JPS58217331A - Method and apparatus for preventing sink in plastic molding - Google Patents

Abstract

PURPOSE:To mold a plastic product, cost thereof is low and which has a boss and a rib of sufficient strength, by pressing a position where the generation of a sink is predicted from the back side of the sink at a proper period of a cooling stage in plastic molding and preventing the sink. CONSTITUTION:In plastic molding, the position where the generation of the sink is predicted in a shape is pressed from the back side of the sink at a proper period of a shrinkage process of plastic as the cooling stage of a molding cycle to prevent the sink. It is preferable that a device in which a sink preventive press pin 4a combining a boss hole forming pin with a projecting section projected to the inside while penetrating a die 3 on the side reverse to the side where the generation of the sink is predicted is set up slidably to said die 3 is used as the titled device at that time, and the position A' where the sink is generated is pressed from the back side of the sink by said press pin 4a to prevent the generation of the sink.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reliably preventing sink marks in plastic molding and an apparatus used therefor.

In plastic molding, when a product having a die or ribs on the back side is molded, there is a problem in that sink marks occur on the front side. To give a concrete example of the drawings, for example, as shown in Fig. 1, a plastic product 1 having a groove 1a on the back side is made, and as shown in Fig. When molding is performed using a mold consisting of the lower mold 3, a sink mark A as shown in the figure occurs on the surface side corresponding to the boss 1aK.

Conventionally, as a measure to prevent the occurrence of such sink marks or to alleviate sink marks, the pace has been made thicker than the thickness of the ribs. Alternatively, there are methods such as reducing the thickness of the ribs compared to the thickness of the pace, but these methods have the following problems. That is, in the former case, the amount of material used increases and the cost increases, and in the latter case, problems may occur due to insufficient strength. others,
Manual use with less shrinkage at the product and mold design stage,
Alternatively, there is a search for better conditions by manipulating various molding conditions at the molding stage, but so far no sufficient effect has been obtained. Furthermore, recently, olefin-based plastics are often used, and even olefin-based plastics made into composite materials have the problem of sink marks, which has hindered the introduction of products.

The present invention has been made in view of the current situation, and an object of the present invention is to reliably prevent the occurrence of sink marks in plastic molding as described above without causing any problems, and to make it possible to manufacture high-quality plastic molded products. With the goal.

In other words, the method for preventing sink marks according to the present invention is achieved by properly controlling the shrinkage process of the plastic during the cooling and pressure stage of the plastic molding process: mold clamping, plastic injection, cooling and pressure retention, mold opening, and product removal. This product is designed to prevent the occurrence of sink marks by applying pressure from the back side of the part of the product where sink marks occur at certain times of the year.

FIG. 3 shows an embodiment of the sink prevention device of the present invention developed to carry out the sink prevention method described above, using a mold for molding the same plastic product 1 shown in FIG. 1 as an example. This is a clear sign. In FIG. 3, 1 is the above-mentioned plastic product, 2 is an upper mold, 3 is a lower mold, and 4 is a pressurizing member consisting of a boss hole forming bottle front sink prevention pressurizing bottle 4a and a bottle holding plate 4b. Member 4 is lower mold 3
That is, the boss hole forming pin front sinking prevention pressure bottle 4a penetrates the mold on the side opposite to the side where sinking is expected to occur, and
It is slidably provided.

The sink prevention device of the present invention is constructed as described above, and by applying a pressing force to the pressure member 4 at an appropriate time during the cooling and pressure holding stage of the molding process, the sink prevention pressure girder 4a in front of the boss hole forming pin is It moves forward and moves to the state shown in FIG. 4, thereby pressurizing the material in the area A' where sink marks are expected to occur.

This pressurization is maintained in parallel with the cooling pressure holding process.

In this embodiment, only the bottom of the sink hole is pressurized, but it is also possible to pressurize the entire sink hole by changing the shape of the sink prevention pressure bottle 4a as shown in FIG. FIG. 6 shows an example in which the entire rib is pressurized.

As explained above, the method for preventing sink marks of the present invention prevents the occurrence of sink marks by applying pressure from the back side of the area where sink marks are expected to occur at an appropriate time during the cooling stage of plastic molding. This is what I did.

Therefore, according to the method for preventing sink marks of the present invention, it is possible to use a conventional method in which the pace is thicker than the wall thickness of the boss and ribs, and in which the wall thickness of the boss and ribs is reduced compared to the wall thickness of the pace. Plasti is inexpensive because there is no need to do so, and has sufficiently strong bosses and ribs. Can mold products.

In addition, the sink prevention device of the present invention utilizes the hydraulic function of conventional molding machines, so there is no need to provide additional functions for each mold, and the knockout structure of the boss part is similar to that of conventional molds on the mold surface. It is possible to prevent the occurrence of sink marks with a slight increase in cost.

In particular, by applying it to olefin-based composite materials, which have recently been widely used, it becomes possible to more easily and expand the use of the inexpensive resin.

[Brief explanation of the drawing]

Figure 1 is a sectional view showing an example of a plastic molded product, Figure 2 is a cross-sectional view showing an example of a plastic molded product.
The figure is a sectional view showing an example of molding the plastic molded product shown in Fig. 1 using a conventional mold, Fig. 3 is a sectional view showing an example of the sink mark prevention device according to the present invention, and Fig. 4 is a sectional view showing an example of the plastic molded product shown in Fig. 3. FIGS. 5 and 6 are partial sectional views showing other embodiments of the present invention. 1...Plastic product, 1a...Boss, 2...
Upper mold, 3... Lower mold, 4... Pressure member, 4a...
Boss hole forming bottle front sink prevention pressurized bottle, 4b...bin holding plate, A...sink. Figure 1 Figure 3 Figure 5 Figure 1 Figure 2 Figure 4 Figure 6

Claims (1)

[Claims] (Re) In plastic molding, the shrinkage is reduced by applying pressure from the back side of the part of the molded product at an appropriate time during the shrinkage process of the plastic, which is the cooling stage of the molding cycle. A method for preventing sink marks in plastic molding, characterized by preventing the occurrence of sink marks. (2) A pressurizing member having a protrusion that penetrates the mold and projects inward on the mold on the side opposite to the side where sink marks occur. 1. An apparatus for preventing sink marks in plastic molding, characterized in that the pressurizing member is slidably provided so that pressure can be applied to the area where sink marks occur from the back side of the pressurizing member.