Features

Device functionality is usually the starting point when designing devices. Another element that needs to be considered when designing devices and their subsequent components is their manu-facturability. 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 manufacturabil-ity: part geometry, location, and size and shape of critical surfaces, among others. These may seem like more obvious characteristics, but there are a few others that can be overlooked, but should be considered just as important. These are material selection, dimensioning/tolerancing, and the selection of critical dimensions. To better understand the impact of these factors, this article explores each characteristic in detail.

Material Selection

Having a well-defined list of requirements at the beginning of the design process helps the with DFM review and subsequent mold design. (Credit: ProMed)

Choosing the correct material for an application is important and can affect 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 (LSR) or high consistency rubber (HCR) — durometer (hardness), and even color. Each of these can affect manufacturability.

Liquid Silicone Rubber vs. High Consistency Rubber

Both LSR and HCR are available in a variety of durometers (see the sidebar “HCR vs. LSR Injection Molding: Which is Right for Your Project?”;) 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 2.5-4.0 percent, and HCR has a typical shrink rate of 1.5-3.0 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.

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 man-ufacturability, using a silicone with a durometer between 30 and 70 Shore A is recommended.

Although the performance and mechanical characteristics of both types are nearly identical, choosing between HCR and LSR boils down to how the part will be made. (Credit: ProMed)

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 a part can benefit component manufacturing and assembly in several ways:

  • Similar, hard to distinguish parts can be colored differently, making them easier to distinguish visually.

  • Coloring very small or micro-size parts can make them easier to see and handle, especially against a white background.

  • 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 percent of the dimension or ±0.003 in., whichever is greater.

HCR vs. LSR Injection Molding: Which is Right for Your Project?

Given their excellent chemical inertness, toughness, ample operating range, flexibility, and wide range of available durometers, silicone rubbers find uses in a wide variety of industries. One industry with a growing use of these elastomers is medical device manufacturing, where their low toxicity, great biocompatibility, and ability to repeatedly withstand steam, gamma ray, EtO, e-beam, and UV sterilization only add to their appeal.

Medical device companies must determine whether high consistency rubber (HCR) or liquid silicone rubber (LSR) is the best for their product. Although the performance and mechanical characteristics of both types are nearly identical, choosing between HCR and LSR boils down to how the part will be made.

High Consistency Rubber

HCR is produced as gummy, high-viscosity sheets of various thicknesses that are partially vulcanized. This form makes HCR a natural fit for compression and transfer molding as well as extrusion (it's the go-to material for flexible components like rubber tubing). Fabricating parts out of HCR requires many steps that are labor-intensive, albeit simple (relative the injection molding process used with LSR): mill softening, preform preparation, extrusion/molding, vulcanization, and finishing.

Although HCR can in theory be injection molded, the material's high viscosity and long cure times result in cycle times that are often too long to be practical. HCR's reliance with these fabrication methods means producing a part out of HCR generates substantial waste material, incurs high labor costs, and requires much floor space, tools, and equipment to accommodate the many required steps. It should be noted, however, that those equipment costs are less than the steep design and machining costs required by LSR's injection molding process.

Liquid Silicone Rubber: Perfect for Injection Molding

LSR, by contrast, starts out as a two-part liquid that cures into a solid form when mixed together. Mixing is performed by a metered mixer that precisely combines the two parts in a 1:1 ratio (mixing in additives if needed) right before pumping the fluid into the mold, which is heated to accelerate the vulcanization process.

The mixed LSR is pushed into the mold under pressure. LSR's low viscosity results in a quick mold fill and pack time, while the elevated mold temperature ensures a short cure time. Since all curing takes place inside the mold, there is less wasted material compared to HCR. The use of “cold drop” or “cold runner” tooling reduces this waste even further by the keeping the LSR cool inside the sprue and runners, which mean vulcanization only occurs inside the hot part cavities, resulting in no LSR lost to sprue and runner volume and no additional step of trimming these sections from the molded part.

LSR's injection molding process produces consistent parts, cycle to cycle. Another advantage to the LSR process is the ability to reproduce intricate and complex shapes, as LSR's low viscosity permits the material to fill even the tiniest of spaces. Because this is such a highly automated process, once the LSR process is up and running, very little labor is required to produce large quantities of parts, making LSR the dominant choice for high-volume production.

LSR is also gaining popularity among medical device OEMs and their contract manufacturers. Making parts via injection molding with LSR does require specific expertise, including mold design, mold performance analysis via simulation software, and very high precision machining of the mold materials. In fact, most of the cost of making silicone parts this way is incurred by the design and production of the tooling itself.

Scale is Key to Selection?

Deciding whether HCR or LSR injection molding is the better choice for a project largely depends on the production volume required. Generally, HCR's methods are better suited for smaller production runs, while LSR is a better fit when making hundreds or thousands of parts.

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 0.010 in.

  • Referencing theoretical points, such as the transition point from a flat surface to a radius.

  • Referencing theoretical planes/surfaces in the use of geometric dimensioning and tolerancing.

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. 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 manufacturing a part with complex geometries, a material with a low viscosity is recommended so that detailed features are consistently and accurately captured. (Credit: ProMed)

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.

Conclusion

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.

This article was provided by ProMed Molding, Minneapolis, MN. For more information, visit here.