Question: When you buy a certain durometer of silicone from the manufacturers, what is the actual durometer are you getting?
Answer: Silicone is a very useful material that has been widely used in both non-medical and medical application for decades. For example, silicone has a wide range of operational temperatures (between -150 °F and -600 °F). Moreover, silicone has another interesting property that enables it to be widely used. It has a wide hardness range without additional additives. Shore A scale is typically used to identify the silicone’s hardness and liquid silicone rubber (LSR) is typically available from 01 to 80 durometers (typically in increments of 10 durometers). Nevertheless, there is a variation in hardness when you buy silicone from the manufacturers. The industry standard tolerance for silicone is ±5 durometers. This is due to the variation in the vinyl gums and treated filler used to make them plus the ±2 potential test error in determining the durometer. As a result, you normally don’t get the exact durometer of silicone as listed from the manufacturers; you only get a range of durometers that is close to the desired durometer.
How is the “softness” (durometer) of medical grade silicone measured?
This is a question that comes up almost every day when discussing new project ideas with perspective clients. The softness or durometer of all polymers/elastomers/rubbers is measured essentially the same way and the term while commonly used to describe the material hardness, actually originated from the device used to take the measurement a duro-meter. There are lots of different designs for this device, click here to compare pricing and options and get a general feel for what they look like and how they work.
Click here to read the rest of our October Newsletter!
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.
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.
Last year we ran a contest asking for your silicone related questions and had a great response! We then had our engineers answer all of your questions and posted them here. In order to give the best service and information possible, we are again asking for industry professionals to submit their silicone questions.
Click here to find out more and submit a question!
*The winning name will be chosen on October 1, 2012.
Question: What is the typical flash tolerance for a silicone part?
Answer: Silicone has a relatively low viscosity compared to other TPE (Thermoplastic elastomeric) or thermoplastic. The silicone’s viscosity enables us to more easily mold micro parts, but it also presents a challenge with flash. The typical flash tolerance for a silicone part is .005”. However, the tolerance can vary widely depending on the individual application. For example, a simple non-medical gasket may have a flash tolerance of .025” because the priority of the gasket is to provide adequate sealant in its application. On other hand, micro silicone components of a catheter may have a flash tolerance as low as .001” or less because a couple thousandths of flash or more may irritate the patient when in contact with the patient’s body parts. In addition, larger flash is a potential cosmetic defect since the components are very small. Finally, the flash tolerance can affect the cost of a part; usually the smaller the flash tolerance is, the more expensive the part is. This is due to manufacturing and inspection challenges.
Question: Is medical silicone more expensive than thermal plastic elastomers?
Answer: In short answer, yes, typically medical grade silicone is more expensive than thermoplastic elastomers. But there are many factors that need to be taken into account, that all have an effect on the cost of material.
Silicone rubber (especially medical grade) excels in biocompatibility, however there are different degrees to which the raw material is biocompatible. Some medical grade silicones are acceptable for human contact, others are acceptable for blood and bodily fluid contact, others still are acceptable for implantation, and implantation is further broken into short term (29 days and under) and long term (indefinite). With every additional step up the biocompatibility later, the cost of the material goes up. A short term implantable material may cost five to ten times as much per pound as a contact grade material, and a long term implantable material may cost ten or more times the cost of the short term implantable. And by the time you get to drug eluting silicones, you might be talking tens of thousands of dollars per pound.
Fortunately, most implantable medical devices are quite small, and molding processes are highly controlled to deliver accurate shot sizing, so while the price of material is prohibitive, silicone molders do all they can to minimize waste material, while simultaneously maximizing the level of part quality.