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 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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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

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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Question: What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?

Question: What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?

Answer: The processing conditions can be optimized to match the tool design, part geometry, and material but each presents limitations. Injection process times are driven by material. Silicone parts can often be released with no draft angle or even undercuts due to the high elongation and low modulus that save time on actions but knock out pins may often damage parts. Silicone part removal is often done by automated brush, hand pull, compressed air, or another way that may take additional time compared to plastic.

Curing time can be reduced in silicone by increasing processing temperature until filling fails or surface quality diminishes. Plastic cooling rates may be more limited by internal stresses causing warping or property changes from rapid cooling. Other contributions to the cycle time include heating and cooling rates, curing or solidification time of the material, injection time, mold travel time, and other smaller contributions increase the cycle time.

Direct comparisons between cycle times of medical plastics and medical silicones are not readily available. Your molder may be able to go into detail for your application if cycle time is critical.

What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?

Question: What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?

Answer: The processing conditions can be optimized to match the tool design, part geometry, and material but each presents limitations. Injection process times are driven by material. Silicone parts can often be released with no draft angle or even undercuts due to the high elongation and low modulus that save time on actions but knock out pins may often damage parts. Silicone part removal is often done by automated brush, hand pull, compressed air, or other way that may take additional time compared to plastic.

Curing time can be reduced in silicone by increasing processing temperature until filling fails or surface quality diminishes. Plastic cooling rates may be more limited by internal stresses causing warping or property changes from rapid cooling. Other contributions to the cycle time include heating and cooling rates, curing or solidification time of the material, injection time, mold travel time, and other smaller contributions increase the cycle time.

Direct comparisons between cycle times of medical plastics and medical silicones are not readily available. Your molder may be able to go into detail for your application if cycle time is critical.

Are micro medical silicone parts easier to mold than medical thermal plastic parts?

Question: Are micro medical silicone parts easier to mold than medical thermal plastic parts?

Answer: The difficulty of molding micro parts is driven by challenges in tooling, processing, demolding, and handling. Silicone and plastic have a wide range of advantages and disadvantages and should be chosen for properties that best serves your application. The following is a few micro molding points to consider that may not be addressed in your standard data sheets.

Micro tooling for both plastic and silicone parts require small features and the mismatch between the plates is very accurate. For example, a 0.003” mismatch or tolerance in a 0.500” part represents a 0.6% difference compared to a 20% difference for 0.015” part. Most silicones have excellent formation of micro features and surface finishes even as small as or smaller than 0.0001”. The pressures and melt viscosity for plastics tend to be very high compared to silicone and may cause greater core or feature deflection compared to silicone but silicone may flash more easily.

Medical molding does run into some extra challenges because materials are more limited. Also releases and additives tend to be restricted in many medical applications so release and processing may require alternative approaches. The handling processes may be challenged by little space or no options for knock out pins or push plates but brushes, mechanical, or hand pull may be an option depending on the size and scale of the molding.  Many times, molding a micro part onto a large runner or with a puddle can ease handling issues by providing a handle and address parts approaching the minimum shot size of molding equipment.

When molding medical silicone parts in the same mold will changing the durometer change the dimensions of the molded part?

Question: When molding medical silicone parts in the same mold will changing the durometer change the dimensions of the molded part?

Answer: The final part dimensions will depend on inherent shrink rate and processing conditions. Different materials molded in the same mold with the same shrink rate should produce the same size parts.

Durometer changes require different materials or additives which often changes the shrink rates. Higher durometer materials often have a greater raw viscosity and may achieve greater cavity pressures that can sometimes produce slightly larger parts compared to lower durometer materials. The difference has been shown to be less than 1% for a 40 durometer change without resetting the optimal processing conditions. Generally, changing durometer in the same series for most suppliers will not significantly affect the final dimensions but a first article is recommended. If you are considering trying multiple materials then I would recommend letting your molder know as early as possible.

What are the differences between designing a medical silicone part vs. a medical plastic part?

Question: What are the differences between designing a medical silicone part vs. a medical plastic part?

Answer: The largest difference in design between silicone and plastic is the properties. Understanding the application and the material properties can help designers better utilize silicone in all applications. Silicone and plastic both have implantable and non-implantable grades and your application will dictate what grade you need. Applications may be medical in nature but not contact people but devices may also be implantable for a few days, weeks, or years or just contact skin or blood.

Designers trying to determine tolerances and fitting may want to keep in mind that silicone can deform more readily than most plastics to fit an assembly so your tolerance level may be dependent on the level of performance. For example a strain relief or compressed seal may not need as tight of tolerances as an optical lens or a pressure membrane. Silicone generally has high elongation and low hardness compared to most plastics so specifications may need to be adjusted accordingly. For example regrinding of silicone is not practical but straight walls and undercuts that may be difficult in plastics may not pose an issue due to the low hardness and high elongation of silicone. Flash and cuts which may not be a significant issue with most plastics is common in silicone.

 

What is the difference between part removal in medical silicone injection molding vs. medical plastic injection molding?

Question: What is the difference between part removal in medical silicone injection molding vs. medical plastic injection molding?

Answer: Part removal is dictated by the size, shape, geometry, material, and surface finish. In both processes the use of an automated handler or manual handling can be used to pull some types of parts directly from the mold.

Plastics tend to be significantly more rigid and lower elasticity than silicones. So plastic often requires drafted sides to prevent binding and undercuts in most plastics are not possible without actions. In silicone, the rigidity does not generally allow the use of knockout pins inside the cavity to push the part out. Silicone can often deflect or stretch during removal and then snap back to original shape meaning that draft is usually not needed and parts can often have undercuts without needing actions. The drawback is that knockout pins may cut or fail to remove parts since they stretch around pins. Compressed air, external brush, or sometimes push plates that contact a greater surface area than knockout pins can be used to remove silicone parts. High hardness, low tear strength parts tend to have more issues in release than those parts with high tear strength and high elongation. Silicone release generally improves with a sand blasted finish while the release may be more difficult with highly polished surfaces.

How small of a micro feature can be molded on a medical silicone part?

Question: How small of a micro feature can be molded on a medical silicone part?

Answer: Silicone feature size is limited by the geometry, tooling, and process. Silicone will replicate surface finish features with less than 0.0005 inches in size. Features larger than 0.005 inch can often be manufactured with some considerations. Part geometries such as undercuts, large L/D pins and bosses can cause challenges in demolding, maintaining tool integrity, and forming full features without bubbles.

Shrink in micro parts is often not as much of an issue compared to larger parts since 2% on 0.005 inches is pretty small compared to 2% on a 3” part. Other issues may be magnified such as the difficulty in part removal from a mold, core deflection, and mismatch. Large undercuts in parts made with a high durometer and low tear strength material tend to tear more easily during removal. Large L/D parts holes (made by pins) may deflect during molding causing tool damage and/or features to be irregular. For perspective take a caliper or micrometer and set it to 0.005 inches. Simple features that are smaller than 0.005 inches may be possible but they are inherently difficult.