Justin Strike/Program Manager/Sil-Pro
Does your device have sufficient mechanical interfacing to meet performance requirements? Can plasma or corona treatments enhance product performance? Will mechanical surface prepping, chemical adhesion or a combination of both processes be necessary for your product? Overmolding or insert molding is ideal for projects with complex final molded component designs. Overmolding offers extended functionality and aesthetic enhancements that can differentiate your products from the competition. In medical devices, overmolding adds complexity and development time to new products. There is often an increase in up-front tooling costs and handling steps, but overmolding can be a more favorable solution with minimal impact to overall price. Overmolding is defined as the placing of a previously formed component (molded, machined, extruded, etc.) into an injection molding tool and the process to form a composite part assembly once both parts are removed from the mold. The forming methods for the inserts in overmolding applications are limitless. Typical overmolding uses machined metal, injection molded components or extruded components.
Molders use many different materials for several different applications such as radio-opaque inserts in silicone, glass-filled nylon over stainless-steel inserts, silicone over PEEK, polyurethanes over PC, silicone on PC and silicone over silicone.
There are requirements to consider when identifying the materials and method of manufacture for the insert of an overmolded product. The requirements are defined by the secondary material and molding processes. These requirements include temperature of tool, structural deformation around shut-offs and deformation from secondary material shrink. Several bonding methods are less costly to develop but they each have their own challenges. Adhesive bonding often results in a compromise of aesthetic control versus quality of bond. Sonic welding can limit your choice of materials and restrict design options. Bonding or sonic welding of silicone or soft/flexible polymers is very difficult to achieve. Overmolding with silicone or flexible thermoplastic materials is much more common.
When designing for an overmold solution it’s important to understand that the second-shot steel does not move, so the shut-off surfaces of the first shot will need to be repeatable. For a successful overmold, you may need to design the second-shot tool and process first. Items to consider are the second-shot material, shut-off surfaces, areas of flash and gate location. There are benefits to increasing the surface area contact of the second material’s flow to minimize the steel-to-part interface. All surfaces should be clean and free of debris and chemicals. Cleaning with isopropyl alcohol (IPA) can enhance the overmolding process resulting in a repeatable bond. Other treatments such as media blasting, corona, plasma and priming have also proven effective in the cleaning process. Overmolding can add complexity to development, but it can be overcome with proper tooling and design. The proper selection of materials, gate locations, shut-offs and locating features in the second-shot tool design can help reduce the impact of variation of the first shot and reduce the variable stack up that delays product development.