What is the best test to determine the sealing capability of a silicone compound or design?

Two common tests are (1) to put the device or component under pressure using regulated compressed air and then submerging in water or other fluid. The leaks will show as bubbles or (2) use a colored die solution that contrasts with your part colors under pressure and the die will highlight leaks.

Ultimately whenever possible pressure testing of the final assembly under the working load or more, in as close to the final environment as possible can help identify failures caused by condition stack up.

Alternatively for many applications there may be published standard test methods that have been shown to be effective. Medical devices and aerospace both have test standards and you may find some relevant test standards under ASTM.

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Life Expectancy of a Silicone Part

While it depends on the specific application, generally molded silicone parts will have a considerable life expectancy. Once fully cross-linked a silicone rubber part will be virtually inert, meaning it won’t degrade or react chemically with most anything in the environment, aggressive solvents can break silicone down. Compared to thermoplastic elastomers and other rubbers silicone tends to retain its physical properties for much longer periods of time, and over numerous cycles of use, hundreds, thousands, millions (again this is somewhat dependent upon the application).

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Albright Technologies President to Receive EMERGE Award

After recently being named President of Albright Technologies, Bob

BobWaitt will be receiving an EMERGE Award this week. This award is given to 10 young professionals who work in North Central Massachusetts for their outstanding work professionally and in the community. Each nominee was scored by a small selection committee on his or her career history and trajectory, community involvement, community dedication and recognition by others.

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What criteria should people consider in selecting the hardness of a silicone compound?

Question: What criteria should people consider in selecting the hardness of a silicone compound?

Answer: Criteria for hardness may be best answered by considering what it means. Durometer is a measure of the resistance to indentation by a probe tip. Most silicones fall into shore A range although there are exceptions. Lower durometers feel softer like pressing a rubber band versus harder like an eraser. Neither of which are silicone but make the point.

The durometer does not necessarily indicate a materials elongation, modulus, tear strength, chemical resistance, or opacity but in the same product lines higher durometer does often but not always come with greater strength and modulus but lower elongation and viscosity. From one supplier to another and from one product line to another doesn’t always apply.

The criteria to use depends on your need, you may need to look at other properties for highly engineered products with strict performance requirements such as diaphragms, valves, gaskets, etc. On the other hand, products such as handles, covers, and skin contact components may lend themselves to the feel by durometer. Samples of different durometers can be helpful for both types of products to give you a sense of the differences between materials

What is the risk of a piece of silicone breaking off and becoming a choking hazard?

Question: What is the risk of a piece of medical silicone breaking off and becoming a choking hazard? How much force to tear?

Unfortunately like many problems, the answer to your question depends on the application, loads, and part size and geometry.

The risk of choking is difficult to quantify but a quick search suggests that thin sheets or hard round shapes like coins are significant choking hazards as we all know. The question becomes how to quantify risk of a deformable body with very low wet friction being released into the windpipe. It may be worthwhile to consult a physician specializing in respiratory to better understand the physiology and failure modes associated with choking.

To answer the second part of the question, factors affecting the tear strength may include temperature, degree of cross linking (how complete the curing reaction is), and sterilization but let’s assume a perfect material. If we choose a 70 durometer implantable material such as Nusil’s MED-4870 and we look up the tear strength of 1350psi on their website. The referenced standard uses the original cross sectional area. So using simple rod geometry in tension at 1mm (0.0394in) tearing occurs at 1.6lb while a 1/8” rod would tear at 16.5lb. As the complexity increases so does the challenge of estimating failure loads such as in balloons or composite products. This is where prototyping and early testing offers the ability to drive a product to failure without risk to life.

Differences in Molding Silicone vs. Plastic

The primary difference between silicone and plastic properties drives the differences in mold design. The largest difference is the temperature profile between silicone and plastic. The mold design and construction will also be dependent on the product geometry, size, materials, equipment, quantities, cycle times, undercuts, flash limitations, and parting line and gate restrictions.

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

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.