Five Key Elements of Injection Mold Design

Table of Contents

1. Product Analysis and Preliminary Preparation

2. Parting Surface Design

3. Runner System Design

4. Core Pulling Mechanism and Ejection System

5. Mold Cooling System Design

6. Summary and Considerations

Injection mold design is a highly technical field in manufacturing, and its design quality directly affects product precision, production efficiency, and cost control. As an injection mold manufacturer with 23 years of rich experience, Alpine Mold understands that high-quality mold design directly impacts product quality and production efficiency. This article will explore five core elements based on Alpine Mold’s years of practical experience, including product analysis, parting surface design, and runner system optimization, providing a systematic design guide for professional clients.

1. Product Analysis and Preliminary Preparation

Product analysis and preliminary preparation are fundamental steps in injection mold design that directly determine the direction and quality of subsequent designs.

Product Structural Feature Analysis

Before starting mold design, a comprehensive structural analysis of the product is needed:

• Wall thickness distribution and uniformity

• Geometric features of the product (e.g., ribs, bosses, blind holes)

• Dimensional accuracy requirements

• Aesthetic quality requirements

Material Selection and Performance Requirements

Rational material selection is the foundation of injection mold design. Different product usage requirements dictate different material performance needs, such as heat resistance, toughness, wear resistance, or chemical stability. When selecting materials, it is essential to consider the flowability, shrinkage rate, and compatibility with mold steel.

Plastic Type	Recommended Mold Steel	Application Scene	Heat Treatment Requirements	Shrinkage Rate (%)
PEEK	S136 Stainless Steel	Aerospace	Quenching + Polishing	1.2~1.5 (unreinforced)
ABS	NAK80	Consumer Electronics	Pre-hardening	0.4~0.7 (unreinforced)
PC	H13	Optical Devices	Vacuum Quenching	0.5~0.7 (unreinforced)
PA (Nylon)	8503 Mold Steel	Automotive Parts	Quenching + Tempering	1.5~2.0 (unreinforced) 0.3~0.8 (30% glass reinforced)
PP (Polypropylene)	P20	Daily Goods Packaging	Pre-hardening	1.0~2.5 (unreinforced)  0.5~1.2 (20% glass reinforced)

Production Volume and Economic Evaluation

The production volume of the product directly affects the cost and complexity of mold design. For small batch production, simpler mold types with lower manufacturing costs, such as aluminum molds, can be chosen. For large batch production, high-strength steel is required to ensure the durability and efficiency of the mold. Additionally, economic evaluation includes the cycle cost of mold design, maintenance costs, and expected lifespan. These factors must be fully considered during the preliminary assessment to achieve a balance between product quality and cost.

Production Scale	Typical Output Range	Core Cost Conflicts	Mold Design Priority
Small Batch Trial	1,000 - 50,000 units	High mold development cost	Quick mold change, modular design
Medium Batch Production	50,000 - 500,000 units	Balance of labor cost and efficiency	Semi-automated, simplified maintenance structure
Large Batch Mass Production	> 500,000 units	Maximizing output per unit time	Fully automated, high lifespan desig

Mold Type Selection

Choosing the right mold type based on product characteristics and production needs is crucial. Common mold types include single-cavity molds, multi-cavity molds, hot runner molds, and cold runner molds. Single-cavity molds are suitable for low-output or high-precision products, while multi-cavity molds are used for large-scale production to improve efficiency. Hot runner molds reduce material waste and enhance production efficiency but have higher initial costs and should be chosen based on specific circumstances.

• Two-Plate Mold: Suitable for simple structures, low cost.

• Three-Plate Mold: Suitable for multi-point gate designs, enabling automatic cutoff of runners.

• Hot Runner Mold: Reduces waste, suitable for high-precision medical components.

2. Parting Surface Design

Parting surface design is one of the most critical aspects of mold design, directly affecting product appearance quality and production efficiency.

Reasonable Layout of the Parting Surface

• Consider the natural parting line of the product.

• Avoid unnecessary intersections of parting surfaces.

• Ensure the sealing performance of the parting surface.

• Optimize the manufacturability of the parting surface.

Determination of Draft Angles

Product Feature	Minimum Draft Angle (°)	Recommended Draft Angle (°)
Outer Wall	0.5	1-2
Inner Wall	1	2-3
Deep Hole	1.5	3-5
Ribs	1	2-3

Sealing Design of the Parting Surface

When designing the sealing of the parting surface, it is necessary to ensure that the parting surface contacts tightly after the mold is closed to prevent overflow and flash during the injection process. Therefore, the machining precision and flatness of the parting surface require high standards, and the sealing effect can be improved by increasing the clamping force or using locating pins. In multi-cavity mold design, precise adjustments of the sealing performance of each cavity's parting surface are essential to ensure product consistency.

Measures to Prevent Product Deformation

Parting surface design should also consider how to prevent product deformation. During injection molding, material shrinkage, internal stress release, or improper mold design may cause warping or deformation of the product. To address this, the position of the parting surface can be optimized to reduce uneven stress during demolding, or reinforcing ribs can be added in weak areas. Additionally, accurately setting the cooling system's position during parting surface design ensures uniform cooling rates across the product, thereby reducing deformation risks.

3. Runner System Design

Selection of Gate Types and Locations

Gate Type	Applicable Scene	Advantages	Disadvantages
Pinpoint Gate	Small thin-walled parts	Minimal marks, auto cutoff	High pressure loss
Fan Gate	Wide flat parts	Even filling	Difficult to clean
Hidden Gate	High aesthetic requirements	Concealed gate marks	High processing costs

Design of the Runner System

• Round Runner: Highest efficiency, recommended for materials with good flow like PA and PP, diameter 3.5~7.0 mm.

• U-Shaped Runner: Convenient for demolding, suitable for dual parting surface molds, cross-sectional area 15% larger than round.

Layout of Temperature Control Systems

Main runner heated to 10~15°C above the material melting point, branch runners use zoned temperature control with a temperature difference of <3°C.

Consideration of Filling Balance

For multi-cavity molds, a balanced runner layout ensures simultaneous filling of each cavity with a filling time difference of <0.5 seconds.

4. Core Pulling Mechanism and Ejection System

A reasonable core pulling mechanism and ejection system design ensure smooth demolding of products and improve production efficiency.

Principles of Core Pulling Structure Design

• Motion interference analysis

• Guaranteeing guiding accuracy

• Reasonable planning of stroke

• Choice of drive method

Layout of the Ejection System

Ejection system design parameters:

Parameter	Recommended Value	Influencing Factors
Ejector Pin Diameter	2-8 mm	Product wall thickness
Ejector Pin Spacing	30-50 mm	Product structure
Ejection Stroke	15-40 mm	Product dept
Ejection Force	3-5 times product weight	Material characteristics

Stroke and Force Calculations

Core pulling force formula: F = μ × P × A (where μ is the friction coefficient, P is the plastic shrinkage pressure, and A is the contact area).

Guaranteeing Guiding and Positioning Accuracy

Using ball guide pillars with a gap of 0.01~0.02 mm and linearity error <0.005 mm/m.

5. Mold Cooling System Design

An efficient cooling system design can significantly improve production efficiency and product quality.

Layout of Cooling Channels

• Cooling channel diameter: commonly 8-12 mm

• Channel spacing: 3-5 times the channel diameter

• Distance to cavity surface: 2-3 times the channel diameter

• Cooling water flow rate: recommended 0.5-2 m/s

Optimization of Cooling Efficiency

To improve cooling efficiency, optimization should be performed in the following areas:

Channel Size: The diameter and cross-sectional area of the cooling channels should match the flow rate of the cooling liquid. Channels that are too small increase flow resistance, leading to decreased cooling efficiency, while channels that are too large may compromise the strength of the mold structure.

Cooling Liquid Flow Rate: The flow rate of the cooling liquid should be moderate; too low flow decreases heat transfer efficiency, while too high flow may cause turbulent states and increased energy consumption.

Cooling Liquid Temperature: The initial temperature of the cooling liquid should be set based on the material characteristics and cooling requirements, usually maintained within a stable range to ensure consistent cooling effects.

Control of Temperature Uniformity

The core goal of mold cooling system design is to achieve uniform cavity temperatures to avoid product defects caused by uneven cooling. Some common measures to achieve temperature uniformity include:

Localized Cooling Design: For complex shapes or uneven wall thickness products, the cooling system can be divided into multiple zones, with each zone independently controlling the flow and temperature of the cooling liquid for precise temperature control.

Auxiliary Cooling: For difficult-to-cool areas (such as deep cavities or thick wall regions), auxiliary cooling methods such as pin cooling or localized cooling pipe designs can be adopted to ensure that these areas maintain temperatures consistent with other areas.

Thermal Balance Analysis: Using CAE (Computer-Aided Engineering) for thermal balance analysis of the mold cooling system to predict temperature distribution during cooling and optimize the layout of cooling channels.

Measures for Controlling Deformation

The cooling system must also consider how to control product deformation. Uneven temperature distribution during cooling may cause inconsistent material shrinkage, ultimately leading to warping, deformation, or stress concentration in the product. Common methods for controlling deformation include:

Uniform Cooling: Ensure that the cooling system design provides uniform cooling effects to avoid uneven shrinkage due to differences in cooling speed.

Optimized Channel Design: In areas where thin walls meet thick walls, cooling channel distribution should be denser to balance cooling speed differences.

Material Selection: Choosing mold materials with good thermal conductivity can enhance overall cooling uniformity, thereby reducing deformation risks.

Post-Processing Techniques: For products prone to deformation, appropriate post-processing techniques such as annealing or flattening can be performed after demolding to release internal stresses and further reduce deformation.

6. Summary and Considerations

Interrelationship Between Design Elements

The runner system and cooling system need to be designed in coordination, for example, high shear rate gates should be paired with rapid cooling to prevent flash.

Problem	Cause	Solution
Product sticking to the mold	Insufficient draft angle	Increase draft to above 2°
Visible weld lines	Poor venting	Add 0.03 mm vent slots to the parting surface
Ejection deformation	Uneven pin distribution	Use balanced grid layout, add buffer springs

Key Points for Quality Control

Mold flow analysis validation: Use Mold flow to simulate the filling process and predict shrink marks and air trap locations.

First article inspection: Measure key dimensional tolerances, such as automotive part fit gaps needing <0.1 mm.

Design Optimization Suggestions

To improve the efficiency and quality of injection mold design, the following optimizations can be made:

Introduce CAE Technology: Use Computer-Aided Design (CAD) and Computer-Aided Engineering (CAE) technologies to simulate filling, cooling, and deformation during the injection process, optimizing design solutions.

Modular Design: Adopt a modular design approach to standardize and modularize various parts of the mold, facilitating manufacturing, maintenance, and replacement.

Energy-Saving Design: Optimize cooling and runner systems to reduce material waste and cooling time, improve production efficiency, and lower energy consumption.

Collaborative Design: In the early stages of mold design, closely collaborate with the product design team to ensure that the structural features of the product align with mold design requirements, reducing subsequent modification costs.

By meticulously designing and optimizing these five elements, Alpine Mold helps clients significantly improve mold lifespan, ensure product quality, and enhance production efficiency. In actual design processes, we flexibly adjust various parameters based on specific product characteristics and production requirements to achieve optimal design solutions. Please feel free to contact Alpine Mold for professional injection mold design solutions.