Category Archives: Silicone Design

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What shrinkage value should be used when designing a silicone mold?

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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.

How Micromachining Patience Speeds Prototype Production

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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.

Can medical grade silicone be bonded to thermal plastic parts?

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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.


 

Learn more about implantable silicone

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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 psoo@albright1.com.

If quoted a tolerance of +/- x, what is the piece-to-piece tolerance?

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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 kevin@albright1.com.

 

 

Does it make a difference if the undercut features are on the core side or cavity side of the mold?

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Question: When designing a molded medical silicone part that will have some undercut features, does it make a difference if the undercut features are on the core side or cavity side of the mold?

Answer: Assume that one side of the mold has no undercuts, and the medical silicone part can pull away easily.  After the mold has opened, then the part can be stripped off the mold half that forms the undercut features.

When designing liquid silicone parts, you may want to consider manufacturing issues such as tool design and fabrication, and part removal. Undercuts in a mold may have risks associated with damage to the liquid silicone part during part removal or even damage to the mold. Sharp corners tend to have the worst issues with tearing because either the mold has a sharp edge that slices the part as the part slides past or the part has a sharp inside edge that constitutes a stress point where tears can start.

The difficulty of undercuts is more dependent on the size and geometry of the undercut in relation to location than if the undercut is
specifically on the core or cavity. Cavity undercuts have more metal
to support inserts and laminate plates them than cores but
conferencing with your molder may provide beneficial feedback for your specific project.

One last consideration is material selection. The properties of
medical silicone does have some effect on the ease of part removal, high durometer liquid silicones with low tear strength (200% elongation) tend to tear more easily than low durometer materials with high tear strength (1000% elongation), But harder liquid silicone materials may be easier to handle or provide the necessary strength needed for your device.

If you have any other questions, please comment or ask me directly at mbont@albright1.com.

 

 

What options are on the market for conductive silicone?

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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.
http://siliconesolutions.com/elec_therm_conductive.html

If you have any other questions, please comment or ask me directly at mbont@albright1.com.

 

 

What methods do you recommend to help eliminate silicone being stuck in the tool?

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Question: I recently completed a project and took it into production.  The production has produced less than stellar yield.  The main issue is the silicone sticking to the tool.  The tooling is fairly complex with some thin material undercuts.  However, my question is: What methods do you recommend or material formulation to help eliminate material (silicone) being stuck in the tool?

Answer: Sticking can be reduced by changing materials, changing the tool surface, adding a release, or seasoning the tool. Changing material with regard to the issue may improve release. For example, undercuts may be dealt with by finding a softer material with higher tear strength. Rougher surface finishes tend to promote less sticking compared to highly polished tools for silicones. Alternatively some plating companies offer nonstick coatings that for some materials may be effective in improving release. Some molds will improve with increased number of cycles. The quick and easy option may be to use a release agent which many suppliers offer. Some releases may be specified for your material or for general use and Soap may be an alternative for those trying to control contamination risks. Releases usually only last some number of cycles before the effect diminishes and some may build up and require periodic cleaning.

If you have any other questions, please comment or ask me directly at mbont@albright1.com.