How does plastic injection molding work?

Feb 02, 2022

How does plastic injection molding work?
To design a complete set of plastic injection molds, the Mold Design Engineer must understand the structure of the plastic injection mold. Depending on the product requirements, we will have different mold structures such as: 2-plate mold, 3-plate mold, slide mold.

In the brochures Mold analysis a lot, in the sky and under the sea of ​​knowledge, if you study and very time consuming & unnecessary practice. So I share the most necessary knowledge for you to make molds.

1. Plastic injection mold construction.

Structural decomposition of the mold set

More or less you already know that Mold includes core and cavity (male mold, female mold). In addition, there are many other parts in the mold. These parts are assembled together to form the basic systems of the mold set, including:

How does plastic injection molding work? The system of guidance and positioning: includes all the guide pins, guide bearings, core positioning, mold shell positioning, etc., which are responsible for keeping the working position of the two mold parts when coupled together to create the cavity of the mold exactly.

How does plastic injection molding work?

The system of leading plastic into the mold cavity: includes the nozzle silver, the plastic channel and the injection port responsible for supplying plastic from the injection machine nozzle into the mold cavity.

Slide system (sliding platform): including side cores, core cheeks, guide bars, oblique cams, hydraulic cylinders, etc., which are responsible for removing the parts that cannot be removed (undercut) right in the direction of the mold opening.

Product ejection system: includes push pins, return pins, support pins, support pins, push plates, retaining plates, support blocks, etc., which are responsible for pushing the product out of the mold after pressing is complete. How does plastic injection molding work?

Exhaust system: consists of air vents, the exhaust valve is responsible for bringing out the remaining air in the mold cavity, creating conditions for the plastic to fill the mold cavity easily and keeping the product free of air bubbles or air bubbles burnt, lack of materials.

Cooling system: includes water lines, grooves, heat pipes, connectors, etc., which is responsible for regulating the mold temperature and cooling the product quickly. There is also a heating system to keep the mold temperature stable at high temperatures for products using engineering plastics.

Hot runner system: The hot runner system is also known as the hot channel system. ( Read more articles about Hot runner system design )

Bolts - nuts: Common to fix mold plates, components in the mold together...

18 basic parts in plastic injection mold structure

Structure of the basic plastic injection mold set.

1: Front clamp plate 7: Support plate 13: Pin guide silver

2: Front mold plate 8: Support block 14: Pause

3: Positioning ring 9: Rear clamp plate 15: Silver Expansion

4: Silver spray stem 10: Push pin 16: Support pins

5: Products 11: Holder plate 17: Rear mold plate

6: Locator 12: Push plate 18: Silver guide

How does plastic injection molding work? Depending on the shape and requirements of the product, we will choose different types of mold structure. Each production unit also follows different mold design directions. However, in order to improve the design level, the designer needs to learn to improve by studying as many mold structures as possible.

If you have the opportunity, you should open the actual molds to study and expand your design thinking.

Functions of components in the plastic mold

  1. Front clamp plate:

The function is common to clamp to the fixed part of the machine wall. As you can see in the picture, this plate has a protruding width compared to other mold plates. That protrusion is common for clamping.


  1. Fixed plate (female mold plate):

This plate is the fixed mold part

  1. Silver sprayer:

The function of leading plastic from the injection head of the injection machine into the mold (first is to lead the plastic into the channels)

  1. Positioning ring:


Common to position the mold with the machine wall, it helps the nozzle of the press to be accurately positioned with the corresponding position of the nozzle bush. This part has a circular shape and protrudes above the top of the front clamp to fit into a corresponding hole in the machine wall.

5. Hexagonal screw in Plastic injection Molding Machine:

Fix clamp plate and fixed mold plate together

  1. Water line:

It is cooling system of the mold. It also has the function of keeping the mold temperature during the heating process for low-melting plastics.

  1. Movable plate (male mold plate):

Movable side mold plate

  1. Backing plate:

Common to increase the rigidity of the movable side mold, this plate is only common in case the movable plate is too thin.

  1. Pillows:

How does plastic injection molding work? Pillows consisting of 2 panels on both sides are called a pair. The support is common to support the movable plate and create the necessary clearance in the middle to arrange the push pin clamp and push plate and the battery system.

  1. Battery clamp plate:

Keeps the push pin system from sliding out during mold operation

  1. Pin ejection plate:

This plate connects to the ejector core of the press; it has the function of pushing the push pin system through which indirectly pushes the product out.

  1. Rear clamp plate:

Common to clamp on the movable part of the injection molding machine

  1. Push pin:

Common to push the product out of the mold

  1. Loxo:

Push the clamp plate assembly and ejector plate backwards to pull the ejector battery pack to prepare for the next injection molding cycle


  1. Return latch:

How does plastic injection molding work? It guides the clamp and ejector plate assembly in a reciprocating straight line to keep them from sliding out and to protect the battery pack from bending during product ejection and retraction.

  1. Guide Bearing:

Precision machined with guide pin makes the guide pin easy to move and position

  1. Guide pin:

Helps the 2 parts of the mold to be precisely positioned during the molding process