Injection molding whitening (top pop) is a common defect during the demolding process of plastic parts. It manifests as white marks or localized cracks at the ejection site, which not only affects the product’s appearance but can, in severe cases, lead to the scrapping of the plastic part. Whitening typically occurs at the contact points between the ejector pin, ejector plate, and the plastic part, such as the base of ribs and the bottom of bosses, where stress concentration occurs. Essentially, it occurs when the ejection force exceeds the local bearing capacity of the plastic part, causing plastic deformation or fracture of the material. For example, when the ejector pin area is too small and the ejection pressure is too high, the contact area of the plastic part will whiten due to stress concentration. If the ejection speed is too fast, the plastic part may not fully cool, and the material’s toughness is insufficient, causing it to pop. The occurrence of whitening (top pop) is closely related to the mold structure, ejection mechanism design, process parameters, and material properties. A systematic analysis is required to identify the root cause and implement targeted measures.
Improper mold ejection mechanism design is one of the main causes of ejector whitening (or ejector popping). The size and distribution of ejector pins directly impact the transmission of ejection force. If the ejector pin diameter is too small (e.g., less than 3mm) or the number is insufficient, the force per unit area will be excessive, causing localized stresses to exceed the material’s yield strength. For example, during the ejection of a mobile phone casing, significant whitening occurred at the ejection site due to the use of only four 2mm diameter ejector pins. However, after increasing the number of 3mm diameter ejector pins, the whitening phenomenon disappeared. The contact point between the ejector pin and the plastic part must also be carefully selected, avoiding thin-walled areas or sharp corners. Prioritize placement within thick walls, at the base of ribs, or near the core to ensure uniform force distribution. Furthermore, the clearance between the ejector pin and the mold plate should be controlled within 0.02-0.05mm. If the clearance is too large, the ejector pin will experience radial runout during ejection, resulting in uneven localized force distribution in the part. If the clearance is too small, the ejector pin’s movement is hindered, potentially causing ejector popping due to excessive impact force. For plastic parts with deep cavities or complex shapes, a top plate ejection or inclined ejection mechanism should be used to increase the ejection contact area and reduce local stress.
Improper process parameter settings are another major cause of ejector whitening (ejector popping). Defects must be reduced by optimizing injection, cooling, and ejection parameters. Insufficient cooling time can result in excessively high temperatures during demolding, leading to lower material strength and hardness. This can easily cause plastic deformation during ejection, resulting in ejector whitening. For example, the optimal demolding temperature for ABS parts is 60-80°C. If the cooling time is shortened from 20 seconds to 15 seconds and the demolding temperature is raised to 90°C, the incidence of ejector whitening will increase from 5% to 30%. Therefore, sufficient cooling time is crucial to fully solidify the part. Typically, the cooling time is 2-3 times the injection time. Ejector pressure and speed should be adjusted gradually. Ejector pressure should be kept to the minimum value for smooth demolding, generally 5-15 MPa. Ejector speed should be controlled in stages, starting with a slow initial speed (5-10 mm/s) and then increasing to 15-25 mm/s after contact with the part to avoid excessive impact. In addition, too high a holding pressure will cause the plastic part to adhere too tightly to the core, increasing demolding resistance. It is necessary to appropriately reduce the holding pressure (usually 60%-70% of the injection pressure) and shorten the holding time to reduce the internal stress of the plastic part.
The impact of material properties and part structure design on ejection whitening (or ejection popping) cannot be ignored, and risks must be mitigated during the product design phase. Brittle materials (such as PS and PMMA) are more susceptible to ejection whitening or ejection popping than ductile materials (such as PE and PP). Therefore, for parts made of brittle materials, the ejector contact area should be increased and the ejection speed should be reduced. For example, transparent PMMA parts require a large ejector plate, with an ejection speed controlled at 5-10mm/s to avoid localized excessive forces that can cause cracking. Regarding part structure, sharp corners or sudden changes in wall thickness should be avoided at the ejection site. Instead, rounded corners (radius ≥ 0.5mm) and reinforced ribs can be used to increase local strength. For deep-cavity parts, the core surface should be polished (Ra 0.4μm or less) and a release agent (such as a silicone-based release agent) should be sprayed on to reduce ejection resistance. In addition, insufficient draft angle of plastic parts will increase the difficulty of demolding. Generally, the draft angle should be ≥1°. For plastic parts with low surface roughness, the draft angle can be appropriately reduced, but it must be guaranteed to be ≥0.5°.
Addressing ejector whitening (or ejector popping) requires a comprehensive approach combining mold improvement, process optimization, and material adjustments. For molds, the ejector system can be optimized by increasing the number of ejector pins, increasing their diameter, or replacing the ejector mechanism (such as using a push tube or ejector plate). For areas with localized stress concentrations, adding bosses or increasing the wall thickness at the ejector pin contact point can improve load-bearing capacity. For process adjustments, extending the cooling time, reducing ejector pressure and speed, and optimizing holding pressure parameters are effective. If necessary, mold flow analysis software (such as Moldflow) can be used to simulate the ejection process to predict areas of stress concentration and enable proactive improvements. For materials, modified plastics (such as toughened PS or impact-resistant PMMA) can be used to improve toughness, or lubricants (such as zinc stearate) can be added to the raw materials to improve demolding performance. For minor ejector whitening that has already occurred, annealing (such as holding the part in an oven at 60-80°C for 2 hours) can eliminate some internal stress and reduce the whitening. If the ejection popping is severe, the part should be scrapped to prevent it from entering subsequent processes. Through multi-dimensional optimization, the top white (top burst) defect rate can be reduced from more than 20% to below 1%, significantly improving product quality.
