While it depends on the specific application, generally molded silicone parts will have a considerable life expectancy. Once fully cross-linked a silicone rubber part will be virtually inert, meaning it won’t degrade or react chemically with most anything in the environment, aggressive solvents can break silicone down. Compared to thermoplastic elastomers and other rubbers silicone tends to retain its physical properties for much longer periods of time, and over numerous cycles of use, hundreds, thousands, millions (again this is somewhat dependent upon the application).
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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.
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.
Question: Is medical silicone more expensive than thermal plastic elastomers?
Answer: In short answer, yes, typically medical grade silicone is more expensive than thermoplastic elastomers. But there are many factors that need to be taken into account, that all have an effect on the cost of material.
Silicone rubber (especially medical grade) excels in biocompatibility, however there are different degrees to which the raw material is biocompatible. Some medical grade silicones are acceptable for human contact, others are acceptable for blood and bodily fluid contact, others still are acceptable for implantation, and implantation is further broken into short term (29 days and under) and long term (indefinite). With every additional step up the biocompatibility later, the cost of the material goes up. A short term implantable material may cost five to ten times as much per pound as a contact grade material, and a long term implantable material may cost ten or more times the cost of the short term implantable. And by the time you get to drug eluting silicones, you might be talking tens of thousands of dollars per pound.
Fortunately, most implantable medical devices are quite small, and molding processes are highly controlled to deliver accurate shot sizing, so while the price of material is prohibitive, silicone molders do all they can to minimize waste material, while simultaneously maximizing the level of part quality.
Question: Why is a medical silicone molded part more stable than a thermal plastic part?
Answer: Molded medical silicone parts are actually more stable in three different respects of the word, so I would like to provide three different answers to your question.
In the first respect silicone molded parts are more stable than plastic parts due to the fact that molded plastic parts have inherent stresses molded into them. These high stress regions can be viewed by holding a molded plastic part up to a light source and looking through a polarizing filter. It is not an uncommon practice to anneal plastic parts after molding them to remove these molded in stresses and create a stronger, more stable part that is less likely to crack under pressure. Silicone parts do not exhibit these inherent molded in stresses due to the way that silicone parts are molded. Silicone parts are molded by injecting “cold” (55°F – 70°F) liquid silicone material into a heated mold, this allows the silicone to already be in its desired shape before curing.
In the second respect silicone is more stable as a medical device due to the bio-inert nature of silicone material. In fact, molded silicone parts are nonreactive with just about everything; it takes some pretty aggressive cleaning agents to remove dissolve fully cured silicone, chemicals designed to remove paint, corrosion, and powerful industrial adhesives. On the other hand, plastics are frequently in the media headlines and shrouded in scares regarding toxicity in materials that people eat off of and interact with every day, BPA, Teflon, and PVC to name a few. These polymers have received attention for leeching into foods and even drinking water, under the right conditions.
And in the third respect of the word silicone is more thermally stable than thermoplastic. While thermoplastics and TPEs have come a long way in terms of temperature resistance, to put it simply, they will eventually reach glass transition point at elevated temperatures and at further elevated temperatures they will melt. Once silicone has been fully cross-linked it can’t be returned to liquid form. Molded silicone rubber behaves more like wood when exposed to extreme temperatures and will eventually smolder and burn, however these temperatures are considerably greater than those at which plastic will reach liquid state.
Question: Why are medical silicone parts post cured?
Answer: Post curing or post baking is a simple heating process that takes place after silicone parts have been molded. Although post baking parameters will vary from material to material, the post baking process typically involves heating parts to 300 – 400°F for 2 – 4 hours. This process is performed for a variety of different reasons.
The first reason parts were post baked was due to the initial chemical composition of the earlier HCR (High Consistency Rubber) materials. HCR materials typically cure as the result of a peroxide based chemical mechanism (where as the more commonly used LSR (Liquid Silicone Rubber) materials cure as the result of a platinum based chemical reaction). A peroxide based cured silicone will leave undesirable, low molecular weight byproducts that must be cooked out of the silicone matrix, typically these byproducts are various forms of benzoic acid. For this reason, silicone vendors required that all HCR molded parts must be post baked prior to shipping.
The other reasons to post bake, which were likely discovered later, are the completion of silicone matrix cross-linking, and standardization of part size and physical and mechanical properties, namely compression set.
All silicone parts will shrink (slightly, typically 1-4% on overall size), while they may not all come out of the mold with the same degree of shrinkage, after a post baking process, all of the parts will reflect the same degree of shrinking. Most molders factor in a value for shrink based on requested material data when designing the mold initially, this could mean that a shrink value is called out on the material of 2% but only approximately 1% of the shrink occurs in the molding process, were parts not post baked prior to shipping, the customer would receive parts that were roughly 1% too big.
The same is true of other physical and mechanical properties, compression set and percent elongation have been witnessed to fluctuate drastically if the parts are not post baked. For this reason design engineers should always inquire with silicone molders as to whether or not they perform a post baking process, prior to embarking on large production runs with parts that have critical physical property requirements.
Question: Does the shrink for medical silicone values change with materials and with different medical silicone manufacturers?
Answer: Percent shrink will vary from one medical silicone manufacturer to another, and from one Liquid Silicone Rubber material to the next. Though typical shrink values range consistently from 1% to 4%. Although this range may not cause a part to fall outside of tolerance spec (especially in silicone micro parts), it is important when molding silicone that the end user of the part understand that changing materials (or material providers) may require redesigning the tool, especially in larger silicone parts with tight tolerances or mating surfaces.
Question: With all the new options today, could you please tell me what is the best and least expensive way to prototype a part (about 3″ x 2″ x 2″) made in Liquid Medical Silicone Rubber?
Answer: For low volume prototyping of medical silicone parts your best option is typically to create a single cavity aluminum (faster and cheaper than steel) compression or transfer molding tool. While others may swear by jumping straight into an injection molding tool, it could take a machinist and technician longer to set up and process a liquid silicone injection molding tool than it would to make 25 parts in a compression mold.
If cost is your most critical concern, and your target first run of medical silicone volume is extremely low, you may also wish to look into getting an acrylic mold made (instead of aluminum). The costs of fabricating an acrylic mold is significantly less than a metal mold. But be forewarned, if you purchase an acrylic tool, you will likely be stuck running an RTV (Room Temperature Vulcanization) material (so your cycle times will be long), you won’t be able to put any real pressure on it (so your dimensions could vary significantly), and your mold is going to wear out FAST. You might only get 10 parts out of an acrylic tool, vs. 10,000 out of an aluminum tool, and 100,000 out of a steel tool.
While many can be seduced by the $500 – $1,000 price tag on an acrylic mold, it might take 10 of them to make 100 parts, at this point, an aluminum tool would already have been paid for, and well into a small scale production run. It is for this reason that Albright Technologies does not fabricate acrylic molds.
If you have any other questions, please email Kevin Franzino directly at firstname.lastname@example.org.
Questions: If quoted a tolerance of +/- x, what is the piece-to-piece tolerance? i.e. does tolerance change within a batch? I assume that since the mold is the same, piece to piece variation should be negligible, but maybe there is something to do with the silicone handling/curing that may affect different pieces from the same batch?
Answer: Assuming the processing parameters are maintained throughout the batch, the medical silicone parts should all have consistent dimensions. However, if temperature and pressure are permitted to fluctuate (significantly) during the curing cycle, the parts will exhibit different overall dimensions. These deviations will be slight and typically difficult to measure (especially in micro parts).
The main factor in this is shrink, Liquid Silicone Rubber typically shrinks 1-3%, depending upon the material and the processing parameters (particularly operating temperature). If shrink of the medical silicone isn’t properly accounted for, and processing parameters are not properly controlled, you could theoretically see a swing as drastic as an eighth inch over a twelve inch diameter gasket.
To ensure that you are creating liquid silicone parts repeatably, fine tune your processing parameters, and keep them tight, try not to fluctuate on temperature by more than a few degrees Fahrenheit, and try to keep your pressure within a few hundred PSI. Postbaking medical silicone parts is also crucial in assuring that part dimensions repeat. Most silicone distributors will recommend a postbaking cycle for completed silicone parts, this cycle helps to ensure that the molecular matrix of the medical silicone is fully cross-linked. While it will vary based on material, it is typically a 2 – 4 hour period of cooking at around 350°F – 450°F.
If you have any other questions, please email Kevin Franzino directly at email@example.com.