Two common tests are (1) to put the device or component under pressure using regulated compressed air and then submerging in water or other fluid. The leaks will show as bubbles or (2) use a colored die solution that contrasts with your part colors under pressure and the die will highlight leaks.
Ultimately whenever possible pressure testing of the final assembly under the working load or more, in as close to the final environment as possible can help identify failures caused by condition stack up.
Alternatively for many applications there may be published standard test methods that have been shown to be effective. Medical devices and aerospace both have test standards and you may find some relevant test standards under ASTM.
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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 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.
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.
Read our March newsletter for silicone molding information and a chance to download a free silicone molding design manual!
Article From: Modern Machine Shop, Derek Korn, Senior Editor
Albright Technologies has become adept at micromachining molds for silicone parts such as the one to the right. This has enabled the company to become effective in quickly generating prototypes for medical device manufacturers pressured to speed new products to market. Many of the silicone components it creates are either tiny themselves or have miniscule features measuring just a few thousands of an inch. What’s interesting is that the company has found it can produce prototypes faster by taking a slower, more conservative approach to micromachining molds using end mills that measure just a few thousands of an inch in diameter.
Plus, while one might assume that very high spindle speeds are needed to effectively mill molds using such small tools, the machine that performs micromachining at Albright—a 30-taper VMC—typically spins 0.005-inch-diameter tools at just 9,000 rpm. Although that means feed rates and cycle times are relatively slow, there are a number of reasons why a company focused on quickly turning prototyping work finds this acceptable. David Comeau, Albright’s president, and Robert Waitt, vice president, explained why during a recent visit to the New England-area molder.
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Question: Can medical silicone be bonded to thermal plastic parts?
Answer: Medical silicone can be bonded to some thermal plastics such as polycarbonate, nylon, polybutylene terephthalate (PBT), and others. Bonding medical silicone can be improved with material selection, surface treatments, adhesives, and primers but many of these fail biocompatibility limiting their medical applications. Many thermal plastics don’t have enough thermal resistance to withstand over-molding temperatures while others bond poorly with liquid silicone such as polyethylene and polypropylene. Surface treatments like heating or flame, scratching, corona, plasma, and others can assist bonding. Adhesives and primers can be effective but they can add significant cost and cause problems with biocompatibility. There are some vendors that make implantable self adhesive silicones, adhesives, and primers but there aren’t a lot of options. One additional consideration is that materials (medical silicone, plastics, etc) from different vendors have different formulations and may result in significant differences in bond strength of the liquid silicone.
Question: I’d like to know more about implantable medical silicone.
Answer: Implantable medical silicone has the capability of being implanted in a living body without the risk of rejection. Commonly, the implantable medical silicone is categorized into two types: short term (restricted) and long term (unrestricted) implantable silicone.
The short term implantable medical silicone is used for a temporary medical application – normally ranging from 1 to 29 days. For example, a suture sleeve is made of short term implantable silicone to hold parts of a medical device to keep them in place during a suture. Once the suture is done, the suture sleeve is removed from the patient’s body. The long term implantable silicone should be able to remain inside the patient’s body for 30 days or more. A good example of long term implant application is the Left Ventricular Assist Device; this device helps the patient maintain the pumping ability of a heart that can’t sufficiently pump blood throughout the body on its own. This device isn’t removed until the patient has a donor.
Each medical silicone implant application requires certain implantable silicone. A medical device containing implantable medical silicone or other biomaterials must be carefully evaluated according to ISO 10993 before it is implanted into a patient’s body; the ISO 10993 contains a series of standards for evaluating the biocompatibility of the device. Also, it sometimes is tested according to ASTM (American Society for Testing and Materials) depending on individual application.
There are commercially implantable medical silicone materials available in high consistency silicone rubber (HCR) and liquid silicone rubber (LSR). Color additives can be added to meet the requirement of a medical application, but it is recommended that the color additives should have the same class and manufacture as the implantable silicone to prevent defects. The implantable medical silicone can also be mixed with additives such as tungsten and barium that allows the implants to be viewed easily with medical imaging equipment.
Therefore, selecting an implantable medical silicone for a medical device should be thoroughly evaluated prior to implantation. If you have any other questions, please email Phayhean Soo 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.