Insert Molding Design Guide: A Reference Post

Plastic injection molding remains the most common method of fabricating plastic parts globally. Insert molding is a popular injection molding technique, and for good reason. It facilitates the integration of two different materials, such as plastic and metal, to create highly functional parts for a wide range of industries.

Overmolding is a related technique where a second layer of material is molded over a substrate, often used to create ergonomic grips or add additional functionality to parts. Having a thorough understanding of the insert molding process is crucial to avoiding common design issues. Engineers and part designers rely on an insert molding design guide to help them create quality, defect-free parts at a reasonable cost.

An injection molding press is essential in the insert molding process. This machine is significant in the manufacturing industry due to its cost-effectiveness and importance in part design.

In this article, you’ll learn about the best practices for designing insert molding products. We’ll also guide you through the insert molding process, materials, and capabilities.

What Is Insert Molding?

Insert molding is a manufacturing process that forms plastic parts with an embedded non-plastic insert. First, the insert is placed into a mold cavity. Molten plastic is then injected into the mold around the insert. As the plastic cools and solidifies, the insert becomes a permanent part of the component.

The mold consists of two sides: the A-side and the B-side. The A-side, also known as the cavity side, is where the show surfaces and features are placed, while the B-side, or core side, is where the part stays during mold open and ejection. Proper design ensures the part transfers correctly from the A-side to the B-side.

Everyday insert molding examples are scissors, screwdrivers, knobs, and knife blades. These are just a few examples — insert injection molding has a wide range of applications including extremely complex parts. Insert molding applications are diverse and include automotive components, medical devices, and consumer electronics.

Engineering plastics are known for being lightweight, flexible, and having great wear resistance. On the other hand, metals offer strength and good conductivity. Therefore, a plastic part with a metal insert combines desirable characteristics such as strength, weight reduction, and flexibility.

Materials for Insert Molding

Insert molding is compatible with a diverse range of plastic materials. The ideal choice will vary depending on the desired specifications of the end product. In this section, we’ll discuss the most common insert molding materials. Material compatibility is essential to ensure that the insert and the plastic material bond well and perform as expected in the final product.


Thermoplastics are a class of plastic polymers that melt at a certain high temperature and solidify upon cooling. Their distinguishing feature is recyclability — they can be remelted and remolded over and over again. Thermoplastics are generally versatile, flexible, and impact-resistant. They include;

Thermoplastic elastomers are a type of thermoplastic that combines the properties of rubber with the recyclability of plastics, making them ideal for flexible and durable parts.


Polystyrene is commonly used in the creation of molded parts thanks to its advantageous physical and mechanical properties. It is transparent, lightweight, and rigid making it suitable for numerous applications across various industries.


Polyethylene is the most extensively used plastic on earth. It offers good chemical stability and electrical insulation. Polyethylene comes in varying densities, each with unique physical and mechanical properties. Common polyethylene densities include low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE).


Nylon material, also known as polyamide (PA), is another thermoplastic material that is lightweight, durable, resistant to wear and abrasion, and has excellent tensile strength. It is often used for high-performance insert molding components and applications that require exceptional strength.


Thermoset plastics and polymers share some similarities with thermoplastics. The key difference is that thermosets can not be remolded once they’ve solidified. In other words, they are not recyclable. Generally, thermosets offer robustness, excellent structural integrity, and high-temperature resistance. Cross-linking is a process where the polymer chains in thermosets form a three-dimensional network, giving them their rigidity and heat resistance. Here are common thermosets used in insert molding.


Epoxy is a durable and resilient material that has proven beneficial for plastic insert molding. It is preferred for applications that demand corrosion and high-temperature resistance. Epoxy is also frequently used in electrical components due to its insulation properties.


Phenolic is another high-quality thermoset material you can consider when producing molded plastic parts. It excels in creating durable components that are wear and heat-resistant. This material is a good insulator and can be used in electrical parts.


Elastomers are a class of rubber-like materials that have elastic properties. They deform under stress and regain their original shape once the stress is removed. Elastomers can either be natural (such as rubber) or synthetic (such as polyurethane). Compression set is a measure of the permanent deformation of an elastomer after it has been compressed and then allowed to recover, which is important for applications requiring long-term elasticity.

Natural Rubber

Rubber is best known for its flexibility. It is commonly used in insert molding projects where durability and high tensile strength are critical.


Polyurethane is a versatile polymer that is elastic and highly durable. It offers remarkable abrasion and impact resistance and can also withstand high temperatures. Thanks to its desirable features, polyurethane can be found in many household items and machinery.

Design Guidelines for Insert Molding

When it comes to insert molding, most of the complicated steps are conducted before the actual process begins. The whole manufacturing process becomes easier once you have the optimal mold and part design.

Here is an insert molding design guide to help you create quality parts.

Avoid Sharp Corners

Avoid Sharp Corners

Sharp corners can spell trouble for insert molded parts. For starters, sharp corners can lead to stress concentration, which may cause reduced strength and even breakage. Further, sharp corners interfere with the smooth flow of melted plastic in the mold cavity causing appearance defects.

A general rule of insert molding is to add radii to all sharp corners. This facilitates better flow of the molten plastic during molding, reducing the risk of part defects. Further, rounded corners are critical for better stress distribution.

Optimize Draft Angles

Avoid Sharp Corners

A draft angle refers to a slight taper that is applied to the vertical faces of molds. Positioned to run in the same direction as the parting line, these features allow for the smooth ejection of the plastic parts from the mold, preventing defects caused by scraping. This prevents defects that are caused by scraping as the part is removed from the mold. A good example of this type of surface flaw is drag marks.

A draft angle of 1 to 2 degrees is recommended for standard insert molding situations. Light textured surfaces will require a draft angle of 1 to 3 degrees while heavy textures call for 5 degrees or more. The CAD system simplifies draft angle optimization. Considering draft angles in the design phase is crucial as they can significantly impact surface finishes, ensuring the desired texture and reducing tooling costs.

Design Inserts

Insert Molding Services

Generally, inserts should be smaller compared to the plastic parts that they are embedded in. At least 0.4 mm of the insert should lie within the mold cavity to prevent sink marks in the final product. Also, it’s recommended that you maintain the thickness of the molding beneath the insert at one-sixth of the insert’s diameter or more.

Suitable Boss Diameter

Injection molding Bosses

At its core, a boss is a structural feature — a cylindrical protrusion with holes to accommodate threaded inserts and screws, improving the strength of fracture-prone features. This common design element primarily improves the strength of fracture-prone features. It also aids in the assembly of insert molded parts.

The dimensions of a boss will affect its performance. Its diameter should be 1.5 times the insert’s diameter for standard applications. Opting for a smaller boss can compromise part strength while larger bosses may lead to unnecessary expenses.

Use Durable Resins

Inserted molded parts are highly susceptible to delayed splitting in the plastic bordering the metal inserts. One half of the component may separate from the other as the injected plastic cools and then shrinks. An easy way of mitigating this issue is opting for a stronger, more durable resin.

Consider Metal Bonding

With insert molding design, you need to take metal bonding into account. Unlike plastic resins which can form chemical bonds with each other, metal inserts cannot fuse with the molded plastic on a chemical level. Therefore, engineers and designers utilize mechanical bonds between metal inserts and molded plastic to form a functional part.

Wall Thickness of the Mold

injection molding wall thickness

The wall thickness of the mold impacts crucial aspects such as production costs, aesthetics, and part performance. If insert molding walls are too thin, you’ll need to apply excessive injection pressure. Also, overly thin walls can cause air traps that affect the quality and functionality of molded parts. On the other hand, overly thick insert molding walls will lead to additional manufacturing costs. The ideal wall thickness for an insert molded part is between 0.08 and 0.16 inches.

Insert Molding Process Guide

Insert molding is a specialized manufacturing technique within the standard injection molding process. As we’ve mentioned, it installs an insert within melted plastic forming an integrated part. Here is a step-by-step breakdown of the insert injection molding process.

Preparing the Tools

The first vital step is the preparation of the necessary machinery and tooling. This involves setting the machining parameters and modes based on the part design. Insert molding machines typically have tight tolerances that yield high-precision components.

Loading the Inserts

Once the machine is ready, the operator places a pre-fabricated insert into the mold cavity. This process can be either automated or done by hand. Automated placement is more efficient, less prone to human errors, and ideal for mass production. Conversely, manual placement increases the cycle time and is more suited to small-volume production.

Inserts can be made from a variety of materials ranging from metals to ceramics to wood. The choice of material will depend on the intended application of the product. Common inserts used in plastic molding are metals such as stainless steel, brass, and aluminum. The inserts are typically threaded, an aspect that improves adhesion.


With the insert in place, molten plastic is injected into the mold cavity under high pressure. The mold closes once the cavity is full. In the cooling phase, the plastic resin solidifies around the insert, securing its position firmly. Next, the mold opens and the molded part is retrieved.

Molded parts may require post-production processes such as deburring, heat treatment, and laser engraving to satisfy final product requirements.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.