What is Medical Silicone?


Customer Questions & Casco Responses:


What is Medical Silicone?


Medical Grade Silicone generally means one of two things confirmed by this Wikipedia link:

  • Medical-Long Term Implantable Silicone designed to stay in the body normally as part of an implantable device. This grade is generally too expensive for Menstrual cups and food grade applications.

  • Medical-Healthcare Grade, Class VI Silicone, tested for Biocompatibility. Medical Class VI is the material choice for Casco Bay Molding’s Menstrual Cups, Baby Bottle Nipples, food & skin contact, Scuba & Athletic Mouthpieces.

Both Medical-Healthcare Grade, Class VI Silicone and Medical-Long Term Implantable Silicones are considered safe for food contact.  Both grades are almost always LSR (liquid silicone rubbers) that are normally processed via injection molding.

What types of Silicone are available? 


There are four major types Silicone commercially available:

LSR- Liquid Silicone Rubber: LSR is designed for injection molding and is almost always Medical Grade Silicone delivered to processors in two sealed drums or 40 lbs. buckets. One drum/bucket has catalyst, the other has the cross linker. LSR advantages are that it is pumped directly from the drum/bucket into the molding machine with little to no possibly of modifying the material with the exception of silicone colorant.  Most medical parts are LSR.

HCR- High Consistency Rubber: Premixed, playdough-like that starts with a “base stock” of silicone then a catalyst and fillers/additives are added to silicone base stock on a two roll mill. Once this step is complete, the HCR is brought to the compression molding press (to make parts) or extruder (to make tubes).
HCR has the advantage of being able to mix in different additives or fillers to change the Silicone’s physical characteristics. This can also be a disadvantage if the additive & filler amounts and ingredients are not shared with the end user. It is also tempting to use extra filler (filler is much cheaper the silicone base) to make more parts with the same amount of silicone base material. Frequently evidence of filler can be seen in wrist bands, children’s toys and even menstrual cups that turn white when stretched.  HCR can be a Platinum, Peroxide or Tin cure system, see below for more info on cure systems.

FSR- Fluorosilicone Rubber: flame resistant, brake fluid resistant silicone used mostly in the auto industry.

RTV- Room Temperature Vulcanize Silicone: RTV solidifies or vulcanizes at room temperature. It is used in prototype parts & Silicone caulks fall into this category.

What does Peroxide and Platinum Cure mean?


Silicones cure (chemical reaction) over time and temperature once a Platinum, Peroxide or Tin catalyst is added to silicone.  LSR & HCR cure relatively quickly once exposed to high heat 350F +/-60F. At room temperature it may take days (LSR) or weeks (HCR) to cure.

Three most prevalent silicone curing systems:

  • Platinum Cured: LSR (liquid silicone rubber) is almost always Platinum catalyzed. Properly molded Platinum Menstrual cups do not need to be post cured. Platinum Cured LSR is generally medical silicone and what we use at Casco Bay Molding.

  • Peroxide Cured: A good deal of HCR Silicone is Peroxide Cured and used for extrusion, transfer or compression molding. Peroxide cured HCR need to be Post-baked in order to remove the volatiles, this can be a time consuming & expensive step. If a company tries to cut cost by shortcutting the necessary post-bake step, volatiles can remain in the silicone. Peroxide cure systems are generally much less expensive that Platinum systems but require a time-consuming post-bake.

  • Tin Condensation Cure Systems: Not recommended in healthcare or medical applications because the Tin (Sn) remains in the compound after curing.

Why does Silicone turn white when stretched?


Fillers are less expensive that Medical silicone.  HCR (High Consistency Rubber) is frequently loaded with filler that turns white when stretched. However, stiffer durometers (60, 70 and 80 Durometer) has some amount filler required to stiffen the material.  Generally this is still not noticeable when stretching LSR parts.

Will all vulcanized silicones in compression molding turn white?

"That was a company's reply when asked about their “Medical Grade Silicone” Menstrual Cup (made in China). One retail company said that it’s a flaw in the material due to high curing temperatures. Another said that it depends on which molding process is being used; compression molding or injection molding, because the molding process changes the silicone structure."


Vulcanization is a chemical process for natural rubber and man-made silicone.  For both LSR and HCR the silicone processor should not to try to mix in extra filler.  The compound should be used as received.

Is Vulcanized Compression Molding the best?  Why or why not?  Do you feel another process is superior to this one?


 Different part manufacturing methods (compression, transfer, injection) should be selected on a cost vs. performance basis.  In our opinion, Menstrual Cups are best produced with as few volatiles and smallest amount unknown fillers and additives (Platinum LSR).  We believe strongly in full material traceability the same procedures we use for medical device component manufacturing.

With Peroxide cure HCR, extra filler should not be added and a proper post-bake should be conducted to remove all volatiles.

Do you have any thoughts about silicone and high heat?

"Some companies suggest boiling a menstrual cup to sanitize it, while others suggest against it. I was under the impression that a silicone (Food and/or Medical grade) were created to withstand high temperatures for cooking and/or autoclaving."


We strongly recommend boiling your cups for sterilization, especially for women in nations where clean water is an issue.  We also recommend boiling our Casco Case made from Medical Silicone and plastic.

Western women have the luxury of using sanitary wipes, clean damp paper towels or their sink when changing their cup during menstruation.

Real silicone has no issue with boiling water. We do not recommend a hot griddle or frying pan because temperature is hard to monitor.  Over 400F may cause discoloration but may not change physical properties of the silicone. Silicone is flammable; it should never be exposed to an open flame.

What about Ash Testing?

"Another path my search took me, was through silicone sex toys. People have claimed that they test their toys by doing the “ash test”. This is the process of burning a good portion of the silicone item to see the outcome. They claim that a Medical Grade Silicone will not continue to flame, will not melt, and will leave behind a pale gray ash and show no or minimal signs of damage."


The Ash Test is a plastics laboratory test to check for volatiles in the plastic.  There was a difference of opinion on silicone.  For any skin contact or short term-internal body application we recommend Platinum cured Medical grade ClassVI Silicone.

Crystal clear, extreme stretch and jelly look of the material means it’s not silicone?

"This came up on a sex toy site that “pure silicone will never be crystal clear and see-through." I have always preferred a clear cup. It allows me to see if there are any flaws inside of the silicone. However, if it’s true that a “pure silicone” will never be clear, then I’ve been going about this all wrong! Do you know if this is true in Medical Grade Silicone since the site states “Pure Silicone”?"


Not necessary true, silicones can have very high elasticity and regarding clear silicone, with the advent of LED lights there are a few optical grade silicones on the market. With injection molding grade LSRs, mold surface finish has a lot to do with how translucent the Menstrual cup appears. The Casco Cup has slight texture for a frosted look, while we polished the soft tip of the Menstrual cup.  HCR generally is less translucent, especially if they use fillers.

Sex toys are relatively unregulated, not a lot of material certifications or traceability.

I don’t doubt that some menstrual cups are made with lesser silicones/materials or “fluffed” with fillers to cut the cost of making them, but how are we to know?

"With Menstrual Cups becoming more popular all over the world, there have been a lot of cups being made that are questionable. Some are very thin or flimsy. Several are even too soft to seem usable."


We could not agree more which is why we see a place for ourselves in the menstrual cup market.  We want to assist women here and in developing nations around the world with a safe menstrual cup they can trust.

Our cup was primarily designed by two women, Liz at Casco Bay Molding with input from Chelsey of Green Cup of Maine.  We spent a lot of effort finding the right combination of part thicknesses, overall design and proper material selection.

Our cup is FDA Registered, has full material traceability, is Platinum cured, Medical Class VI Liquid Silicone Rubber, produced in an ISO Certified factory, sold factory direct to consumers, US stores, NGOs, foreign distribution partners. We also private label cups for other companies under the same manufacturing and quality conditions.

I wanted to inquire these questions to a silicone company and/or manufacturer directly for some concrete answers. 

"I feel that retail companies can and will say anything to ease our mind to sell us their products. We are inserting these items into our bodies for long periods of time. My concern is for my health and safety as well as others that use these products, with my own daughter and nieces in mind."


We could not agree more! My partner, friends, colleagues and daughter use our cup. We want it to have a superior design, produced from the best material possible made by people who care!


Note from Casco Bay Molding: The information above was in response to a series of questions I received by a truly independent Menstrual Cup expert.  I discussed these questions with three trusted silicone chemists that I have known for years.  The chemists have all worked for decades at least two silicone manufacturers including: Momentive/GE, Wacker, Dow and Blue Star.