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The Making, Processing And Selection Tips Of A Mold Base

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We mentioned the “mold base” in the article about “injection mold components“, which represents a major component of the injection mold. Today, we will do some research on some materials for mold base units and provide details on their process and selection.

The content discussed in this article is not something that product designers need to master but is rather aimed at those working in the mold-making and injection molding industries. Please judge for yourself based on your learning needs.

What Exactly Is A Mold Base?

The mold base is a crucial part of the mold, often made of metal and serves to support and secure various components of the mold.

The function of the mold base is to assemble and position all parts of the mold according to design requirements, ensuring the stability and precision of the mold during processing or use.

It mainly includes parts like the top clamp plate / rear clamp plate, A plate/B plate, and spacer block, etc. These components collectively ensure the stability and reliability of the mold. Some mold bases also include ejection systems, guidance systems, pre-reset systems, etc., to meet different mold processing needs.

Although the mold base is a fundamental part of the mold structure, it is indispensable. It not only ensures the processing precision of the mold but is also key to the normal operation of the mold.

Composition Of A Mold Base

The composition of a mold base is crucial due to the significant differences in processing between mold bases and the molds themselves. Mold manufacturers often opt to order mold bases from specialized producers, leveraging the production strengths of both parties to enhance overall production quality and efficiency.

Over the years, the mold base manufacturing industry has matured significantly. Mold makers now have the option to purchase customized mold bases tailored to specific mold requirements or choose from a variety of standardized mold base products. Standard mold bases offer diversity in design, shorter delivery times, and are ready for immediate use, providing mold manufacturers with greater flexibility. As a result, the use of standard mold bases is becoming increasingly popular.

In simple terms, a mold base includes a pre-molding system, a guiding system, and an ejection system.

Typically, a mold base is equipped with a top clamp plate, A plate, B plate, C plate (spacer block), rear clamp plate, ejector pin plate, and ejector retainer plate(The ejector pin plate and ejector retainer plate are often classified as ejector systems), along with guide pins(often classified as guiding system), return pins, and other components.

Top clamp plate:

Supports and secures the mold base and the feed system.

A plate:

Fixes the mold core or is made from high-quality material to directly form the part.

Stripper plate:

Used for products that are not suitable for ejection with ejector pins or flat ejector pins. Stripper plate ejection is used for transparent and appearance parts.

B plate:

Fixes the mold core and can also extend the runner.

Support plate (not present in every mold base):

If the lower mold needs to withstand significant pressure and its thickness may not be sufficient, a support plate is used to enhance mold strength.

C plate(spacer block):

Supports the mold base and is related to product ejection. The height of the spacer block relates to the product’s height.

Rear clamp plate:

Used to mount the mold onto the injection molding machine.

Ejector pin plate:

Secures the ejector pins.

Ejector retainer plate:

Controls the movement of the ejector pins.

Return pin:

When the mold cavity and mold core close, the ejector pins retract. If the ejector pins do not retract, they could collide with the product/mold cavity, causing damage.

Composition of the mold base FirstMold
Composition of the mold base FirstMold

Commonly Used Mold Base Materials


Steel is one of the most commonly used materials for making mold, including types such as 45# steel, 55# steel, 65# steel, and 50Mn. It offers high strength, toughness, and wear resistance, capable of withstanding significant loads and impacts. It is suitable for large, high-precision plastic injection mold processing. However, steel also has its drawbacks, such as being relatively heavy and prone to rust.

Aluminum Alloy:

Aluminum alloy is known for its lightweight, high strength, and corrosion resistance, making it suitable for small to medium-precision mold manufacturing. Common aluminum alloys used include aluminum-silicon alloy, aluminum-magnesium alloy, and aluminum-manganese alloy. With its low density and good thermal and electrical conductivity, aluminum alloy is ideal for situations where reducing the mold’s weight and improving production efficiency are desired.

The selection of material for a mold base should consider the mold’s size, precision, and load requirements comprehensively. Steel materials should be used for applications where the mold base’s strength and stability are critical. For applications that require a reduction in the mold’s weight and have high thermal conductivity requirements, such as small precision molds, aluminum alloys or plastic materials can be chosen for the mold base.

Manufacturing Of The Standard Mold Bases

In our previous discussion, we mentioned that most mold manufacturers focus their efforts on the design and processing of mold cavities and cores, often outsourcing standard mold bases to enhance manufacturing efficiency in the factory. Custom precision machining of mold bases is undertaken only under special circumstances by mold suppliers.

Let’s focus on the processing of standard mold bases.

Main Processing Steps for Standard Mold Bases

1. Bottom surface processing to ensure dimensional accuracy.

2. Alignment of workpiece datum plane, checking for 2D and 3D surface allowances.

3. Rough machining of 2D and 3D surfaces, including non-installation and non-working planes (e.g., safety platform surfaces, buffer installation surfaces, and side datum plane).

4. Before semi-finish machining, ensure the accuracy of the side datum plane.

5. Semi-finish machining of 2D and 3D surfaces, precision machining of various installation and working surfaces (including stop installation and contact surfaces, insert installation and back surfaces, spring installation and contact surfaces, various stroke stop working surfaces, etc.). Semi-finish machining of guide surfaces and holes. Leaving allowances for precision machining of process datum holes and height reference surfaces, and recording data.

6. Inspection and review of machining accuracy.

7. Fitter assembly operations.

8. Check for insert allowances before finishing machining and aligning process datum hole bases.

9. Finish machining of 2D and 3D surfaces and hole positions, precision machining of process datum holes and height references, and finish machining of guiding surfaces and holes.

10. Inspection and review of machining accuracy.

Key Features of Mold Base Processing

1. High precision requirements:

A mold typically consists of a mold cavity, mold core, and mold base, with some being multi-piece assembly modules. Thus, the combination of cavity and core, inserts and cavities, and module assembly all require high precision. Precision molds often achieve dimensional accuracy at the M level.

2. Complex surfaces:

Some products, like automotive covers, aircraft parts, toys, and home appliances, have surfaces composed of multiple curves, making mold cavity surfaces complex. Some curves require mathematical methods for processing.

3. Small batch:

Molds are not produced in large batches; often, only one piece is produced.

4. Numerous processing steps:

Mold processing typically involves milling, boring, drilling, reaming, and threading.

5. Repetitive production:

Molds have a lifespan. When a mold exceeds its lifespan, it needs to be replaced, so mold production often has repetitiveness.

Excellent mold materials, high hardness:

The main materials for molds are usually high-quality alloy steel, especially for high-life molds, which have strict requirements from blank forging, and processing to heat treatment. Therefore, the formulation of processing techniques cannot be overlooked, and heat treatment deformation is a serious concern in processing.

Equipment Factors Affecting Mold Base Processing

Given the many features mentioned above, machine tool selection must meet processing requirements as much as possible, such as strong CNC machine functions, high machine precision, good rigidity, good thermal stability, and profile modeling capabilities.

1. Foundation and Support:

The mold clamp plates serve as the foundation for mold installation, offering support, protection, and connecting critical components (mold cores).

2. Standardization of Mold Bases

Since the mold base does not participate in molding, its shape does not change with the product but is related to the product’s size and structure. This allows for the standardization of mold bases, facilitating processing. Mold base manufacturers can prepare various sizes and specifications of mold base parts (templates, guide posts) in advance and assemble them according to customer needs. Well-known standards include DME, Futaba, HASCO, etc.

3. Selecting the Right Processing Equipment for Mold Bases

Standard mold base processing equipment mainly includes milling machines, grinders, and drilling machines, with milling and grinding providing six polished surfaces to specified dimensions. Drill machines handle precision-unrequired holes, like screw holes and lifting holes, drilling, and tapping. A basic requirement for a standard mold base is to facilitate mold opening smoothly.

4. Precision in Guide Pin Holes:

Smooth mold making is directly related to the precision of the four guide pin holes, typically requiring CNC vertical machining centers for rapid drilling followed by boring to achieve precision.

5. Durability and Safety: Designing Mold Parts with Care

Various parts in mold processing should be strong and stiff enough to prevent damage and deformation during use, and fastening parts should include anti-loosening measures to avoid accidental injuries.

6. Maintaining a Safe Processing Environment

The processing should not result in scrap or projectile workpieces distracting or injuring operators.

7. Reducing Noise and Ensuring Safety in Mold Handling

Efforts should be made to minimize noise and vibration during injection molding. The design should indicate the mold weight for easy installation and safety. Parts weighing over 20 kg should have lifting and handling measures to reduce labor intensity. Installing or disassembling processed parts should be convenient and safe to prevent injuries.

Factors Affecting Mold Base Structure

Cavity layout:

Determined by the geometric characteristics, dimensional accuracy requirements, batch size, manufacturing difficulty, and cost of the mold base, including the number and layout of cavities.

Parting line determination:

The position should facilitate mold base processing, venting, demolding, and molding operations, as well as the surface quality of plastic parts.

Gating system and venting system:

The gating system (main runner, sub-runner, and gate shape, position, size) and venting system (venting method, vent slot position, size) determination.

Ejection method:

Ejection method selection (ejector pins, ejector sleeves, push plates, composite ejection) and deciding on side recess processing methods, core pulling methods.

Temperature system:

Cooling and heating method determination, including the shape, position of cooling/heating grooves, and installation locations for heating elements.

Wall thickness and Dimensions:

Based on the material, strength calculations, or empirical data of the mold base, determine the thickness and outer dimensions of mold base parts, the outer structure, and all connecting, positioning, guiding component positions.


Determine the structural form of the main molding parts and structural components.

Analysis and calculation:

Careful analysis and calculation of the strength of each part of the mold base are necessary to ensure the working dimensions of the molded parts.

Some mold bases can be processed within 10 days, while others might take a month, not necessarily completing the initially specified quantity on time. However, the general trend in the mold base processing industry is towards simplification of processes, with many operations not requiring manual intervention and instead utilizing high-tech production methods. This approach saves time and ensures that the processed mold bases meet production design standards more accurately.

The more complex the mold base processing, the higher the sales price of the produced product, as the CNC machining cost is significantly higher than for other products.

Since mold manufacturers generally do not process mold bases themselves, it’s crucial to have a solid understanding of how to choose a mold base correctly. While there are certain standards in the mold base industry, there are many types of mold bases, making it impossible to introduce them all here. Instead, we can only provide a brief overview of the methods and steps for selecting a mold base.

Selection Methods and Steps of Standard Mold Base

Methods for Choosing

Test the adaptability of the selected mold base model:

For the chosen mold, you should check its relationship with the injection molding machine, such as the closed mold height, open mold stroke, etc. If it’s not suitable, you’ll need to choose again.

Standard mold bases come in many sizes, so it’s important to select an appropriate size. If the size is too small, the mold base might not have sufficient strength or stiffness, or there might not be enough space for screws, pins, and guide bushings (guide pillars). If the size is too large, it could not only increase costs but also require a larger model of injection molding machine.

Types of two-plate standard mold bases provided by mold base suppliers
Types of two-plate standard mold bases provided by mold base suppliers

Steps for Selecting

1. Determine the mold base assembly form

Based on the structural requirements of the plastic part, determine the structural assembly form of the mold.

2. Determine the cavity wall thickness

The wall thickness of the cavity is calculated using a wall thickness formula or based on experience. Calculate the circumference size of the cavity plate.

3. Plates circumference size

The calculated circumference size of the cavity plate should be close to a standard size, usually requiring significant adjustment. Additionally, when making corrections, ensure there is enough space at the wall thickness location to install other parts, such as guide bushings. If there isn’t enough space, you should increase the wall thickness dimension.

4. Determine the thickness of the plate.

Calculate the thickness of the bottom plate based on the cavity depth and correct it according to standard sizes.

5. Select the mold base size

Based on the determined cavity plate circumference size and the required plate thickness, select the mold base.


Discussing mold bases might seem a bit dry, but for those in mold making or injection molding, it’s both fundamental and indispensable. Just as a skyscraper must start from a solid foundation to stand tall and strong, building a solid base is crucial for creating an excellent structure.

I’m Lee Young, a senior mold and injection molding expert from FirstMold. I welcome you to contact me for knowledge exchange in mold making or injection molding. We’re also looking forward to collaboration orders with our company~ Next, we will delve into the details of other components of molds. Stay tuned~

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