Principles of Plastic Injection Molding (Plastic Processing Process)

Plastic is everywhere around us. In our daily life, more than 60% of daily necessities are made of plastics. The most common production method of plastic products is "plastic injection molding."

What is plastic injection?
Simply put, plastic injection molding is composed of three main categories: "plastic material", "plastic mold" and "plastic injection machine". These three main categories then expands into many different sub-categories due to different manufacturing requirements.

(The 3 main categores of injection molding)
→Plastic material
Crystalline material: PP, PE, PA…
Amorphous material: PS, ABS, PC…

→Plastic mold
Two plate mold, three plate mold and hot runner

→Plastic injection machine
Hydraulic crankshaft, two-plate direct pressure, all-electric

Before understanding the process of injection molding, let's begin with a breif introduction of the structure of the injection machine:

  • Barrel: where the raw materials are stored
  • Material pipe: the passage of raw materials to the mold
  • Screw: the material tube is covered with a "screw". The function of the screw is to melt and mix the plastic and inject a certain amount of plastic into the mold.
  • Mold: It is like making a chocolate model, in which raw materials are poured into it to form a product with a specific shape.
  • Thimble: "thimble" is a part of the mold, and its main function is to remove the molded product from the mold

The 5 steps of plastic injection molding

The plastic injection molding process can be simplified into a "5 stages, continuous cycle" process, which are:
Plasticizing → filling → holding pressure → cooling → ejection

Manufacturing process of plastic injection

• Plasticizing
The raw materials are melted in the material pipe (the screw works to evenly mix the raw materials)

• Filling
"Filling" the raw material into the mold through the screw

• Holding pressure
After the raw materials are filled into the mold, in order to avoid backflow of the raw materials, the space in the mold is continuously pressurized to ensure the compactness of the raw materials.

• Cooling
The product is quickly cooled and shaped in the mold.

• Ejection
Push the cooled product out of the mold through the thimble on the mold As we mentioned in the episode of "3D Printing VS Plastic Injection", the principle of plastic injection molding is like making chocolate. The raw materials are poured into the machine, heated and melted, and then injected into the model. After the product is cooled, demolded, and the finished product is completed.

Plastic injection is an easy and rapid prototyping processing method, but because the plastic itself has the characteristics of "cooling shrinkage", it requires the technology and experience that the product designer and processing manufacturer holds to be able to produce the required size and appearance of the product, hence guaranteeing the precision and quality of every single mass produced product.