Causes and solutions of indentations and pores in injection molding products
In the process of product production, product dents and pores are the highest frequency of adverse phenomena, the plastic injected into the mold occurs volume shrinkage when cooling, early cooling part that is, the surface is first hardened, the interior will produce bubbles;
The so-called dent is the slow part of the cooling in the direction of the bubble contraction produced a conspicuous concave; The so-called porosity refers to the material in the mold from the surface to cure, the whole volume of the mold in terms of material is relatively insufficient, for this reason, the vacuum state of the hole, generally occurs in the product thick place and the injection port.
Materials with large shrinkage are also easy to produce dents. When changing the molding conditions to remove dents, the setting conditions should be set in the direction of retraction and shrinkage. That is, the mold temperature and barrel temperature are reduced, and the injection pressure is increased, but it should be noted that residual internal stress may be caused.
Because the indentation is inconspicuous, it does not affect the appearance, and it is processed into a corrosion pattern on the mold, such as striate, granular, etc.
If the molding material is impact resistant polystyrene HIPS(a kind of polystyrene PS), it is also effective to reduce the mold temperature to reduce the finish. However, once these methods have dents, it is difficult to repair the polished product.
Transparent products have pores is a problem, opaque products have pores there is no barrier to use and the product should not be seen.
Due to the moisture and volatiles produced by the stomata, generally spread to all parts of the product, the shape of the stomata is generally small.
A solution
Instant: increase the injection pressure, extend the injection pressure holding time, reduce the cylinder temperature and mold temperature, water and volatiles caused by the material should be fully dry, forced cooling in the place where the dent.
Short-term: fill the upper edge in the place where the dent is produced. Thicken this section when the material in the area where the dent was made passes through there are tight spots.
Long-term: Differences in the thickness of the design part should be completely avoided. Easy to produce a dent reinforcement, narrow shape should be as short as possible. The gate, main channel, diverter channel and nozzle hole should be increased. Improve the exhaust.
Two, reference
1 Molding shrinkage of large material dents are also large, such as polyethylene PE, polypropylene PP, even if only a little reinforcement, will produce dents.
| material forming shrinkage rate | |
| PS | 0.002 to 0.0 06 |
| PP | 0.01 0.02 |
| PE | 0.02 0.05 |
2. When the temperature is reduced to no dents, if the material in the mold cavity is still under pressure, it should be considered that no dents will be produced. The pressure of the material around the mold in the mold, that is, the static pressure, no matter where it is not necessarily.
The pressure close to the gate part is high, if the pass edge of the material is wide, because of the transfer of pressure to each corner, the pressure difference near the gate and away from the gate is very small compared with the total pressure will not produce dents, and you can get no residual internal stress products.
When some of the material flows into a difficult place, there is high pressure in this place, and the pressure in other places is reduced, which will produce a dent. The residue of this part of high pressure is also large in the internal stress of the product. In the ideal state, the material temperature rises with the mold temperature, the material fluidity is better, and the injection is also low in the static pressure state.
3. When changing the forming conditions, the combination of temperature, pressure and time should be made in advance, in order, you can know the result as early as possible. First of all, after the time becomes very long, every small change in pressure is also easy to know. It should be noted that the results obtained when the temperature changes should be obtained after the injection of the material, and the results of the reproduction after the temperature drops.
4. To determine the cause of pores, as long as the bubbles of plastic products appear instantaneously when opening the mold or after cooling, if they appear instantaneously when opening the mold, most of them are material problems, and if they appear after cooling, they belong to mold or injection molding conditions.
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