Injection Molding Process Overview

Injection molding is a process that takes advantage of the flowability and moldability of plastics. First, granular or powdery molding materials are fed from the hopper of the injection molding machine into a heated barrel, where they are melted and plasticized to a flowable state. This molten material is then injected at high speed, under high pressure, through a nozzle at the front of the barrel into a closed mold at a lower temperature. After a period of pressure holding and cooling, the mold is opened, and the molded plastic part with a specific shape and size is ejected. The process can generally be divided into three stages:

Material Preparation

Before molding, the appearance, color, particle condition, and the presence of any impurities in the material should be inspected. It is also necessary to test the material’s thermal stability, flowability, and shrinkage rate. For plastics with high moisture absorption, pre-drying may be necessary according to the allowable moisture content for the injection molding process. If inserts are used, the thermal expansion coefficient of the inserts must be known. The mold may need to be preheated to avoid shrinkage stress and cracking. In some cases, a release agent may be used to facilitate the ejection of the molded parts.

Injection Molding Process

Once the plastic is heated to a flowing state in the barrel, it is injected into the mold cavity. The flow of the plastic inside the mold can be divided into four stages: injection, pressure holding, backflow, and cooling.

Post-Molding Treatment

Due to the complex deformation and flow behavior of the plastic melt under temperature and pressure during the molding process, as well as uneven plastification and varying cooling rates after filling the mold, molded parts often experience uneven crystallization, orientation, and shrinkage. This can lead to residual stresses in the molded part, causing warping, degradation of mechanical properties, optical properties, and surface quality, and in severe cases, cracking. Therefore, some plastic parts require post-treatment. Common post-processing methods include annealing and moisture conditioning.

• Annealing is used to eliminate or reduce residual stresses in the molded part. It can also relax the orientation of the plastic molecules, lower the hardness, and increase the toughness of the part. The annealing temperature typically ranges from 10-20°C above the normal operating temperature of the part, but below the heat distortion temperature by 10-20°C.
• Moisture Conditioning is a post-processing step to adjust the moisture content of the part. This is particularly important for hygroscopic and easily oxidized plastics, such as polyamides. The conditioning process typically uses boiling water or a potassium acetate solution (boiling point of 121°C). The temperature for this treatment is usually between 100-121°C, and the duration of conditioning depends on the thickness of the part, typically ranging from 2 to 9 hours.