Category Archives: Silicone Manufacturing

This category contains useful articles on Silicone manufacturing related questions or topics

Modern Machine Shop: Tempering Urgency Within Your Shop

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Modern Machine Shop Online

By: Derek Korn

Like many businesses, machine shops operate under an atmosphere of urgency. Part programs need to be created quickly so jobs can be released in a timely manner to the shop floor. First-article inspection must to be carried out promptly so necessary adjustments can be made and the production run can proceed. Once a job is completed, machine setups need to be torn down in short order so a new setup can begin for the next job.

These types of things happen at a New-England-area company I recently visited and will profile in next month’s issue. The company, Albright Technologies, has to move at a decent clip. A fast pace is necessary because its niche is rapidly creating prototype molds and small batches of tiny silicon parts for medical customers pressured to speed their new products to market.

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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.

 

 

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 medical acceptable liquid silicone lubricant can be used with a piston with a silicone ring seal?

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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 kevin@albright1.com.

How thin can liquid silicone be molded?

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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?

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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.

 

How long does it take to perform over-molding?

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Question: How long does it take to perform the over-molding? What are the typical curing durations inside the mold?

Answer: Curing time of over-molding process depends on two major factors:

  1. Although an increase in temperature will reduce the cycle time, it can deflect the secondary over-molded piece such as plastics. Temperature should be adjusted according to both materials’ processing specifications.
  2. Larger part dimensions require longer cycle time. An increase in wall thickness of the part geometry will consume more curing time.

The majority of molding time in each cycle is spent during a curing process. Once the part is cured, it can be immediately ejected. Similarly to silicone curing time, thermoplastics cooling time is also the longest phase in the molding cycle.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

What types of injection molding polymers can medical silicone be over-molded on?

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Question: What types of injection molding polymers can medical silicone be over-molded on?

Answer: Medical silicone-plastics bonding has increasingly become a hot topic in the bonding industry and very limited research has been conducted to prove compatibility of the two materials. A ShiEtsu adhesion strength graph below illustrates a limited amount of polymers used in this experiment.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

Do rougher mold surfaces release medical silicone parts better or worse than smoother surfaces in shear?

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Question: There is some confusion here regarding mold surface finish. We don’t care about the surface finish of our molded part, but we need best possible release. Do rougher mold surfaces (like bead-blasted) release medical silicone parts better or worse than smoother surfaces in shear?

Answer: We’ve had our best release results using textured (bead-blasted, etc.) surfaces on our molds. Smooth surfaces (highly polished silicone lenses) tend to be the most difficult to release from the molds. If the surface finish on the final medical silicone part can be rough, I would definitely recommend bead-blasting the mold (number 6 glass beads work well), it’s inexpensive, quick and easy to do and if you make lots of smaller molds, purchasing a bead-blaster to use in-house, is a minor investment that will pay itself off very quickly.

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