Two shot, silicone-thermoplastic molding works well when placing a soft silicone cover over or in combination with a hard thermoplastic. A medical example might be the soft-grip silicone handle on a surgical instrument that has a base of rigid thermoplastic. In two-shot molding, the silicone and thermoplastic parts are molded as one. This cuts part costs, improves part performance, increases bond strength between device components, and allows freedom to design without assembly.

Molding a thermoplastic over thermoplastic is quite common. But molding a silicone over the thermoplastic has been more difficult until now. Our company has completed an extensive material matrix for the two shot, silicone-thermoplastic process and can test the compatibility of proposed materials. Furthermore, it can custom formulate materials to meet application demands.

These few tips will be useful when designing a product that might use two-shot molding.

Know your materials. Material compatibility is key in the two-shot process because not all materials bond well. The first injection places the thermoplastic material often around an insert such as the shaft of a surgical tool. It is followed by an injection of silicone, which requires high heat to cure. So the thermoplastic material must be able to withstand temperatures of 300F or more. Candidate thermoplastics that can handle the heat and bond to silicone include polycarbonate, polyester, nylon, PEEK, and ABS alloys.

Optimize the bond between the two materials. Creating opportunities for the silicone and thermoplastic parts to bond generates the best covalent-bond strength. When possible, use material interlocks that don't inhibit material flow and provide a large area for the silicone and thermoplastic materials to meet and join.

Gate locations and wall thickness. The gates for thermoplastic parts should be located at their thicker sections. This avoids sink marks and voids. It's often useful to visualize the flow path before selecting gate locations on the thermoplastic part. The thermoplastic and silicone use separate runners.

Silicone can then be used to aesthetically improve the look of gates or hide them all together. Note that the gate location for thermoplastic must allow for silicone to flow.

Limit the variations to wall thickness in the thermoplastic part. Thickness variations on a thermoplastic part increase the possibility of warping and sink marks. Silicone, on the other hand, allows for more varied but still gradual changes to wall thickness.

Know the material's shrink rate. Common rigid-thermoplastic resins shrink from 2 to 5%. Silicone shrinks about 2 to 3%. Mold temperature, cavity pressure, cure time, and flow direction all affect shrinkage. Understand the shrinkage of your materials and account for it in the tooling and part design.

Get an expert second opinion of the design work. It is critical to work with an experienced two shot, silicone-thermoplastic manufacturer from the design stage. The right manufacturer can assist in a design that will function as a device component and lend itself to two shot, silicone-thermoplastic molding. The manufacturer should also be experienced in silicone molding and material compatibility.

How a few self-bonding silicones stack up

Manufacturer	Advantages	Disadvantages
Shin-Etsu	Bonds to wide range of substrates (PC, PEEK, polyester)	• Does not bond to Nylon • Limited durometer
Momentive (formerly GE Silicones)	Bonds to polyester	• Does not bond to PEEK or Nylon • Post cure needed for PC • Poor physical properties
Wacker	Bonds to wide range of substrates (PC, Nylon)	Does not bond to PEEK or polyester
Saint-Gobain	• Bonds to polyester and selected grade of PC and PEEK • Wide range of durometers	Does not bond to certain grades of PC or PEEK