JPH0788910A - Injection molding method - Google Patents

Abstract

PURPOSE:To efficiently mold a molded product generating no warpage by performing dwelling by back pressure after the release of the pressure of a resin charged by injection. CONSTITUTION:A resin is injected and charged into a cavity by the advance of a screw and the injection pressure at the point of time when the resin is charged into the cavity is set to a zero value to release the pressure of the resin. Thereafter, the weighing of the resin is started by the rotation of the screw and dwelling is performed by the back pressure at the time of weighing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method in which a material resin is measured and then injected into a cavity in a mold by advancing a screw in an injection heating cylinder to form a desired molded product.

[0002]

2. Description of the Related Art In injection molding with an injection device having a screw for injection in an injection heating cylinder, a screwing process in which a screw is rotated while applying a back pressure and plasticized resin is stored in the tip of the injection heating cylinder. After completing the above, the process moves to an injection process of filling the cavity in the mold with the measured resin by advancing the screw.

The injection process usually consists of a process of filling the cavity with the resin by the primary pressure and a pressure-holding process which is continuously performed by the secondary pressure. Generally, the filling process is performed under the control of speed. Although it is performed, the pressure holding step is performed by pressure control. This is because the molten resin flows backward from the gate until the gate is cooled and solidified when the filling pressure is immediately removed and cooling is started, so that this is prevented and the resin shrinks when it solidifies. This is to compensate for the contraction.

Therefore, since the holding pressure is not necessary after the resin of the gate is solidified, the holding pressure time is set for the time until the gate is solidified. Therefore, the holding pressure time depends on the design dimension of the molded product. On the other hand, in thick-walled products with thick runners and gates, the holding time is inevitably set longer. Then, after the holding pressure time has passed, the process is started to measure the resin.

The back pressure at the time of measurement is applied by the screw by hydraulic pressure as a resistance against the backward movement of the screw due to the forward movement of the molten material. When the back pressure is high, the screw backward speed becomes slow and the plasticizing occurs. Since the capacity decreases, it is usually set lower than the holding pressure.

The cooling process of the cavity is timed from the completion of holding pressure, and when the set time is up, the process is completed and the process of opening the cavity and taking out the molded product is started. This cooling time varies depending on the thickness and shape of the molded product, but is generally longer than the measuring time. However, although some of them may be short, even if they are excessively cooled, they do not particularly affect the molded products, and therefore cooling may be performed until the measurement is completed and the process may be shifted to the next step.

[0007]

If the resin is weighed according to the conventional molding method and the pressure is immediately transferred after injection and filling, the molded product may be partially warped or sinked. Although this phenomenon can be prevented to some extent by the shape of the molded product and the design of the cavity gate, there has been no means for preventing it in the case of a molded product whose shape and gate design are limited.

Therefore, as a solution to the above problem, the present inventors set the injection pressure at the time when the resin is filled in the cavity to a value of 0 to depressurize the resin in the cavity, and then hold the pressure. The molding method was first found. This new molding method has the advantage of being able to prevent the occurrence of partial warpage of the molded product without significantly changing the molding process. It has a new problem of taking a long time.

The present invention has been conceived in view of the above circumstances, and its purpose is to shorten the time required from the start of injection to the completion of measurement and start the measurement of the cooling time by combining the pressure holding process and the measuring process. An object of the present invention is to provide an injection molding method capable of shortening the molding time even if the molding method requires forward depressurization time in the injection process by allowing advance feed of time.

[0010]

The feature of the present invention according to the above-mentioned object is to inject the resin into the cavity by advancing the screw, and to hold the pressure after the completion of the filling, to set the injection pressure at the time when the resin is filled in the cavity. The value is set to 0 to depressurize the resin in the cavity, and then the screw is rotated to start the measurement of the resin, and the back pressure at the time of measurement is used to maintain the pressure.

The time for setting the zero value of the injection pressure is in the range of 0.1 to 10 seconds, and the range time is set in consideration of the shape and wall thickness distribution of the molded product, the gate design, the cavity volume, etc. Holding pressure is performed by controlling the back pressure in multiple stages.

[0012]

[Operation] In the above molding method, the energy in the cavity is removed once before the holding pressure is set by setting the injection pressure to 0 value, or at the beginning of the holding pressure, and the cooling of the downstream portion filled earlier is promoted. As a result, the resin flowable region in the cavity is narrowed.

[0013] With the holding pressure in such a state, the resin in the flow region is solidified under a predetermined pressure, and the energy at that time is larger than that in the case of the holding pressure in the wide flowable region. Therefore, the energy in the molded product is balanced as a whole and warpage and sink marks are less likely to occur.

Further, due to the combination of the holding pressure and the measuring, the resin is measured by the rotation of the screw even while the back pressure acts as the holding pressure. Since the back pressure as the holding pressure is set higher than the normal back pressure, the plasticizing capacity during holding is smaller than in the normal case, so the weighing time is longer, but overall The time is shorter than when weighing is started after holding pressure.

Further, since the cooling can be started at the same time as the start of the measurement, the start time of the time measurement is earlier than the case where the time measurement is started at the time when the holding pressure is completed, and the time measurement is completed earlier by the forwarded time.

[0016]

EXAMPLES The present invention will be described in detail below by taking injection molding of a molded article (box connector) having a shape shown in FIG. 4 as an example. Further, in injection molding, there are cases where the filling step immediately shifts to the pressure holding step, and cases where the filling step-compression step-pressure holding step shifts in this order. However, since the compression process is included as a part of the pressure holding process, all the pressure holding processes will be described.

When the above-mentioned molded product 1 is molded by a conventional injection molding method, a warp occurs in a hatched portion. This portion is a thin side plate portion 4 molded on the side of the cavity gates 3 and 3 branched from the runner 2, and it is difficult to remove by the injection molding means so far.

Also in this embodiment, the resin is injected with the side plate portion 4 as the gate side. FIG. 2 is a waveform data diagram in which the operating state of the molding machine at the time of actual molding of this embodiment is drawn on the time axis from the start of injection. The injection speed, injection pressure, mold pressure, and back pressure corresponding to each process are shown. Shows the state of.

First, injection and filling of resin by speed control is started (see FIG. 1). As the injection fills, the resin fills the cavity, and the mold pressure rises as shown in Fig. 2.
The pressure in the mold reaches a peak after c. In the cavity,
As shown in FIG. 3 (A), the solidification of the resin at the flow front (downstream side) is progressing, and the rest is in a flowable state except for the contact portion with the cavity surface, and the energy in the cavity is The upstream side where the gate is located is higher than the downstream side.

When the pressure in this state is completed and the pressure is immediately transferred to the holding pressure, the resin is solidified to the upstream side with its energy balance, and the energy greatly exceeds the warpage generation level. Therefore, the molded product is likely to warp. Molded product 1 is shown in Figure 4.
In the case of the box-shaped connector having the shape as shown in (1), the part indicated by diagonal lines is warped. This portion is a thin side plate portion 4 formed on the side of the cavity gates 3, 3 branched from the runner 2.

However, in this embodiment, the injection pressure is preset so as to have a zero value at the time when the pressure in the mold reaches a peak, and thereafter the injection pressure does not act on the resin in the cavity. Therefore, the pressure in the mold is 0 in less than 3 seconds.
Falls to the value. If this state is maintained for 3.3 seconds, the pressure inside the mold is equalized during that time, and as shown in FIG. 3 (B), the energy in the flowable region is removed below the warpage occurrence level, and the downstream side Solidification is also promoted and the flowable area is further narrowed.

Next, the injection process is completed when the above time elapses, and the screw is rotated to shift to the resin measuring process. In this measuring step, as shown in FIG. 1, first, the back pressure is set high as the back pressure primary pressure, and the back pressure is used as the holding pressure to measure the resin and simultaneously hold the pressure. After the holding time has elapsed, the pressure is reduced to the secondary pressure of the back pressure, and the measurement is continued until the set time. The cooling time starts to be measured at the same time when the measurement is started.

1.5 se for increasing the holding pressure by the back pressure
The mold pressure rises after a delay of c. With this holding pressure, the energy is applied only to the narrowed flowable region, and even if the energy exceeds the warpage generation level, it is a very small region as a whole as shown in FIG. 3 (C). Become.

Therefore, as compared with the case of solidification by holding pressure in a state where the flowable region is wide, the energy in the cavity is small, and the energy in the molded product is generally balanced, so that warpage and sink marks are less likely to occur. Become. In the box-type connector 1, as a result of the analysis, in combination with the prevention of distortion due to low-pressure injection, the pressure distribution is remarkably sharp, and the shear stress, volume shrinkage, etc. Even if the same, there is little shear stress and almost no warpage is observed. In addition, the pressure-holding and metering, and the time required to complete the cooling are also shortened as compared with the conventional method in which the pressure-holding step is completed and then the measurement step is performed.

FIG. 5 shows the molding under the setting conditions of the conventional method,
It is a comparison figure which shows the time difference of the case when it performs by this invention. According to this, the weighing time of the embodiment is 1.74 sec longer than that of the conventional method. This is because the back pressure was set to 300 kg / cm ² for keeping the pressure, and the decrease in the plasticizing capacity due to the back pressure inevitably lengthened the measuring time. However, in comparison with the time from the pressure holding step of the conventional method, it is shortened by 1.56 seconds. The cooling time is the same for both, but due to the difference in the measurement start time, the embodiment completes earlier by about 3.3 seconds (the amount of advance).

Therefore, when the molding process is completed at the time when the measurement is completed and the mold opening process is started, the molding time is 1.5.
The molding time is shortened by 6 seconds, and the molding time is 3.3 seconds when the molding process is completed when the cooling is completed and the mold opening process is performed.
It will be shortened. As a result, even if it takes time to release the pressure, the time can be offset.

[0027]

As described above, according to the present invention, when the cavity is filled with the resin, the resin in the cavity is depressurized to prevent the molded article from warping or sinking due to injection molding. Even molded products that are restricted in design can be molded without causing warpage,
Further, it is advantageous that it can be adopted in the conventional injection molding machines without requiring any special means only by setting the pressure so that the pressure becomes 0 when the resin is filled in the cavity.

Further, since the holding pressure is performed by the back pressure and the holding pressure process and the measuring process are combined, the depressurizing time can be offset, and the cooling time is also advanced in advance. The time is shortened in any of the case where the molding step is completed and the case where the cooling completion is completed.

Furthermore, the molding time can be shortened by combining the pressure-holding process and the measuring process and advancing the cooling time start point in advance, and no special equipment or complicated molding operation is required. It has advantages such as being economical.

[Brief description of drawings]

FIG. 1 is a process explanatory view of an injection molding method according to the present invention.

FIG. 2 is a waveform data diagram illustrating an operating state of the molding machine according to the present invention.

FIG. 3 is an explanatory diagram showing a state of energy with respect to resin in a cavity.

FIG. 4 is a perspective view of a molded product and a runner in which a warp occurrence portion in a case of a conventional method is shown by diagonal lines.

FIG. 5 is a comparison diagram of the time reduction by the injection molding according to the embodiment of the present invention and the conventional method.

[Explanation of symbols]

 1 Molded product 2 Runner 3 Cavity gate 4 Side plate

Claims (1)

[Claims] 1. When the resin is injected and filled in the cavity by advancing the screw and the pressure is maintained after the filling is completed, the injection pressure at the time when the resin is filled in the cavity is set to 0 value to depressurize the resin in the cavity. Then, the screw is rotated to start measuring the resin, and the back pressure at the time of measuring is used to maintain the pressure, which is an injection molding method.