3 Tips For Specifying Silicone Tolerances

By Phil LaRoy, Vesta

The best part of my job is helping medical device manufacturers use molded silicone components in their innovations. After years of working on these collaborative projects, I’ve noticed that many medical device designers think silicone molding is basically the same as thermoplastics molding, but the two processes are actually quite different. Specifying optimal tolerances for silicone molding requires special consideration. In this article, I’ll offer three tips to help you understand silicone tolerances and facilitate the success of your next silicone medical device component.

1. Refer to the Rubber Manufacturing Association (RMA) manual when specifying tolerances. The RMA manual provides general guidelines for specifying the tolerances for molded components made of silicone rubber. Medical product tolerances most often fall under the association’s A1 high-precision tolerance category. Using these tolerance ranges as a starting point in your designs will help you keep your tolerance requirements achievable and economical. Tolerances outside this range may be difficult to meet, causing delays and adding expense to product development and production.
2. Find out how your design and the manufacturing process may impact your tolerances. Understanding the potential issues before you begin production will save you time, expense and frustration down the line. If necessary, consult with an expert who can tell you how the following factors will impact the ability to meet your tolerances:
   • Molding technique
   • Geometry of the component
   • Material shrink rate
   • Impact, if the product is overmolded on a substrate
3. Use the right tooling. Surprisingly, silicone is an abrasive material and causes significant wear on your tools during the manufacturing process. Good tool maintenance practices are essential for consistently meeting your tolerance requirements. To maximize the life of your tooling, you must use the right type for your product and manufacturing needs. Use the following categories as a general guideline when selecting the best tooling for your next silicone molded component:
   • Soft Tooling
     • ~24RC
     • Typically used for prototype products
     • 1,000 cycle guarantee
   • Pre-hard Production Tooling
     • 33RC–50RC, depending on the steel grade
     • Used for production tooling
     • 10,000–100,000 cycle guarantee, depending on the geometry of the component
   • High-precision / High-yield Production Tooling
     • Pre-hard base (33RC–50RC)
     • Full hard cavity inserts (60RC–61RC)
     • 250,000+ cycle guarantee

I hope these tips help you the next time you begin designing a silicone medical device component. If you have any questions along the way, I’d be happy to answer them for you. After all, helping customers is what I love best about being a project engineer at Vesta. Feel free to contact me anytime at plaroy@vestainc.com

Ask an Engineer

Question:
What do I need to consider when determining tolerances for my thermoplastic component?

Answer:
To specify optimal tolerances for thermoplastic extrusion, evaluate the following five factors:

1. Critical Features
   Consider which features of your component are the most critical to the functionality of the medical device. Component dimensions directly impact the final product’s performance, so be sure to set appropriate tolerances for these dimensions to ensure functionality at both the low and high end of the tolerance.
2. Secondary Operations
   Consider any secondary operations or automated manufacturing processes that may be required after extrusion. Assign tolerances that allow for the repeatability and reproducability of these additional operations.
3. Tolerance Stack-up
   Consider any tolerance stack-up issues that could lead to yield loss. For example, a marker band with an ID of 0.040"±0.002" that fits over a shaft will require the shaft to have an upper-end tolerance that is less than the lower-end tolerance of the marker band.
4. Inspection Method
   Consider the metrology equipment available to measure your component dimensions. If you specify a tolerance that is too tight and doesn’t have the appropriate inspection equipment to accurately measure, you risk false acceptance or false rejection of your component.
5. Tolerance Range
   Consider the range of tolerances you specify. Overly tight tolerances may affect the yields or possibly require the supplier to conduct inspections, both of which will increase the cost of the product. You should assign the widest tolerance range the product can accept.

Question:
What is post-curing and when should I consider it for my product?

Answer:
Post-curing is the process of “baking” a product for a specific time and temperature in an industrial oven once the molding process is complete. Post-curing times can range from 2–16 hours at temperature of 250–400°F.

Post-curing is most often used to remove volatiles from the silicone material and to stabilize the properties of the silicone material. Post-curing is not required for all molded silicone components. Product specifics such as part geometry, end-use, material composition, process settings and physical property requirements should be considered when deciding if post-curing is necessary for your product.