Causes And Solutions of Dents And Pores in Injection Molding Products

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In the process of product production, product dents and pores are the most frequent adverse phenomena. The plastic injected into the mold shrinks in volume as it cools. The surface hardens first when it cools earlier, and bubbles form inside.


The indentation is the slow cooling part of the bubble in the direction of the contraction of the concave surface; The so-called stoma refers to the material in the mold solidifies from the surface, which is relatively insufficient for the total volume of the mold. Because of this reason, holes in the vacuum state are generated, which generally occur in the thick parts of the product and the filling port.


Materials with high shrinkage are also prone to indentation. When changing the forming condition to eliminate the indentation, the setting condition should be set in the direction of shrinkage. That is, mold temperature and barrel temperature decrease, injection pressure increases, but it should be noted that this may cause residual internal stress.


Because the indentation is inconspicuous, so it does not affect the appearance of the process in the mold into corrosion, such as striated, granular and so on.


Reducing die temperature to reduce finish is also effective if the molding material is impact resistant polystyrene HIPS (a type of polystyrene PS). But once a dent occurs in these methods, it is difficult to repair the polished product.


Transparent products with air holes is a problem, opaque products with air holes do not have obstacles to use and should not be seen in the product.


Due to the water and volatiles produced by the stomata, are generally diffused to all parts of the product, the shape of the stomata is generally small.



First, the solution


Instant: increase injection pressure, extend injection pressure holding time, reduce barrel temperature and mold temperature, moisture and volatiles caused by the material should be fully dry, in the place of indentation forced cooling.


Short term: Fill the upper edge where the indentation is made. Where the dent is made, the material is thickened as it passes through the narrow space.


Long term: the thickness difference of design products should be completely avoided. Easy to produce dent reinforcement, long and narrow shape should be as short as possible. Should increase the gate, main channel, shunt, nozzle hole. Improved exhaust.


Second, reference matters


1 molding shrinkage of large material indentation is also large, such as polyethylene PE, polypropylene PP, even as long as a little reinforcement, will produce indentation.



Mold   shrinkage rate


0.002   ~ 0.006


0.01   ~ 0.02


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 surrounding the mold in the mold, that is, the static pressure, is not necessarily everywhere.


Close to the gate part of the pressure is high, if the material wide edge, because of the transfer of pressure to all corners, near the gate and away from the gate of the pressure difference with the whole pressure compared to a small difference will not produce dents, also can get no residual internal stress products.


When some material flows into a difficult place, there is a high pressure in this place, and the pressure decreases in other places, resulting in dents. This part of the high pressure residue is the internal stress of the product is also large. In the ideal condition, the material fluidity is better when the temperature of the material rises with the temperature of the die, and the injection in the static pressure state also becomes lower.


3. In the change of forming conditions, the combination of temperature, pressure and time should be made in advance of the table, in order to know the results. First of all, when the time becomes very long, it is easy to know every small change in pressure. It should be noted that the results obtained when the temperature changes should be produced after the injection material and after the temperature drops.


4. To determine the reasons caused by pores, as long as the observation of the bubble of plastic products in the mold is instantaneous or after cooling, if when the mold is instantaneous, most of it is a material problem, if it is after cooling, it belongs to the mold or injection conditions.


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