TPE|TPR injection molded products produce shaded surface solution

  • Time of issue:2023-04-15
  • Views:313

TPE|TPR injection molded products produce shaded surface solution

  • Categories:Elastomer knowledge
  • Author:SPP
  • Time of issue:2023-04-15
  • Views:313

  Definition of shaded surface of injection molded products

  A TPE|TPR injection molded product surface appears to have a difference between bright and dark colors, while the corresponding mold surface does not have this rough and detailed difference.

  Analysis of the position of the appearance of shaded surface of injection molded products

  The difference of light and dark near the gate and far from the gate may be the difference of light and dark in the local and nearby area. This defect generally appears on large products, and the other is that it appears more on the surface of the skin sand pattern, the material with large shrinkage is the main material, and the parameters are mainly low and medium mold temperature.

  Understanding of the three types of injection molded products shaded surface defects

  1 A product, different parameters will have a bright and dark color difference. For example, the high speed and high mold temperature is bright, and the opposite is dull. 2.

  2. If a product has a vertical tendon on the reverse side, the front surface of this tendon is dark and dark.

  3. a product on some places bright and some places dark, yin and Yang surface (product thick place Yin, thin place Yang).

  The root cause of the shaded surface of injection molded products - the lack of replication of the product to the mold in injection molding

  When the parameters are suitable, including high material temperature, high mold temperature, fast injection speed and high holding pressure, the mold is glossy and the product looks bright, which is because the product has good replication of the mold.

  The good replication of the mold is actually when the hot material fully replicates the surface finish of the mold during the cooling process. When is the replicability not good? The epidermis of the hot material is not completely cold solid when it is separated from the surface of the mold, that is, "collapse", once it is separated from the surface of the mold, the thermal conductivity of the material is greatly reduced, the epidermis of the material becomes hot again => blistering => shrinkage => increase in roughness, that looks like the product is oozing.

  As above left, microscopic plastic product surface is this, can not be ooze strange, the right picture of the product effect than the left picture is better, we look at the product is shiny.

  When will "the skin of the hot material is not completely cold solid when it is out of the mold surface"? When the pressure inside the mold tends to squeeze the material outward than the hot material tends to shrink inward, the material will come off the mold surface. This can explain why:

  1. the material with high shrinkage rate is easy to be oozed

  2. when the injection pressure and holding pressure are small, it is easy to ooze, in addition

  3. The mold temperature and the material temperature are easy to be oozing when the material/mold contact is a moment. See the following chart:

  The second of the above 3 reasons can explain the yin and yang surface defects.

  When the hot material is flowing, the pressure inside the mold cavity is a little place big and a little place small, the place where the pressure is big causes the possibility of "collapse" to be small, and the product is bright, and vice versa, it is shiny, which is manifested as the yin and yang surface.

  What place pressure is big what place pressure is small!

  1. The same product, near the gate where the pressure is high, the distal pressure is small.

  2. For the same product, the pressure is high at the place with thin wall and low at the place with thick wall, except for the place with "stagnant effect".

  3. The same product, when there is a bend in the material flow to the rib causing "diversion", the pressure in this area is small.

  For large products with long range and high shrinkage material, the following places are prone to shading:

  1. Yang at the sprue, Yin at the distal end.

  2. Yang at the thin wall, Yin at the thick one.

  3. Yang at the plane, Yin at the corner.

  4. Yang at the plane, Yin with tendons behind.

  5. Yang at the normal place, Yin at the place with "stagnation effect".

  6. For multi-stage injection molding products, the area with fast glue injection is positive and the area with slow glue injection is negative.

  The solution of the main location of the yin and yang surface

  1. Near the parting surface

  The location of the parting surface is the main channel for exhaust, exhaust is not smooth, it will lead to a sharp rise in gas temperature, resulting in the temperature of plastic materials, which will lead to the parting surface around the location of the product surface shiny. The formation of shaded surface. Therefore, we should strengthen the exhaust near the parting surface. Appropriately reduce the injection speed and increase the injection pressure can also facilitate the gas discharge. Reduce the formation of shaded surface.

  2 The distance between the gate and the yin and yang surface

  Generally, near the gate is the positive side, far from the gate is the negative side. Strengthen the melt flow end exhaust, increase the glue mouth can dilute the yin and yang surface.

  3.Uneven thickness of the product yin and yang surface

  The product is thick at the yin and thin at the yang. The solution is to add the exhaust slot at the position of thinner size and change the material flow speed and pressure. In addition, the mold design should pay attention to the uniform transition when the thickness changes sharply.

  4. The product structure turns two sides of the corner at the yin and Yang

  Attention should be paid to the uniform transition of thickness and angle at the corner to avoid sharp changes in thickness and to avoid designing sharp product angles. (The following figure)

  Summary: Product structure design (such as flow direction product thickness changes unevenly, gate design, product corner, etc.), as well as molding process injection speed pressure improperly caused by the exhaust is not smooth, the temperature of the rubber is too high or low, etc., may lead to products produced by the yin and Yang surface. In addition, from the perspective of TPE|TPR materials, the greater the shrinkage of the material, the greater the possibility of shaded surface. Choosing TPE material with good fluidity and high filling ratio will reduce the chance of shaded surface of the product.

  Sometimes due to the material contains more water, and color matching dispersion and other factors, may also lead to TPE products surface material flowers, pockmarked surface and other appearance quality uneven problems. But this is different from the conventional sense of shaded surface!

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