JPH0338967B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a foamed thermoplastic synthetic resin molded article from raw resin all at once using a mold.

Typical methods for producing foamed thermoplastic synthetic resin molded articles using a mold include so-called bead molding method and foaming injection molding method. However, the bead molding method involves a two-step foaming process in which beads obtained by pre-foaming raw resin particles are heated in a mold and fused while secondary foaming occurs, making the operation complicated. In addition, it is difficult to feed the beads into narrow gaps within the mold, making it impossible to mold complex shapes. In addition, although the injection molding method allows a foamed molded product to be obtained in one step, it is not as easy to carry out as ordinary injection molding of non-foamed resins. That is, in this method, an amount of foamable molten resin is injected into the mold cavity in an amount smaller than the volume of the mold cavity,
In the shot shot method, the mold cavity is filled with resin that expands and increases in volume immediately after injection.
The foaming ratio does not increase as expected, and the molded product tends to become rough. Furthermore, in the case of the full shot method, in which the mold cavity is filled with foamable molten resin and then one side of the mold is moved to expand the mold cavity and foam the resin, the molding equipment becomes complicated and expensive. Not only is this unavoidable, but the shape of the molded product is restricted due to the need to move the mold during molding. Moreover, there is a difference in the expansion ratio between parts that are strongly affected by cavity expansion due to mold movement and parts that are not, and as a result, the molded product tends to have uneven surface characteristics and physical properties. For the above reasons, when injection molding is used, there is a problem in that it is not possible to increase the expansion ratio in the case of molded products that require sufficiently high strength (for example, foam molded products for structural materials).

An object of the present invention is to provide a foam molding method that does not have the drawbacks of the conventional synthetic resin in-mold foam molding methods.

The present invention, which has succeeded in achieving the above object, will first be summarized as follows: a female mold having a cylindrical portion surrounding the forming surface; A female die of a mold consisting of a male die and a male die having a part is supported in a position capable of holding a molten molding resin on its molding surface, and a molten thermoplastic synthetic resin containing a blowing agent is placed in the female die. The foam is supplied onto the molding surface of the mold and foamed to a foaming ratio of 50% or more of the maximum foaming ratio that can be achieved at normal pressure and larger than the foaming ratio of the intended molded product, and then the male mold is inserted into the cylindrical part of the female mold. The method is characterized in that a predetermined mold cavity is completed, and in the process, the foamed resin melt in the female mold is compressed, the mold is then cooled, and the hardened foamed resin is taken out from the mold. .

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to drawings schematically showing an example of an apparatus for carrying out the invention.

In FIG. 1, 1 is an extruder, 2 is a female mold, and 3 is a male mold. The female mold 2 has three female mold constituent units 4 each having a convex cross-section on a bottom plate 23 of a shallow box-like body 22 having an upright wall 21, and their surfaces serve as molding surfaces 41. On the other hand, the male mold 3 has a male mold constituent unit 5 having a concave cross section on a flat substrate 31.
They are fixed in close contact with each other. The female mold 2 and the male mold 3 are connected to the outer periphery 32 of the latter male mold constituent unit 5 group.
and the inner circumferential surface 24 of the former upright wall 21 are tightly fitted over the entire circumference in a piston-cylinder relationship (tightly enough to allow air to escape but not to leak molten resin). Then, in the state in which the above-mentioned fitting is completed, a box-shaped mold cavity (the part filled with resin melt 6 in FIG. 3 described later) is formed between each female mold component unit 4 and male mold component unit 5. is formed, and three boxes are molded at one time. At the start of molding, the female mold 2 is located below the extrusion die 11 of the extruder as shown in the figure, but is supported by a support mechanism (not shown) that is movable in the horizontal direction.
Supported by On the other hand, the male mold 3 is provided with a support means (not shown) equipped with a lifting mechanism at a position where it can be lowered and combined with the female mold 2 when it moves to the left in the drawing.
Supported by Each mold can be heated (or cooled) using a heating medium (or cooling medium),
It is heated at the start of molding.

When molding is carried out using this apparatus, a molding resin is supplied to the extruder 1 through a hopper 12, and a solvent-based blowing agent is press-fitted through a blowing agent injection port 13, and both are mixed under heat and pressure. The molten resin for foaming, which is made up of molten resin and a foaming agent, is extruded into the air from an extrusion die 11 having a discharge port with a circular cross section in the form of a thick thread, and is immediately applied to the molding surface 41 of the female mold 2 below. It is accepted. At this time, the extrusion die 11 attached to the bendable extrusion die attachment pipe 14
and the female mold 2 little by little to apply a sufficient amount of foamable molten resin to the molding surface 4.
Apply it to the entire surface of 1 as much as possible.

Immediately after supplying the resin, the female mold 2 is moved directly below the male mold 3.

Since the foamable molten resin immediately starts foaming on the heated molding surface 41, the molding surface 41 is soon covered with the foamed molten resin (this state is shown in an enlarged manner in FIG. 2. , 6 is the foamed molten resin).

When the foaming of the resin reaches 50% or more of the maximum foaming ratio that can be achieved at normal pressure, preferably 60 to 80%, and the foaming ratio is greater than the foaming ratio of the intended molded product, the male mold The male die 3 is lowered while the piston-shaped outer circumferential surface 32 of the constituent units 5 groups is aligned with the cylindrical inner circumferential surface 24 of the female die 2, and the male die 3 is fitted into the female die 2 and combined. As shown in Figure 3, the mold cavity formed by the combination is made of molten resin 6 that is foamed during the process of formation.
However, since the fit between the female mold 2 and the male mold 3 is a tight fit that does not leak the molten resin as described above, unless the amount of resin supplied is too small, it must be filled before the mold cavity is completed. Compression of the foamed molten resin occurs. When the foamed molten resin is compressed, the foaming force increases for reasons that are not clear, so the foamed resin spreads to every corner of the mold cavity, producing a foamed molded product with a uniform structure. Furthermore, the surface is strongly pressed against the cavity molding surfaces of both molds, forming a dense skin layer. Note that, by performing the compression, the expansion ratio of the finally obtained foamed molded product becomes lower than the maximum expansion ratio reached by the molten resin before the compression by the male die is started.

Thereafter, the supply of heat medium to the mold is stopped, and a coolant is supplied to cool the mold, thereby cooling and hardening the foamed resin 6. After curing, the male mold 3 is raised and separated from the female mold 2, and the cured and molded foamed resin is taken out from the mold.

When carrying out the manufacturing method of the present invention as described above, the foamable molten resin prepared in an extruder is not only directly supplied to the mold as in the above example, but also stored in a storage tank kept under pressure and then sequentially poured into the mold. It may also be fed into a mold. In addition, it is desirable to extrude the foamable molten resin into a relatively thin string or tape shape and apply it in the required amount to the entire molding surface of the mold using an industrial robot. When forming the mold, the resin may be extruded in a relatively thick flow and adhered to a part of the molding surface of the mold in dots or lines.

From the start of extrusion of the resin to the joining of the mold, the mold itself is not only heated by a heat medium or an electric heater, but also placed in an atmosphere at a high temperature comparable to the temperature of the resin to prevent the resin from cooling. desirable. This is because if the molten resin placed on the molding surface of the mold is rapidly cooled from the surface, the foaming of the resin near the surface will not be sufficient, and the surface quality of the resulting molded product will deteriorate. .

Examples of thermoplastic synthetic resins that can be made into foam molded products by the method of the present invention include polyethylene, polypropylene, copolymers of ethylene and other monoolefins (e.g. propylene), and copolymers of ethylene and vinyl monomers (e.g. propylene). Examples include polyolefin resins such as copolymers with vinyl acetate); styrene resins such as polystyrene and ABS resins; other polyvinyl chloride resins, various nylon resins, polycarbonate resins, and polyhydroxyether resins.

The blowing agent for foaming the above resin may be either a decomposable blowing agent or a solvent-type blowing agent, but a solvent-type blowing agent is preferable, and particularly preferable examples include cycloaliphatic hydrocarbons such as cyclobutane and cyclopentane. aliphatic hydrocarbons such as propane, butane, pentane, hexane, and heptane; halogenated hydrocarbons such as trichlorofluoromethane, dichlorodifluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride. In addition to these foaming agents, foaming aids such as metal salts or hydrogen metal salts of organic acids, metal salts or hydrogen metal salts of carbonic acid, talc, etc. may be used in combination.

In the production method of the present invention, the material compounding ratio and extrusion conditions when preparing a foamable molten resin by adding other materials as necessary to the raw resin, blowing agent, and foaming aid and melting them in an extruder are as follows: There is no essential difference from conventional foam injection molding, except that the extruder requires a low pressure extruder. Therefore, a detailed explanation of these conditions will be omitted.

The method for manufacturing a foam molded article of the present invention as described above has the following features compared to the conventional foam injection molding method.

Unlike injection molding, in which foamable molten resin is forced into a closed mold cavity, the resin is supplied onto the open molding surface of one mold, so there is no need for high-pressure extrusion equipment, and the metal The mold does not need to be pressure resistant either. Therefore, not only can the entire molding apparatus be inexpensive, but also large molded products, which were conventionally difficult to mold, can be easily produced.

Since the foamable molten resin is freely foamed and then compression molded, it produces molded products with more uniform foaming, a denser surface, and a higher expansion ratio than injection molding, which foams in a closed cavity. (In the case of injection molding, it is difficult to increase the expansion ratio to 2 times or more, but according to the method of the present invention, foaming can be easily increased from 2 to 10 times.)

Based on the unique resin supply method to the mold, the flow of resin along the molding surface can be minimized, and the foamed molten resin that is actually molded is smaller than the unfoamed (or immediately after foaming) molten resin. Because it has good fluidity, the resulting molded product has a beautiful surface without roughness or streaks, and has no unevenness in terms of physical properties, so it is stronger than an injection molded product with the same expansion ratio.

For the same reason as mentioned above, it is possible to mold even complex shapes that cannot be molded by injection molding.

The present invention will be explained below with reference to Examples.

Example Using a device similar to that shown in Figure 1,
A rectangular plate-shaped foam molded product measuring 125 mm, width 190 mm, and height 24 mm (depth of concave portion 20 mm, length 100 mm, and width 150 mm) was manufactured.

High-density polyethylene with a melt index of 11 is used as the raw material resin, and a mixture of 0.1% by weight (relative to resin weight) of sodium citrate as a foaming aid is supplied to the extruder from a hopper, and a foaming agent injection port is separately provided. 5% by weight (based on the weight of the resin) of a blowing agent (dichlorodifluoromethane) was injected into the
A foamable molten resin was prepared by heating to 160°C and kneading. The obtained molten resin was sent through a bendable tube to a die with a diameter of 8 mm, from which it was extruded in the form of a thread and suspended from the molding surface of the female mold below. The mold was a composite mold (5 rows x 8 columns arranged) for molding 40 molded products of the above shape at once, and was heated to 120° C. with an electric heater. After approximately 200 g of molten resin was uniformly deposited per 1 m ² of the horizontal portion of the molding surface, the supply of resin was stopped and the female mold was moved horizontally. Approximately 2 minutes after extrusion, when the molten resin has expanded to approximately 5 times the foam (60% of the maximum foaming ratio that can be achieved at normal pressure), the male mold corresponding to the female mold is lowered from above to separate both molds. Immediately after combining, heating of the mold was stopped. After that, cooling water was sent to the mold cooling pipe to cool the mold and the resin, and after 10 minutes, the male mold was separated from the female mold and the hardened foamed resin was taken out.

The resulting foamed molded product had a dense, uniform, and very thin skin layer, and the foaming in areas other than the skin layer was extremely uniform, and the average expansion ratio was 3.1 times.

[Brief explanation of the drawing]

FIG. 1: Schematic representation of an example of an apparatus for carrying out the invention. FIG. 2: A sectional view showing a state in which the resin melt foams on the female mold 2 of FIG. 1. FIG. 3: A sectional view showing a state in which the female molds 2 and 3 of FIG. 1 are combined to mold a foamed resin. 1: Extruder, 2: Female mold, 3: Male mold, 41: Molding surface, 6: Foamed resin.

Claims (1)

[Claims] 1. A female die of a mold consisting of a female die having a cylindrical portion surrounding the molding surface and a male die having a piston-like portion capable of being fitted in a liquid-sealing manner to the cylindrical portion of the female mold is placed on the molding surface. The molding resin is supported in a position that can hold the molten molding resin, and a molten thermoplastic synthetic resin containing a blowing agent is supplied onto the molding surface of the female mold to achieve the maximum expansion ratio that can be achieved at normal pressure. The foaming is carried out to a foaming ratio of 50% or more and larger than the foaming ratio of the intended molded product, and then the male mold is fitted into the cylindrical part of the female mold to complete the specified mold cavity, and in the process, the inside of the female mold is A method for manufacturing a foamed thermoplastic synthetic resin molded article, which comprises compressing a foamed resin melt, cooling a mold, and taking out the hardened foamed resin from the mold.