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.
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.
Click here to read the rest of the article.
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: What is the range of compression set for liquid silicone?
Answer: The compression set range for liquid silicone is generally between 5% and 30%. Where 5% displays very little permanent deformation and 30% is relatively high permanent deformation. The grade of liquid silicone and the degree of completion of cross linking generally effects the final compression set. If compression set is a major issue for your project, it may be worth the time to post cure (A.K.A. Post Bake) your parts in order to complete cross linking of your parts. Your liquid silicone material supplier should be able to provide the compression set range for their liquid silicone.
Question: What durometers are available in liquid silicone rubber?
Answer: Liquid medical silicone rubber is typically available from 01 to 80 durometers. Durometer is a measure of the indentation hardness of a material. The procedure for determining indentation hardness of a polymer material can be found in ASTM D2240. There are twelve different durometer types: A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Nevertheless, type A and type D are the most commonly known and used in the industry.
Type A is generally used for soft and flexible polymer materials and type D is used for hard material. Therefore, liquid medical silicone rubber is generally labeled in type A durometer. Silicone durometer is normally categorized in an increment of 10, but the distribution of the durometer can range from minus to plus 5 durometer from the labeled value; industrial standard allows +/- 5 durometer that includes +/- 2 potential test error.
Nevertheless, the durometer of liquid medical silicone may be customized to meet the specifications of individual applications. The hardness of a silicone can be modified by changing the crosslink density, filler concentration, or both.
If you have any other questions, please email Phayhean Soo directly at email@example.com.
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.
Question: What options are on the market for conductive liquid silicones?
Answer: Many liquid silicones are made conductive by adding in some conductive medium like iron or silver particles. As concentrations of additive increases, physical properties tend to decrease. The texture also changes significantly with concentration. A quick Google search does provide some options from companies like Dow Corning. Silicone
Solutions has a few options that we have been able to utilize.
If you have any other questions, please comment or ask me directly at firstname.lastname@example.org.
Question: What medical acceptable liquid silicone lubricant can be used with a piston with a silicone ring seal?
Answer: Most silicone distributors offer a variety of silicone lubricants. Some distributors, such as NuSil, offer medical grade lubricious silicone fluids such as MED 360 and MED 361. NuSil also offers both restricted and unrestricted implantable lines of self-lubricating silicones, MED 1-4800 and MED 1-4900. Material like this would allow for the silicone ring/gasket/seal, to be created, and not require the use of additional silicone lubricant.
If you have any other questions, please comment or ask me directly at email@example.com.