Strain (strain) and its solution
Scratches (or “drag marks”) are common surface defects in injection molding. They occur when the mold cavity or core surface scrapes the surface of the part during the demolding process, resulting in scratches, burrs, or partial material loss. These defects not only affect the product’s appearance but, in severe cases, can damage the part’s structural integrity, leading to product rejection. Scratches are related to a variety of factors, including mold surface quality, demolding mechanism design, part structure, injection molding process parameters, and plastic properties. Addressing these issues requires comprehensive measures, including mold optimization, process adjustments, and raw material handling.
Poor mold surface quality is one of the main causes of mold scratches. When the mold cavity or core has a high surface roughness (Ra > 0.8μm) and contains defects such as machining marks, scratches, rust, or oxide layers, the surface material will generate significant friction with the rough mold surface during demolding, causing scratches on the part surface. Furthermore, the presence of oil, release agent residue, or char buildup from plastic decomposition can increase adhesion between the part and the mold, exacerbating the scratches. The key to addressing this issue is improving mold surface quality. Finely polish the cavity and core to a surface roughness of Ra ≤ 0.4μm. For parts requiring a high finish, polish to Ra ≤ 0.02μm. Furthermore, the mold surface should be cleaned regularly to remove oil and char. Rusted molds can be repaired by grinding or electroplating (such as chrome plating). Chrome plating, typically 0.01-0.03mm thick, improves surface smoothness and enhances wear resistance.
An improperly designed demolding mechanism can result in uneven force on the plastic part during demolding, causing damage. Insufficient demolding angle is a common problem. If the contact surface between the plastic part and the mold cavity or core lacks sufficient demolding angle (generally required to be ≥1°, and can be appropriately reduced to 0.5° for high-precision plastic parts), the demolding resistance will increase, causing the surface of the plastic part to be damaged during forced demolding. Uneven distribution of ejection force and the layout of the demolding mechanism can also cause damage. For example, if there are insufficient ejector pins, the ejector pin position is offset from the center of gravity of the plastic part, or the contact area between the ejector pin and the plastic part is too small, the plastic part will be subjected to excessive force locally, causing deformation and relative sliding with the mold surface, forming scratches. Solutions include: reasonably designing the demoulding angle and adjusting the angle according to the plastic part material and surface roughness. The better the plastic fluidity, the smaller the demoulding angle can be. Optimize the ejection mechanism, increase the number of ejector pins, or use large-area ejection methods such as ejector plates and ejector tubes to ensure uniform distribution of ejection force. The contact area between the ejector pin and the plastic part should be no less than 80% of the ejector pin’s cross-sectional area. Polish the ejector pin surface to reduce friction.
Defects in the plastic part’s structural design increase the risk of damage. Deep grooves, sharp corners, undercuts, or excessively deep patterns on the part’s surface can create strong friction or hooking with the mold during demolding, causing surface material damage. For example, if the depth-to-width ratio of a deep groove exceeds 2:1, the mold core will exert significant compressive pressure on the inner wall of the groove during demolding, potentially causing damage. Stress concentration at sharp corners can cause localized deformation of the part during demolding, leading to scratches on the mold surface. The solution is to optimize the part’s structural design, avoiding deep grooves and sharp corners as much as possible and rounding them (radius ≥ 0.5mm). For undercuts that must be retained, core pulling or tilting mechanisms can be used to facilitate demolding and minimize damage caused by forced demolding. Pattern depth should be minimal, generally no more than 0.3mm, and the pattern direction should align with the demolding direction to reduce frictional resistance.
Improper injection molding process parameter settings are also a significant factor in causing strain. When the mold temperature is too low, the plastic part cools too quickly, creating strong adhesion to the mold cavity surface, increasing demolding resistance and easily causing strain. Excessive mold temperature leads to insufficient cooling of the plastic part, resulting in lower strength and easy scratching by the ejector mechanism during demolding. Excessive injection and holding pressures can cause the plastic part to overfill the mold cavity, resulting in a tight fit between the part and the cavity wall, making demolding more difficult. Excessive internal stress can also cause the part to deform during demolding, exacerbating friction damage. Solutions include: reasonably setting the mold temperature according to the type of plastic (such as 40-60°C for PE molds and 80-120°C for PC molds) to ensure that the plastic parts can be fully cooled and shaped without excessively adhering to the mold; appropriately reducing the injection pressure and holding pressure (by 5%-10% each time) to reduce the tightness of the fit between the plastic part and the cavity, while extending the cooling time to increase the strength of the plastic part during demolding. The cooling time is generally 2-3 times the filling time.
Improper plastic raw material properties and processing can also affect the occurrence of scratches. Impurities (such as metal debris or other plastic particles) in the plastic raw materials or insufficient lubricant can increase friction between the melt and the mold surface, leading to scratches on the part during molding and demolding. For reinforced plastics (such as glass-fiber-reinforced PA), if the fibers are too long (over 0.5mm) or unevenly dispersed, protruding fiber tips can form on the part surface. These tips can easily scrape against the mold surface during demolding, causing scratches. Solutions include: improving raw material filtration by installing a filter (60-120 mesh) at the front of the injection molding machine barrel to remove impurities. For plastics requiring improved demolding performance, an appropriate amount of internal lubricant (such as zinc stearate, at a dosage of 0.5%-1%) can be added to the raw materials to reduce the friction coefficient between the part and the mold. For reinforced plastics, fiber length should be controlled within the 0.2-0.4mm range and evenly dispersed to avoid agglomeration. Additionally, appropriately increasing the mold temperature can reduce direct friction between the fibers and the mold surface.
