Insert Molding vs. Overmolding: What’s the Difference?

The custom injection molding process starts with a conceptualized design and ends with a uniquely manufactured product. 

If your product needs to be aesthetically pleasing or ergonomically friendly, injection molding techniques like insert molding or overmolding might be used to achieve those needs. 

It might be difficult to figure out which process is best for your custom plastic part. Here’s a break down of the insert molding and overmolding processes. 

Insert Molding

Insert molding is a process where a rigid plastic resin is injected over another material, typically metal. The metal piece is placed in a tool for the plastic to be injected around. Encapsulating the insert with plastic creates a single molded piece that’s typically stronger than one created by assembling separate pieces together. 

An example of insert molding (a white blower wheel)

This process is most commonly used for custom-machined metal parts like threaded fasteners and electrical parts. There is no chemical bonding between metal inserts and plastic, so the insert and resin components must be designed for mechanical bonding. 

Insert molding is controllable and allows better encapsulation than other methods used to mold plastic around metal parts. Molded inserts eliminate the need for a secondary insert installation process, making it more time-and-cost-efficient. 

Benefits of Insert Molding: 

  • Eliminates secondary assembly operations like gluing and fastening. 
  • Creates design functions and features that are not feasible by plastic alone. 
  • The encapsulating process can add strength and durability to parts. 


Overmolding is the process of adding an additional layer of resin to the existing plastic part. This process adds characteristics that a single piece of plastic can’t provide. 

An example of overmolding (a red caster wheel)

One common reason to use this technique is to add a soft, functional, ergonomic layer of rubber-like material (typically a thermoplastic elastomer) over a hard surface. This improves the grip of a hand-held item. 

One example of this is toothbrushes. Toothbrushes are often made from a hard plastic center with a soft plastic grip around the handle. 

Another use for over-molding is to change or enhance the appearance of a part by overmolding material of a different color or finish. 

There are two primary methods of overmolding: 

  • Two-shot molding uses a single production mold.
  • Pick-n-place molding uses two production molds where an entire batch of parts are molded. Then, they are manually placed into a second mold where the overmold resin is injected to produce the completed parts. 

Thousands of possible combinations exist for over-molded material. Resins have to be adhesive and compatible with each other in order for the process to work. 

If the goal of using overmolding is to enhance grip or increase cushioning in your product, make sure your injection molding company knows those goals. Factors like cushioning, flexibility, and friction will play into the type of resins that are used in the product.

Benefits of Overmolding:

  • It can provide a soft, non-slip grip to your product. 
  • It acts as an environmental barrier to shock, vibrations, and noise.
  • It creates colorful, visually attractive surfaces. 
  • It reduces the number of secondary steps and costs associated with them, in turn reducing the complexity of assembly. 
  • It can provide adhesion between different materials and eliminate the need to assemble different materials by hand. 

Why Choose Pioneer Plastics?

We specialize in all aspects of the process- from concept to consumer. We can design, build, produce, and distribute your part. From insert molding to overmolding and so much more, we can help you produce your custom plastic part.

Contact us today to get a quote for the production of your part. 

Injection Molding Defects and How to Avoid Them

Just like the design of the part, the mold design & manufacturing process is important to the integrity and function of the part. If not executed properly, the injection molding process can cause cosmetic defects and form defects, sometimes compromising the safety and performance of the product. With that in mind, be sure that your plastic injection molding partner takes the proper precautions with your product so that none of these types of defects occur.

Here is a list of common injection molding defects and how to avoid them:

Flow Lines

Flow lines (sometimes called flow marks) are lines commonly caused by a variation in the cooling speed of the molten plastic as it flows through the mold. They often appear in a wavy pattern and might be a slightly different color than the rest of the piece. Flow lines don’t typically impact the integrity of the piece, but they can be unsightly and unacceptable in certain products (like high-end frames for glasses).

How to Avoid Them:

Flow lines can be avoided by increasing the injection speed and pressure to ensure the material fills the mold before cooling. You can also try rounding the corners of the mold to increase wall thickness and keep flow rate consistent.


Burns can be caused by trapped air or overheated resin in the mold during injection. Excessive heat or increased plastic injection speeds can cause overheating, in turn causing the resin to burn. Burns can appear as black or brown colored spots on the edge or surface of the part.

How to Avoid Them:

To prevent burns, lower the melt temperature and mold temperature to prevent overheating, reduce the injection speed to limit the risk of an air trap, and shorten the mold cycle time so that trapped air and resin doesn’t have time to overheat. 


Most commonly heard when referring to wood that has dried unevenly, warping is a molding defect that appears when different components of the plastic part shrink unevenly. During the cooling stage of the molding process, uneven shrinkage can put stress on parts causing bending or twisting. Most often, the cause of warpage is the cooling process happening too quickly. 

How to Avoid It:

Warping can be prevented by ensuring the cooling process is long enough to prevent uneven stresses on the material and by lowering the temperature of the plastic material or mold. 


Sink marks are depressions on an otherwise flat surface of a part. These can occur when the inner part of the injection mold shrinks (or cools too slowly), pulling the outside inward before it has a chance to completely cool. 

How to Avoid It:

To avoid sinking, increase cooling time and increase holding pressure to allow the material at the part’s surface to cool. 

Air Pockets

Air pockets are trapped air bubbles in the finished part. Air bubbles are commonly caused by inadequate pressure to force air out of the mold cavity. Larger air pockets can weaken the part. 

How to Avoid Them:

To avoid air pockets in the mold, raise the injection pressure to force out trapped air pockets.

Weld Lines

A weld line is a result of weak material bonding. They appear on the surfaces of parts where the material has come back together after splitting into two or more directions in the mold. If the material bonding is weak, the overall strength of the part is lowered. 

How to Avoid Them:

Weld lines can be avoided by increasing the material temperature to prevent uneven solidification and increasing injection speed and pressure to limit cooling before the mold is filled. 


Jetting occurs in a molded part when the initial jet of molten material starts solidifying before the rest of the material fills the mold. Jetting looks like squiggly lines on the surface of the molded product. 

How to Avoid It:

Jetting can be avoided by reducing injection pressure (to prevent rapid movement of the material into the mold) and increasing material and mold temperature (to keep the material from solidifying too early). 

While there are many potential problems that can occur, partnering with an experienced custom injection molder like Pioneer Plastics should relieve your worries.

If you have an idea for a plastic product and would like to partner with a company that has been manufacturing them for over 35 years, contact us today!