In our daily lives, You may see that some plastic parts have different colors joined together and some have plastic and rubber joined together, these products look very nice and have clean joint areas, do you know how these products are manufactured? it is an injection overmolding process. Today, in this article, we will share a guide and explain the everything about overmolding process.
What Is Overmolding?
Overmolding uses a multi-step injection molding process to make one component or layer over top of another component or layer. it is sometimes called two-shot molding because it use two step to inject the final product.
How Does Overmolding Work?
Manual Insert Overmolding
In manual insert overmolding, the base component is injection molded at first by a traditional single-shot mold. then, put the base component into another mold and fix it in the mold for overmolding. Lastly, inject the second component over the top of the base component, you will get your final product.
Two-Shot Overmolding
Two-shot overmolding uses rotated molds to inject the base component at one side and then rotate it 180 degrees to transfer it to the other half to inject the second component over the top of the base component. this method can make the final product quickly because it can make the two shots at the same time, below is a picture of two-shot overmolding.
Advantages Of Overmolding
Design Flexibility
Overmolding can save much space between two components which can give more room for the design. without clip structure, designers can use this area for better function use.
Cost Savings
Overmolding Joined the two components together can reduce the cost of producing two separate components.
Improved Strength
Traditionally, two components need to be fixed together by a clip or screws which is not robust for the whole product’s strength, overmolding uses chemical bonding to stick the two components together, which can have a much better strength of the final product.
Enhanced Appearance
there will be no gap between the two components after using the overmolding process, this will make the products look much better.
Materials Guide for Overmolding
- Choose materials with good bonding compatibility for the base component and the second component. below is the materials combination matrix guide.
- The difference in melting temperature between the two component materials should not exceed 30~60°C.
- The coefficient of thermal expansion (CTE) of materials of the two components needs to be similar.
- The shrinkage rate of the two components of the materials must be closed.
- The heat deformation temperature (HDT) of the base component material needs to be higher to avoid deformation when injecting the second component.
Design Guidelines for Overmolding
Bonding Surfaces
- Increase the area of the bonding surface to improve bonding strength, as shown in the picture below.
- When the bonding surfaces are too small, use mechanical interlocking structures to improve bonding strength.
Design Matching Surfaces
When the joint area between two components is visible, to avoid burrs in the joint area during the injection, make a step as below pictures to let the product surface match with tooling steel perfectly to avoid burrs.
Keep Uniform Wall Thickness
Make uniform wall thickness in all the areas including corners to avoid shrinkage. this guideline works for all injection molding processes including overmolding.
Ensure a sufficient draft angle
Ensure a sufficient draft angle as in the below picture, this is to allow the part to adhere to the rotating core during the overmolding process and easy to depart from the cavity.
Common Examples Of Overmolding
- The automotive lighting outer lens as below uses an overmolding process to make it look nice.
- Handheld products such as toothbrush handles, power tool grips, wrenches, and thermos cups use overmolding to let you feel better when you hold them.
- Products with different color appearance requirements such as keyboards, use an overmolding process for both aesthetics and durability purposes.
- overmolding is also commonly used for plastic and rubber joining for those Products that require waterproofing.
Overmolding vs Insert Molding
Similar to overmolding, insert molding requires a part to be placed in the cavity of an injection molding tool so the plastic can be molded around it. The main difference between these two processes is that overmolding means molding a plastic layer over an existing plastic component while insert molding means molding plastic around a metal component.
You may also interested in our article: What Is Insert Molding Process?
Conclusion
Overmolding is popular, but you have to know how it works before starting a project with this process. KUSLA has many years experiences of injection molding prototyping process including overmolding. you can trust us at KUSLA for the best injection molding prototyping service.
FAQ
What is the difference between overmolding and 2K molding?
The main difference is the number of Materials Used. 2K molding generally uses two different materials for one part while overmolding can use only one material or two materials for one part.
What is the thickness of overmolding?
Generally from 2mm to 4mm to avoid sink marks in the injection molding process.
