Tool Material Options
Author: Krissy Dempsey - Marketing Manager
Silicone molding tooling can be made from a variety of materials depending on the quantity and quality of parts needed, among other criteria. The typical materials used are aluminum or steel. Steel tooling is capable of being produced out of pre-hardened or fully hardened material. Factors to evaluate include: price, time, processing conditions, and part geometry.
Price: As far as price, aluminum tends to be a cheaper option than its steel counterpart. However, you will get a smaller, limited number of total parts from aluminum tooling while steel tooling can produce much higher quantities. Depending on material, aluminum molds can produce <10K shots before needing a complete refurbishment. Average quality steel mold can produce ~500K shots and high quality steel molds can produce more than 1M shots. Specific consideration is needed with regards to part geometry. As the part reduces in size, especially within the micro range, steel tooling is recommended. Often times with prototype tools, the part will be molded laying down (customer permitting or geometry requiring) which may require the tool to shut off around a core pin. Maintaining shutoff around a core with aluminum would be very difficult and not last many cycles.
Time: With today’s advancements in tool manufacturing equipment such as high speed machining and cutter technology, both materials can be machined in approximately the same amount of time. However, when considering mold texture/finish, steel can be finished-machined while aluminum may require hand-polishing thus taking more time.
Processing Conditions: Silicone can flash at three ten-thousands of an inch and thus tooling must shut off very precisely to prevent flash from occurring. In the event that flash does happen, it can sometimes remain at the parting line of the tool. Aluminum is a softer material and should it close up on remnant flash, it can crush or deform. This will continue to perpetuate the poor shut off condition as molding continues. Steel is much harder, typically 50-54 Rc versus 20Rc with aluminum. Steel does not deform and can help to keep a crisp parting line, keeping flash to a minimum over many shots. Also, aluminum and steel have different values of thermal conductivity. The shrink will be different; the cure temperature and time will be different, etc. Therefore in a prototyping phase, if aluminum is used for prototyping while steel is planned for production, the molder will not have the opportunity to learn as much as he could if he was utilizing the same tool material for both phases. Finally, aluminum can only handle the heat and pressures required of silicone molding for a limited period of time. This is okay if you’re only interested in a small number of parts; however, if you are hoping for a larger number of parts, steel may be the better choice.
All in all, there are a few factors to consider when deciding which material to use to create your tooling for silicone molding. Lead time to machine the tooling is similar for both aluminum and steel. Aluminum is typically cheaper. Steel will be more robust creating larger quantities of parts with better quality.