Solution for Sink Mark Defect Analysis of Plastic Products

Sink marks and sink marks refer to local contraction caused by lack of material after gate sealing or during injection, which leads to depression or tiny dents on the surface of plastic products. This is a common problem in the injection molding process, usually occurring at the place where the wall thickness of the plastic product increases, especially at external sharp corners, wall thickness mutations, protrusions, behind ribs or supports, and sometimes in uncommon places. The root cause of sink marks is the thermal expansion and contraction of materials. The thermal expansion coefficient of thermoplastics is high, and the degree of expansion and contraction is affected by many factors, including plastic properties, temperature range, cavity holding pressure, size and shape of injection molded parts, as well as cooling speed and uniformity.

Analysis and troubleshooting of dents and shrinkage defects

1. Improper molding condition control

– Too low injection pressure, too short injection and holding time, too slow injection rate, too high material and mold temperature, insufficient cooling of plastic parts, too high temperature during demolding, too low insert temperature or insufficient material supply will all lead to dents or orange peel-like fine unevenness on the surface of plastic parts. To address these problems, plastic injection molding services can take the following measures:
– Increase injection pressure and injection speed, increase the compression density of the melt, extend injection and holding time to compensate for melt shrinkage, and increase the injection recoil volume, but be careful not to hold too high to avoid convex marks.
– If dents and shrinkage marks occur near the gate, they can be solved by extending the holding time. If dents occur at the wall thickness, the cooling time should be appropriately extended.
– Depression around the insert is usually caused by too low insert temperature, and the insert temperature should be increased.
– Check whether the nozzle hole is too small or partially blocked, and replace or clean the nozzle if necessary.
– If insufficient feed causes dents, increase the feed amount.
– Ensure that the plastic part is fully cooled in the mold, which can be achieved by adjusting the barrel temperature, lowering the melt temperature or strengthening the cooling system.

2. Mold defects

– The mold runner and gate cross-section is too small, the mold filling resistance is too large, the gate setting is asymmetric, the mold filling speed is uneven, the feed port position is unreasonable, and the mold venting is poor, etc., will cause dents and shrinkage marks on the surface of the plastic part. Solutions include:
– Appropriately expand the gate and runner cross-section, ensure that the gate position is as symmetrical as possible, and set the feed port in the thick-walled part of the plastic part.
– If the dent occurs far away from the gate, it may be that the melt flow in a certain part of the mold is not smooth, and the structural size of the mold casting system needs to be expanded, especially the “bottleneck”.
– For thick-walled plastic parts, wing gates are preferred, and the gates are set on the small wings to reduce the dent defects on the plastic parts.
– Regularly check the mold wear, replace the wearing parts in time or improve the mold venting conditions.

3. Raw materials do not meet molding requirements

– Excessive shrinkage of raw materials, poor flow performance, insufficient lubricant or damp raw materials will lead to dents and shrinkage marks. The following measures should be taken:
– Select resin grades with low shrinkage, especially for plastic parts with high surface requirements.
– If the melt does not flow smoothly, lubricant can be added to improve fluidity, or the structural dimensions of the gating system can be increased.
– For damp raw materials, pre-drying treatment is required.

4. Unreasonable design of plastic part shape structure

– If the wall thickness of the plastic part varies greatly, the thick-walled part is prone to dents due to insufficient pressure. The wall thickness should be kept consistent as much as possible during design. For special cases, the problem can be solved by adjusting the structural parameters of the gating system.

Through the above analysis and solutions, combined with the professional technology of plastic injection molding services, the dents and shrinkage problems of injection molded products can be effectively improved, and product quality can be improved.