JPS6124972B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an injection molding method for reducing the shrinkage rate of injection molded thermoplastic resin products and increasing the dimensional accuracy of the injection molded products, and a mold used therefor.

Conventionally, it has been known that in order to reduce the shrinkage rate of injection molded thermoplastic resin products, the molding pressure can be increased to increase the injection pressure and the holding pressure. However, in reality, the resin first cools and solidifies at the gate of the mold, so the molding pressure is stopped at the gate and cannot be sufficiently transmitted into the cavity. For this reason, the pressure in the cavity is not sufficient, and as a result, the dimensional accuracy of the injection molded product cannot be improved. Regarding this point, it is possible to improve the transmission of molding pressure into the cavity to some extent by suppressing cooling solidification of the resin in the gate portion to some extent by adopting a gate with a large cross-sectional area. However, in this case, large marks from the gate remain on the injection molded product, and post-processing is required to remove the marks.

The present invention utilizes an injection molding method and a mold which do not have such drawbacks, and the cavity is fixed by retracting the movable member to increase the volume of the mold cavity just before the resin at the gate of the mold solidifies and the gate is closed. The resulting large difference between the resin pressure inside the cavity and the molding pressure causes the resin in the gate area that is about to solidify to be pushed into the cavity, and the gate area is replaced with new molten resin. This transmits the molding pressure into the cavity through the gate, increases the pressure inside the cavity, and as a result, obtains a molded product with high dimensional accuracy.

That is, the injection molding method of the present invention involves injecting molten thermoplastic resin through a gate made of a narrow through hole into the cavity of a mold having a movable member inserted into the cavity so that the volume of the cavity can be changed. In the first step of filling the cavity, the molten thermoplastic resin in the cavity is cooled by the mold, and immediately before the gate is substantially closed by the cooled and solidified thermoplastic resin, the movable member is a second step in which the resin pressure in the cavity is reduced by increasing the volume of the cavity; a third step in which new melted thermoplastic resin is introduced into the cavity through the gate; and a third step in which the heat in the cavity is reduced. The method is characterized by comprising a fourth step of taking out the thermoplastic resin molded article from the mold after the plastic resin has solidified.

Furthermore, the mold used for this injection molding method is
A mold body having a cavity into which the thermoplastic resin is injection molded, a gate consisting of a thin through hole for introducing the molten thermoplastic resin into the cavity, and a movable mold provided on a part of the mold surface forming the cavity. a first locking device for holding the movable member in a reduced volume of the cavity; a movable device for moving at least the movable member in a direction to increase the volume of the cavity; and a second locking device that maintains the increased volume of the device.

The injection molding method of the present invention and the mold used therein will be explained below.

The thermoplastic resin molded by the method of the present invention is polyethylene that melts when heated and solidifies when cooled.
Refers to thermoplastics such as ABS, thermoplastic composites such as glass fiber-reinforced thermoplastics, and organic materials such as lost wax. Hereinafter, it will be simply referred to as resin.

The mold used in the present invention has the function of increasing the cavity volume during molding. Specifically, a cylindrical hole is formed in the cavity wall of the mold, and a movable member corresponding to a piston is inserted into the hole. A cylinder/piston type that increases cavity volume by moving it to the back.
Then, a part of the wall surface of the cavity is formed with a film of elastic material such as rubber, a chamber is formed with the mold body and the film of elastic material, and a part of the fluid in the chamber is moved outside of the mold to cause the elastic material to flow. A membrane type that is deformed in the direction of the room and increases the cavity volume is considered.

Figure 1 shows a cross section of a typical cylinder/piston type mold. This mold consists of a rocket ring 1, a sprue bush 2, a fixed mounting plate 3, a runner stripper 4, a fixed mold 5 having a gate 51 consisting of a thin through hole, and a movable mold similar to a typical conventional injection mold. It consists of a mold 6, a receiving plate 7, a spacer block 8, an ejector pin 9, an ejector plate 10, a movable mold mounting plate 11, a slide pin 12, a wedge 13, a wedge plate 14, and a hydraulic system 15, which characterize this mold. . Note that the cavity 17 is formed between the fixed mold 5 and the movable mold 6. Here, the fixed mounting plate 3 and the movable mounting plate 11 are normally incorporated into an injection molding machine. A rocket ring 1 and a sprue bush 2 are fixed to the outer surface (the surface facing the injection molding machine) of the fixed mounting plate 3. The fixed mounting plate 3 and the movable mounting plate 11 are opposed to each other, and their relative positions can be changed while facing each other along a guide (not shown). This is normally driven left and right in FIG. 1 by a hydraulic device (not shown) installed outside the movable mounting plate 11. Movable mounting plate 1
A spacer block 8, a receiving plate 7 and a movable mold 6 are fixed to the inner surface of the mold 1. A runner stripper 4 and a fixed mold 5 are provided between the fixed mounting plate 3 and the movable mold 6 so as to be movable left and right along the guide. Further, an ejector plate 10 and an ejector pin 12 are provided in association with the receiving plate 7 and the movable mold 6. The above-mentioned parts of this mold and their functions are the same as those of a typical conventional injection mold. In addition to these parts, this mold also includes a hydraulic device 15 fixed to the movable mold mounting plate 11, and a hydraulic device 1.
The wedge plate 14 is attached to the tip of the piston 5, and the wedge 13 and wedge 1 are driven left and right by the vertical movement of the wedge plate 14.
A slide pin 12 is fixed to the movable mold 6, passes through the receiving plate 7, and is inserted into a through hole of the movable mold 6. Here, the slide pin corresponds to a movable member of the mold used in the present invention. The tip end surface of the slide pin 12 forms a part of the mold surface of the cavity 17 of this mold, and the volume of the cavity 17 changes due to the relative movement between the movable mold 6 and the slide pin 12.
That is, the slide pin 12 is driven left and right in the drawing by the hydraulic device 15, and the volume of the cavity 17 is changed. This hydraulic device 15 is a normal hydraulic piston cylinder that is composed of a piston and a cylinder, and the piston is driven relative to the cylinder by hydraulic pressure from a hydraulic source, and the movement and stop of the piston is controlled by a hydraulic valve (not shown). That is, the reduction in the volume of the cavity 17 is achieved by opening the hydraulic valve and supplying hydraulic pressure to the hydraulic device 15 to drive the wedge plate 14 up and down in the drawing, thereby moving the slide pin 12 to the left. . On the contrary, the increase in the volume of the cavity 17 is
This is achieved by allowing hydraulic pressure to flow in the opposite direction with a hydraulic valve. That is, the hydraulic system 15 includes a first locking device, a movable device, and a second locking device for the mold used in the present invention. The shape of the cavity 17 of this mold is a disc with a center hole and a thick wall at the outer periphery and the center hole, and its dimensions are: outer diameter 70 mm, inner diameter 8 mm, thick part 8 mm, The thin part is 4mm. Further, there are four ejector pins 9 and four slide pins 12, and they are arranged alternately. The outer diameter of the slide pin 12 was 5 mm, and the maximum movement distance of the slide pin was 2.5 mm (the cavity volume increase rate was 0 to 1.03%).

In this mold, the tip of the slide pin 12 is projected a predetermined amount into the cavity 17 before molding.
During molding, the slide pin 12 is moved backward to increase the volume of the cavity 17 and obtain the desired cavity shape. Furthermore, with this mold, it is also possible to move the slide pin 12 back beyond the desired cavity shape during molding, and then move the slide pin 12 forward to form a predetermined cavity shape. In this case, the volume of the cavity 17 first increases during molding and then decreases.

In the mold of this example, slide pin 1
By arbitrarily changing the degree of protrusion of No. 2 into the cavity 17, an arbitrary cavity volume increase rate can be achieved. Furthermore, with this mold, a molded product of the same shape can always be obtained even if the volume increase rate is set arbitrarily.

Next, an example of the injection molding method of the present invention will be described using this mold. Polyacetal was used as the resin.

First, the mold shown in Figure 1 was attached to an injection molding machine with a clamping force of 80 tons and a plasticizing capacity of 5 ounces. Next, the wedge plate 14 and wedge 13 are driven by the hydraulic device 15 so that the tip of the slide pin 12 protrudes 1.0 mm to 2.5 mm into the cavity 17, and then the valve of the hydraulic device 15 is closed and the slide pin 12 is moved into the cavity 17. Locked in position. Next, the plasticized resin was injected from an injection molding machine, and the cavity 17 was filled with the resin. Thereafter, after 1.5 seconds to 7 seconds have elapsed, the hydraulic device 15 is retracted, and the slide pin 12 protrudes into the cavity 17.
Retract the tip of the cavity to make it a flat cavity plane, and fix the slide pin 12 in that position, and in this state, sufficiently cool the resin in the cavity 17,
The movable mounting plate 11 was driven to the left in the drawing, the movable mold 6 was opened, and the molded product was taken out. Thereafter, the weight and outer diameter of the molded product obtained were measured. FIG. 2 shows a relationship diagram between the weight of the molded product obtained and the time required to return the slide pin 12 after filling the cavity 17 with resin. In FIG. 2, the horizontal axis shows the time (seconds) until the slide pin 12 returns, and the vertical axis shows the weight (g) of the molded product obtained. Note that the value of the slide return time of 0 seconds means that the slide pin 12 is not protruded and injection molding is performed in the position returned from the beginning, and corresponds to conventional injection molding. Also, the symbol 〇 in the figure indicates the slide pin 12.
The results are shown when protruding by 2.5 mm and returning by 2.5 mm. In addition
By moving the slide pin 12 of 2.5 mm, the volume of the cavity 17 increases by 0.196 ml, or 1.03% in proportion. In addition, the symbols △, □, and × indicate that the slide pin 12 was protruded by 2.0 mm, 1.5 mm, and 1.0 mm, respectively, and then returned to its original position.
0.157ml (0.82%), 0.118ml (0.62%), 0.079ml
(0.41%). From Figure 2, the cavity volume increase rate is 0.62% in the injection molding of this example.
As mentioned above, the volume increase time is 3 to 4 seconds after filling the resin into the cavity, and the weight of the molded product is 0.1 to 0.5 g.
A significant increase was observed. Looking at the shrinkage rate of the outer diameter of the molded product, the weight of the molded product is 0.1 to 0.5.
The external shrinkage rate of the molded products with an increase in weight was 2.2% to 2.3%, while that of the molded products with no increase in weight was 2.4%. From these results, in the case of this example, the volume increase rate is 0.62%.
If the volume increase time is set to 3 to 4 seconds after filling the resin into the cavity as described above, the dimensional shrinkage of the molded product will be small, and the area corresponding to sinkage will be small.
It was confirmed that better molded products could be obtained.

From a different perspective, since the resin in the gate part has not solidified within 3.5 seconds, there is no effect of replacing the resin in the gate part with new resin due to the increase in cavity volume. On the other hand, if the resin in the gate part is solidified for 4.5 seconds or more, the pressure inside the cavity is lowered due to the increase in the volume of the cavity, and even if a large pressure difference is created between the molding pressure and the molding pressure, the gate part will be completely solidified. It is not replaced by molten resin, and as a result, the holding pressure inside the cavity does not increase.

Next, an example will be described in which a molded product is obtained by increasing the cavity volume during molding and then decreasing the cavity volume. The same mold as in the above example was used, and the tip of the slide pin 12 was made to protrude 2.0 mm into the cavity 17. Next, the same polyacetal resin as in the above example was injection molded, and the inside of the cavity 17 was filled with molten resin. 3.5 seconds after filling, the slide pin 12 was returned 2.0 mm to increase the cavity volume by 0.157 ml (0.82%). Furthermore, 5 seconds to 20 seconds after filling, slide pin 1
The tip of 2 was made to protrude into the cavity 17 by 2.0 mm to reduce the cavity volume by 0.157 ml (0.82%) and cooled in that state to obtain a molded product. FIG. 3 shows the relationship of resin pressure within the cavity over time when injection molding is performed using various methods. In FIG. 3, the vertical axis shows the resin pressure (Kg/cm ² ) measured by a load meter installed on the wall of the cavity, and the horizontal axis shows the time (seconds) since the cavity was filled with resin. Also, the dotted line (...) shows an example when molding is performed without any change in cavity volume as in the conventional method, and the solid line (-) shows an example when the cavity volume is changed to 0.157 after 3.5 seconds of resin filling as in the above example.
An example of the present invention in which ml is increased, dotted line (-・-・
−), two-dot dashed line (−・・−・・−・・−), three dot dashed line (−・・−・・−・・), and dashed line (−
-----) are respectively 5 seconds and 10 seconds from the time of resin filling.
Examples are shown in which the cavity volume is reduced after 15 seconds, 15 seconds, and 20 seconds, and the cavity volume is returned to the cavity volume at the time of filling. Further, FIG. 4 shows a relationship diagram between the weight (g) and outer diameter shrinkage rate (%) of the obtained injection molded product and the time for reducing the cavity volume. In FIG. 4, the horizontal axis shows the time (seconds) from filling the resin to reducing the cavity volume, the left vertical axis shows the weight of the molded product, and the right vertical axis shows the outer diameter shrinkage rate.
Further, in the figure, the 〇 mark indicates the outer diameter shrinkage rate, and the × mark indicates the weight value. For reference, the weight and outer diameter shrinkage of the molded product when there is no change in cavity volume as in the conventional method are expressed as × ¹ , 〇 ¹ , and in addition, in the case where the cavity volume is only increased as in the previous example. The weight and outer diameter shrinkage of the molded product are × ² , 〇 ²
It is shown in FIG. As is clear from FIG. 4, if the time required to reduce the cavity volume and return it to its original state is as fast as 5 seconds, a product with a lower shrinkage rate can be obtained by not reducing the cavity volume. This is thought to be because the resin in the gate part is not completely solidified within 5 seconds, so the more solidified resin in the cavity is sent to the gate part and the gate is closed without increasing the pressure inside the cavity. It will be done. However, when the time is increased to 10 to 20 seconds, a good molded product with a shrinkage rate lower than that without reducing the cavity volume is obtained. This is because if the time is longer than 10 seconds, the resin in the gate portion will completely solidify, and therefore, the decrease in cavity volume will directly increase the cavity pressure, contributing to molding. As is clear from comparing FIGS. 3 and 4, when a good molded product with low shrinkage rate is obtained, the resin pressure within the cavity is maintained high for a long time.

As is clear from these examples, the timing at which the cavity volume is increased varies depending on the size and shape of the cavity, the size and size of the gate portion, the type of resin to be molded, etc. Therefore, before starting full-scale molding, it is first necessary to decide when to increase the cavity volume in accordance with the molding conditions. If the optimum timing is selected and molding is carried out according to the method of the present invention, an excellent injection molded product with high dimensional accuracy and few "sink marks" can be obtained.

[Brief explanation of the drawing]

Figure 1 is a sectional view of the main parts of the mold used in the present invention, Figure 2 is a diagram showing the relationship between the slide pin return time of the mold shown in Figure 1 and the weight of the molded product obtained. Figure 3 is a diagram showing the relationship between resin pressure and time after filling the cavity of the mold shown in Figure 1, and Figure 4 shows the relationship between the time for increasing the volume of the cavity and the weight of the molded product obtained. It is a figure showing the relationship with shrinkage rate. In the figure, 1 is a rocket ring, 2 is a sprue bush, 3 is a fixed mounting plate, 4 is a runner stripper, 5 is a fixed type, 6 is a movable type, 7 is a receiving plate, 8 is a spacer block, and 9 is an edge. Ector pin, 10
is the ejector plate, 11 is the movable mounting plate,
12 is a slide pin, 13 is a wedge, 14 is a wedge plate, and 15 is a hydraulic system.

Claims (1)

[Claims] 1. Using a mold having a moving member inserted into the cavity so that the volume of the cavity can be changed, molten thermoplastic resin is injected into the cavity through a gate consisting of a narrow through hole, a first step of filling the cavity; the movable member being retracted immediately before the molten thermoplastic resin in the cavity is cooled by the mold and the gate is substantially closed by the cooled and solidified thermoplastic resin; a second step of increasing the volume of the cavity and reducing the resin pressure within the cavity; a third step of introducing the new melted thermoplastic resin into the cavity through the gate; and a third step of reducing the heat within the cavity. An injection molding method comprising a fourth step of taking out a thermoplastic resin molded article from the mold after the plastic resin has solidified.