Plastic Over-Molding
Plastic over molding is a multiple-step injection molding process, that adds additional plastic layers (thermoplastic elastomer ) over the molded thermoplastic substrates. This provides a combination of characteristics that no single plastic material can offer. Additionally, the same plastic materials with different appearances or cosmetics are also easy to finish in plastic overmolding.
The overmolding manufacturing process provides excellent adhesion between two different plastic materials and eliminates the need for hand assembly.
Common Overmolding Plastics
There are thousands of possible combinations of plastic substrates and overmolded resins. There are a few common possibilities in the following chart:
| Overmolded Materials | Substrate Materials | ||||
| ABS | PC | PBT | PP | Nylon | |
| TPU | C | C | C | M | M |
| TPV | M | M | M | C | M |
| TPE | M | M | M | C | M |
| TPC | C | C | C | M | M |
| M: Mechanical Bonding C: Chemical Bonding | |||||
However, if you require special characteristics, there are also other combinations for selection. To maximize the adhesion of overmolding, plastic substrates should be handled carefully to prevent contamination of the surface.
Bonding in Overmolding
There are two main bonds in the plastic overmolding process: one is chemical bonding at the interface of two resin layers, and the other is the mechanical bonding by the physical geometry at the interface. These two bonds are achieved by the combination of material selection, part design, tooling design, and molding process.
Chemical Bonding is at the molecular level, and it is affected by different factors. The first is the wetting ability of the substrate by the injected overmolded materials. Better wetting provides more contact between the two plastic materials for bonding. Wetting is affected by the material temperatures, viscosity of overmolded resins, texture, and porosity of substrate surfaces.
Mechanical Bonding can replace or be combined with chemical bonding. The overmolded resins can flow into the holes with dovetails in the substrates, and then the cooled resins are locked to the substrates. There are different methods to enhance mechanical bonding: wrapping the overmolded resins, increasing the surface area of the interface with grooves, pickets, posts, or bosses.
Benefit of Plastic Overmolding
Cost Reduction
Plastic overmolding can produce a single part with different materials without assembly requirements. Although mold production in overmolding is more complicated, it also eliminates the assembly cost of thousands of parts.
Unique Properties
- Creating a waterproof seal for combined parts, which is difficult to achieve by separate molding.
- Improve grip and overall ergonomics for products.
- Vibration dampening for absorption of both vibrations and sounds.
- Significant customization within color and aesthetic themes.
- Flexible mechanical features for fitment and function.
Overmolding Applications
Overmolding is applied widely in different industries, from consumer products to electronic and automotive components. Additionally, it is particularly suitable for medical and healthcare applications.
- Consumer goods: multiple-color objects, bowling pins, storage bins, plastic containers, toothbrushes, phone cases, thumb drives, and plastic housings.
- Automotive industry: electrical sensors, fasteners, door panels, dash panels, handles, knobs.
- Medical devices: surgical instruments, diagnostic devices, implantable devices.
- Beauty industry: cosmetic packaging with unique surface texture and multiple colors, compacts, makeup brushes, perfume bottles.
