The primary difference between silicone and plastic properties drives the differences in mold design. The largest difference is the temperature profile between silicone and plastic. The mold design and construction will also be dependent on the product geometry, size, materials, equipment, quantities, cycle times, undercuts, flash limitations, and parting line and gate restrictions.
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Question: What shrinkage value should be used when designing a silicone mold?
Answer: Shrinkage is defined as “the amount or proportion by which something shrinks” (http://www.thefreedictionary.com/shrinkage). A material’s shrinkage must be accounted for when designing a mold to produce a silicone part that meets all required dimensions. Silicone normally can shrink from 1% to 4%. The shrinkage analysis is sometimes not provided when we buy silicone from manufacturers. Based on my personal opinion, 2% can usually be used for a standard shrinkage value when designing a silicone mold. Nevertheless, variation between material lots can significantly affect the shrinkage percentage as well as the part’s geometry. For example, a long hollow cylinder part that has a thin wall is going to shrink differently on different axes. Specifically, the long section of the part is going to shrink more than other axes. In this case, the part must be scaled differently on different axes.
The suggested shrinkage value will work most of the time. However, in a case where the material’s shrinkage doesn’t meet the standard shrinkage allowance or a part has a similar geometry to the one described above, educated estimation on shrinkage value should be made when designing a silicone mold.
Question: What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?
Answer: The processing conditions can be optimized to match the tool design, part geometry, and material but each presents limitations. Injection process times are driven by material. Silicone parts can often be released with no draft angle or even undercuts due to the high elongation and low modulus that save time on actions but knock out pins may often damage parts. Silicone part removal is often done by automated brush, hand pull, compressed air, or another way that may take additional time compared to plastic.
Curing time can be reduced in silicone by increasing processing temperature until filling fails or surface quality diminishes. Plastic cooling rates may be more limited by internal stresses causing warping or property changes from rapid cooling. Other contributions to the cycle time include heating and cooling rates, curing or solidification time of the material, injection time, mold travel time, and other smaller contributions increase the cycle time.
Direct comparisons between cycle times of medical plastics and medical silicones are not readily available. Your molder may be able to go into detail for your application if cycle time is critical.
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While it is difficult to provide one general answer to your question, if I had to limit my response to one word, “yes”, because the cost of parts is typically very dependent upon the cost of the raw materials, and the cost of raw silicone is usually more expensive than the cost of plastic resins. Even the least expensive silicones may cost $5 – $10 per pound, while plastic resins run closer to a couple dollars.
However, in the medical device market, lots of parts are very small or microscopic. Micro molding is much less dependent upon the cost of the materials since there is so little physical volume. The cost of a micro part is driven more by the processing that goes into demolding, quality control inspection, and packaging, as all of these activities will require the use of microscopes that would not be required for larger parts.
But in general, molded or extruded silicone medical parts will cost more than plastics, thermoplastic elastomers, and petroleum based rubbers. The tradeoff is the biocompatibility and implantability that you get with silicone products, as well as other contributing factors like the physical, chemical and mechanical properties required of the device. Silicone may not always be the best material for the job, but when it comes to soft, flexible parts for medical applications it is a very good place to start.
Don’t forget to visit us at booth 1091 at MD&M West this week from February 12-14. Stop by to find out how to get a free silicone sample and ask us questions about your silicone molding projects.
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In a few weeks Albright Technologies will be exhibiting at MD&M West from February 12-14 in Anaheim, CA. Stop by booth 1091 to find out how to get a free silicone sample and ask us questions about your silicone molding projects.
Silicone, especially liquid silicone will conform well to just about any mold shape that can be dreamed up, the difficulty is typically in the mold making process. In the prototyping sector, we see a lot of cutting edge design concepts that push the envelope with features that are very small and precise. Every molder has their own specialty and also their own limitations. At Albright we specialize in quick-turn micro medical device parts molded in liquid silicone rubber. One design limitation that we frequently contend with is the size and geometry of micro features, we like to use “The 3X Rule”, which basically means that if you have a microscopic opening in your part design (which would be reflective of a raised boss in the mold) the overall height of that feature shouldn’t exceed three times the diameter (or side length) of the feature. That would mean that if you wanted to form a 0.005″ diameter hole in your molded part, you wouldn’t want to go much more than 0.015″ deep into the part, from a tool making standpoint.
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