The Casco Bay team enjoys developing two-component or over-molded silicone parts and offers the advantage of being able to injection mold both the thermoplastic and silicone rubber component parts in one factory, quickening the product development cycle while avoiding “two-vendor finger pointing.”
Our team’s liquid silicone rubber (LSR) over-molding / two-component experience includes: silicone spatulas: silicone over-molded onto a plastic handle, two-component silicone military products, silicone molded onto a precision metal plate for diode fabrication, food processing parts: silicone seal over-molding onto a polysulfone base, labware applications with silicone rubber gaskets over-molded onto Ultem and Valox parts, medical and dental handles with silicone over-molded onto a stainless steel tube, filtration applications with silicone gaskets molded to a plastic screen, and silicone over-molded onto a glass slide.
The over-molded LSR product development cycle starts by understanding the application and part’s environment. Below please find a few tips for a successful two-component / over-molding project outcome:
- Part Design is #1: Understanding the LSR and the thermoplastics processes, and both material's capabilities. The simplest designs are the most elegant.
- Material Selection: Work with Casco Bay Molding to define which thermoplastic can handle the heat of an LSR injection mold. A few acceptable thermoplastic examples are injection-molded Nylon, SPS – Syndiotactic Polystyrene: Valox – PBT, Polycarbonate, Radel & Polysulfone. Glass filled versions of these resins aid in silicone’s chemical bond.
- Bonding Type: The most reliable results are achieved with both a mechanical and chemical bond. Mechanical design is dictated by a good part design. Chemical bonding is facilitated with a priming agent and/or a surface treatment with an ample amount of surface area.
- Tooling: We frequently get our silicone molding team and tooling engineers involved early in the process once an order is received. Care must be taken to address potential variation in shrinkage in the X-Y-Z directions.
- Production Approach: High or Low volume? The answer can dictate the approach. High volumes can permit a fully automatic production with a two-component molding machine, while with a low volume approach, most frequently, machine operators insert plastic or metal part manually.
- Prototyping Over-molded Parts: If a project is of a technical nature and high volume, it behooves the customer to invest in a single cavity silicone short run / prototype injection mold. Casco Bay frequently will build a single cavity in a four cavity mold base, enabling us to adjust the customer part design on a single cavity before blindly investing in high volume production.
- Manufacturability & Tolerances: Design a part with real world tolerances, define non-critical areas were silicone and plastic can be gated and vented.