Silicone Fabrication Q&A
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It’s been a number of years since I’ve worked with silicone, so forgive my ignorance. Are there any major providers of systems—i.e., Stratasys, etc.—that manufacture rapid prototyping systems, other then subtractive, that are capable of producing/curing medical-grade silicones with thick or thin walls and in multidurometer configurations? The idea is to eliminate my molding process altogether. If such systems are not available, why? If they are, names would be greatly appreciated.Statasys and other companies, I believe, do offer rubber or “siliconelike” materials called TangoPlus, Tangogray, and TangoBlack. The resolution and accuracy available are generally good enough to help validate a design, but they are not a replacement for an actual silicone part. As far as why they have not made a rapid prototyping system for silicone, I am not entirely sure. I am guessing that it is due to the peanut butter consistency of silicone in its uncured state. I am sure that it would be very difficult to lay down a thin enough layer to accurately create a part, not to mention then having to heat it up to cure it.
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I am currently working on a medical device in which a needle pierces through a silicone septum. I can’t use silicone oil. Is there any other medical-grade lubricant that can be used to aid the needle through the silicone?
I am not aware of any other medical-grade lubricant. One additive we sometimes use in applications is Zonyl. Teflon can also be loaded into the silicone prior to molding (~15% by weight). This creates some added lubricity throughout the part.
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Is there a good way to surface treat silicone in order to make the surface more dirt resistant so that it will not pick up fibers and other debris easily?
A mat finish versus a polished/shiny finish will pick up slightly less FM or “dirt.” There are also surface treatments such as a parylene coating that create a lubricity on the surface and coat the silicone part. The use of ionizing fans to reduce static when handling the part will also reduce FM instances.
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I have an application in which I have fully cured silicone molded parts. These are parts of a subassembly. The next operation of the manufacturing procedure involves passing this subassembly through a UV-cure cycle (for curing another part that has been glued on to this assembly). My question is whether exposing the silicone parts to this secondary UV-cure cycle might affect their properties in a certain way. Is there any harm in doing this?
The short answer is, I do not “believe” it would affect the molded component of any significance or noticeable amount. However I don’t have experience with this and would refer you in this instance to a chemist at Dow Corning, Nusil, or Momentive silicones for a more in-depth response on how the chemistry may react or be affected within the material.
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When trying to swell a silicone extrusion utilizing heptane on a percentage basis, what can one expect for a radial expansion and a longitudinal change? Does swelling the silicone compromise its strength and weaken it? Is the degree to which an extrusion will swell when exposed to heptane a function of time or exposure? To dissipate the heptane out of the silicone, what is the best way to process this—exposure to ambient room temperature, in an oven, etc.?
Swelling is dependent on the type of silicone, the size of the part, and how long it is exposed to heptane. Ultimately, there is no specific formula to calculate this. Over time and with extended exposure, heptane will degrade the silicone rubber. Each unique application requires its own trial and error procedure and process development—in other words, heptane exposure.
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We are having problems adhering silicone rubber to our aluminum substrate. There is a bond check after molding, but we find this to be ineffective, since we are finding failures at customer sites. The problem does not affect some of our HCR materials, but only injection-molded silicone parts. We have investigated our aluminum substrate cleaning process many times. Applying bonding agents also seems to be working, since we are seeing problems with bonds to the silicone rubber. Any help will be appreciated.
Our experience shows Nusil MED 6-161 to be the best primer for this type application. Use of this primer and making sure you have a clean surface is the most important factors in achieving a strong bond. Also, wiping off excess dried primer residue is important.
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Can you please recommend a silicone product suitable to form masks? It is important that the final product have the density of water.
I am not sure of any silicone product with the density of water. But we have commonly used Wacker 3003/40 and Momentive LIM 6050— to name a few LSRs—on respirator and other mask applications.
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Could you discuss the methods that produce the most transparent liquid injection-molded (LIM) silicone device? Could dip-molded silicone be as transparent as the LIM process?
I know that Nusil Technology has some optically clear liquid silicone rubber materials, and we use a couple of them at Sil-Pro—Nusil MED 6033, for one. The other key is a highly polished mold finish. I am not certain how dip molding would compare, since that process is not my field of expertise. But I would guess that it wouldn’t be as optically clear as LIM molding could be.
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In developing medical-grade silicone prototypes, there are certain medical-grade products (RTV or two-part elastomers) that will not fully cure in molds made from medical-grade rapid-prototype material (light-cured resin), even with moderately elevated temperatures. It appears that there is lower inhibition resistance in medical-grade silicone (e.g., Dow MDX-4210). Any suggestions for improved inhibition resistance in medical-grade silicone?
Some things to make sure you are doing that can affect curing:
- Mixing A and B properly and equally per manufacturer’s specification.
- Not poisoning the material by allowing it to contact IPA or other chemicals in the process.
- Making sure to use the material at room temperature in a moderately humid environment.
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My question relates to the effect of radiation (gamma, E-beam) on a silicone material, specifically LSRs and HCRs. What technologies are available to combat radiation-induced methyl-to-methyl cross-linking between surfaces in contact with each other? The end result is that molded parts can stick to themselves. Are there any additives or processes that could be employed to minimize or eliminate the free radical generation or the effects of the free radicals?
Sticking parts can definitely be an issue, especially with lower-durometer molded parts. The first thing I would suggest trying is to perform a postcure at ~350°–400°F for two to four hours after molding. This finishes most of the cross-linking that may be occurring after molding and is occurring during your gamma-radiation period in your application. An additive that helps with lubricity but may also be an issue in your application is a filler to add to the silicone. I know that Zonyl (Teflon), which is sometimes used to create lubricity within molded silicone components, would reduce sticking and cross-linking between two like materials. Generally a mixed ratio of ~10–15% Zonyl by weight is a sufficient loading. But I would try the postcure first.