In this seemingly simple overmolding application, the TPE material provides safety, ergonomics, and sealing functionality for the lid and handles of a sippy cup.
GLS has been a major TPE supplier for more than 25 years, providing standard and custom compounds targeted at applications across a variety of markets. According to the companyâs âOvermolding Design Guide,â the technique is defined as the injection molding process where one material (usually a TPE) is molded onto a second material (typically a rigid plastic). Excerpts from the Guide that follow detail the considerations designers should give when specifying this process. Overmolding can be used to enhance many features of product designs, including:
Safety. Improved grip in dry and wet environments; vibration damping.
Ergonomics. Increased comfort.
Functionality. Water-resistant seals; sound absorption; electrical insulation.
If properly selected, the overmolded TPE will form a strong bond with the plastic without the need for primers or adhesives between the two materials.
The most common word used to describe a soft-touch overmold is âfeel,â but the term itself is difficult to describe. When a designer wants the product to feel âgrippyâ or âsquishy,â what exactly does this mean in terms of material properties?
Basically, the âfeelâ of a soft-touch overmold depends on a combination of material properties (hardness, modulus, and coefficient of friction), texture, and TPE wall thickness.
Thickness effects. When choosing a soft-touch TPE, designers usually ask for the softest material available. What they may not know is that the soft durometer of a TPE adds little value to the concept of âcushionâ when the thickness of the TPE is less than a certain amount (typically less than .040 inch). This means that the thinner a TPE overmold is, the harder it feelsâthe actual hardness felt depends on the thickness of the TPE. One way of getting around this issue is to incorporate multiple ribs that are placed closely together to create the perception of thickness without using a large amount of material. This technique is used often in personal care grips.
Hardness vs. modulus. One common myth in the TPE industry is that the durometer (or hardness) of a material is directly related to its flexibility. This is not always true; for example, a 65 Shore A SEBS material is much more flexible than a 65 Shore A TPU. Instead of using Shore hardness, a better way to gauge flexibility is the flexural modulus, which measures a materialâs resistance to bending. A higher flexural modulus typically means that a material will feel more stiff and unyielding.
Coefficient of friction. When two surfaces are dragged flat against each other, the resulting resistance is characterized as friction. The coefficient of friction (COF) measures the degree of force required to move one surface across anotherâeither from a complete stop (static friction) or when the surface is already moving (kinetic friction). Typically, TPEs are described as rubbery or âgrippy,â ranging from smooth and silky to extremely tacky.
One area that product designers often misunderstand is the relationship between durometer and COF. Most believe that the softer the TPE, the greater the COF, but this is a very general statement and is not true in all cases. Products in the same Shore hardness range can have varying COFs.
Adhesion requirements. When selecting a TPE for an overmolding application, the substrate type should be considered. Not all TPEs bond to all types of substrates; for example, a Dynaflex TPE that bonds to polypropylene will not adhere to polycarbonate. The table on the next page indicates which TPEs will adhere to a number of rigid plastic substrates.
Ready for the short course on overmolding design? There are several factors that designers should consider when approaching an overmold project. Both part and mold design are related, so make sure these two processes are collaborative.
Watch wall thickness. The wall thickness of the substrate and overmold should be as uniform as possible for optimum cycle time. Transitions between wall thicknesses should be gradual to reduce flow problems such as backfills and gas traps.
- Wall thickness in the range of .060 to .120 inch ensures good bonding in most overmolding applications.
- If the part requires the use of thick sections, they should be cored out to minimize shrinkage problems and reduce part weight.
- The use of radiuses (.020-inch minimum) in sharp corners helps reduce localized stress. Deep, unventable blind pockets or ribs should be avoided.
- Long draws should have 3Â° to 5Â° draft to help ejection.
- Properly designed, deep undercuts are possible with the right TPE compounds. Care should be taken to minimize sharp corners. An advancing core should be used when the mold opens and the elastomer should be allowed to deflect as it is ejected.
Minimize shrinkage and warpage. Most styrenic TPEs have fairly high mold shrinkage that causes the overmolding compound to contract more than the substrate, which can result in warpage or cupping of the substrate part. This is especially true for long, thin parts, where the substrate is thinner than the overmold, and when a low-modulus substrate material is used. To manage:
- Use a higher-modulus substrate.
- Add ribs to the substrate part.
- Minimize the soft TPE thickness.
- Use a softer TPE.
- Relocate the gate to minimize the flow-length-to-thickness ratio.
Use texture. Adding texture to the TPE overmold surface is a good way to impart a unique surface feel to the product and minimize the appearance of surface defects. It should be emphasized that certain textures create a perceived hardness that may be greater (or less) than the actual hardness of the TPE. As a result, the TPE surface texture should be taken into consideration during the material selection phase of the product development process.
Reduce flash and peel. To reduce the chance of flashing the mold, or of peeling the TPE from the rigid plastic upon ejection, the overmold should be designed as follows:
- Provide a .015- to .030-inch-deep (.38- to .76-mm) groove on the substrate along the edge of the TPE overmold (Figure 1). The steel should have positive shutoff in the groove. In addition, shrinkage of both the TPE and substrate should be considered.
- When using metal or other noncompressible substrates, provide springs underneath the steel sections that shut off on the substrate to prevent flashing from a steel insert with a poor fit.
Be aware of shear. Overmolding compounds are shear-responsive, so their viscosity is reduced when they are processed at high shear rates. This allows for easy flow into thin-walled sections commonly encountered in overmolding applications. This should be considered when designing molds and setting process conditions:
1. Start with small TPE injection gates to obtain best TPE fill with minimum cosmetic gate vestige. Large gates should be avoided.
2. Gates should be located at the thickest TPE wall section.
3. Thought should be given to proper component ejection to minimize marks on the soft elastomer surface.
4. Provide adequate cooling to the TPE cavity through proper mold cooling techniques to minimize cycle time.
5. Flow ratios should not exceed 150:1 L/T as an absolute maximum for most overmolding applications.
GLS Corp., McHenry, IL