Injection shrinkage marks are common surface defects on plastic parts, often appearing in areas of uneven wall thickness, at the base of ribs, or near the gate. These marks not only affect the product’s appearance but may also indicate stress concentration within the internal structure, reducing the part’s mechanical properties. These defects are caused by the lack of sufficient melt replenishment during the cooling and solidification process due to volumetric shrinkage in the plastic melt, resulting in localized depressions. For example, a PP washing machine panel produced by a home appliance company exhibited significant shrinkage marks near the ribs, resulting in a lower product appearance rating, a 30% increase in customer complaints, and necessitating rework.
Improper mold design is one of the main causes of shrinkage marks. When plastic parts have varying wall thicknesses, thicker areas cool more slowly and shrink more than thinner areas, easily forming depressions at the junction. Furthermore, improper gate placement can lead to an excessively long melt filling path, resulting in shrinkage marks at the end due to insufficient holding pressure. For example, an automotive parts manufacturer produced ABS instrument panels. Because the gate was located at the edge, the melt pressure decayed when it reached the thick-walled center, forming a shrinkage mark approximately 5mm in diameter. By moving the gate to the center and adding an auxiliary gate, the holding pressure was effectively transferred to the thick-walled area, reducing the shrinkage mark depth from 0.3mm to below 0.05mm.
Improper process parameter settings are also a key factor in the formation of shrinkage marks. Insufficient holding pressure or too short a holding time, which fails to provide sufficient replenishment to the cooling and shrinking melt, is a common cause of shrinkage marks. Too low a melt temperature will reduce its fluidity, making it difficult for the melt to enter the shrinkage area during the holding phase; while too low a mold temperature will cause the surface of the plastic part to solidify rapidly, and the surface will not be able to deform accordingly during internal shrinkage, thus forming a depression. When an electronics factory produced mobile phone cases made of PC material, the holding pressure was only 50% of the injection pressure and the holding time was less than 2 seconds, resulting in continuous shrinkage marks on the edge of the case. By increasing the holding pressure to 70% of the injection pressure and extending the holding time to 4 seconds, the shrinkage marks were basically eliminated.
The characteristics of the raw materials also have a significant impact on the formation of shrinkage marks. Different plastics have different shrinkage rates. Crystalline plastics (such as PE and PP) generally have higher shrinkage rates than non-crystalline plastics (such as PC and PMMA), making shrinkage marks more likely to form during the molding process. In addition, additives in the raw materials (such as plasticizers and fillers) will also affect shrinkage behavior. Excessive plasticizer content will increase the shrinkage rate, while fillers (such as glass fiber) can reduce the shrinkage rate. When a toy factory produced dolls made of PE material, the shrinkage marks on the doll’s abdomen were significantly deepened due to the replacement of raw materials with a higher proportion of plasticizers. By switching to PE raw materials with a low shrinkage rate and adding 5% calcium carbonate filler, the shrinkage mark problem was effectively alleviated.
Resolving injection molding shrinkage marks requires comprehensive optimization of the mold, process, and raw materials. Regarding the mold, the wall thickness of the plastic part should be kept uniform to avoid sudden thickness variations. The gate should be positioned appropriately to ensure that the holding pressure is evenly distributed across all areas. Overflow chutes should be added in locations prone to shrinkage marks to collect excess space caused by shrinkage. Regarding the process, the holding pressure and holding time should be appropriately increased to ensure adequate melt replenishment. The mold temperature should be increased to slow the surface cooling rate so that internal shrinkage can be released through surface deformation. The injection speed should be optimized to prevent premature cooling of the melt. Regarding raw materials, plastics with low shrinkage should be selected, and appropriate fillers should be added when necessary. Through these measures, a medical device manufacturer reduced the defective rate of shrinkage marks in plastic parts from 12% to 1.5%, significantly improving product quality.
