The challenge with material selection is that there are literally thousands of materials to choose from, and new ones are developed practically every week. That’s why when we are evaluating a new overmolding project, we go straight to our suppliers and make sure we have the best materials for the job. Most plastics manufacturers have application engineers to evaluate each application and stay in touch with the latest developments. There is no reason not to leverage their experience and knowledge. At the same time, we want to understand some general factors in material selection (like adhesion), as well as having a solid grasp of material properties (like hardness and friction).
When selecting material, both the substrate and the overmold material should be considered. TPE plastic comes in several variations based on physical properties like elasticity and hardness, as well as its ability to adhere to other plastics. For example, one TPE could “feel” the same as another but may not adhere well to a polypropylene substrate. Here are some factors to consider:
- Bonding– Some Materials will bond together when overmolded, and others will not. A complete bond is the ideal situation, although it is not always necessary. If the materials can’t bond together or don’t need to, mechanical features like undercuts and indents can be designed to ensure a long-lasting connection.
- Temperature– Since you are molding plastic over plastic, the substrate must not deform under the heat and pressure of the injection molding process.
Material properties are important for applications requiring specific functions (like vibration absorption or grip). For overmolding TPE materials, there are a few properties that are key:
- Tensile Related Properties – Most data sheets will have several properties related to tensile strength, elongation, tensile modulus, etc. Tensile properties address how well the material performs under stretching conditions. If your project requires the overmolded features to stretch and return properly, this is an important property. You will also want to look at the gate location and the direction of plastic flow as there could be a drastic difference in tensile properties with and against the plastic flow.
- Compression Properties – Since TPE plastics are most commonly used because of their flexibility, it could be important for your project to understand how much the part can be compressed without permanent damage.
- Friction Coefficient – Friction is the amount of force it takes to move one surface across another. The coefficient of friction varies with different types of TPE. The surface texture of the mold is also a factor.
- Hardness – The hardness property is a material’s ability to resist indentation. Because the hardness of different materials can vary so drastically (think about TPE vs. hardened steel), different scales are used. If you are comparing two materials, make sure that they are on the same scale. While the “hardness” of a material is commonly associated with how a TPE plastic feels, adding in the flexural modulus gives the full picture.
- Flexural Modulus – A material’s resistance to bending is the flexural modulus. This property is the second factor in the “feel” of TPE and should be considered for grip features.
- Material Thickness – From a design perspective, the thickness of the material will alter the effectiveness of the material properties to perform. A thicker wall section of TPE will absorb more vibration and feel softer. Having a thin wall section of TPE will diminish the material’s ability to absorb vibration.
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