STRUCTURE AND OPERATIONAL PRINCIPLES OF PLASTIC INJECTION MOLDS

In this article, let’s explore Plastic Injection Molds together with Hanoi Mould, including general technical requirements for molds, classification, structure, and operational principles of plastic injection molds and plastic molding molds.

Plastic injection molds or plastic molding molds are widely used to produce plastic products for various industries, including daily life, transportation, electronics, construction, defense, aerospace, automotive, and more. Based on the mold structure, plastic injection molds can be categorized as two-plate molds, three-plate molds, multi-cavity molds, etc. Besides high precision requirements, plastic injection molds also need to possess aesthetic appeal, high durability, and ensure productivity.

What is a plastic injection mold?

Finishing on the outside of the mold

A plastic injection mold is a tool used to shape plastic products. It is designed based on the shape of the product and consists of multiple assembled components to create a hollow space where the liquid plastic is injected and then cooled to produce the final product.

The shape and size of the product determine the dimensions and structure of the plastic injection mold. Productivity and output greatly influence the mold design. For small-scale production requirements, complex and multi-cavity molds may not be necessary. However, for large-scale production, more complex mold designs are required.

The production of a plastic injection mold involves two main processes: mold design and mold machining. To calculate the mold design, a complete design drawing of the product is required.

General technical requirements of plastic injection molds include:

• Achieving precision in terms of size and shape of the product.
• Ensuring the necessary surface gloss for both the mold cavity and the core.
• Maintaining accuracy in the relative positioning between the two halves of the mold.
• The mold should have sufficient hardness to prevent deformation or displacement of its components under high pressure.
• The mold should have a cooling system to ensure stable temperature within the mold cavity, facilitating easy material filling and quick shaping.
• The mold should be made from materials with high corrosion resistance and ease of machining.
• The mold structure should be reasonable, not overly complex, and suitable for practical technological capabilities.
• Ensuring that the product can be easily released from the mold.

The structure of a plastic injection mold.

A plastic injection mold consists of two main parts: the movable part (male mold) and the fixed part (female mold).

• Fixed mold half (female mold): This part is attached to the injection molding machine and remains fixed in position throughout the injection process. The hot molten plastic material is injected into the mold cavity through the nozzle system, which is connected to this fixed part of the mold.
• Movable mold half (male mold): This part performs the mold closing motion to mold the product and opens the mold to release the product. A system of ejector pins is designed on the movable mold to push the product out.

The two main parts of the injection mold are composed of 17 basic components in the mold structure (as shown in the image above), with the following functions:

1. Upper clamp plate: Used to clamp onto the fixed part of the machine frame. The drawing clearly shows that the front clamp plate has a protruding width compared to other mold plates, and this protruding part is used for clamping the mold.
2. Fixed plate (female mold plate): This plate is the fixed part of the mold.
3. Sprue bushing: It guides the flow of molten plastic from the machine nozzle into the mold (first directing the plastic into the runner channels).
4. Locating ring: Used to position the mold with the machine frame, ensuring that the machine nozzle is accurately aligned with the corresponding position of the sprue bushing. This component is circular in shape and slightly higher than the top surface of the front clamp plate to fit into corresponding holes on the machine frame.

5. Hex screws: Used to fasten the clamp plate and mold plate together.

6. Water lines: These are the cooling system of the mold and also help maintain the mold temperature during the heating cycle for plastics with low melting temperatures.
7. Movable plate (male mold plate): This plate is the movable part of the mold.
8. Support plate: Increases the rigidity of the movable mold part. This plate is only used when the movable plate is too thin.
9. Support pillars: Consisting of two plates on each side, they provide support for the movable plate and create empty space for the ejector system.
10. Pin retainer plate: Keeps the ejector pin system from sliding out during mold operation.
11. Ejector plate: This plate connects to the ejector core of the injection machine and is responsible for pushing the ejector pin system.
12. Lower clamp plate: Used to clamp the movable part of the injection molding machine.
13. Ejector pins: They push the product out of the mold.
14. Springs: Push the ejector system back to prepare for the next injection cycle.
15. Return pins: Help guide the clamp plate and ejector plate in a straight-line motion to prevent them from sliding out. They also protect the ejector pin assembly from bending during the product ejection and retraction process.
16. Guide bushing: Facilitates easy movement and positioning of the guide pins.
17. Guide pins: Ensure accurate alignment of the two movable and fixed parts of the mold throughout the mold closing process.

The above parts are assembled together to form the basic systems of the mold, including:

• Guiding and positioning system: This includes guide pins, guide bushings, core locators, cavity locators, etc. They function to maintain the correct working position of the movable and fixed mold parts when they are pressed together to create precise mold cavities.
• Plastic inlet system: This includes sprue bushings, runner channels, and injection gates. They are responsible for delivering the molten plastic from the injection nozzle of the molding machine into the mold cavity.
• Slide system: This includes side cores, core plates, guide bars, angled locking cams, hydraulic cylinders, etc. They are used to remove the undercuts or complex features in the mold that cannot be directly pulled out in the mold opening direction.
• Ejection system: This includes ejector pins, return pins, support pins, ejector bushings, ejector plates, retaining blocks, etc. They are responsible for pushing the finished product out of the mold after the molding process is completed.
• Venting system: This includes vent grooves and vent valves. Their task is to release trapped air from the mold cavity, ensuring that the cavity is properly filled with plastic and that the products are free from gas bubbles or burn marks.
• Cooling system: This includes water channels, cooling grooves, heat transfer pipes, connectors, etc. They are designed to regulate the mold temperature and cool down the products quickly.
• Hot runner system: Also known as the hot manifold system. It is a specialized injection molding system that uses heated channels to deliver molten plastic directly to the mold cavities, eliminating the need for sprue and runner systems.

These systems work together to create a functional and efficient plastic injection mold.

The mold components will require bolts and screws to secure the mold plates and mold components together.

For a 3-plate plastic mold, an additional intermediate plate is used to connect the movable and fixed parts. A 3-plate mold is commonly used in systems with a cooling runner. The runner system is arranged on two planes, and when the mold is opened, there are two openings. One opening is used to remove the product, and the remaining opening is used to remove the runner. The product is ejected using a pin ejection system located on the side of the movable mold. The runner is removed using a stripper plate arranged on the fixed part to separate the runner from the product.

A 3-plate plastic mold is used when it is necessary to arrange the gate(s) in the center or multiple gates for separate flow paths into the mold cavity. For thin-walled details with wide and long plastic flow, a point gate is used.

Due to its complexity and cost, 3-plate molds are less commonly used. Designers will always optimize the mold production process by designing 2-plate plastic molds whenever possible.

Classification of plastic molds:

Two-Plate Mold:

This is the most commonly used type of mold, and its structure is as mentioned above. A two-plate mold is also known as a single parting line mold because there is only one parting line when the product is taken out. When the product is removed from the mold, it remains attached to the plastic runner and gate, requiring an additional step to separate the product. Two-plate molds have the advantages of being easy to use and material-saving due to the short plastic runner.

Three-Plate Mold:

A three-plate mold has two parting lines when opened: one for product removal and one for runner extraction. The plastic runner and gate are automatically separated from the product when the mold is opened. Three-plate molds consume more material due to the longer runner.

Multi-Cavity Mold:

The structure of a multi-cavity plastic mold typically consists of three mold sets, with the middle set having both sides as the mold cavity. When the mold is opened, it creates two empty spaces, and both spaces are used for the products to drop out. Multi-cavity molds are suitable for mass production of products, and they help reduce the clamping force of the machine. However, the ejection system becomes more complex.

Side Core Mold:

Typically, plastic products are ejected from the mold through the mold’s opening and closing motion. However, if the products have horizontal holes or recesses, they cannot be ejected in the same way. To remove the products, the components creating the horizontal holes or recesses need to be withdrawn first. To assemble and disassemble side cores, various methods can be used, such as using angled pins or hydraulic cylinders to create independent horizontal movements while the mold is opening. Due to this mechanism, it is called a side core mold.

Operating principles of plastic injection molds, plastic molding molds

Regarding the operating principle of plastic molds, firstly, the mold is closed in the correct position thanks to the guiding and locating system. Once the mold is in the closed state, the molten plastic is injected into the mold cavity through the sprue bushing via the runner system, filling the cavity and shaping the product.

Once the plastic has filled the mold cavity, the machine remains in the closed mold state to allow the product to cool using the cooling system. Once the plastic has solidified and taken the shape of the product, the movable mold part is pulled out to open the mold, creating space for the product removal process.

At this point, the ejection system comes into operation, and ejector pins apply force to push the product out of the mold. Once the product is ejected, the ejection system partially returns due to the action of springs, although there is still a distance from the initial position.

At the end of the plastic molding process, the mold is closed again, and the movable part applies force against the return pins, which in turn exert force on the ejector plates, pushing the ejection system back to its initial position. The mold is closed in the correct position and ready for another molding cycle.

Plastic molds need to be installed in plastic injection molding machines to operate. There are various types of injection molding machines, such as tilt machines, vertical machines, or mini injection molding machines. The production efficiency of plastic products through injection molding also depends on the capacity of the injection molding machine.