Select the Right TPE Materials for Your Overmolding Process
Small touches of overmolded soft-touch plastic can make a big impact on product value. Getting it right depends on the strength of the bond between hard and soft plastic – largely determined by the compatibility of the two materials. This is why material selection is so important, as well as optimizing the injection molding process. Browse our resources to help you achieve overmolding success.
FAQs
Overmolding is the process where one material, usually a TPE, is injection-molded onto a second material, usually a rigid plastic also known as the substrate.
Overmolding adds value by enhancing product features:
- Ergonomics – Provide a cushion / increase comfort level
- Safety – Improved grip, vibration damping
- Functionality – Provide a tight seal to air/water, absorb sound, insulate
Insert Molding
- Pre-molded insert is placed into a mold & TPE is shot over it
- Uses a conventional single shot injection molding machine
Advantages: (1) lower tool costs (2) uses conventional equipment
Multi-Shot or 2-Shot (2K) Molding
- Requires a specialized injection molding machine with 2 or more barrels, allowing more than 1 material to be shot during the same molding cycle
Advantages: (1) shorter cycle times (2) superior part quality (3) lower labor costs
Typically PE-based color concentrates are used with standard TPEs. However, PE may not be compatible with specialty TPEs that are formulated specifically for overmolding, and may adversely affect adhesion. Consult Teknor Apex on the best carrier resin to use.
1. Time Delay Between First and Second Shot
We recommend minimizing the delay time between the first and second shot, in order to maintain the high temperature of the substrate.
2. Injection Speed of TPE
Inject TPE as fast as possible without warping or moving the substrate.
3. Holding Pressure and Holding Pressure Time
The ideal holding time is however long it takes for the gate to freeze off. We recommend using a “Gate Freeze Analysis” - Weigh parts with different hold times and determine when the part weight doesn’t change. Determine the correct tool temperature of hard and soft component to reduce cooling time.
In the industry, several different test methods are used to determine the resistance-to-peel strength of adhesive bonds between flexible and rigid materials, so when looking at data, make sure you’re comparing apples to apples. At Teknor Apex, we use a 90°C peel test, ASTM D6862, which determines the resistance to peeling in lbf/in or N/cm. Our research shows that excellent adhesion is achieved when the peel force is greater than 15 lbf/in, and poor adhesion is exhibited by values of 10 lbf/in and under.
There are two types of failure modes:
1. Adhesive failure occurs when the TPE peels from the substrate (Material Strength > Peel Strength)
2. Cohesive failure occurs when the TPE material breaks before it peels from the substrate
(Material Strength < Peel Strength)
Cohesive is the more desired mode of failure.
Teknor Apex is your
Partner in Overmolding
The most critical challenge in overmolding is poor adhesion between the thermoplastic elastomer (TPE) and the substrate, which is typically an engineering thermoplastic (ETP). The bond is largely determined by the compatibility of the two materials, which is why material selection is so important. Take advantage of our experience in manufacturing both TPEs and ETPs, and adhesion-modifying these materials to work together for the most optimal bond strengths.
