Silicone materials can be compounded to create materials that are both, bio-inert and fully compatible with biological systems. Medical silicones are compounded in a similar manner as general purpose silicones to obtain certain characteristics, such as durometer, elongation and tear strength. However, medical silicones undergo additional processing to remove unwanted residuals. This allows for the purest silicones to be manufactured for use in medical devices.
Silicone manufacturers have created systems to determine which materials should be used for a variety of bio-contact applications. These suppliers have created in-house certifications that are based on the positive results of a series of USP (U.S. Pharmacopeial Convention) Tests. There are six separate USP tests, from Level I to Level VI. Materials that meet each of the six tests, are often referred to as “Grade 6″ materials.
To better understand the requirements of each USP test, please see the table below.
Silicone manufacturers have created several groups in which to place these various medical silicones based on their anticipated use.
- Food Grade Silicones: Silicones of this group must meet USP tests, as well as FDA and USDA guidelines and criteria. Materials from this group would commonly be found in food processing equipment in the form of seals or even spatula blades.
- Medical Non-Implantable: Silicones of this type do not see use inside the body. Typical applications include tubing and one-time use disposables.
- Medical Short Term Implantable: These particular silicones can be implanted into the body for up to 29 days. A few applications include catheters and surgical tools.
- Medical Long Term Implantable: Silicones from this group can be implanted for a period greater than 29 days. Typically these materials are used in cardiovascular implants and remain in the body until end of life.
- Pharmaceutical: Silicone materials used in the pharmaceutical space, must pass all six USP tests and must be compatible with any drug coming into contact with the part. Drug delivery devices, such as punctal plugs, are typical applications in this group.
Determining which medical grade silicone material is required is based on the function and use of the medical device to be manufactured. Albright’s staff of engineers can assist you in material selection so that your medical application’s requirements are met. Please remember that although many silicone suppliers have conducted USP bio-compatibility tests on their respective materials, all finished medical devices will still need further USP testing. To learn more about medical silicone materials visit our Silicone Material page.
Before manufacturing silicone parts, it is important to select a finished durometer for the part or product that you will be molding. Depending on the requirements of the project, the silicone finished part may be near gel-like or very stiff. Durometer is a measure of hardness that is used in elastomers, polymers and rubbers. Hardness may have many definitions, but in our case with regards to Shore durometer, hardness is defined as a material’s resistance to indentation.
The Shore durometer scale was created by Albert F. Shore in the 1920’s. Originally, the result was just signified by a number, for example 50 durometer. As time progressed, multiple durometer scales were created. The Shore durometer scales that are most commonly used in plastics, rubber and silicone are the A, D and 00 shore scales.
There are two main differences amongst these scales. First, the configuration of the indenter that is pressed into the material is different with each scale. Second, the hardness range is different with each of the three scales. The Shore 00 Scale measures rubbers and gels that are ultra soft. The Shore A Scale measures rubbers that range from soft and flexible to hard with almost no flexibility. The Shore D Scale measures the hardness of hard rubbers and hard plastics.
Below is a table of common finished products and their respective durometers.
Here at Albright, we use the Shore A scale to quantify our molded silicone durometers. The Shore A scale ranges from 0 to 100. An example of a “0” durometer Shore A silicone molded part would be a very soft shoe insole. Near the opposite end of the scale, an example of an “80” durometer Shore A silicone molded part would be an o-ring seal or a stiff silicone kitchen spatula. Every day, Albright molds products in materials that range in finshed durometer from 10 to 80 on the Shore A Scale.
What materials and durometers have you molded parts in? What durometer Shore scale do your materials come in? How do you determine what durometer of material to use?
Please remember that the durometer selected will be based on the requirements of your project. If you need assistance determining a durometer for your next silicone project, Albright engineers and technical staff are here to help. In addition, we can provide samples of silicone in various Shore A durometers. Want samples of silicone durometers? Visit our free durometer sample page on the Albright web site.
Silicone is being used as a material in wearables with computer sensors. The silicone is overmolded on to the computer sensors and then attaches to the skin like a band aid. Learn more here: http://nyti.ms/1wzdh0J
Now for some quick questions regarding wearables. Were you aware of the predominate usage of silicone in wearable applications? Silicone is great for overmolding applications, have you worked on any of these types of applications? If you have worked with silicone overmolding in the past, what advantages or disadvantages did you experience?
1. Our standard turnaround is 15 business days.
2. Three, five and ten business day expedites are available when you need parts fast.
3. Injection, compression and transfer molding processes are available.
4. Albright designs and manufactures all tooling in-house for complete control.
5. We have an ISO Class 7 hard walled clean room for medical and biopharmaceutical devices.
6. We stock a massive inventory of silicones to accommodate any application.
7. Albright’s Quality Management System is ISO 13485:2003 certified by TÜV SÜD.
The main benefit of Albright’s QMS for non-medical customers is the overall Quality System emphasis on improving compliance to repeated processes. The Quality System is especially important in the integrity of the silicone material Albright uses and in the consistent, high-tech quality inspection applied to finished parts.
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Silicone over-molding is a process that is used to cover, bond or encapsulate an existing part with silicone material. For example, a silicone over-molding process may be used for providing a grip surface to a smooth plastic handle, a flexible septum on a hard plastic surface or for encapsulating an electronics assembly for mounting or protection from the environment.
The motivation for over-molding silicone may include a means of final assembly, enhancing physical surface characteristics and feel, providing a self-healing internal access (ie, an integrated septum), creating the finished product enclosure (such as a key-ring LED light), creating a thermally conductive path for heat-sinking or perhaps applying elastic properties to mounting components.
Important things you should know about silicone over-molding:
1. Over-molding bonding strength typically increases over time.
2. Polymers that are ideal for over-molding include: polycarbonate, nylons and other high temperature resins.
3. In order to have a clean silicone over-mold, the over-molded part must be made with the requisite tight tolerance and part samples of exact size are needed. A stable-dimensioned part is needed to develop and maintain the desired clean over-mold outcome.