(1) Improper control of molding conditions

If the injection pressure is too low, the injection and holding time is too short, the injection speed is too slow, the material temperature and mold temperature are too high, the cooling of the plastic part is insufficient, the temperature is too high during demolding, the temperature around the insert is too low, or there is insufficient material supply, it can cause the surface of the plastic part to show dents or an orange peel texture with fine unevenness. To address this, the injection pressure and speed should be appropriately increased, the compression density of the melt should be enhanced, the injection and holding time should be extended to compensate for melt shrinkage, and the injection back pressure should be increased. However, the holding pressure should not be too high, as this may cause sink marks.

If the dents and shrink marks occur near the gate, extending the holding time can help resolve the issue. When dents appear in areas with thick walls of the plastic part, the cooling time in the mold should be appropriately extended.

If local shrinkage around the insert causes dents and shrink marks, this is mainly due to the insert being too cool; efforts should be made to raise the temperature of the insert.

If the nozzle hole of the injection machine is too small or there is a local blockage at the nozzle, significant local pressure loss can cause dents and shrink marks. In this case, the nozzle should be replaced or cleaned.

If surface dents are caused by insufficient material supply, the feed amount should be increased.

Additionally, the cooling of the plastic part in the mold must be sufficient. On one hand, this can be achieved by adjusting the barrel temperature and appropriately lowering the melt temperature; on the other hand, modifications to the mold cooling system can be implemented to lower the cooling water temperature or to strengthen cooling in areas prone to dents, while maintaining uniform cooling of the mold surface and various parts as much as possible. Otherwise, if the plastic part is demolded under insufficient cooling, it can easily lead to shrinkage dents and, due to hard demolding, cause local dents in the part at the ejector pin.

(2) Mold Defects

If the cross-sections of the mold’s runners and gates are too small, the resistance to mold filling becomes too high, or if the gate is asymmetrically placed, leading to uneven filling speed. Improper positioning of the feed point, poor mold ventilation affecting material supply, shrinkage compensation, and cooling, or mold wear causing pressure relief can all result in surface sink marks and depressions on the molded part. To address this, specific measures should be taken based on the situation, such as enlarging the gate and runner cross-sections, positioning the gate symmetrically, and placing the feed point in the thick-walled section of the molded part.

If the sink marks and depressions occur far from the gate, it’s usually because the melt flow in a certain part of the mold structure is hindered, obstructing pressure transmission. In this case, the structural dimensions of the mold gating system should be appropriately enlarged, especially at "bottleneck" areas that impede melt flow, where the cross-section of the runner must be increased. Ideally, the runner should be extended to the area where the sink marks occur.

For thick-walled parts, a wing gate should be used as a priority. This is useful when it’s not appropriate to place the gate directly on the molded part, or if residual deformation is likely to occur at the gate area after molding. In such cases, an auxiliary wing-shaped structure can be added to the part, and the gate can be placed on the small wing. The gate on the small wing can be either a reverse gate or a pinpoint gate. This way, the sink mark defect is transferred to the small wing, which can be removed after molding.

Additionally, the mold should be frequently checked for wear that might cause pressure release or poor ventilation, and consumable or easily worn parts of the mold should be replaced in a timely manner, or the mold's ventilation conditions should be improved.

(3) Raw Materials Not Meeting Molding Requirements

If the shrinkage rate of the raw material is too high or the flow properties are too poor, or if there is insufficient lubricant in the raw material or the material is moist, sink marks and depressions will occur on the surface of the molded part. Therefore, for molded parts with high surface requirements, resin grades with lower shrinkage rates should be selected.

If under-injection sink marks occur due to poor melt flow, an appropriate amount of lubricant can be added to the raw material to improve the melt’s flow properties, or the structural dimensions of the gating system can be increased.

If the surface depressions are caused by moisture in the raw material, the material should be pre-dried.

(4) Unreasonable Structural Design of the Molded Part

If the wall thickness varies greatly across different areas of the part, the thick-walled sections are prone to sink marks and depressions during molding due to insufficient pressure. Therefore, when designing the structure of molded parts, wall thickness should be as uniform as possible. In special cases where there are significant differences in wall thickness, the issue can be addressed by adjusting the structural parameters of the gating system.

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