Injection molding is one of the most widely used manufacturing processes for producing complex plastic parts in high volumes. A significant aspect of this process is the design and construction of the injection mold itself, which serves as the cavity into which molten plastic is injected to form the desired part.
Before discussing the components of injection molds, it’s essential to understand the three primary types: two-plate mold, three-plate mold, and hot runner mold. These types differ mainly in their gating system design, which determines how molten material is injected into the mold cavity.
3 Types of Injection Molds
| Mold Type | Description | Key Features | Best For |
|---|---|---|---|
| Two-Plate Mold | Simplest and most common mold, divided into core and cavity. | – Single gate system – Simple design – Gate marks near parting line | – Simple parts – High-volume production |
| Three-Plate Mold | Includes a stripper plate, enabling more complex gating. | – Multiple gates – No visible gate marks – More complex structure | – Large or complex parts – Aesthetic requirements |
| Hot Runner Mold | Keeps molten material heated with no cold runners. | – No material waste – Multiple injection points – Faster cycle times | – High-precision parts – High-volume, efficient production |
Please also read: 2 Plate Mold vs. 3 Plate Mold: Which is Right for Your Project?
Key Components and Structure of an Injection Mold
In this section, we will systematically explore the various components that make up an injection mold system.
An injection mold includes the ejection system, venting system, core and cavity, cooling system, guiding structure, mold base, and feed system—all work in concert to ensure efficient, precise, and high-quality injection molding.
While some components, such as the mold base or cooling system, are fairly straightforward, others—like the gating system or ejection system—require a deeper examination due to their complex functions and critical impact on part quality and cycle time.
Mold Base
The mold base serves as the core framework of the injection mold, providing the necessary structural support to hold all other mold components in place. he mold base is usually made of standard steel components that can be customized based on the specific requirements of the mold. It is designed to fit the injection molding machine and to withstand the clamping forces during operation.
Given the complexity and precision required in mold base manufacturing, many mold makers choose to source these components from specialized manufacturers. This allows both the mold base suppliers and the mold makers to focus on their areas of expertise, leading to improved overall efficiency and product quality.
Typically, a mold base consists of several critical components: the clamp plate, A plate, B plate, C plate (spacer block), rear clamp plate, ejector retainer plate, and ejector plate.
Key elements of the mold base:
| Component | Description |
|---|---|
| Rear Clamp Plate | Connects the mold to the injection machine and secures the B plate. |
| B Plate | Supports the mold core and extends the runner system. |
| Spacer Block (C Plate) | Supports the mold base and adjusts part ejection height. |
| Top Clamp Plate | Secures the mold to the injection machine and attaches to the A plate. |
| A Plate | Holds the mold cavity and is made from strong materials. |
Key Functions:
- Provides structural integrity to the mold.
- Houses the core and cavity inserts.
- Supports the cooling and ejection systems.
Molding System
The molding system is the heart of the injection mold, responsible for shaping the molten plastic into the desired product. This system directly influences the accuracy, quality, and consistency of the molded parts. It includes a variety of components such as the mold cavity, mold core, slides, lifters, and inserts.
| Component | Description |
|---|---|
| Lifters | Help eject parts with internal undercuts. |
| Inserts | Removable components for complex features or geometry modifications. |
| Mold Core | Shapes internal features and moves to create undercuts. |
| Mold Cavity | Forms the external shape of the part and bears pressure. |
| Slides (or Sliders) | Create undercuts and withdraw before ejection. |
Venting System
During the injection process, air trapped in the cavity can cause defects such as voids and short shots. A venting system is designed to allow the air to escape from the cavity. This can be achieved through vents machined into the parting line, the use of porous inserts, or other specialized venting techniques.
Common methods of venting include parting line vents, vent grooves, and vent pins.
| Venting Method | Description |
|---|---|
| Parting Line Vents | Located at the parting line to allow air to escape during injection. |
| Vent Grooves | Small grooves cut into the mold to allow air to escape. |
| Vent Pins | Pins used in specific areas to release trapped air. |
Please also read: Key Design Considerations for Injection Mold Venting Success
Key Functions:
- Prevents defects caused by trapped air or gases.
- Ensures that the mold fills properly and completely.
- Improves part quality and reduces cycle time.
Feeding System
The feeding system ensures that the molten plastic is delivered efficiently from the injection molding machine’s nozzle to the mold cavity. The system comprises several components: the sprue, cold slug well, gates, runners, sub-runners, and manifold. Each part of the feeding system is designed to control the flow of plastic into the mold, ensuring uniform filling and minimizing defects.
| Component | Description |
|---|---|
| Sprue | Directs plastic from the nozzle into the mold, connecting to the gate. |
| Cold Slug Well | Captures cold plastic at the end of the sprue to ensure smooth flow. |
| Gate | The opening where molten plastic enters the mold cavity, affecting quality. |
| Runner | Distributes molten plastic from the sprue to multiple cavities. |
| Manifold | Distributes plastic in hot runner systems, ensuring consistent temperature and pressure. |
Cooling System
To solidify the molten plastic quickly and efficiently, an injection mold is equipped with a cooling system. This usually involves cooling channels drilled into the mold plates near the cavity and core. These channels are connected to a coolant supply, typically water or oil, which circulates through the channels to extract heat from the mold.
The design of the cooling system affects the cycle time of the injection molding process and the quality of the part. Efficient cooling helps reduce part warping and speeds up production cycles.
Key functions of the cooling system:
- Ensures that the mold remains within the desired temperature range to prevent overheating and premature wear.
- Regulates the cooling of the molten plastic to speed up the solidification process.
- Helps control part shrinkage and warping by maintaining uniform temperature distribution.
- Optimizes cycle times and improves the overall efficiency of the molding process.
Ejection System
After the plastic part has solidified in the cavity, the ejector system is used to remove the part from the mold. A well-designed ejection system prevents part damage during the ejection process and ensures that parts are easily removed without defects such as cracking or deformation.
Common components of the ejection system include ejector pins, ejector plates, and return pins, and ejector sleeves.
| Component | Description |
|---|---|
| Ejector Pins | Push the part out of the mold, preventing damage during removal. |
| Ejector Plates | Hold and guide the ejector pins during ejection. |
| Return Pins | Reset the mold after ejection, ensuring correct alignment. |
| Ejector Sleeves | Used for cylindrical parts to apply even force and prevent deformation. |
Key Functions:
- Ensures safe and efficient removal of the part from the mold.
- Prevents damage to the molded part during ejection.
- Allows for quick cycle times in high-volume production.
Guiding System
The guiding system ensures precise alignment of the mold halves during the clamping and opening actions of the injection molding machine. The guiding system typically consists of guide pillars and guide bushings, which work together to ensure accurate mold alignment and prevent misalignment during injection
Guide pillars and bushings are used to ensure the proper alignment of the moving and stationary parts of the mold during opening and closing. They reduce friction and wear, and maintain the accuracy of the mold’s operation. The guide pillars are usually fixed to one of the mold plates, and the bushings are installed on the corresponding mating plate.
Core-Pulling Mechanism
The primary function of the core-pulling mechanism is to move the core (or movable part of the mold) during or after the injection process, allowing the part to be ejected smoothly without any interference from undercuts or other intricate features. This is essential for parts with internal holes, complex shapes, or undercuts that would otherwise trap the molded product in the cavity.
The core-pulling mechanism is a critical design consideration for molds with complex geometries, and its design can vary from simple to highly sophisticated, depending on the nature of the part and the mold.
Types of core-pulling mechanisms:
| Mechanism | Description | Best for |
|---|---|---|
| Sliding Block | Moves horizontally/at an angle to disengage core. | Simple undercuts, low complexity. |
| Hydraulic | Uses hydraulic cylinders for high force and precision. | Complex molds with deep undercuts. |
| Pneumatic | Uses compressed air to move the core. | Small to medium molds. |
| Mechanical | Uses cams, levers, and gears for core movement. | Simple designs without complex moves. |
| Multi-Stage | Combines multiple actuators to move the core in stages. | Complex parts with intricate undercuts. |
Conclusion
Building an injection mold requires careful attention to detail and precision in the design and assembly of various components. The mold base, cavity and core inserts, cooling system, ejector system, runner and gate channels, and other components must work together seamlessly to ensure the efficient production of high-quality plastic parts.
If you would like to learn more information or start your next injection molding project, please visit our “Injection Molding Services” page.
FAQ
An injection mold consists of two halves: the core and the cavity. These halves are aligned during each cycle to ensure product quality. The alignment is maintained by guide pillars and guide bushes.
High – strength mold steels such as P20, H13 are commonly used. These materials can withstand high injection pressures and temperatures during the molding process. Additionally, some specialized molds may use other alloys or even beryllium – copper inserts for specific applications.
Based on part shape, size, draft angles, and ejection direction. CAD and mold – flow analysis help.
Less waste, shorter cycle times, better melt flow control, and better part quality.
This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.
