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Causes and solutions of dents and Blowholes in injection molded products
- Aug 08, 2018 -

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In the process of product production, the notch and hole of the product are the most frequently occurring in the bad phenomenon. The plastic of the injection mould takes place in volume contraction when cooling, and the early cooling part is the first hardening of the surface, and the air bubbles will be produced inside.

The so-called "dent" is the slow cooling part that produces a prominent concave surface in the direction of the bubble contraction; the so-called pores in the mold begin to solidify from the surface, and the material is relatively inadequate for the full volume of the mold. For this reason, the vacuum state holes are produced, usually in the thick parts of the products and at the filling mouth.

Large shrinkage materials also easily produce dents. When changing the molding conditions to eliminate dents, the setting conditions should be set to narrow down.  That is, the mold temperature and the barrel temperature decrease, and the injection pressure increases, but we should pay attention to the residual internal stress.

Because the indentation is not conspicuous, so do not affect the appearance of the time, in the mold processing into corrosion patterns, such as grain, etc.

If the molding material is impact-resistant polystyrene HIPS (polystyrene PS is a kind of), with lower mold temperature to reduce the smoothness is also effective. But once these methods have dents, it is difficult to repair the polished products.

Transparent products have problems with pores, opaque products have holes, no barriers to use and products are not suitable to see.

Because moisture and volatiles produced by stomata generally spread to all parts of the product, the shape of stomata is generally small.

One, solution method

Instant: increase the ejection pressure, extend the injection time, reduce the temperature of the barrel and the mold temperature. When the water and the volatiles are caused, the material should be dried enough, and it is forced to cool in the place where the dent is produced.

Short term: fill up the edge in the place where the dent is produced. The material is thickened when there is a narrow place in the place where dents are produced.

Long term: the difference in thickness of design products should be avoided completely. It is easy to produce dent reinforcement, and the long and narrow shape should be as short as possible. Gate, main stream, diversion channel and nozzle hole should be increased. Improve the exhaust.

Two, reference matters

1. Material with large molding shrinkage also has large dents, such as polyethylene PE, polypropylene PP, even if only slightly reinforced, will produce dents.


2. When the temperature drops to no dent, if the material in the cavity is still under pressure, it should be considered that no dent will occur. The pressure around the mold inside the mold, which is static pressure, is not necessarily anywhere.

The pressure near the gate is high. If the side of the material is wide, because the pressure in every corner is transmitted, the difference between the pressure difference of the near gate and the place away from the gate is very small compared with the pressure of the whole, and the indentation will not be produced, and the products without residual internal stress can be obtained.

When some of the material flows into a difficult place, there is high pressure in this place, and other places where the pressure drops will produce dents. This part of the high pressure residue is the internal stress of the product is also large. Under the ideal condition, the material fluidity is better with the increase of material temperature and mold temperature, and the injection is also lower in the static pressure state.

3. When the molding conditions are changed, the combination of temperature, pressure and time should be made in advance, and the results can be known early. First, after a long time, little changes in pressure are easy to know. It should be noted that the results of temperature change should be obtained after the injection material, and the results of reproduction after the temperature drop.

4. to judge the cause of the stoma, as long as it is observed that the bubble of the plastic product appears instantaneously or after the cooling, if it appears instantaneously when the mold is opened, it is mostly material problem. If it is cooled after cooling, it belongs to the mold or injection molding condition.