid-assist injection molding method, and oligomer-assist injection molding method of the present invention, and injection molding with a polyimide coated mold. Then, the molded product has improved gloss, sink mark, and roughness mark, and has the inherent advantages of low-pressure injection molding, such as low required mold clamp, reduced warpage of molded product, and molded product dimensional system. Can be maintained.

Example 4

 Using the mold body 1 and the modified epoxy resin-coated molds having various thicknesses of the mold body 1, the same injection molding as in Example 2 is performed.

(Hereinafter the margin) Table 5

[Notes] l) C r mold: “C represents the crimp mold J _c 2) PI:“ represents the volume j

<Industrial applicability> The low-pressure injection molding method of the present invention improves glossiness, sink mark, and heaviness mark, and improves the appearance and stress resistance of molded products. Injection molded products with excellent performance (SCR) can be obtained ο

Claims

Scope of the request 1. A mold obtained by covering the wall of the metal mold body, which constitutes the cavity, with a heat-resistant resin eyebrow with a thickness of more than 0.1 mm and less than 0.5 mm. A low-pressure injection molding method for synthetic resin in which injection is performed including low-speed injection in which the flow velocity of synthetic resin in a mold cavity is 100 mm or less.  2. The low-pressure injection molding according to claim 1, wherein the synthetic resin is a synthetic resin containing a resin reinforced material, and the molded article after molding the resin has substantially no protrusion of the resin reinforced material on its surface. Law.  3. The low-pressure injection molding method according to claim 2, wherein the synthetic resin containing the resin reinforcement is a rubber-reinforced synthetic resin.  4. The low pressure injection molding method according to claim 3, wherein the rubber reinforced synthetic resin is a rubber reinforced styrene resin.  5. The low-pressure injection molding method according to claim 1, wherein the thickness of the heat-resistant resin layer is in a range of 0.15 to 0.45 mm.  6. The low-pressure injection molding method according to claim 2, wherein the thickness of the heat-resistant resin layer is in a range of 0.15 to 0.45 mm.  7. The low pressure injection molding method according to claim 3, wherein the thickness of the heat resistant resin layer is in the range of 0.15 to 0.45 mm.  8. The low pressure injection molding method according to claim 4, wherein the thickness of the heat resistant resin layer is in the range of 0.15 to 0.45 mm. 9. Low-pressure injection molding methods include gas-assist injection molding method, liquid-assist injection molding method, oligomer-assist injection molding method, injection compression molding method, and sun-molding method where the core resin is a foam resin. Doi 9. The low-pressure injection molding method according to claim 1, which comprises at least one of the group consisting of a punch injection molding method and a Hettinger low-pressure injection molding method.