Overmolding is a process where one can join two different plastics during the molding process without the use of adhesives or primers. Good examples of overmolded products are the soft-handled toothbrush or razors and soft grip power tools. In these applications one desires a soft, pliable outer surface; however, a rigid interior structure is necessary for proper strength. These two different properties come from two different materials that are adhered together in a molding operation.
The driving factor for overmolding is not technical or even economic, but it is related to consumer preference. Many consumers prefer the "warm" soft touch feel over a hard plastic feel. The soft feel is often perceived to be of higher value. Overmolded parts can also provide a safe grip in wet environments, vibration damping, gasketing, and a variety of other functions.
Since an adhesive is not used, overmolding operations are somewhat limited in that the two plastics must be chemically compatible. However, there is an incredible range of thermoplastic elastomer (TPE) materials that can be used as the outer surface. The TPE is injection molded over or around a compatible substrate using either insert or multi-shot molding processes.
Although adhesives are not used, good adhesion is necessary between the TPE and the interior plastic or else problems such as peeling and delamination could occur. In early overmolded products this was not an issue since TPE over polypropylene was the most common material used. Today, overmolding has expanded to many different materials. These include different formulation of TPE, thermoplastic polyurethane, copolyesters, copolyamides, and styrene block copolymers. The rigid interior structure can be ABS, polycarbonate, nylon, or a variety of other materials.
The adhesion between soft elastomer and hard engineering plastic depends on many variables. Matching the surface energy of both materials is critical for obtaining good wetting and adhesion. The soft elastomer should have a low melt viscosity so that it wets out the hard plastic surface and fills-in any micro-roughness on the hard plastic surface. Melt temperatures must be maintained at a level that optimizes adhesion. Manufacturers of molding compounds can formulate overmolding elastomers for adhesion.
Good bond strength is also achieved via macro-mechanical interlocking. This requires appropriate part designs. Air entrapment at the interface must be avoided by appropriate design of the part and venting of the mold.
So what if one wants to use two materials that are incompatible with regard to adhesion. Fortunately, there have been several developments in this area. If the overmolding process is an insert molding operation then a primer can be used. The primer will have functional groups that provide good bonding to both the hard and soft materials.
A more recent development is the use of plasma surface treatment with overmolding. Three German companies have worked together with the Institute of Polymer Technology at the University of Erlangen-Numberg to develop a process that uses plasma treatment inside of a mold to create a strong bond between multi-material thermoplastics that otherwise had shown poor or no adhesion. Using PlasmaTreat GmbH's patented Open-Air Plasma system to apply a plasma coating, a bond can be achieved with many low surface energy polymers without the use of a primer or adhesion promoter.
Plasma treatment is performed directly in the manufacturing process, at atmospheric pressure. For a two-material part, after the first material is injected and the mold cavity raised for injection of a second material, the mold is opened momentarily. Precisely directed plasma is then applied to the still warm surface. After the mold is reclosed, the second material is injected onto the treated surface. The plasma treatment is claimed to expand multi-material combinations, creating new design possibilities and cost savings potential.
As a result of these developments, the future for overmolding looks exceptionally positive. Expect to see more overmolded products in the sports and leisure, hand and power tools, appliance / electronics, office equipment, and personal care sectors of the economy.