JPH079453A - Mold for molding synthetic resin - Google Patents

Abstract

PURPOSE:To provide a mold markedly enhancing the appearance of a molded product while suppressing an increase in a molding cycle time to a slight degree by coating the surface of a mold with a thin heat insulating layer composed of a selected synthetic resin. CONSTITUTION:The material quality of a main mold is composed of a metal whose heat conductivity is 0.05cal/cm.sec. deg.C or more generally used in a mold for a synthetic resin such as iron, an iron-containing steel material, aluminum, an aluminum alloy, a zinc alloy or a copper alloy such as beryllium-copper alloy. A heat insulating layer with a thickness of 0.02-2mm composed of a heat insulating layer whose heat conductivity is 0.002cal/cm.sec. deg.C or less is applied to the molding wall surface of the mold. The heat insulating material is composed of a polymer with a glass transition temp. of 150 deg.C or higher. The coefficient of thermal expansion of the heat insulating material is 3-0.5 times that of the main mold. By this constitution, the heat insulating material can be adapted to the mold whose molding cavity has a complicated shape. This mold is reduced in the increase of a cooling time and can withstand repeated molding accompanied by heating and cooling of ten thousand times.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synthetic resin molding die. More specifically, it relates to a mold for injection molding or blow molding that can endure molding of tens of thousands of times.

[0002]

2. Description of the Related Art In order to improve the reproducibility in imparting the shape condition of the mold surface to a molded product and to improve the gloss of the molded product, it is usually necessary to inject a thermoplastic resin into a mold cavity for molding. Can be achieved to some extent by selecting molding conditions such as higher injection pressure and injection pressure.

The mold temperature has the greatest influence among these factors, and the higher the mold temperature, the more preferable.
However, if the mold temperature is increased, the cooling time required for cooling and solidifying the plasticized resin becomes longer, and the molding efficiency decreases.
Improves mold surface reproducibility without raising mold temperature,
There is also a demand for a method which does not lengthen the required cooling time even if the mold temperature is raised. There is also a method in which heating and cooling holes are attached to the mold and heating and cooling of the mold are repeated by alternately flowing a heat medium and a refrigerant, but this method also consumes a lot of heat, Cooling time becomes longer.

A method for improving mold surface reproducibility by coating a mold wall forming a mold cavity with a substance having a small thermal conductivity is disclosed in US Pat. No. 3,544,518 and the like, and a substance having a small thermal conductivity is disclosed. As such, polyethylene terephthalate, polyphenylene sulfide, etc. are shown. Further, there is WO89 / 10823 as a method of providing a heat insulating layer near the surface of the mold. In this specification, when the injected resin is cooled in the mold, the outermost surface of the mold is made of metal such as aluminum or nickel, and the next layer is a heat insulating layer The mold structure is shown as the mold body. The purpose of providing the heat-insulating layer is to significantly reduce the cooling rate of the injected heating resin, and as the heat-insulating layer, a plate of liquid crystal polymer having a thickness of several mm, Vespel (molded polyimide, Du Pon)
A plate (trade name of company t) is shown. JP-A-62-37
No. 107 gazette attaches a breathable heat insulating layer to the mold surface,
It describes how to prevent the occurrence of silver streaks.

[0005]

In the molds disclosed in these known documents, in which the surface of the mold is covered with a heat insulating layer, the coefficients of thermal expansion of the mold and the heat insulating layer are largely different, and therefore, the molds cannot withstand long-term use. Had a problem. Generally, the thermal expansion coefficient of the polymer used in the heat insulation layer differs from that of the mold by an order of magnitude, and it is generally difficult to bring the metal and the heat insulation layer into close contact with each other and to maintain the close contact in a thermal cycle test performed tens of thousands of times. is there.

Injection molding has the greatest advantage in that a molded product having a complicated shape can be obtained by one-time molding. While maintaining this advantage, the cooling time in the mold does not become long, and the mold surface is kept. It is required to mold a mirror-like molded article with good reproducibility. An object of the present invention is to provide a mold having a mold surface covered with a heat insulating layer, which is applicable to a mold having a mold cavity having a complicated shape, and 2) a small increase in cooling time,
3) Withstands repeated molding involving heating and cooling tens of thousands of times,
4) To provide a mold having excellent mold surface reproducibility, for example, a high-gloss molded product can be obtained.

[0007]

That is, the present invention is
Thermal conductivity at room temperature is 0.05 cal / cm · sec
・ The heat conductivity is 0.002cal / cm ・ s on the mold wall that forms the mold cavity of the main mold made of metal above ℃
In a mold coated with a heat insulating layer made of a heat insulating material of ec · ° C or less, (1) the heat insulating layer has a thickness of 0.02 to 2 mm,
(2) The heat insulating material is a polymer having a glass transition temperature of 150 ° C. or higher, and (3) the thermal expansion coefficient of the heat insulating material is 3 times or less and 0.5 times or more the thermal expansion coefficient of the main mold. It is a resin molding die.

The present invention will be described in detail below. As described above, there are many known documents about a method of improving the mold surface reproducibility of a molded product by coating the surface of a main mold made of metal with a thin heat insulating layer made of synthetic resin. However, although these molds are conventionally simple molds and can be used for molding with a small number of molding times, it is possible to form a mold cavity with a strong material such as steel in a full-scale mold that can withstand tens of thousands of moldings. It is common sense so far. In injection molding, a synthetic resin is injected into a thin mold cavity of about 2 mm at high speed, so it is essential to form a mold cavity with a strong material such as steel for a full-scale mold that performs tens of thousands of times. It is believed that.

We have conducted a deeper study on this, and even if the surface of the main mold is covered with a thin synthetic resin, if a heat insulating layer and a main mold made of synthetic resin satisfying certain conditions are used, the The present invention has been completed by discovering that it can withstand injection molding of 10,000 times. That is, in injection molding, the thermoplasticized resin injected into the mold contacts the wall surface of the cooled mold to immediately form a solidified layer on the contact surface, and the subsequently injected resin is between the solidified layer and the solidified layer. And the flow front (flow front)
When it reaches t), it faces toward the mold wall surface and comes into contact with the mold wall surface to form a solidified layer.

That is, the injected resin flows so as to press the mold wall surface from above, and does not flow like dragging the mold wall surface. Therefore, if the surface of the mold is covered with a thin heat insulating layer made of a selected synthetic resin, the heat insulating layer will not be directly worn by the injected resin and can withstand tens of thousands of injection moldings. Generally, a polymer having a large coefficient of thermal expansion is bonded to a polymer, and in order to withstand repeated cooling and heating cycles, it is necessary to use a polymer having a large elongation at break,
For example, it is recommended to use a material such as a rubber material having a significantly high elongation at break. In particular, coating the surface of a metal mold with a polymer having a breaking elongation of less than 10% as in the present invention seems to be difficult to coat the mold of the present invention involving tens of thousands of cooling and heating cycles. There is. The present invention aims to make it possible to use a hard and heat-resistant resin having a breaking elongation of less than 10%.

The main mold material used in the present invention has a thermal conductivity of 0.05 cal / cm · sec · ° C. or higher, and iron or a steel material containing 50% by weight or more of aluminum, 50% by weight of aluminum or 50% by weight of aluminum. Metals generally used for synthetic resin molds such as alloys, zinc alloys, copper alloys, such as beryllium copper alloys, contained above are included. Particularly, aluminum or aluminum alloy, copper or copper alloy having a relatively large thermal expansion coefficient can be favorably used in the present invention. In order to improve the adhesiveness with the heat insulating layer, the surface of the main mold is appropriately provided with appropriate irregularities and various plating.

The polymer used in the present invention is a heat-resistant polymer having a glass transition temperature of 150 ° C. or higher, preferably 200 ° C. or higher, more preferably 250 ° C. or higher. Further, the coefficient of thermal expansion is preferably 5 × 10 ⁻⁵ or less, more preferably 4 ×.
It is a polymer having a small coefficient of thermal expansion of 10 ⁻⁵ or less. The thermal expansion coefficient of the polymer that can be used in the present invention is 3 times or less, 0.5 times or more, preferably 2 times or less and 0.6 times or more, more preferably 1.5 times that of the main mold. 0.8 times less than double
More than double. The coefficient of thermal expansion used here is from room temperature to 100.
The coefficient of thermal expansion of ° C shall be used. The closer the thermal expansion coefficient of the heat insulating layer and the main mold are, the more preferable.

In the present invention, a polymer and a mold having a similar coefficient of thermal expansion are combined, so that even a polymer having a breaking elongation of less than 10% can withstand a thermal cycle. Table 1 shows the approximate values of the thermal expansion coefficients of various metals and various resins generally used for molds. Among these, a combination of a metal having a large coefficient of thermal expansion and a resin having a small coefficient of thermal expansion,
That is, a combination of metal and resin having similar thermal expansion coefficients is favorably used.

[0014]

[Table 1]

Polymers which can be favorably used in the present invention are heat-resistant polymers having an aromatic ring in the main chain, mainly polybenzimidazole (hereinafter abbreviated as PBI) and low thermal expansion polyimide. The polymer etc. can be favorably used. PBI shown in (Chemical Formula 1): Poly- [2,2 ′
-(M-phenylene) -5,5'-biben
Zimidazole] can easily be applied as a solution to the wall surface of the mold, and has good adhesion to the wall surface of the mold, and can be favorably used in the present invention. This PBI has a small coefficient of thermal expansion, has a coefficient of thermal expansion almost equal to that of aluminum, and has a glass transition temperature of about 400 ° C., so that it can be favorably used in the present invention.

[0016]

[Chemical 1]

Various additives can be added to PBI. Other polymers, thixotropy-imparting agents and the like can be blended as necessary. For example, another polymer is blended in order to increase the elongation at break of PBI. PB
Since I has a small elongation at break, blending various polyimides soluble in a solvent to increase the elongation at break can be favorably used in the present invention. As various soluble polyimides that can be blended with PBI, Ultem1000 (Gen
eralElectric), XU-218 (Giba
-Geigy), PI2080 (Upjohn), La
rc TPI (Mitsui Toatsu Chemical) or the like can be used favorably.
These polyimides are compatible with PBI,
It provides the intermediate properties of BI. 8 for polyimide and PBI
Blended in the range of / 2 to 2/8. PBI has a breaking elongation of about 1 to 3%, and a blend having a breaking elongation of 5% or more by blending can be particularly favorably used.

As the polymer, a low thermal expansion type polyimide can also be favorably used. The low thermal expansion type polyimide described here refers to one having a thermal expansion coefficient of less than 3 × 10 ⁻⁵ / ° C. Table 2 shows the thermal expansion coefficients of various low thermal expansion polyimides. In the table, "Bixix" and "Free" are made to allow the film to shrink freely when the polyimide precursor is imidized to form a polyimide film (Free), or fixed to a square frame to prevent shrinkage that occurs during imidization. Is the polymer chain in-plane oriented with the stress suppressed (Bixix)
Is the difference. The thermal expansion coefficient of the polyimide formed by applying the polyimide precursor solution to the main mold and then heating is Fre.
It is an intermediate value between e and Bifix.

[0019]

[Table 2]

Of these, the biphenyltetracarboxylic acid type polyimide is soluble in a solvent and can be easily applied to the mold wall surface, and thus can be used favorably. Various polyimides having a slightly large thermal expansion coefficient can be used as long as the combined mold has a large thermal expansion coefficient. This low thermal expansion type polyimide and the like can be blended with other various polyimides and the like as necessary. Also, various polyimides having a breaking elongation of less than 10% can be used, as shown in the present invention, if the coefficients of thermal expansion of the mold and the polymer are close to each other. Various cross-linked polyimides,
For example, a bismaleimide type polyimide or the like has a small elongation at break, but can be used by selecting a main mold having a close thermal expansion coefficient.

The adhesion between the heat insulating layer of the present invention and the main mold is preferably 500 g / 10 mm width or more at room temperature,
More preferably, the width is 1 kg / 10 mm or more. This is the peeling force when the adherent heat insulating layer is cut into a width of 10 mm and pulled at a speed of 20 mm / min in the direction perpendicular to the adhesive surface. Although the peeling force varies considerably depending on the measurement place and the number of times of measurement, it is important that the minimum value is large, and it is preferable that the peeling force is stable and large. The adhesion force described here is the minimum value of the adhesion force of the main part of the mold. In order to improve the adhesion, the surface of the main mold can be appropriately treated with a primer.

The smaller the thermal conductivity of the heat insulating layer is, the more preferable it is.
Generally, the thermal conductivity of the polymer is 0.002 cal / cm · s.
Since it is ec · ° C or less, a general polymer can be used. The thickness of the heat insulating layer is appropriately selected within the range of 0.02 to 2 mm. If the thickness is less than 0.02 mm, the effect of improving the surface of the molded product is small, and if it exceeds 2 mm, the cooling effect of the mold is reduced.
Molding efficiency decreases. It is necessary to decrease the thickness of the heat insulating layer as the mold temperature is higher, and to increase the thickness of the heat insulating layer as the mold temperature is lower. The thickness is appropriately selected within the range of 0.02 to 2 mm. Further, the preferable thickness of the heat insulating layer varies depending on the molding method in which the mold of the present invention is used. In the injection molding in which the mold of the present invention can be used most preferably, 0.02-0.5 mm
Is preferable, and more preferably 0.05 to 0.2 m.
The thickness is m. On the other hand, in extrusion blow molding, a thickness of 0.1 to 1 mm is preferable.

It has been practiced to coat the surface of the mold with a resin layer having excellent releasability to improve the mold release during molding. The resin layer in this case is much thinner than the resin layer of the present invention, and the thickness thereof is 0.01 mm or less, generally 0.001.
mm is sufficient, and in the case of such a thin wall, various problems to be solved by the present invention are not targeted.

The strength and elongation of the polymer used in the present invention are preferably high, and the elongation at break is generally about 2% or more, preferably 3% or more. However, when the thermal expansion coefficient of the polymer is very close to that of the main mold, the polymer can be used even if the breaking elongation of the polymer is smaller than this. It is a great feature of the present invention that the polymer can be used even when the breaking elongation of the polymer is 10% or less, and thus the selection of the polymer can be widely performed. The measuring method of breaking elongation is ASTM
Perform according to D638.

Injection molding has the greatest merit that a mold having a complicated shape can be formed by one molding, and therefore, the mold cavity generally has a complicated shape. It is extremely difficult to apply a coating material to the surface of this complicated mold cavity in a mirror-like manner, and therefore it is the best method to polish the applied coating layer to a mirror-like finish. Therefore, it is preferable that the heat insulating layer of the present invention can be polished and mirror-finished. Generally, a polymer having a lower breaking elongation is more easily polished, and from this viewpoint, a polymer having a lower breaking elongation is better. In the present invention, even a polymer having a breaking elongation of less than 10% can be used, so that a favorable smooth surface mold can be obtained.

In order to further improve the smoothness of the surface of the heat insulating layer or to further improve the scratch resistance of the surface, another substance having a thickness of 1/10 or less, preferably 1/20 or less of the thickness of the heat insulating layer is used. It can be applied to the surface of the heat insulating layer as needed and is included in the present invention. The surface of a synthetic resin sheet or mold may be coated with a paint generally called a hard coat agent, which is used for improving scratch resistance. For example, a thermosetting silicone-based hard coat agent can be favorably used.

The synthetic resin that can be molded with the mold of the present invention is generally a thermoplastic resin that can be used for injection molding, blow molding and the like. For example, a general thermoplastic resin such as a styrene polymer or a copolymer thereof, an olefin polymer such as ethylene or propylene or a copolymer thereof, a vinyl chloride polymer or a copolymer thereof, a polyacetal, a polyamide, a polyester or the like can be used. .

When these resins are mixed with various reinforcing materials and various fillers, or when they are polymer alloys, a particularly great effect is obtained. For example, resin, rubber, glass fiber, asbestos, calcium carbonate,
One or two of talc, calcium sulfate, foaming agent, wood flour, etc.
More than one species can be blended. Recycled resin containing foreign matter such as dust and paint powder can also be favorably used in the present invention.

Although the present invention has been mainly described by injection molding, the mold of the present invention can be used in blow molding as well.

[0030]

[Example] The main mold is aluminum (heat conductivity of about 0.2c
al / cm · sec · ° C, thermal expansion coefficient 2.2 × 10 ^-5
/ ° C.), has side gates on the sides of a 100 mm × 100 mm square, and has a flat mold cavity with a thickness of 2 mm. PBI is shown in Chemical formula 1. This PBI has a glass transition temperature of 400 ° C. and a thermal conductivity of 0.0009 ca.
1 / cm · sec · ° C., elongation at break 2.5%, coefficient of thermal expansion 2.3 × 10 10 ⁻⁵ / ° C.

The mold surface of the main mold is polished to a mirror surface by sandpaper or the like to be sufficiently degreased, and then a PBI dimethylacetamide solution is applied and heated in the order of 120 ° C. to 210 ° C. Is repeated 5 times to form a PBI layer. Next, a diamond paste is applied to the buff and polishing is performed with an electric grinder to form a 0.05 mm thick mirror-like PBI coating layer. Cut the coating layer to a width of 10 mm and pull it at a speed of 20 mm / min in the direction perpendicular to the coated surface,
The adhesion was measured. The adhesion was 1 kg / 10 mm width or more.

Using a PBI-coated mold and a main mold, rubber-reinforced polystyrene # 495 (manufactured by Asahi Chemical Industry Co., Ltd.) is injection-molded, and the glossiness of the molded product is measured. Coated with PBI. The gloss of the molded product was remarkably good.

[0033]

By performing injection molding or blow molding of synthetic resin using the mold of the present invention, the increase in molding cycle time can be slightly increased and the appearance of the molded product can be remarkably improved. .

Claims (4)

[Claims] 1. The thermal conductivity at room temperature is 0.05 cal.
The heat conductivity is 0.002c on the mold wall forming the mold cavity of the main mold made of a metal having a temperature of / cm · sec · ° C or higher.
In a mold coated with a heat insulating layer made of a heat insulating material having an a / cm · sec · ° C or less, (1) the heat insulating layer is 0.02 to 2 m.
m thickness, and (2) the heat insulating material has a glass transition temperature of 150 ° C.
A synthetic resin molding die, which is the above polymer, wherein (3) the thermal expansion coefficient of the heat insulating material is 3 times or less and 0.5 times or more the thermal expansion coefficient of the main die. 2. The mold according to claim 1, wherein the heat insulating material is a polymer mainly containing polybenzimidazole. 3. The mold according to claim 1, wherein the heat insulating material is a polymer mainly composed of low thermal expansion type polyimide. 4. The mold according to claim 1, 2 or 3, wherein the thermal expansion coefficient of the heat insulating material is 1.5 times or less and 0.8 times or more the thermal expansion coefficient of the main mold.