All about liquid silicone rubber with medical devices

Liquid silicone rubber (LSR) injection molding has been around for years. Its use has significantly expanded recently, especially in medical devices and wearable technology. LSR cures faster and offers properties not obtainable with traditional rubber materials, especially heat-resistance, extreme low-temperature flexibility, chemical resistance, biological inertness, and an intrinsic capacity for reducing friction. The material’s expanded use has resulted in the development of new LSR process equipment, especially technology that optimizes LSR injection molding machines to provide the greatest value and ease of use.

LSR basics

The basic raw material for silicone rubber is sand, or silicon dioxide. The material is processed into pure silicon. It is then reacted with methyl chloride, after which a range of processing steps create a variety of silicone types, including liquid.

LSR is a two-component reactive chemical with a thick, almost paste-like consistency, which has been compared to peanut butter. The two components are usually shipped in separate containers. Some medical-grade silicones are shipped in small disposable plastic cartridges. The two components are mixed in a 1:1 ratio to produce a reaction. Accelerated by heat, the two liquids then change to a rubber.

LSR injection molding is an inherently clean production process, because the component chemicals are sealed within a closed system. No ambient air contacts the parts until they are removed from the mold, eliminating issues with dust and moisture. This also improves part quality, because contaminants can diminish the cured rubber’s physical properties.

Medical, wearable benefits

Use of LSR is growing in both traditional rubber applications and those where traditional rubber materials had not previously been used. Key examples include medical devices, wearables, automotive, industrial, and even home goods (see sidebar).

Medical devices – LSR cures completely and quickly. This is especially critical when medical devices are placed in a patient’s body, because it means the device will not leach chemicals and cause potential adverse reactions. By contrast, latex, a material long used in the medical industry, does not fully cure during production, and can lead to adverse patient reactions.

Due to LSR’s chemical makeup, it does not degrade until heated to very high temperatures – much higher than most other polymers could tolerate. So LSR can handle sterilization processes, contributing to its effectiveness for medical and baby care uses.

A final (and critical) advantage is the ability to use LSRs to manufacture drug-eluting devices (DEDs). For example, hormones used in the NuvaRing contraceptive product are injected as an additive in the LSR dosing process. LSR DEDs can also be placed in pacemaker heart catheter leads, enabling the leads to introduce anti-inflammatory medication directly into heart tissue for improved results.

Wearable technology – Wearable fitness trackers, such as FitBit and Jawbone, are largely responsible for the expansion of the flexible wearables category. With its ability to handle both high and low temperatures, ultraviolet (UV), and ozone without degrading, LSR is a better fit than traditional materials for wearable technology used under constant sun exposure. Unlike other rubber, products manufactured with LSR are unlikely to cause adverse skin reactions when worn by users, even for extended periods of time.

Optimized production process

To achieve LSR’s benefits, injection molding machines must be optimized for value and ease of use.

While LSR equipment is similar in many ways to that used in the plastics industry, manufacturing LSR tools in the same manner as a plastic tool can lead to production failures. It is essential to use tool makers with a history of making LSR tooling. Also critical is working with an injection molding machine company that can assist with processing challenges, since successful LSR manufacturing requires that all components work properly together.

The most common pain points in LSR manufacturing are managing waste and controlling color changes and additives. Excess material is wasted because it is difficult to reclaim due to air bubbles, loss of certification, and a lack of lot tracking. Color changes can pose production down time, because extensive cleaning processes between colors can take as long as 4 to 6 hours. In addition, control of color or additives is a concern, especially controlling functional additives in the medical device industry.

Waste and increased additive control can be addressed through closed-loop control system technology. For example, Graco Fluid Automation F4 series systems use a dosing valve and a high-resolution flow meter to provide a closed-loop control for third- and fourth-stream additives, such as color and medications. The system monitors and adjusts to ensure the additive is being dispensed in the appropriate amount. If there is an out-of-tolerance condition, the system stops production.

Controlling the flow of the two primary material components in a closed-loop system allows the machine to react to changes in the material viscosity and the presence of air bubbles. Operators can vary the ratio to ensure the correct amount of material is used.

Closed-loop-control of two-component LSR dispense ratio is achieved by monitoring the material flow using high-resolution, helical gear-style flow meters. The helical gear uses multiple gear teeth to measure the flow in small increments. Flow meter data is fed back to the controller, which operates the valve to alter the flow of material to the flow meter, forming the closed loop.

The increased number of measurements provides more assurance that the machine is running on-ratio, and significantly reduces waste and rework caused by off-ratio dispensing.

The system offers a calibration routine that can be performed by the end user as necessary for a particular process, which also has a significant impact on product quality. The sample is collected and weighed, and resulting data is entered into the display module, calculating the current actual dispense ratio and calibrating the control system.

Other controls monitor processes to reliably manage the LSR system for its entire life cycle. The Graco Control Architecture (GCA), for example, provides longer life cycles than standard PLC products, and has a faster response time than other control architecture types.

LSR at the leading edge

In a state of rapid expansion, LSR continues to offer new and improved materials to replace older technologies with longer-lasting, more effective solutions. Improvements to LSR physical properties for individual applications mean LSR will likely continue replacing traditional rubber materials in existing industries and possibly others. With the advanced dispense and production technology currently on the market, manufacturing of LSR products can be managed to minimize problems and take full advantage of this material’s wide-ranging potential.

About the author: Mike Pelletier, business development manager at Graco Inc., can be reached at mpelletier@graco.com or 248.635.8817.