PCB News - PCB process The reasons and solutions for the shrinkage and depression of the finished product after plastic injection

PCB process The reasons and solutions for the shrinkage and depression of the finished product after plastic injection

First of all, we must understand that most objects have the so-called "thermal expansion and contraction" phenomenon. The same is true for plastic products. High temperature must be used to melt the resin when the plastic is injected. After the injection is injected into the mold, it is cooled and shaped. Therefore, after plastic products are molded from the mold, there will be shrinkage. This is a natural matter, and all the plastic materials specifications will also indicate their shrinkage rate. More professional ones will also provide horizontal And the longitudinal shrinkage rate.

Therefore, the plastic shrinkage problem that we generally see should be the appearance phenomenon caused by uneven local shrinkage. The other two phenomena: "unsatisfied injection" and "wind" are due to the appearance phenomenon and "partial uneven shrinkage" Similar, so it is often classified as a "shrinkage" problem.

The following explains the reasons and solutions for the shrinkage and depression caused by these three different reasons (some of the data here are collected from the Internet, and some are compiled from my own experience):

1. Shots that are not full

Shrinkage caused by insufficient filling in the plastic mold cavity is usually caused by insufficient feed, which is generally caused by injection pressure, or the profile of the plastic product is too thick or the difference in thickness is too large.

The usual solution is usually as follows, the purpose is to increase the amount of glue injection:

Increase the injection pressure and prolong the injection time. (The same mold is set up, and the injection pressure is small, and it is easier to shrink than the injection pressure.)

Increase the feed volume.

Increase the holding pressure and extend the holding time. (Assuming the same mold structure, shrinkage is more likely to occur far away from the injection gate, because the farther away from the injection gate, the lower the injection pressure, and it is difficult to maintain pressure during injection. Naturally, it will be more powerful. .

Increase the size of the sprue gate.

Move the spout to a place where it is easy to shrink

Speed up the shot. Some molds do not have a preheating system. At this time, it is necessary to increase the speed of plastic injection to prevent the plastic from cooling prematurely in the mold, so as to block the subsequent injection volume.

Reduce injection speed. It is suitable for molds with preheating system, the purpose is to make plastic injection molding completely into every place in the mold base.

Slow down when passing through thick parts.

2. Pack wind

"Winding" is the lack of material caused by the inability of air to effectively escape the mold cavity so that the plastic cannot be effectively injected into a corner of the mold. Generally, the most commonly used solution for "wrapped wind" is to increase vents, but the vents will easily generate flash after a long period of time.

The following methods can also change the position of the "wind" or reduce the phenomenon of wind:

Reduce the speed of injection. Allow time for the air to escape from the mold cavity, and it can also shoot the problem of insufficient material at low corners. It is suitable for molds with preheating systems.

Change the position of the glue injection crossing. The path for the plastic material to flow in the mold changes, and the location of the air remaining will also change.

3. Uneven cooling rate after plastic injection

Most objects have the so-called "thermal expansion and contraction" phenomenon, and the same is true for plastic products. According to a certain shrinkage rate, the thicker the plastic flesh, the larger the shrinkage size.

Imagine that after a round plastic product with a thickness of 2.0cm is injected, the outer part of the plastic will be cooled because it is exposed to air, and the volume will be reduced first, but the temperature in the middle of the plastic product is wrapped in it, so it cannot be cooled for a while. At this time, the outer part that has already begun to cool down in size will pull the plastic part that has not been cooled inside, so the later the part is cooled, the smaller its volume will become. The result is that there are shrinkage marks in the two circles inside and outside the middle of the upper and lower sandwich.

Apply this concept to the design of some plastic products with uneven thickness and great difference. For example, a piece of 5cm square finished product can be given a thickness of 2cm, and the outer circumference has a thickness of 0.5cm in the center. 0.5cm wide flange. The thinner part (0.5cm * 0.5cm flange) will be cooled first, and the volume will be reduced and finalized, but the thicker part (middle 5cm * 5cm) will slowly cool down later, and the volume will also shrink. Shrinking will occur on the surface in the middle of the upper and lower sides, because the outer flange has cooled down first, and there is no place to steal the meat, so I have to steal the meat on the upper and lower surfaces.

For this kind of shrinkage problem caused by uneven cooling rate after plastic injection, the solutions are as follows:

Extend the cooling time. Some molds will be designed with cooling water channels. The location of the water channels is best to be placed in a place where the plastic flesh is relatively thick, and the cooling time in the mold can also be extended. The purpose is to balance the cooling rate of the plastic everywhere.

Design grooves where the shrinkage occurs. Since the shrinkage is caused by the slower cooling, stealing the plastic meat (groove) can increase the area of contact with the air, which can speed up the cooling rate, and the shrinkage will not occur in this place.

Reduce mold temperature. In this way, the temperature difference between the temperature of the finished product after injection and the room temperature after cooling can be reduced.

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