JPH0550971B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to the treatment of dents formed on the surface of injection molded products;
The present invention relates to an injection molding method that can prevent so-called sink marks.

[Conventional technology]

A conventionally known injection molding machine has an injection mechanism that melts and injects an injection molding material (hereinafter simply referred to as resin), such as a thermoplastic raw material or a thermosetting resin, using a heating cylinder equipped with a screw (and/or plunger). It consists of a mold clamping mechanism that clamps the mold under high pressure, and a control mechanism that automatically moves these.The resin is dropped from a hopper into a heating cylinder, plasticized and kneaded, and this heated and molten resin is moved forward by a screw. The structure is such that a molded product is obtained by injecting (injecting) into a mold at high pressure, cooling and solidifying, and then releasing from the mold.

By the way, when heated and molten resin is injected into a mold, the molten resin 1 near the mold surface (resin layer outside the molded product) is rapidly cooled, as shown in FIG.
The molten resin (resin layer inside the molded product) 2 inside the mold is gradually cooled, and the resin shrinks.

On the other hand, as shown in FIG. 5, the injection pressure 3 is highest immediately after the start of injection, and sharply decreases when resin filling is completed (when switching to holding pressure). On the other hand, the mold internal pressure 4 is
Although it is very high immediately after resin filling is completed, it becomes extremely low when the molded product is taken out (when cooling is completed). In other words, the molten resin undergoes very complex changes (behaviors), such as being compressed and contracted by the high injection pressure immediately after the start of injection, and expanding by the decompression effects of the various pressures mentioned above.

In this way, when the molten resin is injection molded, a difference in volume, so-called molding shrinkage, occurs due to the above-mentioned complicated behavior. This molding shrinkage of the molten resin causes sink marks, which are said to occur most frequently among molded product defects.

FIG. 6 is a sectional view for explaining the principle of generation of sink marks. In FIG. 6, reference numeral 5 indicates a sink area, and 6 indicates a rapidly cooled resin portion outside the molded product, which exhibits a temperature change in the molten resin 1 near the mold surface as shown in FIG. 7 is the slowly cooled resin part inside the molded product (inside the thick wall part), and the part exhibiting temperature changes in the molten resin (resin layer inside the molded product) 2 inside the mold shown in FIG. It is. The sink mark 5 is caused by the slowly cooled resin part 7 pulling the rapidly cooled resin part 6.

[Problem that the invention seeks to solve]

In short, the sink marks are caused by molding shrinkage caused by a wide variety of behaviors of the molten resin, as described above, and are considered to be defects that are extremely difficult to remove.

In order to prevent this, conventional methods include, for example, increasing the injection pressure and lowering the temperature of the injection heating cylinder while applying sufficient holding pressure, or increasing the diameter of the sprue, runner, and especially the gate.
Alternatively, various measures are taken, such as creating a design that is less likely to cause sink marks at the molded product and mold design stage.

However, all of these methods involve technical difficulties and require complicated control means, and cannot be said to be a satisfactory and effective means for preventing sink marks.

Although it is not directly related to sink prevention measures,
Molding methods are known in which a foaming agent is added to a resin to produce sponge-like plastic molded articles, such as urethane foam or foamed polyethylene. When using this method, the air pressure during foaming prevents the occurrence of sink marks, but since the foaming extends to the surface layer, there is a problem in that the appearance becomes dirty.

In addition, the so-called sanderch molding method is also known, in which the resin that will become the skin layer is first press-fitted, and then the inner resin (containing a foaming agent) is press-fitted, but when using this method, two molding machines are required. The structure is complex,
Another problem is that it is limited to thick-walled molded products.

Therefore, the present invention provides an injection molding method that can prevent sink marks and provide a beautiful finished appearance to molded products by using novel means that are completely different from conventional means such as applying excessive injection pressure and holding pressure. The purpose is to provide

[Means for solving problems]

In order to achieve the above object, the present invention injects into the mold a foaming agent that does not foam at the temperature of the molten resin in the parts of the mold where the cooling rate is fast, but foams at the temperature of the molten resin in the parts where the cooling rate is slow. , molding is performed by giving different foaming reactions due to differences in the cooling rate of the molten resin.

[Effect]

In the process of cooling and solidifying the molten resin mixed with the foaming agent filled in the mold, as shown in Figure 4,
The molten resin (resin layer outside the molded product) 1 near the mold surface is rapidly cooled, and at this rapidly cooled resin temperature, the foaming agent mixed therein does not foam. Therefore, in this portion, the resin solidifies without the foaming agent foaming. As a result, a molded product can be obtained that has a clean surface, that is, a molded product appearance, and has no decrease in strength.

On the other hand, as shown in FIG. 4, the molten resin (resin layer inside the molded product) 2 inside the mold is slowly cooled and its temperature is not easily lowered. In other words, the resin temperature in this portion is maintained higher than the resin temperature in the vicinity of the mold surface. At this high resin temperature, the foaming agent reacts and foams. The molten resin in the mold is expanded from the inside by the foaming pressure during this foaming. This expansion makes it possible to prevent the occurrence of sink marks due to contraction of the resin during cooling.

〔Example〕

Hereinafter, the present invention will be explained in detail based on embodiments shown in the drawings.

FIG. 1 is a schematic front view showing an embodiment of an injection molding machine suitable for carrying out the present invention, in which 10 is a hopper for supplying resin, 11 is a heating cylinder for injection equipped with a screw, and 12 13 is a fixed plate, 13 is a fixed mold, 14 is a movable mold, 15 is a movable plate, 16 is an ejector pin, 17 is a sprue, 18 is a runner, 19
is a mold cavity, and a molding machine main body is constructed from each of these known elements.

The injection molding machine applied in this embodiment has a hopper 20 for supplying a blowing agent in the molding machine main body.
, a foaming agent extrusion screw 21 , a foaming agent mixing passage 22 provided vertically from the tip of the screw 21 through a part of the movable mold 14 and communicating with the insertion hole of the ejector pin 16 , and this passage 22 .
This structure is equipped with a blowing agent mixing mechanism consisting of an air supply pipe 23 for stirring the blowing agent, which is connected to the blowing agent, and a motor 24 for rotationally driving the screw 21.

In addition, as the foaming agent, mold cavity 1
Due to the difference in the cooling rate of the molten resin in the parts 9, foaming does not occur at the resin temperature in the part where the cooling rate is fast (the temperature of the rapidly cooled molten resin 1), and the resin temperature in the part where the cooling rate is slow (the temperature of the molten resin 1 that is slowly cooled). A foaming agent with different foaming reactions is used so that the foaming occurs at the temperature of the molten resin 2.

Specifically, for example, as shown in Figure 2, if heat is applied above a certain temperature for a certain period of time, that is, if it is heated above a certain temperature (temperature of the resin in areas where the cooling rate is slow) for a certain period of time, it will melt. Foaming will occur unless the foaming agent 32 (for example, azodicarboxylic acid amide or baking soda) is coated with the foaming material 31 and hardened into granules with the coating agent 30, or the foaming agent itself is at a high temperature (temperature of the resin in the area where the cooling rate is slow). Use something that does not react (for example, hydrazodicarbonamide).

In this embodiment, the coated foaming agent 32 shown in FIG. 2 is used as the foaming agent.

Next, the injection molding operation in this example will be explained.

In FIG. 1, first, after the fixed mold 13 and the movable mold 14 are clamped, the ejector pin 16 is retracted to connect the foaming agent mixing passage 22 to the sprue 17 and the runner 18. Next, the motor 24 is driven to rotate. Then, the blowing agent extrusion screw 21 rotates, and the coated blowing agent 32 in the hopper 20 passes through the mixing passage 22 and enters the movable mold 1.
Fall to 4. At this time, in order to smoothly feed the coated foaming agent 32 into the movable mold 14 through the passage 22, that is, to prevent the foaming agent 32 from stagnation, air is introduced through the stirring air supply pipe 23. It is fed into the passage 22 and the coated foaming agent 32 is stirred. Thereafter, the melted resin is press-fitted into the mold cavity 19 using the heating cylinder 11 . In this case, melting and injection of the resin, and mixing of the foaming agent 32 coated therein into the molten resin are performed coaxially.

Thereafter, in accordance with a conventional general method, the coated molten resin mixed with the blowing agent 32 in the mold cavity 19 is cooled and solidified, and then released from the mold.

In the cooling solidification step, the molten resin near the surface of the mold cavity 19 is rapidly cooled as shown by the solid line 1 in FIG. 4, so that the foamed material 3 in FIG.
The foaming agent 32 solidifies together with the resin without causing a foaming reaction, without melting the coating agent 30 coated with the foaming agent 1. Therefore, the appearance of the molded product is maintained as neatly as that of non-foamed resin, and the strength is not reduced. On the other hand, mold cavity 19
Since the molten resin inside is slowly cooled as shown by the broken line 2 in FIG. 4, the temperature does not easily drop, so the coating agent 30 in FIG. 2 is melted. As a result, the foaming substance 31 reacts and the foaming agent 32 foams. As shown in Figure 6, the foaming pressure during foaming causes sink marks 5, which occur during the molding of normal non-foamed resin, to the molded product, which solidify first but are drawn in by shrinkage due to cooling of the resin inside. To eliminate dents in the rapidly cooled resin part 6 appearing on the surface.

FIG. 3 is a front view of main parts showing another embodiment of an injection molding machine suitable for carrying out the present invention.

The injection molding machine used in this example has an injection heating cylinder 1 equipped with a screw similar to that shown in FIG.
1, fixed plate 12, fixed mold 13, movable mold 14,
Movable plate 15, ejector pin 16, sprue 17,
A molding machine body consisting of known elements such as a runner 18 and a mold cavity 19 has a hopper 40 for supplying a foaming agent, a screw 41 for extruding the foaming agent, and a part of the fixed mold 13 that passes through and communicates with the sprue 17. A blowing agent mixing piston 42 provided vertically as shown in FIG.
The blowing agent mixing mechanism is provided with a blowing agent mixing mechanism consisting of a motor (not shown) for rotationally driving the screw 41 and a motor (not shown).

In this embodiment, first, the blowing agent mixing piston 42 is retracted, and the coated blowing agent 32 shown in FIG. 2, for example, is fed from the hopper 40.
Molten resin is injected from the heating cylinder 11 while moving the piston 42 forward, and the coated foaming agent 32 is mixed into the molten resin.

That is, in this embodiment, the injection of the molten resin and the supply of the foaming agent are carried out on the same axis, and the coated foaming agent 32 is directly mixed into the molten resin flowing on the same axis (the sprue 17) from the vertical direction.

In this way, the coated blowing agent 32
The molten resin mixed with is injected into the mold cavity 19, and thereafter exhibits the same action and effect as in the case of FIG.

[Effects of the Invention] As is clear from the above examples, the present invention uses a foaming agent that does not foam at the temperature of the molten resin in the parts of the mold where the cooling rate is fast, but foams at the temperature of the molten resin in the parts where the cooling rate is slow. As shown in FIG. At the temperature of the molten resin that is rapidly cooled near the mold surface, the foaming agent does not foam, and the foaming agent in this portion solidifies without foaming. Therefore,
A molded product with a beautiful appearance and no decrease in strength can be obtained.

On the other hand, the molten resin inside the mold is slowly cooled and its temperature does not easily drop, so the blowing agent reacts to the temperature of the molten resin in this area and foams, and the foaming pressure during this foaming causes the inside of the mold to Since the resin expands, the quenched molten resin also expands. This can prevent sink marks from occurring on the surface of the molded product.

In addition, since the present invention has the effect of locally foaming only the inside of the thick-walled part of a molded product, the appearance is beautiful for molded products other than transparent molded products, and molded products with mixed thin-walled parts can be used. It can also be applied to

Furthermore, according to the present invention, it is possible to prevent the occurrence of sink marks without applying excessive injection pressure (holding pressure) as in the conventional method, and since this sink mark is eliminated, it is possible to fill the resin to an excessive mold internal pressure. In addition to eliminating the need for molding, burrs are also reduced in precision molded products that combine nested pieces or products with complex shapes. It also prevents warping due to excessive pressure.

In addition, the fact that there is little sinkage even when the mold internal pressure is low
It also helps reduce burrs, making it extremely advantageous for molding precision parts, which are often small parts. Furthermore, it has the advantage that it can be applied to both small and large machines. Particularly, in large-sized injection molding, there are many advantages such as reduction in mold clamping force and injection pressure of the molding machine and extension of mold life.

In addition, the present invention compensates for the weaknesses of existing injection molding, and can be applied, for example, to injection molding, sealing molding, etc. of products other than completely thin-walled products such as transparent products and containers.

[Brief explanation of drawings]

FIG. 1 is a schematic front view showing an embodiment of an injection molding machine suitable for carrying out the injection molding method according to the present invention, and FIG. 2 is a sectional view showing an example of a blowing agent directly used in carrying out the present invention. , FIG. 3 is a front view of main parts showing another embodiment of an injection molding machine suitable for carrying out the present invention, FIG. 4 is a graph showing temperature changes of molten resin in the mold, and FIG. FIG. 6 is a graph showing subsequent changes in injection pressure and mold internal pressure, and is a sectional view for explaining the principle of occurrence of this sink mark. 1... Molten resin near the mold surface, 2... Molten resin inside the mold, 3... Injection pressure, 4... Mold internal pressure, 5... Sinking, 6... Rapidly cooled resin part, 7
... Slowly cooled resin part, 11 ... Heating cylinder,
12... fixed plate, 13... fixed mold, 14... movable mold, 15... movable plate, 16... ejector pin, 17... sprue, 18... runner, 19...
Mold cavity, 21... Foaming agent extrusion screw, 22, 43... Foaming agent mixing passage, 23...
Stirring air supply pipe, 30...Coating agent, 3
1... Foaming substance, 32... Foaming agent, 41... Foaming agent extrusion screw, 42... Piston for mixing foaming agent.

Claims (1)

[Claims] 1 A foaming agent is injected into the mold that does not foam at the temperature of the molten resin in areas where the cooling rate is fast, but foams at the temperature of the molten resin in areas where the cooling rate is slow, and a foaming reaction occurs due to the difference in the cooling rate of the molten resin. An injection molding method characterized in that the foaming agent has a double structure in which a foaming substance is coated with a coating agent.