What are the Types of Injection Mold: A Comprehensive Guide

Injection Mold Classification

If you’re assessing injection molds, it’s essential to grasp the classification system that differentiates their capabilities and applications. In this article, we’ll provide you with a straightforward guide to understanding various mold types:

By Geometry
By Number of Cavities
By Mold Plate
By Ejection Principle
By Plastic Material
By Feeding System

Your choice among these types of injection molds often balances the scale of your project, the complexity of the parts, and the operational cost implications. Let’s explore various kinds of plastic injection molds together!

By Geometry

Simple Geometries:
- Easier to design and manufacture.
- Often have fewer hollow sections.
Complex Geometries:
- Allow for greater design flexibility.
- Can accommodate hollow sections needed for specific functions.

Complex parts often include features like undercuts, which are indentations or projections that can complicate the mold design. Designing for undercuts requires careful consideration to ensure mold components can separate properly during the ejection phase without damaging the part.

For intricate component designs, using advanced mold technologies, like sliders or lifters, might be necessary to create and release the part from the mold successfully. While this approach offers high design flexibility, it also adds to the complexity and cost of the mold.

Your selection between a simple or complex mold geometry will ultimately depend on the desired part design, functionality, the feasibility of the mold manufacturing process, and the project’s budget. Always balance the need for complex geometries with practical production considerations.

By Number of Cavities

Single-Cavity Molds

Produce only one product per injection cycle
Ideal for large, complex, or low-volume parts
Allow more attention to individual parts to minimize defects
Lower tooling costs compared to multi-cavity molds for the same part
Suitable for prototyping and providing out new designs

Multi-Cavity Molds

Contain multiple identical cavities to produce several parts per cycle
Enable shorter lead times and increased production efficiency for high volumes
Reduce per-part costs for large batches
Require careful design to ensure even filling and consistent quality across cavities
May have higher initial tooling costs than single-cavity molds
Typical cavity numbers range from 2 to 64 or more, depending on part size and application

Family Molds

Incorporate multiple cavities with different shapes to produce various parts in one cycle
Allow molding of related components or product variants in a single shot
Useful for prototyping or producing kits with assorted parts
Limited to parts using the same material and color
Often have unbalanced filling due to different cavity geometries, increasing defect risk
Require more post-molding labor to separate and handle different parts

By Mold Plate

Two-Plate Injection Mold

Simplest and most common injection mold design, consisting of two main parts (A side and B side)
Has a single parting line where the mold splits into two halves
Cavity side is fixed, while the core side is movable during the molding process
Runner and part are located on the same parting plane and ejected together
Advantages: lower cost, shorter cycle times, easier to set up and operate
Disadvantages: less flexibility in gate location, requires manual delegating, risk of short shots in multi-cavity molds

Three-Plate Injection Mold

Also known as stripper plate mold, consists of two parting planes and splits into three sections
Has an additional floating plate between the cavity and core to accommodate the runner system
Allows automatic degassing of the runner from the part
Offers more flexibility in gate location compared to two plate molds
Advantages: suitable for large parts requiring multiple gates, enables automatic debating
Disadvantages: more complex and expensive, longer cycle times, less stable due to more moving parts

Stack Injection Mold

Special mold structure with multiple parting surfaces, each accommodating one or more cavities
Consists of a moving mold, middle mold, and fixed mold that open simultaneously
Can double the output of a standard mold without requiring additional machines
Suitable for high-volume production of flat, thin-walled parts
Advantages: significantly increased productivity, reduced costs, shorter manufacturing times
Disadvantages: more complex design, requires careful balancing of cavity filling and cooling

By Ejection Principle

1. Pin Ejection

Uses ejector pins to push the molded part out of the mold cavity
Pins are located in the ejector half of the mold and are designed to withstand ejection forces
Pin diameter, length, material, placement, and shape are critical design considerations
Suitable for most injection molded parts, but may leave visible ejector pin marks

2. Sleeve Ejection

Employs a sleeve-shaped ejector that surrounds the core to eject the part
Provides uniform and stable ejection force without leaving visible marks
Suitable for cylindrical, thin-walled, or shell-shaped products
More complex and costly than pin ejection

3. Stripper Plate Ejection

Uses a stripper plate to push the part off the core
Ejection force is high, uniform, and stable, minimizing part deformation
Ideal for cylindrical parts, thin-walled containers, and shell-shaped products
Avoids visible ejector marks but has a more complex mold structure and a higher cost

4. Blade Ejection

Utilizes rectangular ejector blades instead of pins
Suitable for parts with specific geometries or requirements

5. Air Ejection

Introduces compressed air between the part and mold to eject the part
Simplifies mold structure and allows ejection at any location
Often used to support other ejection methods for large, deep-cavity, or thin-walled parts

6. Lifter Ejection

Employs lifters that move laterally to release undercuts or internal features
Enables ejection of parts with complex geometries or side actions

By Plastic Material

1. Acrylic (PMMA)

Strong, clear thermoplastic with excellent optical clarity and UV resistance
Lightweight and shatter-resistant alternative to glass
Used for lenses, displays, signs, and medical devices

2. Acrylonitrile Butadiene Styrene (ABS)

Tough, rigid, and impact-resistant thermoplastic
Good chemical resistance and is easy to paint or glue\
Used for automotive parts, appliances, toys, and electronics housings

3. Nylon (Polyamide, PA)

Strong, flexible, and wear-resistant thermoplastic
High melting point and good chemical resistance
Used for automotive parts, gears, bearings, and electrical components

4. Polycarbonate (PC)

Tough, transparent, and heat-resistant thermoplastic
Good dimensional stability and electrical insulation properties
Used for automotive components, medical devices, and safety glasses

5. Polyethylene (PE)

Lightweight, flexible, and chemical-resistant thermoplastic
High-density (HDPE) and low-density (LDPE) variants available
Used for packaging, containers, toys, and automotive components

6. Polyoxymethylene (POM)

Strong, rigid, and dimensionally stable thermoplastic
Excellent wear and chemical resistance
Used for gears, bearings, and precision parts

7. Polypropylene (PP)

Lightweight, tough, and chemical-resistant thermoplastic
Good fatigue resistance and electrical insulation properties
Used for packaging, automotive parts, and household goods

8. Polystyrene (PS)

Rigid, transparent, and easy-to-process thermoplastic\
Low cost and good dimensional stability
Used for packaging, disposable cutlery, and toys

9. Thermoplastic Elastomers (TPE) and Polyurethanes (TPU)

Flexible, rubber-like thermoplastics with good impact and abrasion resistance
TPUs offer better chemical and temperature resistance than TPEs
Used for soft-touch grips, gaskets, seals, and footwear

By Feeding System

Cold Runner Injection Mold

Consists of the sprue, runners, sub-runners, gates, and cold slug wells\
Sprue delivers the melt from the nozzle to the runner system
Runners transport the melt from the sprue to the gates
Gates are small orifices connecting the runner to the cavity
Cold slug wells store the initial cold material produced during injection intervals
Can be further divided into side gate and point gate systems
Suitable for most injection molding applications

Hot Runner Injection Mold

Has no main runner and sub-runner
Melt passes through a manifold plate and hot nozzle directly into the cavity via gates
Utilizes insulated runner or hot runner molds
Eliminates the need for a traditional cold runner system
Reduces material waste and cycle times compared to cold runner systems

Runnerless Injection Mold

Melt flows directly from the nozzle into the cavity without a runner system
Simplest feeding system design

Recommended Injection Mold at Moldie

Moldie is a leading provider of high-quality, precision plastic injection molds and molding services. With years of experience and state-of-the-art technology, Moldie offers a wide range of injection mold solutions to meet your specific needs: