Causes and solutions of dents and porosity of injection molded products

In the process of product production, product dents and porosity is the highest frequency of bad phenomena, injected into the mold of plastic in the cooling volume contraction, the early cooling part that the surface first hardened, the internal will produce bubbles;

The so-called dent is the slow cooling part in the direction of the bubble shrinkage produced a conspicuous concave surface; the so-called porosity refers to the material in the mold from the surface to start curing, the entire volume of the mold in terms of the relative lack of material, due to this reason the vacuum state of the hole, generally occurring in the product thick place and injection mouth.

Materials with high shrinkage also tend to produce dents. When you want to change the molding conditions to eliminate the dents, you should set the conditions in the direction of less shrinkage. However, it should be noted that this may cause residual internal stress.

Since the dent is preferable to be inconspicuous, when it does not affect the appearance, it is processed on the mold as an eroded mold pattern, such as grainy, granular, etc.

If the molding material is impact-resistant polystyrene HIPS (a type of polystyrene PS), it is also effective to reduce the finish by lowering the mold temperature. However, once dents occur in these methods, it is difficult to repair the polished product.

Transparent products with air holes are a problem, but opaque products with air holes are not a barrier to use and should not be visible inside the product.

The pores generated by moisture and volatiles are generally diffused to all parts of the product, and the shape of the pores is generally small.

I. Solutions

Immediate: increase the injection pressure, extend the injection holding time, reduce the barrel temperature and mold temperature, fully dry the material when moisture and volatiles are caused, and force cooling at the place where the dent is produced.

Short-term: Make up the flow edge at the place where the dent is produced. If the material in the place where the dent is produced passes through a narrow place, thicken this part.

Long-term: Differences in the thickness of designed products should be completely avoided. The narrow shape of the reinforcement that is prone to dent should be as short as possible. The gate, main flow channel, diversion channel, and nozzle hole should be enlarged. Improve the exhaust.

II. Reference matters

1. The dents of materials with large molding shrinkage are also large, for example, polyethylene PE, polypropylene PP, even if only a slight reinforcement, dents will be produced.

Material

Molding shrinkage

PS

0.002~0.006

PP

0.01~0.02

PE

0.02~0.05

2. When the temperature decreases to the point that no dent is produced, if there is still pressure in the material inside the mold cavity, it should be considered that no dent will be produced. The pressure of the material inside the mold around the mold, i.e. the static pressure, is not certain no matter where it is.

If the pressure near the gate is high, and if the material has a wide edge, the difference between the pressure near the gate and the pressure far from the gate is small compared to the pressure of the whole part, no dent will be produced, and the product will be obtained without residual internal stress.

However, when some of the material flows into a difficult place, the high pressure in this place will cause dents when the pressure in other places decreases. This part of the high pressure residual is the internal stress of the product is also high. In the ideal state the material temperature rises with the mold temperature, the material flows better, and the injection also becomes lower in the static pressure state.

3. When changing the molding conditions, the combination of temperature, pressure and time should be made in advance in a table in order to know the result early. After the time becomes long in the first place, it is also easy to know every time the pressure changes slightly. It should be noted that the result when the temperature changes should be obtained after the injection of the material and when the temperature is lowered and then the result is produced.

4. To determine the cause of air holes, just observe whether the air bubbles of plastic products appear instantaneously when the mold is opened or after cooling, if they appear instantaneously when the mold is opened, most of them are material problems, if they appear after cooling, they belong to the mold or injection conditions.