Insert Molding

insert molding

Manufacturing processes grow more sophisticated all the time, and applying new and innovative methods reduce time and cost for the end product.

Insert injection molding, also called overmolding, can accomplish these objectives, thanks to its uniquely simple technique and vast number of applications. Central to this method are injection mold inserts, which provide a framework for the process.

To remove some of the mystery about insert molding, our plastic molding company provides this explanation of the process, benefits and typical applications of injection mold inserts.

What Is Insert Molding?

You may be familiar with plastic injection molding, which pushes hot plastic into a specially designed mold to form new components. Insert molding involves the injection of plastic into plastic injection molds that contain pre-placed injection mold inserts or substrates. This produces a single molded plastic piece with the insert components permanently molded into the part.

Overmolded inserts or substrates may consist of different kinds of other plastic parts, metal components, or possibly even fabrics. The insert or substrate applications range from simple injection molded products like threaded fasteners to complex items like decorative labels or electronic components.

What is the Difference Between Insert Molding vs. Overmolding?

Many people use the two terms interchangeably. They are both forms of multi-material injection molding, but they each describe different processes suitable for specific applications.

  • Insert molding involves placing an insert, usually made of metal, in the tool. When the injected plastic flows around the insert, it becomes a part of the finished piece. The encased metal is either placed by hand or machine depending on the precision level necessary. This method can help reduce the number of assembly steps required by combining the addition of components of different materials in the injection molding process.
  • By contrast, overmolding combines the plastic part with different materials using an overlaying technique. The overlay is in the form of a rubber-based substance that coats the rigid plastic part during the injection process. This method enables manufacturers to add a skin-tight coating to products without adding steps to the production procedure.

How Does This Process Work?

threaded injection mold inserts for plastic molding

As the name implies, inserts or substrates are placed into the injection mold before the mold closes. The component to be overmolded must be located and held in place properly inside the mold before plastic is injected, and that can be done in various ways including mechanical pockets or bosses, magnets, or even static charging.

After the mold closes the plastic is injected into the mold to encapsulate the intended areas of the insert or substrate. The injection mold is designed to prevent plastic from covering any areas that should be exposed after the injection molding process. The end result is a multi-component assembly that is now a single piece.

What Benefits Does This Method Offer?

Producing insert molded assemblies offers many advantages. It’s a flexible and effective way to blend multiple materials and components into one simple piece in a single step.

Additionally, overmolding with unique combinations of metals and plastics can incorporate the strength of metal with the light weight of plastics. As a result, the end product can be made at lower costs without any loss of performance.

Who Uses Insert Injection Molding?

Insert injection molding is common in a vast number of industries. Its easy to find a lot of applications in Automotive, electronics, industrial, defense, medical and aerospace.

One of the most common applications in these industries are molding around threaded inserts. (See above)

What Are Examples of Insert Overmolded Products?

A commonplace use of insert injection molding is creating plastic-covered metal cylinders to bolt pieces together and reduce stress on joined plastic parts. Threaded metal tubes or bracketed mounts minimize the amount of stress borne by the plastic piece. These simple compression limiters are ideally suited for insert injection molding since, to work correctly, the plastic overmold must tightly couple to the metal cylinder.

When considering the overmolding process as a whole, however, a wide range of insert materials are possible.

  • flexible circuits
  • tubing
  • needles
  • custom stampings
  • connectors
  • bushings
  • rubber parts
  • magnets
  • drill bits

The variety of inserts available can aid in designing many custom and unique products. Creative designers continuously visualize new ways to employ overmolding to improve existing product quality, reduce manufacturing costs and meet customer performance demands.

What Are Some Manufacturing Hurdles For Insert Injection Molding?

Insert injection molding and overmolding are not without challenges. Careful design and working with injection molding experts who have experience with possible pitfalls will help minimize potential problems with the process.

For example, knowing the precise preload shut-off tolerances for various threaded insert types is required to stop plastic from leaking into the threads.

When parts require bosses to act as a receptacle for a screw or threaded insert, warping can occur where different thicknesses meet. A thorough understanding of the materials’ shrink rate is necessary to allow and adjust for these and other molding issues.

Other problems can occur when overmolded inserts cannot be shut-off on both sides of the mold. When multiple inserts are present in a product design, each one represents a potential for flashing of the plastic into the metal. Again, in-depth knowledge of the materials and molding processes can help offset issues at the beginning of the procedure to prevent having to scrap parts.

What Unexpected Costs and Delays Can Occur in Production?

Strictly adhere to standard design guidelines to avoid costly delays. Failure to consider special design requirements for overmolded parts could result in expensive fixes to parts once produced.

  • Materials: First, particular attention to materials is warranted. Substances with low shrink rates perform better and have a greater success rate. Lean toward nylons and polycarbonates, and try to avoid polypropylene and polyethylene. The insert material must be able to withstand the injection process without distorting. Stainless steel, copper, brass and ceramic are all suitable insert materials. Inserts that move under pressure, are too thin or do not have a grip to hold in place can be problematic.
  • Mold Depth: Try to keep the depth of the mold at least 1/6 the diameter of the insert. Bosses should be 1.5 times the insert diameter to avoid warping or shrinkage.
  • Design Guidelines: Failure to follow design guidelines usually result in parts that contain defects. Working with injection mold specialists with extensive experience in heading off problems before they occur is your best chance for a successful run on your insert injection molding project.

Your Custom Parts Solution

Thanks for reading our brief explanation on insert injection molding. At MSI, we specialize in manufacturing high quality custom plastic parts that always meet our customer expectations. We focus on lean manufacturing and 5S methods to maximize productivity, reduce waste, and continuously improve production efficiency and safety.

We look forward to helping you bring your custom parts to life. To get started, please request a quote through our convenient online form or call us.