This article discusses ways to set up new projects for success and covers ideas for material selection, dimension and tolerance, and critical feature selection. Designing a silicone component can be a challenge when trying to balance design for manufacturability and the optimum design for end use. Working closely with your customers on the front end of a project offering your knowledge in silicone processing and best design for manufacturability is the key to success for each new project.
There are three key rules that should be followed before taking on any new project. They are:
- Know your company’s capabilities and know the capabilities of the materials you work with.
- Always understand the specifications and requirements of any new project before starting development.
- Always align specifications and requirements with capabilities before starting development.
By applying these three rules at the start of each new project, you have taken the first step to setting yourself up for success.
Choosing the correct material for your application is important and can have an effect on the performance and cost of the project. There are a few things to consider when deciding on a silicone to use for manufacturing. These include type of silicone (liquid silicone rubber or high consistency rubber), durometer, and clear or colored. Each of these can have an impact on its manufacturability. (See Figure 1)
Liquid Silicone Rubber (LSR) vs. High Consistency Rubber (HCR): Both LSR and HCR are available in differing durometers (hardness). Of the two, LSR is the preferred silicone for manufacturability. LSR can be molded faster due to a few factors. It a lower viscosity than HCR, therefore, it can be injected into the mold faster as well as cured faster. Both of these factors mean that a manufacturing cycle for LSR can be significantly shorter than that of HCR. The majority of HCR parts also need a post cure, which is a secondary operation and can add cost to the price of a part.
When manufacturing a part with complex geometries, a material with a low viscosity is recommended so that the detailed features are consistently and accurately captured. As mentioned above, LSR has a much lower viscosity, making it the more desirable material for these applications.
Useful information for designing with either silicone is the shrink rate. LSR has a typical shrink rate of 2.5 percent to 4.0 percent and HCR has a typical shrink rate of 1.5 percent to 3.0 percent. Some factors that can affect shrink rate are durometer, lot to lot variation in the material, additives/colorants, the manufacturing process, gate/vent size, and material flow. While shrink rates don’t typically affect the manufacturing process, these rates are used in mold design.
Soft or Firm Durometer: Silicones for manufacturing are available in durometers ranging from 5 to 80 Shore A. Durometer has a significant impact on manufacturability at all stages. Parts that are made with very soft or very firm silicones can be difficult to remove from the mold. Soft parts will want to stick to the mold surfaces more while parts made with firm silicone are more brittle and may tear or break during removal. For manufacturability purposes, we recommended using a silicone with a durometer between 30 and 70 Shore A.
Clear or Colored? Colorants come in in a wide variety of colors and can be mixed into LSR or HCR materials at very precise percentages. Adding color to your part can be beneficial for many reasons. Some examples include:
Differentiation: If you have similar parts where it’s hard to tell the difference visually, make them different colors.
Visibility: If you have very small or micro size parts, color can make them easier to see and to handle.
Cosmetic reasons: If you don’t want to reject for very small fully encapsulated bubbles or foreign material in a thick wall area, add color to disguise them.
Measurement: Color can improve accuracy and repeatability when using a non-contact measurement process.
Dimensioning and Tolerances
The dimensioning and tolerances of a silicone part can be a make or break factor in the success of a new project. The application of dimensions, selection of critical dimensions, and size of tolerances are the major categories and will be explained in depth in the following paragraphs. The main things to keep in mind when dimensioning a silicone part are to: apply dimensions to silicone, not the spaces between silicone. Tolerances should be a minimum of 2.5 percent of the dimension or ±.003", whichever is greater. Critical dimensions should be applied to rigid features so they can be easily measured repeatedly. (See Figure 2)
Application of Dimensions: While anything can be dimensioned on paper or mathematically, it may be difficult or impossible to measure accurately and repeatedly. Some examples include, but aren’t limited to:
- Radii that are less than 90° of a circle;
- Angles that have reference surfaces of less than .010";
- Referencing to theoretical transitions, such as a transition point from a flat surface to a radius;
- Dimensioning across spaces between silicone; and
- Referencing theoretical planes/surfaces in the use of GD&T.
If there is a situation when these types of dimensions need to be applied, it is a good practice to make them reference dimensions if possible. This way, the development of a project won’t be slowed when the data doesn’t meet certain statistical standards. Making them reference dimensions also voids the application of tolerances. (See Figure 3)
Selection of Critical Dimensions
Critical features are typically those that will be measured in production to ensure continued quality assurance. They also must meet higher statistical requirements than non-critical dimensions during development. The success of a project can hinge on the selection of critical dimensions. When selecting critical dimensions, there are some important things to consider.
A critical dimension located in a rigid area of a part will prove to be more successful. If a rigid feature is being measured, the methodology will be easier to develop. Typically, less complex fixturing will be required. This can speed up measurement times and decrease the amount of fixturing required. Measuring rigid features will also yield higher repeatability, which will be reflected in improved statistical results.
This article was written by Jason Nelson, New Product Development Tech Center Manager, ProMed Molded Products, Inc., Minneapolis, MN. For more information, Click Here .