Device functionality is usually the starting point when designing devices. Another element that needs to be considered when designing devices and their subsequent components: manufacturability. Part design should also be focused on the ease of manufacturing because it can help reduce cost and lead to a robust and reliable process. Several aspects should be considered regarding manufacturability: part geometry, location and shape of critical surfaces, size, and among others. These may seem like more obvious characteristics, but there are a few others that can be overlooked, but yet should be considered just as important. These are material selection, dimensioning/tolerancing, and the selection of critical dimensions. To better understand the impact, each characteristic will now be explained in detail.
Choosing the correct material for your application is important and can have an impact on the performance and cost of the component.
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 (hardness), and even color. Each of these can have an impact on manufacturability.
(LSR) Liquid Silicone Rubber vs. (HCR) High Consistency Rubber
Both LSR and HCR are available in a variety of durometers. Of the two, LSR is the preferred silicone for manufacturing. LSR can be molded faster due to a few factors. LSR has a lower viscosity than HCR, therefore it can be injected faster into the mold. This means 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 detailed features are consistently and accurately captured. LSR’s low viscosity allows it to quickly and fully fill small and intricate features in a mold, and therefore makes 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% to 4.0% and HCR has a typical shrink rate of 1.5% to 3.0%. 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.
Durometer: Soft or Firm?
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 tend to stick to the mold surfaces more while parts made with firm silicone are more brittle and may tear or break during removal. For optimal manufacturability, we recommended using a silicone with a durometer between 30 and 70 Shore A.
Clear or Colored?
Colorants come in a wide variety of hues and can be mixed into LSR or HCR materials at very precise amounts. Adding color to your part can benefit component manufacturing and assembly in several ways:
- Similar, hard to distinguish parts can be colored differently, making them easier to tell apart visually.
- Coloring very small or micro-size parts can make them easier to see and handle, especially against a white background.
- Very small, fully encapsulated bubbles or incursions of foreign material in a thick wall area are more easily hidden in colored silicone than in clear silicone. This can reduce the quantity of parts rejected solely for these minor cosmetic defects.
- Colored silicone can improve the accuracy and repeatability of measurements obtained from non-contact (i.e. optical) measurement processes.
Dimensioning and Tolerances
The dimensioning and tolerances of a silicone part can make or break a new project. The application of dimensions, selection of critical dimensions, and size of tolerances are all key to manufacturing success. The main things to keep in mind when dimensioning a silicone part are to apply dimensions to silicone (not to the spaces between silicone), and keep tolerances to a minimum of 2.5% of the dimension or ±.003 inches, whichever is greater.
Application of Dimensions
While anything can be dimensioned on paper, not everything may be practical or even possible to measure accurately and repeatably. Examples include:
- 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.
- 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. Although doing so voids the application of tolerances, the development of a project won’t be slowed when the inspection data doesn’t meet certain statistical standards.
Selection of Critical Dimensions
Critical features are typically those that will be measured in production as part of continued quality assurance. They must also meet higher statistical requirements than non-critical dimensions used 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 more likely to be measured within specification than one located in a more pliable region. Also, if a rigid feature is being measured, the methodology used to measure it will be easier to develop. Typically, simpler fixtures will be required, and both the measurement times and number of fixtures required will be decreased. Measuring rigid features will also yield more consistent data, which will be reflected in improved statistical results.
Successful silicone molded components must not only perform as intended, but must also be designed from the beginning to be manufacturable. By making the right material, color, durometer, dimension, and tolerance choices OEMs can develop molded devices and components that can be reliably manufactured in large volume—while minimizing scrap rates and their losses.