Albright Technologies Monthly Insider
|Designing a Medical Silicone Part vs. |
a Medical Plastic Part
By: Matt Bont
The largest difference in design between silicone and plastic is the properties. Understanding the application and the material properties can help designers better utilize silicone in all applications. Silicone and plastic both have implantable and non-implantable grades and your application will dictate what grade you need. Applications may be medical in nature but not contact people but devices may also be implantable for a few days, weeks, or years or just contact skin or blood.
Designers trying to determine tolerances and fitting may want to keep in mind that silicone can deform more readily than most plastics to fit an assembly so your tolerance level may be dependent on the level of performance. For example a strain relief or compressed seal may not need as tight of tolerances as an optical lens or a pressure membrane. Silicone generally has high elongation and low hardness compared to most plastics so specifications may need to be adjusted accordingly. For example regrinding of silicone is not practical but straight walls and undercuts that may be difficult in plastics may not pose an issue due to the low hardness and high elongation of silicone. Flash and cuts which may not be a significant issue with most plastics is common in silicone.
For more information or questions, email Matt at firstname.lastname@example.org.
Think Liquid Silicone Rubber for
Medical Device Molding
LSR O-rings are being released from the mold.
Madison Heights, MI
LSR presses are different from standard plastic injection molding machines in a number of ways. LSR presses use a pipe coupling instead of a hopper, rear seals, hydraulic or pneumatic shut-off nozzles, and a water-cooled jacket on the barrel and screw tip. The shot size is 30% to 80% of barrel capacity, and tonnage requirements are 1.5 tons to 2 tons of clamp force for each square inch of exposed area.
In addition, LSR molds are cartridge-heated and thermocouple- controlled versus steam or oil heated. Their vents measure around 0.0002 to 0.0005 in. instead of a few thousandths of an inch. Mold surface finishes can be vapor honed or straight electric discharge machined (EDM) for easy release. Molds for plastics often have glossy finishes that cause parts to stick. LSR runners are as small as possible, for example, 1/8 in. diameter and 0.050-in. deep.
The most common formula for LSR involves a two-component resin. The raw materials are delivered as an "A" part, which is the catalyst, and a "B" part, which is the cross-linking agent. Mixed at a 1:1 ratio for manufacturing, the "A" and "B" components are stable for long periods when kept separate and sealed. They begin to harden when mixed, but can have extended pot lives of up to several days at room temperatures and even longer if kept cool or cold. However, they set quickly at elevated temperatures (in seconds at 302ºF to 356ºF).