How to Improve 8 Types of Injection Molding Defects

plastic injection molding

In the process of injection molding of plastic products, due to the different processing equipment, different molding properties, different properties of various raw materials, as well as the operation status of the equipment, the cavity structure of the mold and other factors, the internal structure and the appearance of the plastic product often has various injection molding defects. The following provides feasible solutions for 8 common appearance defects.

Solutions to shrinkage and sink marks

Produced by the shrinkage of plastic volume, it is often found in thick areas, such as stiffeners or the area where the column foot meets the surface.

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  • Insufficient injection pressure and holding pressure, and insufficient plastic melt feeding. In general, the injection is carried out in stages. First, about 95% of the product is filled at high pressure and high speed, and then the product is filled at low pressure and low speed, and then the pressure is maintained.
  • Increase the holding pressure time.
  • Adjust the injection speed to improve the plastic melt feeding.
  • Increase the injection volume.
  • The material temperature and mold temperature are high, the cooling is slow, and the plastic shrinks after cooling and shrinking. Adjust the temperature reasonably.
  • The size of the runner and gate is too small, the pressure loss increases, and the gate solidifies too early, resulting in poor feeding. Modify the mold, and increase the runner and injection gate.
  • Area material is too thick. The thickness can be appropriately reduced.
  • When the operation of the injection molding machine is not smooth, the uneven wall thickness of the product will also shrink, and the surface of the product will have waves.

Solutions for flash, burr and batch front

  • High-pressure high-speed injection leads to elastic deformation of the mold, and gaps on the parting surface cause flashing of the product. Adjustment method: use two-stage injection, first high-pressure high-speed and then a low-pressure low-speed injection, in order to achieve the purpose of elastic retraction of the mold at low pressure, avoid flash.
  • The clamping force on the mold is insufficient, and when the high-pressure plastic molten material is injected into the cavity, a gap is formed on the parting surface or the mating surface of the insert, and the plastic melt flows into this gap.
  • Impurities adhere to the parting surface, resulting in a gap in the mold clamping.
  • Try not to get the injection gate too close to the mold inserts.

Solution to weld seam

  • Reduce the number of injection gates.
  • Add an overflow well near where the material is fused, transfer the fusion line there, and then cut it away.
  • Adjust gate position (uneven wall thickness).
  • Strengthen the exhaust in the fusion line area to quickly evacuate the air and volatiles in this part.
  • Increase the temperature of the material and the mold, enhance the fluidity of the plastic and increase the temperature of the material during fusion.
  • Increase the injection pressure and appropriately increase the size of the gating system.
  • Increase the injection speed at the weld line.
  • Shorten the distance between the gate and the weld area.
  • Reduce the use of mold release agents.

Solution to flow marks

  • It is mainly a patterned product or a product with a special structure. When the material flows through this place, a sudden change occurs, resulting in flow marks on the surface of the product. During injection, the injection process is mainly adjusted, and multi-stage injection is adopted. The position determination here is the most critical step. After identifying the position where the injection passes through, drop the pressure and speed sharply to prevent the raw material from flowing here. Fast, so that it flows smoothly here, the product is filled at high speed and high pressure.
  • If the cold material that stays at the front end of the injection molding machine nozzle enters the tool cavity, it will cause flow marks. When injecting, it is necessary to pay attention to the grading set. The first-stage injection is low pressure and low speed, and the cold material is controlled in the flow channel.
  • When the temperature of the plastic melt is lower, the fluidity of the material decreases, the viscosity increases and flow marks appear.
  • The low mold temperature reduces the plastic melt temperature, and the material’s viscosity in the molten state increases, resulting in flow marks.
  • If the injection speed is too slow, the flow time of the material increases, the temperature loss increases, and the viscosity of the material increases, resulting in flow marks.

Solution to deformation

  • Uneven meat thickness and uneven cooling. The cooling speed of the plastic is different. The place where the cooling is fast shrinks less, and the place where the cooling is slow shrinks more, resulting in deformation.
  • The material temperature is high (not easy to cool), the shrinkage is large, and the deformation is large.
  • The injection pressure is large and the holding pressure is large so that the molecular chains in the product are arranged in a rigid direction (under the action of pressure, they are forced to align). The forces within the molecules tend to the material’s natural state, causing the product to shrink and deform.
  • After different material parts are forcibly fused, due to the different shrinkage rates of the materials, the product will generate internal stress at the fusion joint, resulting in product deformation, and in severe cases, irregular cracks will occur.
  • The same material, the stress is relatively small. In addition, the stress will also be reduced after the workpiece is preheated and then fused.

Solutions for sticking mold and gate break

  • Poor polishing.
  • The draft angle is not enough. A good drafting slope is required.
  • Insufficient number of thimbles. The ejection speed is too fast. Ejection was not delayed, resulting in vacuum sticking.
  • Overcharge and overpressure. Or the conversion from injection to holding pressure is too slow, resulting in excessive filling, mold expansion and die sticking or gate swelling.
  • Improve cooling. Avoid the local mold temperature is too high, resulting in uneven shrinkage, and sticking to the mold or gate breakage.
  • In the case of a fixed mold, it can be improved by setting the fixed mold temperature higher or lower than the movable mold temperature.

Solution of water silk

  • The back pressure is too tight, causing the gate to hold the overflowing material, and the raw material cannot enter the cavity smoothly, and the resistance is too large, and it is decomposed by overheating;
  • The injection speed is reduced to avoid the decomposition of raw materials due to overheating;
  • Adopt multi-stage injection method, from slow to fast injection molding;
  • Expand the size of the feeding port (gate);
  • The storage time should not be too long, and the storage speed should not be too fast to avoid overheating decomposition of raw materials;
  • The back pressure is too tight or there is material on the screw, which makes it difficult for the raw material to enter the screw when the screw is storing the material so that the storage time is too long, and the raw material is overheated and decomposed;
  • When the raw material flows through the sudden change of the mold, the flow marks (water filaments) are generated. After the sudden speed reduction here, the flow-through, and then increasing the injection speed can eliminate this defect. The key lies in the injection. After searching for this location;
  • The back pressure is too loose, resulting in the block water filaments generated by the air entering the screw when the material is stored. This defect can be eliminated by adjusting the back pressure of the storage material;
  • The cold material at the nozzle enters the cavity, resulting in the formation of water filaments on the surface of the product. By adjusting the position of the primary injection (low-speed injection), the cold material can just be controlled in the flow channel without entering the surface of the product, thus eliminating the need for Defects on the product surface due to cold material entering the mold cavity.

Solution to cold material pattern

  • The injection pressure of the first stage is too small, so that the cold material is not controlled in the flow channel during the first stage of injection so that it flows into the surface of the product during the second stage injection;
  • A period of speed that is too slow or too fast will also cause the above phenomenon to occur;
  • The end position of the first-stage injection is too large so that the second-stage injection is carried out before the cold material is finished, which causes the cold material to enter the mold cavity under high pressure and high speed (on the contrary, if the position is too small, water ripples will be generated at the gate edge. );
  • Mold temperature or nozzle temperature is too low, resulting in cold material;
  • The cold material hole (runner) is too small; the design is unreasonable.
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