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  • Injection-molded parts surface shrinkage, from seven aspects one by one analysis in place!

Injection-molded parts surface shrinkage, from seven aspects one by one analysis in place!

The depressions on the surface of molded parts are called shrinkage holes, which are caused by the shrinkage of plastic during molding, so most of them are difficult to eliminate. In the process of injection molding, molten plastic is injected into the cool mold cavity, and because of the poor thermal conductivity of plastic, cooling is very complicated.

Especially the thick-walled part is cooler than the thin-walled part, so the thick-walled part is easy to appear shrinkage. In addition, the mold temperature is slightly higher parts of the slow condensation, thus forming the mold local temperature difference, if coupled with the mold itself, the difference in heat conduction, that the mold temperature is high, poor conduction parts will appear shrinkage.

Therefore, the design should consider the use of difficult to appear shrinkage molding parts and mold structure. Such as the tendons, protruding part of the thin, and add rounded corners; or the tendons will be designed as non-solid; the surface design into the pattern to cover this defect.

1. Insufficient compression

Flow channel material (composed of main sprue, sprue, and gate) than the molding part wall thickness (volume is too small) in the mold, because the injection pressure can not fully act on the molten material in the mold cavity, so that the shrinkage increases, and thus a large shrinkage hole.

Especially when the gate is too small, even if the holding time is sufficient, the gate is already solidified, so that the pressure is not transferred to the melt in the cavity. Especially for the solid melting point of inconsistent knotted plastic, it is more likely to produce this phenomenon, and easy to burr the mold, because the closure is not tight and can not add enough molding pressure, also easy to appear shrinkage.

The screw injection machine is equipped with a non-return ring to prevent the melt from flowing back along the screw, but it is more prone to shrinkage than the plunger injection machine. In this regard, it can be said that the plunger-type injection machine is better than the screw cutter-type injection machine. As mentioned above, when the melt or even the melt at the end of the cavity has not yet solidified, adding sufficient holding pressure can prevent the shrinkage caused by insufficient compression.

Therefore, it is effective to increase the main gate, sprue, gate, and especially the gate diameter. It is also important to increase the injection pressure or to add sufficient holding pressure. And insufficient melt also contributes to shrinkage. If the plastic flow is good, if the pressure is increased, it will cause shrinkage due to burr. If necessary, lowering the material temperature or changing to poor flowing plastic can prevent shrinkage.

In the far from the gate, the end of the flowing melt is also prone to shrinkage, which is caused by the pressure loss due to the resistance of the flow path of the melt to the end, so it is effective to open no gate near the easy to appear shrinkage, or to increase the thickness of the part. Therefore, it is more effective to increase the number of gating points or change the location of gating points according to the situation.

2. Improper adjustment of injection volume

At the end of the injection of the screw injection molding machine, there must be a proper amount of molten plastic (about 5MM according to the size of the machine) between the head of the screw and the nozzle, which is used to buffer.

If this buffer amount is zero, and the injection volume is adjusted to the end, the screw at the same time also top to the bottom, so that the screw can not move forward in the pressure, and therefore can not be pressure, plastic shrinkage will become shrinkage and performance.

The solution is to leave a certain amount of buffer so that at the end of the injection screw cuttings can still advance a few millimeters or even a dozen millimeters. The buffer amount is zero (i.e., the end of the injection screw top to the bottom) will shorten the life of the injection machine itself, and must be noted.

3. Shrinkage on the work surface of the part

Some molded parts are sometimes unhindered by the presence of internal shrinkage. In this case, as described at the beginning, the side of the mold with high temperature is easy to have shrinkage, while the side with low temperature is difficult to have shrinkage.

Therefore, the side that does not allow shrinkage should be cooled sufficiently, or on the contrary, the side that allows shrinkage (i.e., the opposite side that does not allow shrinkage) should be molded at high temperature, which is also effective.

4. Uneven cooling

When the wall thickness of the formed part is extremely uneven, the thick-walled part cools more slowly than the thin-walled part, thus producing shrinkage holes in the thick-walled part.

To eliminate the shrinkage due to uneven wall thickness is theoretically difficult, so the design of the part should make the wall thickness uniform. In other words, the focus is on reducing the variation of wall thickness.

For example, when designing a boss, if the outer diameter size is required, a process hole to eliminate the shrinkage should be provided in the center; when the strength of the boss is required, instead of thickening the boss itself, a way to increase the strength using reinforcement should be adopted.

The shrinkage holes that are gently recessed are less conspicuous than those that are sharply recessed. Therefore, for parts that do not require precision, they should be released from the mold when the outer layer is solidified and the central part is still soft enough to be ejected, and then slowly cooled in air or warm water, so that the shrinkage holes are not obvious and do not affect the use.

5. Shrinkage is too large

The coefficient of thermal expansion of the molding plastic itself is large, of course, easy to appear shrinkage.

For example, PE shrinkage 0.02 ~ 0.05, PP shrinkage 0.01 ~ 0.02, PS shrinkage 0.002 ~ 0.006, even with a slight reinforcement, it will produce dents

Therefore, low-temperature molding of this plastic will not be easy to appear shrinkage. If the injection pressure is increased, more plastic can be injected into the mold cavity, so the higher the pressure, the smaller the shrinkage will be.

However, when the temperature drops below the minimum temperature required for the plastic, it is difficult to prevent the shrinkage of knotty plastics even if the injection pressure is increased.

For example, polypropylene, high-density polyethylene, paraformaldehyde, etc., its crystalline solids and the density of the molten state are significantly different, so it is very difficult to prevent shrinkage. At this time, if allowed to use non-crystalline copolymer instead, it can reduce the shrinkage. In addition, if filled with inorganic fillers, such as glass fiber, asbestos, etc. can also make the shrinkage smaller.

6. Surface curing is too slow

Wall thickness of the part of the shrinkage or luster, because the surface layer does not form a solid curing layer when the central part of the shrinkage will pull the surface inward into a shrinkage, and even the heat diffused from the inside out will again melt the surface layer and appear luster.

On the contrary, if the surface layer is strong enough in the thick part of the wall, the shrinkage of the center part will form a vacuum bubble. You can lower the mold temperature, lower the material temperature, lower the speed of the melt through the wall thickness area so that the curing layer is thicker (but easy to appear vacuum bubble); adjust the wall thickness, such as rib thinning, thick and thin slow change; use low shrinkage plastic; add foam in the plastic.

7. For the mold

Fig. A Additional flow edge at the place where the dent is produced

Fig. B Widening of the through-edge of the resin

Fig. C Reinforcement rib, the narrow shape becomes smaller

Adding flow edge at the place where the dent is produced (Fig. A)

If there is a narrow material edge at the place where the dent is produced, thicken this part (Fig. B)

Differences in the thickness of designed products should be completely avoided. Reinforcements that are prone to dents should be as short as possible for narrow shapes (Fig. C)
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