Injection molding annealing treatment
Injection molding annealing is a crucial step in eliminating internal stress and improving mechanical properties. It’s particularly suitable for transparent plastics like polycarbonate and polymethyl methacrylate, which are prone to internal stress. During the injection molding process, the plastic melt fills the mold at high speed and high pressure. During the cooling process, factors such as temperature gradients and molecular orientation generate internal stress, which can lead to warping, cracking, or decreased optical properties. Annealing heats the product to near its glass transition temperature and holds it there for a sufficient time to allow the molecular chains to re-arrange, eliminating internal stress and stabilizing the product’s dimensions and performance, thereby improving its reliability and lifespan.
The process parameters for annealing treatment directly affect the treatment effect, mainly including heating temperature, holding time, and cooling rate. The heating temperature must be controlled between the glass transition temperature and heat deformation temperature of the product material. For example, the annealing temperature for polycarbonate is generally 120-130°C, and that for polymethyl methacrylate is 70-90°C. Excessively high temperatures will cause the product to soften and deform, while too low a temperature will not effectively eliminate internal stress. The holding time must be determined according to the thickness and complexity of the product. Thin-walled products are generally held for 1-2 hours, while thick-walled products require 3-6 hours to ensure sufficient time for internal stress to be released. The cooling rate should be slow, and furnace cooling or step-by-step cooling can be used to avoid the generation of new internal stresses due to large temperature differences. The product should be removed after cooling to room temperature to prevent sudden temperature changes from affecting product performance.
The equipment selected for annealing treatment should be determined based on the scale of production and the characteristics of the product. Commonly used equipment includes hot air circulation ovens, vacuum annealing furnaces, and infrared heating furnaces. Hot air circulation ovens are suitable for batch processing of small and medium-sized products. They have the advantages of uniform temperature and simple operation. The forced convection of hot air ensures uniform heating of the product, avoiding local overheating. Vacuum annealing furnaces are suitable for products with extremely high surface quality requirements. They can prevent surface oxidation or contamination during heating and are particularly suitable for the processing of transparent products. Infrared heating furnaces heat products through infrared radiation, with rapid heating rates and good energy-saving effects. They are suitable for large products or continuous production. The temperature control accuracy of the equipment must reach ±2°C to ensure the stable execution of annealing process parameters and improve the consistency of treatment results.
The operating specifications of the annealing treatment are an important part of ensuring the quality of the treatment. Before the operation, the product needs to be cleaned to remove impurities such as oil and dust on the surface to avoid stains or defects during the heating process. The products must be placed reasonably to avoid squeezing or contact with each other to ensure uniform heating. For products that are easy to deform, special clamps can be used to fix them to prevent warping during the annealing process. During the heating process, the temperature in the furnace must be monitored regularly and the temperature change curve must be recorded to ensure that it meets the process requirements; the furnace environment must be kept stable during the insulation stage to avoid temperature fluctuations; the cooling rate must be strictly controlled during the cooling stage, and it is prohibited to remove the product prematurely. After the treatment is completed, the product is inspected for appearance and performance tested, such as impact strength, heat deformation temperature, etc., to verify the annealing effect and adjust the process parameters according to the test results.
Targeted measures need to be taken for the annealing treatment of special types of products. For example, for injection molded products with metal inserts, due to the different thermal expansion coefficients of plastic and metal, large internal stresses are easily generated. During the annealing treatment, a step-by-step heating method is required to slowly reach the set temperature to reduce stress concentration caused by excessive temperature differences. For fiber-reinforced plastic products, the annealing treatment needs to control the heating temperature and time to avoid damage to the interface between the fiber and the matrix, which affects the mechanical properties of the product. In addition, the annealed products need to be aged and placed at room temperature for 24-48 hours to further stabilize the internal stress and ensure that the product will not undergo dimensional changes or performance degradation due to stress release during use. Through scientific and reasonable annealing treatment, the quality stability and service life of injection molded products can be significantly improved to meet the use requirements of different fields.
