Insert Injection Molding: To Make Custom Insert Molded Parts

insert molding services, plastic injection molding

Insert molding is a form of injection molding that shares characteristics with overmolding. This method is widely applied to produce highly functional components in a variety of industries. The insert injection molding design guide is used by engineers and product designers to guarantee that plastic components and threaded metal inserts are combined in an effective manner.

This article examines the procedures involved in the insert injection molding technique. Then, to assist you in achieving the greatest outcomes from the procedure, we will offer comprehensive design guidelines for insert injection molding. Well, let’s get right to it.

insert injection molding

Insert Molding Capabilities Overview

Insert molding is a technique that enhances the mechanical properties and functionality of plastic components by incorporating threaded inserts. This method provides an efficient means of securing materials together, eliminating the need for repetitive assembly. Below is an overview of the insert injection molding process.

Machinery Preparation

The insert injection molding process necessitates the use of sophisticated molding machines designed for product manufacturing. Typically, these machines are vertical and specifically engineered for insert injection molding. Moreover, the engineering team must adhere to a set of design guidelines for injection molding when operating these machines.

It is crucial to configure the machine according to the desired specifications outlined in the part design. The machinery employed in insert injection molding maintains tight tolerances to ensure the precision of manufactured parts. Even a minor deviation in injection molding tolerances can lead to product failure.

Insert Loading into the Mold

Various inserts can be utilized in the design of molded parts, with materials such as stainless steel, brass, or regular steel commonly employed. These inserts often feature threaded surfaces to enhance adhesion to the product.

The molds are positioned vertically in the machine, either through CNC machining or manual placement. Automated insertion, facilitated by robot arms, ensures efficiency, consistency, and precision. Automated machines operate at higher speeds and can withstand elevated temperatures during molding.

Alternatively, manual insertion is suitable for low-volume production, allowing for continuous operator presence to conduct detailed part inspections. However, manual insertion may impact repeatability and precision, and the high temperatures involved in the molding process can pose challenges for operators.

Once inserted, the mold remains in position through gravity during the insert molding process. Molten plastic is then injected into the mold under high pressure, ensuring complete adherence of the plastic to the inserts.


The molding phase follows the preparation and insert loading. Molten plastic enters the cavity, enveloping the carefully positioned insert. Maintaining a constant temperature during molding is essential to ensure uniform solidification.

Sustaining holding pressure helps minimize the shrinking effect and prevents backflow into the barrels. The molding cycle time depends on the nature of the materials and the size of the intended component.

Considerations before Insert Molding Design

Comprehensive awareness of the insert injection molding design guide is paramount. Here are key factors to contemplate in your product design:

Part Size and Depth

Consideration of part size and depth is crucial in insert mold design. These aspects impact the duration of the molding process. Some parts may necessitate the creation of new molds, extending manufacturing time and costs. Utilizing inserts with rounded knurling is advisable to avoid sharp corners.

Production Volume

The production volume of plastic molded parts dictates the choice between automated or manual loading. Automated loading, while faster and more precise, requires advanced CNC machines, potentially elevating injection molding costs. Selecting the appropriate loading method involves conducting a cost-benefit analysis and understanding production requirements.

Product Application

The product application is a pivotal consideration in insert injection molding design planning. While this process is compatible with a diverse range of materials, identifying the optimal material for each application is essential.

Project Budget

For cost-sensitive components, evaluating the project budget is crucial. This assessment includes insert costs and expenses related to engaging a manufacturing partner. Additionally, it involves considering the impact on the price of a molded part due to the inclusion of inserts.

Design Principles for Insert Molding

When employing injection molding with metal inserts, adhere to the following design principles for optimal results.

Avoid Sharp Corners

Eliminate sharp corners to mitigate stress on components, reducing the risk of failure. Rounded corners facilitate smooth material flow through molds, minimizing stresses on mold walls and reducing differences in shrinkage rates during cooling. Maintain uniform thickness and ensure the inner radius is at least half the wall thickness, with the outer radius being the sum of the internal radius and wall thickness.

Optimize Draft Angles

Draft angles aid in part removal after shrinkage during cooling. Carefully choose draft angles to prevent cosmetic abnormalities like drag marks. Standard mold texture standards recommend 1 to 2 degrees per side for basic texture and 3 to 5 degrees per side for additional texture. For heavier texture, consider 5 degrees or more per side.

Design Inserts

Keep inserts proportionally small relative to the plastic components for effective insert injection molding. Ensure a minimum of 0.016 inches (0.4 mm) of the insert extends into the mold cavity to prevent sink marks. The molding beneath the insert should be at least one-sixth of the insert’s diameter.

Suitable Boss Diameter

Employ bosses with a diameter 1.5 times that of the insert for added strength in slots or holes. Post-mold inserts, cold-pressed into holes, require a larger boss diameter to withstand greater stress.

Use Durable Resins

Choose strong and durable resins to prevent delayed cracking in the plastic surrounding metal inserts. Consider the molded-in hoop stress of the insert to enhance elongation and wear-resistance qualities.

Consider Metal Bonding

Recognize that metal inserts do not establish chemical bonds with over-molded plastic components. Design plastic and metal components to achieve mechanical bonding for a robust connection.

Wall Thickness of the Mold

Carefully determine wall thickness to impact functionality, aesthetics, and manufacturing costs. Aim for a standard wall thickness ranging from 0.080 to 0.160 inches, depending on the material. Maintain uniform thickness to avoid dimensional control issues and prevent the need for constant mold modifications.

Materials for Insert Molding

The insert injection molding process accommodates a variety of molding materials, with engineers and designers selecting materials based on the specific requirements of the intended final products. The following are common materials used in insert injection molding:


These environmentally friendly materials can be reused multiple times and offer excellent chemical resistance, making them well-suited for achieving variation and sheen in insert injection molding. Common types of thermoplastics include:

  • Polystyrene:
    Lightweight and clear with a low melting point, resistant to acids and bases, making it suitable for everyday products like medical devices and consumer goods.
  • Polyethylene:
    Lightweight, tough, chemically resistant, with dielectric and impermeability properties. Variants include LDPE, HDPE, and MDPE.
  • Nylon:
    Naturally solid and sturdy, resistant to chemicals and abrasions, with a relatively high melting point. Widely used as a metal replacement in heavy manufacturing and extraction applications.


Similar to thermoplastics but not recyclable, thermosets find less frequent use due to cost considerations and environmental impact. Commonly used thermosets include:

  • Epoxy:
    Hardens permanently without a melting point, suitable for challenging applications, with excellent adhesion, chemical resistance, and heat resistance.
  • Phenolic:
    Resistant to chemicals and electricity, capable of withstanding heat, and known for exceptional hardness and dimensional stability. Commonly used in the production of electrical equipment and circuit boards.


Natural and synthetic polymers with properties close to rubber, elastomers are chosen for their excellent elasticity and recyclability, making them eco-friendly. Common types of elastomers include:

  • Natural Rubber:
    Highly flexible and durable, ideal for manufacturing tires and various household appliances.
  • Polyurethane:
    Exhibits outstanding abrasion resistance, increased wear and tear resistance, high elasticity, and impact resistance. Its durability and high-temperature resistance make it suitable for a wide range of applications, including automotive products and home appliances.

Select Appropriate Threaded Inserts

Insert molding is a widely employed technique for incorporating threaded fasteners into parent materials, proving beneficial in the production of diverse components such as computer and terminal boards, instrument boxes, automotive items, communication equipment, electronic devices, and aerospace products.

Manufacturers commonly utilize undercuts to secure fasteners to parent materials, and a helical groove pattern enhances tensile and torsional resistance in products. This feature protects against potential defects such as “torque out” and “pull out.”

Threaded insert technology, often constructed from materials like brass, aluminum, or stainless steel, offers purpose-built implants that can be integrated through various methods, including ultrasonic insertion, molding (via insert molding), or application of heat.

Among these methods, insert injection molding stands out, delivering enhanced mechanical properties, accelerated manufacturing rates, and heightened precision. It is imperative to ensure that the selected threaded fasteners and their installations can withstand the loading requirements of the design.

Execute a Comprehensive Testing Program

Conducting a thorough end-use testing program is crucial in the insert injection molding process. This step aids in identifying potential faults or complications during the prototype stage of development. The testing program should encompass parameters representative of the conditions to which the product will be exposed. For instance, subjecting the product to temperature and pressure cycling reflective of its intended applications is essential for a comprehensive assessment.

Advantages of Insert Injection Molding

Insert molding stands out as a widely embraced and specialized technique, finding applicability across diverse industrial sectors such as electrical components, automobiles, kitchenware, and housing furnishings. The appeal of insert injection molding is underscored by several compelling reasons:

Enhanced Design Flexibility and Reliability

Insert molding provides designers with unparalleled flexibility by accommodating a wide array of configurations and shapes. This process allows for the incorporation of features that make plastic parts sturdier than conventional alternatives. The seamless integration of plastic and metal parts enhances design reliability, preventing issues like loosening, improper terminations, and misalignment.

Cost Reduction in Assembly

The single-shot nature of insert injection molding contributes to cost-effective manufacturing. By eliminating post-molding assembly and separate parts installation, this technique minimizes motion waste and optimizes production time. It offers an economically efficient and swift molding process, utilizing vertical injection machines for heightened efficiency.

Environmentally Friendly

Insert molding aligns with environmentally friendly production practices by utilizing only the necessary amount of plastic to create parts. This approach enhances sustainability and supports the recycling of plastic materials, contributing to waste reduction.

Reduction in Size and Weight

The insert injection molding method promotes resource efficiency by using less material, resulting in reduced waste and costs. Furthermore, it eliminates the need for additional fasteners and connectors, contributing to smaller and lighter molded components. This approach is particularly advantageous for enhancing part strength by incorporating the robustness of metal inserts into plastic components.

Make Insert Molded Prototypes and Parts

By eliminating the need for additional processes that could raise production costs, insert injection molding produces substantial parts without the complications of integrating components. This method works with a wider variety of end-use applications and material choices.

We provide a wide range of insert injection molding services to make sure your parts fulfill all specifications without going over production schedule. We are able to examine your design and offer a design for manufacturability analysis. Get in touch with us for prompt and trustworthy feedback. You will receive the best results thanks to our hardworking engineering team, powerful insert injection molding capabilities, and stringent quality inspection standards. Please give us a call or send an email right now to discuss your upcoming project if needed.

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