JPS59165634A - Injection molding method - Google Patents

Abstract

PURPOSE:To equalize the distribution of the residual stress of each section of a shape, and to obtain the injection-molded shape with no sink and warp by completely filling a die with a resin, decompressing resin pressure in the vicinity of a gate through a specific method and applying injection holding pressure again after a fixed time passes. CONSTITUTION:A melted resin 7 stored in the nose section of a screw 1 is injected and filled with high pressure into a die 4 by the advance of the screw 1. When pressure in the die 4 reaches to a set value, a reaching to the set value is detected by a die inside resin pressure sensor 15, and transmitted over a die inside set pressure gage 17. A signal is transmitted over a timer 18 from said pressure gage 17, and a solenoid valve 10 is changed over. Consequently, the screw 1 is retreated, and the distribution of resin pressure in the die, which is higher near a gate 8 and lower in the terminal section of a shape 5, on a filling is reversed while cooling is advanced. When the timer 18 times out, the solenoid valve 10 is automatically changed over again, and pressure oil is forwarded from an injection oil pressure cylinder 9 to apply injection holding pressure, thus completing molding.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an injection molding method that can be applied to an injection molding machine.

In the injection molding process, granular (or powder) raw resin is
The rotation of the screw (1) from the hopper shown in FIG. By sending the molten resin to the screw (1)
is pushed back (to the right in the figure), and the molten resin is stored in the space (7) created thereby. When the amount necessary for molding is plasticized and melted, the screw (1) stops rotating.

The molten resin is injected by the screw (1) through the nozzle (13) into the mold, which is held with a large mold clamping force by a mold clamping mechanism (not shown), to form a predetermined molded product <s>. get.

Molded product (S): If high injection pressure is not applied to the injection ram (6), short shots (unfilled injection) or sink marks (concave state) will occur; if too much pressure is applied, cracks or warpage will occur.

Furthermore, in order to reduce sink marks, it is common to lower the temperature of the molten resin (?) and the temperature of the mold (<'), but this requires high injection pressure. However, a large residual stress is generated near the gate (g), causing warping. Therefore, in conventional injection molding machines, it is necessary to finely adjust the molten resin temperature, mold temperature, injection holding pressure level, and injection holding time, resulting in the following problems.

In other words, sink marks and warpage are incompatible with each other, and a compromise molding condition has to be adopted that suppresses sink marks to an extent that allows for warpage. In addition, in order to reduce warpage, it is necessary to use a refrigerator or temperature controller in combination to create a difference in mold temperature between the fixed mold (lIa) and the movable mold (0), which requires additional equipment costs. was necessary. On the other hand, since the cooling time is increased in order to reduce warpage, productivity is poor and the molded product may have to be placed in a correction frame even after being taken out, which has the disadvantage of requiring manual labor.

The present invention is based on the combination of a lid and a bottom case when molding a case, etc.
This was proposed to satisfy the conditions that warpage must be suppressed within a certain limit and sink marks must be minimized in terms of appearance. Immediately after filling the molded product, the injection pressure is completely lowered. , reduce the resin pressure near the gate of the molded product, and apply injection holding pressure after a while.
The purpose of the present invention is to provide an injection molding method that balances the residual stresses near and far from the gate and produces molded products without sink marks and warpage.

The embodiments of the present invention will be explained below with reference to the drawings. Figure 2 shows an embodiment of the present invention. (1) is a screw that is rotated by a hydraulic motor (:10) to melt the raw citrus fat in the hopper. The molten resin (7) is stored at the tip. (
9) is an injection hydraulic cylinder, and a pump (II) is installed on the (9a) side.
), the screw (1) and ram (/
, 2) are connected, so the screw (1) moves forward (
(towards the left in the figure).

Molten VM resin (7) stored at the tip of the screw (1)
) is injected through a nozzle (/3) and a gate <g> into a mold (ge) which has been clamped and held in advance by a mold clamping mechanism (not shown). (5) is a molded product, and an in-mold resin sensor (lS) using an ejecting bottle (lda) is installed.
It is connected to the in-mold setting pressure needle (17) in the controller (1B). The output signal from the constant pressure inside the mold (/?) is further connected to a timer (ig), and the output signal
Turn the solenoid valve (10) on and off.

The solenoid valve (l) is energized by a sequence controller (not shown) at the ('#7a) part during injection, and when reducing the resin pressure in the mold, the (ta) part is excited from the timer (/g) signal.
b) Department enters. (10b) enters, the ram (l:l) retreats (rightward) due to the pressure oil from the pump (ll), and the connected screw (1) also retreats.The injection height advances by the sequence controller [) The injection holding pressure operated by J, J and the controller (16) is adjusted by an electromagnetic proportional pressure valve (19).

Next, to explain the operation, the fixed pressure inside the mold is the pressure necessary for filling, and the timer time in the resin pressure release process is the time for reducing the resin pressure near the gate and forming the skin of the molded product, both of which are set in advance. It is set. When the mold internal pressure reaches the set value, a signal is transmitted from the resin pressure gauge (l) to the timer (ig), and the solenoid valve (io) starts from (10a) to (l/
rb) and proceed from the injection high-pressure filling process to the resin pressure release process.

In the high-pressure filling process, all the resin flows from g) to the end of the molded product, whereas in the resin pressure release process, the resin flows from the molded product (S) through the gate (&) and through the nozzle [13]. Resin flows back. Therefore, in the resin pressure release step, the resin pressure near (g) is reduced to near atmospheric pressure. Also, when the timer (1g) times out, injection holding pressure is applied again to prevent sink marks.
Also, the sled becomes smaller.

Now, in the embodiment described above, the completion of filling was detected by the in-mold rosin pressure sensor (13), but the screw position sensor (2) was used to detect the completion of filling.
1) can also be substituted. In other words, in injection molding, depending on the size of the molded product, the screw position sensor (2/) detects the screw stroke (in Figure 2, the pressure of the molten resin sent to the tip of the screw by the screw rotation) causes the screw to move to the right. t17+o Therefore, there is a correlation between the screw stroke, in other words, the amount of screw movement and the degree of filling of the molded product. It is possible to know when the filling is complete by electrical signals.

It is also possible to use an injection oil pressure sensor as a filling completion signal. That is, as shown in Figure S, in injection molding, the injection speed is generally lowered near the completion of filling, so the oil pressure on the (9a) side of the injection hydraulic cylinder (9) also drops once, but once the filling is completed, it rises again. . Therefore, in FIG. 5, if this oil pressure once decreases and then changes to increase, it can be detected by the injection oil field sensor and used as a signal indicating completion of filling of the molded product. In addition to controlling the resin pressure releasing process using a timer (7g), it is also possible to set and control the amount of screw retraction using a screw position sensor (xl).

Now, Figure 3 shows the relationship between the in-mold pressure waveform and the amount of warpage (Fig. 3).
The total in-mold resin pressure in the figure is an example of the pressure at the center between the gate and the end of the molded product.) As shown in FIG. 1, the amount of warpage X refers to the value of X at the point where the molded product is placed on a surface plate and has the largest amount of floating.

In conventional molding methods, the resin pressure in the mold increases unilaterally with respect to the injection time, and begins to decrease when the gate cools and solidifies. However, in order to prevent sink marks, the resin pressure in the mold near the gate due to the injection holding pressure is abnormally higher than that at the end of the molded product. Therefore, the residual stress distribution of the molded product after cooling becomes large, and the amount of warpage X becomes large.

In the present invention, the screw is once retracted after filling is completed, so that the resin pressure in the mold, which is high especially near the gate, decreases the most. On the other hand, at the end of the molded product, the in-mold resin pressure during filling remains applied, and during this time, the molded product skin continues to cool and solidify. Since the injection holding pressure is applied again after the molded product skin is formed, sink marks can be prevented. On the other hand, the residual stress distribution near the gate and at the end of the molded product can be balanced.
Warping is prevented. In addition, in the experimental example, the warpage was resolved with the sink marks being the same level.

Further, since the burr near the gate, which tends to occur during the injection holding pressure process, is applied after forming the thin skin of the molded product, burr does not occur and sink marks are also prevented. In the present invention, there is no need to create a temperature difference between the fixed side and the movable side of the mold, so no freezing or temperature control equipment for the mold is required. Therefore, with the present invention, there is no need to take into consideration differences in thermal expansion when designing a mold, which simplifies the design of the mold, and the molding cycle can be shortened from the previous long cooling time due to warping. This has the effect that the time can be shortened.

[Brief explanation of drawings]

Fig. 1 is a side sectional view of a conventional injection molding machine, Fig. 2 is a system diagram of an injection molding machine implementing the method of the present invention, and Fig. 3 is a comparison between the total in-mold resin pressure and time in the present invention and the conventional method. A diagram showing the relationship, Figure 4 is an explanatory diagram showing the amount of warpage, and Figure S is a diagram showing the relationship between injection oil pressure and time (screw position). DA・φ・Mold Mold S...Molded product 7...Melted resin G...Gate
9... Injection hydraulic cylinder 13... Nozzle /
&・Bite・In-mold resin pressure sensor ・In-mold constant pressure gauge Patent applicant Mitsubishi Heavy Industries, Ltd. Churyo Engineering Link Co., Ltd. Figure 1

Claims (1)

[Claims] In injection molding, after high-pressure, high-speed injection, the screw is temporarily stopped or retreated to reverse the resin pressure distribution throughout the mold, which is high near the gate and low at the end of the molded product during the filling process, while cooling proceeds and the injection is held again. An injection molding method characterized by applying pressure to uniform the distribution of residual stress after taking out the molded product, thereby eliminating warpage, cracks, and sink marks.