Liquid Injection Molding (LIM) for Medical Device Manufacturing – Part 2

Silicone is typically used in liquid injection molding (LIM). It is a type of synthetic liquid rubber. There are different manufacturers and durometers of silicone that can be used in LIM, each with benefits and uses for specific applications.

Liquid Silicone Rubber (LSR), typically platinum cured silicone, is probably the most common type of silicone used in liquid injection molding. It is biocompatible and very durable, making it the silicone of choice for medical devices and instruments.

Fluoro Liquid Silicone Rubber (F-LSR), using fluorosilicone, as its name suggests, incorporates the element fluorine as a way to improve chemical resistance in the final product. Fluorosilicone is a popular choice for automotive parts as well as precision parts and tinier parts or parts.

When designing parts and products using silicone liquid injection molding, certain considerations should be kept in mind. Generally, the design constraints are not all too dissimilar from thermoplastic injection molding. Here are some potential issues to be aware of:

Shrinkage: Liquid silicone rubber will not shrink when it is inside the mold as much as thermoplastic does. However, once the silicone is removed from the mold, it can shrink as much as 3% while curing.

Venting: Trapped air can ruin silicone liquid injection molding, so venting must be carefully instituted to avoid air building up in the mold. Complex part designs may require the use of an in-mold vacuum.

Hot tearing: The point at which the silicone component is extracted from the mold is critical, as hot tearing can occur. Working with an experienced LIM manufacturer like Flexan can mitigate this issue.

As mentioned above, two types of silicone are typically used in liquid injection molding, Liquid Silicone Rubber (LSR) and Fluoro Liquid Silicone Rubber (F-LSR). In medical device manufacturing, LSR is commonly used to produce biocompatible components.

To create a part with Liquid Silicone Rubber (LSR), an injection molding machine will typically use a metered dosing unit that delivers two components and Part A and B (one of which has the platinum-based catalyst) are mixed into the homogeneous suspension in the static mixer first before entering the injection molding machine. The silicone does not cure unless both parts are mixed together.

Silicone liquid injection molding machines include injectors, metering Units, supply drums, mixers, nozzles, and mold clamps. In brief, the injection device will move the silicone into the mold via pressure, where the material needs to set. The machine will be cooled to keep the silicone cool and uncured until it reaches the mold where the heat of the mold will set the part to its final geometry. The mold needs to be properly vented, or an in-mold vacuum utilized, to protect the component from bubbles or leaks. Sometimes a cleanroom is necessary, especially when medical devices are being produced.

This is a simplification of a very complex process that involves various moving parts to ensure a good final product.

Prototyping with silicone liquid injection molding can be very helpful, especially in the final states of the prototyping process. Depending on the product or component, 3D printing, aluminum molds, and casting silicone may be a more economical and flexible way to prototype in earlier stages, but often partners cannot achieve the functionality they desire for these types of prototypes.

Prototyping with LIM silicone has some advantages, such as the ability to test and finalize the product before production. The mold used in prototyping, provided the results have been approved, can also be used for production.

Founded in 1946, Flexan delivers the best in contract custom manufacturing solutions for partner medical devices. With decades of successful product component manufacturing as contract manufacturer, we offer unparalleled experience and knowledge in molding, including liquid injection molding. We know the best ways to leverage liquid injection molding to create the best medical devices while saving you time and money.

With our unique Flexacution model – design, develop, deliver – we combine the best in quality and engineering creative product development and manufacturing solutions. We are always looking for ways to incorporate the latest manufacturing technologies to help our clients. We are constantly innovating in areas such as silicone liquid injection molding, thermoplastic molding, micro molding, compression molding, and transfer molding. With ISO 13485 certification, FDA Registration, and a robust Quality Management System (QMS), you can be assured you are in good hands with Flexan.

If your medical device company could benefit Flexan’s experience bringing medical devices to market, contact our sales team today.