Overmolding, Insert Molding, and Bonding Polyurethane
A permanent adhesion between two materials
Why It Matters
Bonding polyurethane to a rigid material offers design options for structural reinforcement, hybrid material construction, or attachment hardware. We call the second material an “insert.” We bond polyurethane on metal, polyurethane on plastic, or polyurethane on composite inserts. Some call it polyurethane overmolding or polyurethane insert molding.
Our bonding utilizes a chemical reaction to permanently adhere polyurethane to the insert. We can also include a mechanical feature to supplement polyurethane chemical bonding. With our bonding process, we achieve bonds that are stronger than the polyurethane itself.
Polyurethane Bonding can be a critical consideration for applications like wheels (polyurethane tread bonded to a metal hub), bumpers/pads (attachment bolts bonded into the polyurethane), and suspension bushings (polyurethane bushing bonded to a metal core).
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The Gallagher Advantage
We precisely control our polyurethane over molding process to ensure bond quality and strength. We measure and control parameters like time, temperature, and surface finish to create ideal bonding chemistry. Preventing contamination is also critical. We place so much emphasis on the process because we’ve found that’s the best way to guarantee great bonding.
We verify our process control by performing non-destructive or destructive testing on a sample of production parts. Destructive tests measure bond strength by physically separating the polyurethane from the insert. Other post-production tests can be helpful, but destructive testing is the truest gauge.
How We Do It
Preparation
We prepare inserts using a sequence of intense cleaning, surface finish enhancement, and priming. As part of our process, we measure and control cleanliness, surface roughness, and primer dry film thickness.
- Cleanliness is the foundation of our bonding process because contaminants – like grease, oxidation, or even skin oils – will prevent strong bonds from forming.
- An insert’s surface finish will also impact bond strength. The right surface texture creates an anchor pattern which adds an additional, and helpful, element of shear to the bond interface. We perform abrasive grit blasting to achieve optimal surface roughness. The blasted surface also increases the bonded surface area between the insert and the polyurethane.
- Priming refers to the application of a chemical bonding agent to the insert. The bonding agent acts as a transitional reactant between the insert’s surface and the polyurethane. A uniform and precise primer thickness contributes to better bond strength.
Bonding
We create the bond primarily through a chemical reaction. As part of our process, we measure and control the time at temperature of the primed inserts.
- The reaction process begins by pre-heating the primed insert. This activates the bonding agent, bonds it to the insert, and readies it to react with the polyurethane during the subsequent molding operation.
- During molding, as the liquid polyurethane cures, it reacts with the primer to fully adhere to the insert. When complete, the bond will be stronger than the polyurethane itself.
Testing
We verify our bonds thru inspection and testing.
- Visual inspection of the bond can be sufficient for some products.
- On the other hand, other products may require that the bond strength be confirmed using special tests. We can sample production parts and perform non-destructive or destructive testing. The most common test that we do is a peel test, which measures the pull force required to separate the polyurethane from the insert. We also regularly do a shear test, where we apply shear stress to the bond line and measure for any separation.
