Question: What criteria should people consider in selecting the hardness of a silicone compound?
Answer: Criteria for hardness may be best answered by considering what it means. Durometer is a measure of the resistance to indentation by a probe tip. Most silicones fall into shore A range although there are exceptions. Lower durometers feel softer like pressing a rubber band versus harder like an eraser. Neither of which are silicone but make the point.
The durometer does not necessarily indicate a materials elongation, modulus, tear strength, chemical resistance, or opacity but in the same product lines higher durometer does often but not always come with greater strength and modulus but lower elongation and viscosity. From one supplier to another and from one product line to another doesn’t always apply.
The criteria to use depends on your need, you may need to look at other properties for highly engineered products with strict performance requirements such as diaphragms, valves, gaskets, etc. On the other hand, products such as handles, covers, and skin contact components may lend themselves to the feel by durometer. Samples of different durometers can be helpful for both types of products to give you a sense of the differences between materials
Question: What is the risk of a piece of medical silicone breaking off and becoming a choking hazard? How much force to tear?
Unfortunately like many problems, the answer to your question depends on the application, loads, and part size and geometry.
The risk of choking is difficult to quantify but a quick search suggests that thin sheets or hard round shapes like coins are significant choking hazards as we all know. The question becomes how to quantify risk of a deformable body with very low wet friction being released into the windpipe. It may be worthwhile to consult a physician specializing in respiratory to better understand the physiology and failure modes associated with choking.
To answer the second part of the question, factors affecting the tear strength may include temperature, degree of cross linking (how complete the curing reaction is), and sterilization but let’s assume a perfect material. If we choose a 70 durometer implantable material such as Nusil’s MED-4870 and we look up the tear strength of 1350psi on their website. The referenced standard uses the original cross sectional area. So using simple rod geometry in tension at 1mm (0.0394in) tearing occurs at 1.6lb while a 1/8” rod would tear at 16.5lb. As the complexity increases so does the challenge of estimating failure loads such as in balloons or composite products. This is where prototyping and early testing offers the ability to drive a product to failure without risk to life.
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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