Working Principle Of Injection Molds
Jul 06, 2026
I. What is an injection mold?
Injection molds can be simply understood as industrial-specific high-temperature and high-pressure "mooncake molds", which, together with injection machines, complete the batch molding of plastic products. Solid plastic particles are heated and melted into liquid melt. Under the pressure effect, the melt is injected into the cavity of the closed mold. After the melt cools and hardens, the mold is opened and the complete plastic product is automatically ejected. The equipment continuously performs repetitive cyclic operations. The core of the entire molding process consists of three steps:Melt injection → Cooling and solidification → Demolding and ejection.
II. Complete Process Flow
Cavity injection：The moving mold and the fixed mold are precisely locked and closed, forming a sealed product cavity. The screw of the injection molding machine heats and melts the plastic, and then injects it into the cavity under high pressure at a fast speed. It only takes a few seconds for the melt to fill the cavity completely.
Pressure retention cooling：After the cavity is filled with the molten material, a constant pressure is continuously applied to compensate for the plastic's cooling shrinkage, preventing the product from shrinking or deforming. The water channels of the mold are connected to a circulating cooling water system to quickly dissipate the heat. The cooling process is the longest part of the entire cycle.
Demolding ejection： After cooling is complete, the mold is separated. The ejecting mechanisms such as the pins and the ejecting blocks push the plastic products forward; the waste materials and burrs on the parting surface are cleaned. Then the mold is closed again and enters the next production cycle.
III. Core Components and Functions of the Mold System
Molding cavity system (moving die + fixed die) After the moving and fixed molds are fitted together, the internal cavity precisely replicates the shape and dimensions of the product, directly determining the appearance and dimensional accuracy of the plastic part.
Pouring + Exhaust system The flow channels and gates serve as the transportation channels for the molten material; the vent grooves discharge the air inside the mold cavity, preventing the molten material from getting trapped and causing bubbles, burning, or material shortage defects.
Auxiliary Function System
Cooling system: The mold is equipped with a built-in circulating water channel, which helps control the temperature and accelerate cooling, thereby enhancing production efficiency;
· Guidance and positioning system: The guide pins and guide sleeves ensure precise alignment during the mold closure, preventing the mold from misaligning and damaging the products;
· Ejection and demolding system: Structures such as the ejector pins, ejector tubes, and push plates automatically remove the solidified products from the mold.
IV. Three Key Process Control Parameters
1. temperature
The barrel is heated sectionally to melt the solid plastic; the temperature of the mold directly affects the flowability of the melt, the internal stress of the product, and the surface gloss. Different plastics (such as PP/ABS/PC, etc.) have corresponding dedicated temperature control ranges.
2. pressure
The injection pressure overcomes the resistance of the melt flow, ensuring complete mold filling; the holding pressure is usually around 85% of the injection filling pressure, counteracting the plastic's cooling shrinkage.
3.time
The injection filling time is extremely short; the cooling time accounts for the majority of the injection molding cycle. Reasonably allocate the injection, holding pressure, cooling, and mold opening/closing times to balance product quality and production efficiency.
Conclusion
The entire injection molding process is closely interrelated. The mold structure and process parameters mutually restrict and cooperate with each other. Only with a properly matched mold structure and precise control of the three core parameters of temperature, pressure, and time can stable production of plastic products with good appearance, meeting dimensions, and without defects be achieved, and achieve low-cost and high-efficiency automated batch production.
