Author Archives: Matt

About Matt

Matt is a project engineer at Albright Technologies, Inc. After interning for two years and earning a Bachelors of Science in Mechanical Engineering from Western New England College Class of 2009, Matt joined Albright Technologies full time.

What is the Coefficient of thermal expansion or CTE for silicone at different temperatures?

Question:  What is the Coefficient of thermal expansion or CTE for silicone at different temperatures compared with other steel or aluminum for a 10 inches long example part?

Answer: Thermal expansion is a change in volume or size of something in response to a change in temperature. The molecular interactions in most materials increase with the increase of temperature causing expansion in the material. Thermal expansion is a direct function of the material and silicones expand significantly greater than other materials like metals for the same change in temperature.

Thermal expansion is usually expressed in one of two ways either volumetric expansion or more commonly linear expansion expressed as a coefficient. For example silicone may expand linearly 5.9E-4 to 7.9E-4 in/in-C compared to aluminum at 23.0E-6 in/in-C or steel at 10.8E-6 in/in-C. So for the sake of comparison, a temperature change from 65 degrees to 150 degrees Fahrenheit for a 10 inches long piece would translate to an expansion for a given material of approximately:

Silicone: 0.279” – 0.374”

Aluminum: 0.010”

Steel: 0.005”

The silicone can expand 34X the aluminum and 73X the steel which for higher temperature applications may be worth noting. Many silicone manufacturers do not include thermal expansion rates on their data sheets but will usually provide them by request.

How is compression testing done for liquid silicone?

Question: How is compression testing done for liquid silicone?

Answer: Compression set measures the permanent deformation of parts that have been subjected to high temperatures for some predetermined amount of time. One standard for compression set used in supplier literature is ASTM D395 A or B. Basically a standard puck is compressed between two plates and subjected to temperatures in the range of 250F at constant force or constant deflection. The sample is allowed to recover for 30 minutes and then measurements are taken. Alternative tests like ISO 815-B heat for 22 hours at 175 degrees C. The test method dictates the calculation but basically the compression set is a measurement of how well the puck returns to the initial thickness, so lower values indicate higher resistance to permanent deformation.

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

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

Answer: Sticking can be reduced by changing liquid silicone molding materials, changing the tool surface, adding a release, or seasoning the tool. Changing medical silicone molding 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 liquid silicone molding. 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 liquid silicone 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.

Can medical grade silicone be bonded to thermal plastic parts?

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.


 

What is the range of compression set for liquid silicone?

 

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.


 

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

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?

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 modules range can be achieved in medical-grade silicones?

Question: What modules range can be achieved in medical-grade silicones?

Answer: The modulus for medical silicone is low compared to traditional materials with a modulus ranging between 150 to 1500 psi for silicone. Many of the properties are temperature dependent and nonlinear but the supplier may provide you with properties and guidance in selecting liquid silicone materials.

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

 

 

 

How thin can liquid silicone be molded?

Question: I may need to mold a liquid silicone element that is extremely thin.  It will be the shape and size of a finger of a glove, but will have a small element extending back from the tip through the center. I would need to be in the .002-.005” thick wall.  So my question is, how thin can liquid silicone be molded?

 

Answer: Parts with wall thicknesses at 0.005” or less are possible but expect to experience some tearing issues of the liquid silicone part in release, part removal, and removal of flash.  Problems with tolerances in the mold geometry become significantly more noticeable when 0.001” corresponds to a 20% or more change in wall thickness of the liquid silicone part. Materials with high elongation, high tensile strength and high tear strengths may provide some benefits but additional geometry like bosses, holes, and undercuts may overshadow many material benefits.

 

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?

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.