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.
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.
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: What is the shelf life of uncured medical silicone?
Answer: The typical shelf life of uncured medical silicone may range from 6 to 24 months; it is normally labeled with a “best use before” date. If the medical silicone has passed the recommended date, it still can be mixed, molded, and used. However, the properties of the medical silicone are likely to be affected. Therefore, the medical silicone parts must be thoroughly inspected and tested to insure that all the necessary requirements are met before using them.
Question: Why do you have to clean the barrel and feeding system in medical silicone injection molding when changing the material?
Answer: Cleaning the barrel and feeding system is essential in medical silicone injection molding because it significantly reduces the risk of contamination during the molding process. In thermoplastics injection molding, the barrel is simply purged a few times when changing from one material to another. However, this approach doesn’t completely remove all the material in the barrel and feeding system in silicone injection molding; varying amounts of mixed silicone and purging agent (typically part B of the silicone) will remain in the barrel and feeding system due to their tackiness and low viscosity. As a result, the residual can ultimately get mixed in with the desired material during the subsequent molding process. In addition to cleaning the barrel and feeding system, they should be purged through at least once to remove the cleaning agents.
Although cleaning the barrel and feeding system is a time consuming processing, it helps to reduce variation in the molding process. Relying on purging the barrel and feeding system without cleaning could be a costly mistake because the resulting contamination is often not visibly detectable. For example, suppose that two different materials are molded in succession with only purging during the transition; the only difference between the two materials is their durometers (hardness). In this case, the contamination from the residual is often visibly undetectable in the finished parts because the residual can simply blend in with the desired material to form a homogenous phase unless the residual is cured before mixing. Imagine that the contamination is eventually found. All the previous molded parts would become suspect and excessive tests would be needed to insure their quality. In addition, the barrel and feeding system would need to be purged again or cleaned. As a result, the time and resource are not effectively used, and a simple procedure becomes longer and more labor-intensive. Therefore, the cleaning the barrel and feeding system is highly recommended when changing material.
Question: Why is aluminum and not steel used to build prototype molds for molding medical silicone parts?
Answer: Aluminum is used to build prototype molds because an aluminum mold is easier to machine than a steel mold. As a result, the aluminum mold can be built faster than the steel mold which leads to a lower cost in building prototype molds and a shorter lead time. Due to the fact that the aluminum mold doesn’t have as long tool life as the steel mold does, the prototype molds don’t require the longevity of tool life due to prototyping purposes. Therefore, the aluminum mold is generally preferred over the steel mold for molding medical silicone parts.
Question: Why should we prototype medical silicone parts?
Answer: Medical silicone’s biocompatibility is well known and silicone has been widely used for many individual medical applications in the medical device industry. However, medical devices made from medical silicone are still required to be tested for their biocompatibility. In addition, the medical devices sometimes need to be redesigned and tested for specified applications several times before manufacturing and introducing the device into the market. Therefore, the device should be prototyped first to determine whether the device can be manufactured and it meets all the requirements of the application. As a result, prototyping is recommended to minimize undesired outcomes.
Question: What durometers are available in liquid silicone rubber?
Answer: Liquid medical silicone rubber is typically available from 01 to 80 durometers. Durometer is a measure of the indentation hardness of a material. The procedure for determining indentation hardness of a polymer material can be found in ASTM D2240. There are twelve different durometer types: A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Nevertheless, type A and type D are the most commonly known and used in the industry.
Type A is generally used for soft and flexible polymer materials and type D is used for hard material. Therefore, liquid medical silicone rubber is generally labeled in type A durometer. Silicone durometer is normally categorized in an increment of 10, but the distribution of the durometer can range from minus to plus 5 durometer from the labeled value; industrial standard allows +/- 5 durometer that includes +/- 2 potential test error.
Nevertheless, the durometer of liquid medical silicone may be customized to meet the specifications of individual applications. The hardness of a silicone can be modified by changing the crosslink density, filler concentration, or both.
If you have any other questions, please email Phayhean Soo directly at email@example.com.
Question: I’d like to know more about implantable medical silicone.
Answer: Implantable medical silicone has the capability of being implanted in a living body without the risk of rejection. Commonly, the implantable medical silicone is categorized into two types: short term (restricted) and long term (unrestricted) implantable silicone.
The short term implantable medical silicone is used for a temporary medical application – normally ranging from 1 to 29 days. For example, a suture sleeve is made of short term implantable silicone to hold parts of a medical device to keep them in place during a suture. Once the suture is done, the suture sleeve is removed from the patient’s body. The long term implantable silicone should be able to remain inside the patient’s body for 30 days or more. A good example of long term implant application is the Left Ventricular Assist Device; this device helps the patient maintain the pumping ability of a heart that can’t sufficiently pump blood throughout the body on its own. This device isn’t removed until the patient has a donor.
Each medical silicone implant application requires certain implantable silicone. A medical device containing implantable medical silicone or other biomaterials must be carefully evaluated according to ISO 10993 before it is implanted into a patient’s body; the ISO 10993 contains a series of standards for evaluating the biocompatibility of the device. Also, it sometimes is tested according to ASTM (American Society for Testing and Materials) depending on individual application.
There are commercially implantable medical silicone materials available in high consistency silicone rubber (HCR) and liquid silicone rubber (LSR). Color additives can be added to meet the requirement of a medical application, but it is recommended that the color additives should have the same class and manufacture as the implantable silicone to prevent defects. The implantable medical silicone can also be mixed with additives such as tungsten and barium that allows the implants to be viewed easily with medical imaging equipment.
Therefore, selecting an implantable medical silicone for a medical device should be thoroughly evaluated prior to implantation. If you have any other questions, please email Phayhean Soo directly at firstname.lastname@example.org.