Injection molding burn (carbonization) and solutions
Injection burn (carbonization) is a common and serious defect in injection molding production. It manifests as black or brown scorch marks on the surface of the plastic part, which not only affects the product’s appearance but can also degrade mechanical properties and even generate toxic gases that harm the production environment. This defect often occurs at the ends of the melt flow, in corners, or in areas with poor venting. Essentially, it’s the result of the plastic melt being trapped under high temperature and high pressure for a prolonged period, causing thermal degradation or oxidative decomposition. For example, when producing ABS washing machine control panels at an electric power plant, poor venting in the corners of the mold cavity caused the melt to stagnate and burn. The resulting black spots led to a product scrap rate as high as 20%, severely impacting production schedules.
Improper mold design is one of the main causes of burns during injection molding. Inadequate or improperly positioned venting structures in the mold cavity prevent the timely expulsion of air within the cavity. This air is compressed by the melt, generating high temperatures that can ignite or decompose the plastic melt. Furthermore, an undersized gate or complex runner design increases the melt’s flow resistance, causing it to reside in the runner for extended periods, making it susceptible to overheating and decomposition. In the case of an automotive parts manufacturer producing PP bumpers, the gate diameter was only 2mm, hindering melt flow and causing it to reside in the runner for three times longer than normal. This resulted in melt decomposition and scorch marks on the surface of the plastic part. By increasing the gate diameter to 3.5mm and adding three venting slots, the burn problem was completely resolved.
Improper process parameter settings can also lead to injection burning. If the injection speed is too fast, the melt will form turbulence in the mold cavity, entraining air and generating severe friction, resulting in a sudden increase in local temperature; and if the injection pressure is too high, the melt will be over-compressed under high pressure, increasing the residence time. Excessive melt temperature is another key factor. When the temperature exceeds the thermal stability limit of the plastic, the molecular chain will break, causing degradation and carbonization. For example, the normal processing temperature of PC material is 270-310℃. If the temperature is set to 330℃, obvious degradation and carbonization will appear in just 5 minutes. When an electronics factory was producing PC lenses, it mistakenly adjusted the melt temperature to 340℃, resulting in large areas of burnt spots on the lens surface. Testing found that these were carbide residues after material degradation.
Problems with raw materials can also cause burns during injection molding. Impurities or different types of plastics mixed in with the raw materials can prematurely decompose during the molding process due to different melting points, forming burn particles. Furthermore, raw materials with excessive moisture content will convert to water vapor at high temperatures, which will mix with the melt to produce bubbles. The localized high temperatures generated when these bubbles burst under high pressure can also cause burns. When a toy factory used recycled materials to produce PE toys, a small amount of PVC (which has a lower melting point than PE) was mixed into the recycled materials. During processing, the PVC first decomposed and carbonized, resulting in black, granular burn marks on the toy surface. By strictly screening the recycled materials, removing impurities, and thoroughly drying them, the burn defect rate has been reduced from 15% to 1%.
Solving the problem of injection molding burns requires a comprehensive approach. First, optimize mold design and add sufficient venting grooves (typically 0.02-0.05mm deep) to ensure smooth exhaust of air from the cavity. Simplify the runner structure, increase the gate size, and reduce melt flow resistance. Second, adjust process parameters, reduce injection speed and pressure, control the melt temperature within the material’s thermal stability range, and appropriately shorten the holding time. Finally, strengthen raw material management, strictly screen raw materials, remove impurities, and fully dry hygroscopic materials (for example, PA materials need to be dried at 80-100°C for 4-6 hours). Through the above measures, a medical device manufacturer reduced the burn rate of plastic parts from 8% to 0.3%, significantly improving product quality and production efficiency.
