Reasons and countermeasures of molding defect

The common defect of Acrypet? products include short shot, silver streak, sink mark, cell, flow mark, weld line, scorch, crack, warp, wrinkle, contamination etc. The countermeasures are as below.

Short shot

It is caused by insufficient filling of resin into the mold cavity. The countermeasures are as below.

  1. Adjust the feeding amount.
  2. Increase the molding pressure.
  3. Increase the molding speed.
  4. Increase the cylinder temperature.
  5. Increase the back pressure of the screw.
  6. In case of thin product, increase the mold temperature.
  7. Extend the hold pressure duration.
  8. Extent the molding cycle.
  9. Enlarge the cross section and shorten the length of the sprue, runner and gate to let resin flows easier.

Silver streak

It is the streak in silver color formed on the product surface which follow the flow direction of the resin in the cavity. The reason is various and the countermeasures are as below.

  1. Pre-dry fully.
  2. Decrease the cylinder temperature and increase the molding pressure.
  3. Increase the molding speed.
  4. Decrease the rotation speed and adjust the back pressure of the screw.
  5. In case of thin product, increase the mold temperature and decrease the cylinder temperature.
  6. Ensure the proper melting duration of resin.
  7. Enlarge the cross section and shorten the length of the sprue, runner and gate to let resin flows easier.

Sink mark

It is the depression occurred on the product surface due to the shrinkage of the product. Normally it is called as sink mark. The countermeasures are as below.

  1. Increase the molding pressure.
  2. Extend the hold pressure duration.
  3. Decrease the cylinder temperature.
  4. Decrease the rotation speed of the screw.
  5. In case of thin product, adjust the mold temperature to avoid over heated.
  6. If the thickness of the product varies part to part, install additional gate at the thick position.
  7. Increase the feeding amount of resin to compensate the shrink portion in the cavity during hold pressure period. Install valve to prevent the backflow of resin will be even better.
  8. Keep the surface temperature of the mold unified.
  9. Enlarge the cross section and shorten the length of the sprue, runner and gate to let resin flows easier.

Cell

It is the cell formed inside the thick part of the product. For transparent products, it causes unrecoverable defect in appearance. Insufficient pre-drying or too high cylinder temperature makes cell formed in the thick part of the product. Moreover, shrinkage concentrating in the thick part also makes cell formed in the thick part. Countermeasures for sink mark listed above are also applicable for this defect. Beside, the below countermeasures are also be effective.

  1. In case of cooling down the product by water after releasing it from the mold, increase the water temperature or shorten the cooling period.
  2. Increase the mold temperature.

Flow mark

It is the streak looks like growth ring formed on the product surface nearby the gate. It can be prevented by the below countermeasures.

  1. Increase the cylinder temperature.
  2. Increase the mold temperature.
  3. Increase the molding pressure and molding speed to make resin feeding up the cavity promptly.
  4. Extend the molding cycle.
  5. Enlarge the cross section and shorten the length of the sprue, runner and gate to let resin flows easier.

Weld line

It is a slight depression on the product surface caused by the re-conjunction of separated resin inside the cavity. Normally it is hard to eliminate it completely by adjusting the molding condition. However, this defect can be eased by the below countermeasures.

  1. Increase the cylinder temperature.
  2. Increase the molding pressure and molding speed.
  3. Increase the mold temperature.
  4. Make the position of where weld line occurs more close to the gate.
  5. Enlarge the cross section and shorten the length of the sprue, runner and gate in order to increase the feeding speed of resin.

Scorch

It is the whitening or black scorch resulting in the burning of resin by highly compressed hot air. The countermeasures are as below.

  1. Set up vent. Set up vent in 5mm-10mm width, 0.04mm-0.07mm depth along the parting line of the scorch.
  2. Decrease the molding pressure and molding speed.
  3. Decrease the cylinder temperature.
  4. Set up the gate in where air cannot reach

Crack

It is the crack occurred on the product surface or among the edge of the inserting. It looks shining from a particular angle. The reason for this kind of defect is relatively complicated. The countermeasures are as below.

  1. Increase the cylinder temperature.
  2. Increase the mold temperature.
  3. Increase the molding speed.
  4. Adjust the molding pressure.

Warp

Because of its sharp, some products warp unavoidably after releasing from the mold. This problem can be fixed by the below countermeasures.

  1. Set temperature difference between the fixed bolster plate and the moving bolster plate. Normally, set temperature higher for the bolster plate on the side in which the product protrudes to.
  2. Decrease the mold temperature.
  3. Extend the cooling time inside the mold.
  4. Avoid unbalanced ejection of product during releasing.
  5. Make correction after release.

Wrinkle

This defect occurs in thick products or products have differential in thickness. Similar to the weld line, it has a slight depression on the product surface. The difference is that weld line is normally straight in direction but wrinkle is in random direction. Wrinkle is formed when resin connects again after jetting inside the mold. It can be avoided by the below countermeasures.

  1. Increase the cylinder temperature.
  2. Increase the mold temperature.
  3. Decrease the molding speed.
  4. Change the gate position.
  5. Make the gate thickness close to the product thickness as possible.

Contamination

Since Acrypet? is widely used in various optical aspects, it is necessary to avoid the interfusion of contamination. Normally, the source of contamination is various and it takes time to find out the reason. It is recommended to pay attention to the below issues.

  1. Is the dust suspension level in factory high or not?
  2. Is the procedure correct or not in opening the package of Acrypet??
  3. Is there any different resin mixed up in the drying machine? Or is contamination occurred because of the fraction between different parts of the drying machine?
  4. Is the hopper well cleaning up?
  5. Is there any metal peeling off from the inner wall of the hopper because of the strong fraction between Acrypet??
  6. Is there any smoke or dust come in the hopper from the vent pipe? Is the hopper produces any contamination itself?
  7. Is there any different resin remained in the cylinder or nozzle?
  8. Is there any fraction between the cylinder and the screw?
  9. Will the fraction between the pin and the other parts of the mold produces any oil or metal powder?
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