Medical ECG Button via LSR Liquid Silicone Molding

The Challenge

Liquid silicone rubber is a different discipline from thermoplastic injection. It is a two-part platinum-cure liquid that is metered, mixed, and injected into a heated mold where it vulcanizes in seconds, then demolds as a fully cured elastic part. The value of the material is its elastic recovery, its inertness, and its skin-safe biocompatibility, but those same properties make it unforgiving on tooling and process control. The ECG button had to be elastic, comfortable against skin for long monitoring sessions, and held to ±0.03 mm on its sealing and engagement features.

The medical context raised the bar. The button sat against patient skin in a wearable ECG monitor, so the LSR had to meet skin-contact biocompatibility and leave no extractable that could irritate. Any silicone oil bleed, uncured fraction, or particulate from the mold would be a compliance failure, not just a cosmetic one. The part also had to seal reliably against the rigid backing so electrode contact stayed clean.

Process control was the hard part. LSR cures in a hot mold (typically 150 to 200 °C) within roughly 10 to 30 seconds, and the two components must be metered at an exact one-to-one ratio with no air entrainment. A ratio error or a cold spot in the mold produces uncured or partially cured silicone, which is a latent field failure. Because the material is liquid at injection, flash and bleed at parting lines are constant risks that the tool must suppress at the design stage.

The assembly combined materials. The elastic LSR button was paired with a rigid thermoplastic member and a CNC-machined backing to form the complete actuator, so the LSR molding cell had to deliver parts whose geometry mated exactly with the rigid and machined components at an annual volume of 300,000 units. The 7-week rapid-tooling window left no slack for mold rework.

Regulatory evidence was part of the job. A skin-contact medical part requires material biocompatibility documentation and a clean manufacturing environment, and the LSR process had to be shown capable and controlled, not just producing good-looking samples. Any change to the material lot, the cure temperature, or the mix ratio had to be treated as a potential requalification event.

Scale and consistency at 300,000 units per year demanded a cell that ran the same way every shift. LSR is unforgiving of variation: a meter ratio drift of even a small amount changes cure and properties, and the elastic part gives little visual warning of an under-cured condition. The process therefore needed closed-loop control and in-line verification rather than end-of-line sampling alone.

The two-part liquid chemistry is unforgiving on ratio. LSR is metered and mixed at one to one, and a deviation of even a small fraction changes the crosslink density, which changes the cure, the hardness, and the long-term elasticity of the button. Because the part is soft and gives little visual cue when under-cured, the metering had to be closed-loop and verified rather than set once and trusted.

Mold venting and flash control decide yield. LSR is injected as a low-viscosity liquid that will find any parting-line gap, so the tool must vent correctly and seal tightly or it flashes and the part fails. For a skin-contact medical button the flash is not just cosmetic; it is a sharp edge against skin and a potential site for material debris, so the tool design carried more weight than for a rigid part.

The program also had to respect the device it actuated. The LSR button sat in a wearable ECG monitor where its travel, its seal, and its feel affected both the measurement contact and the user experience, so the molded dimensions had to hold relative to the rigid and CNC mating parts, not just to the LSR drawing. A button perfect in isolation but off against the backing would still fail the device, so the tolerance was managed across the assembly.

The Solution

We stood up a dedicated LSR molding cell with in-line metering and mixing so the two platinum-cure components were delivered to the nozzle at a verified one-to-one ratio with closed-loop ratio control. The metering pump and static mixer were qualified so that every shot had a fully cured, consistent chemistry, and the machine was run with dehumidified feed to keep the cure stable.

The mold was built for the material rather than adapted from a thermoplastic tool. Tight parting-line fits and vented cavities controlled flash and trapped air, and the mold temperature was held uniformly across the cavity so cure was complete everywhere within the cycle. Post-cure was applied where needed to drive the part to full physical properties and to eliminate any residual volatiles, supporting the skin-contact requirement.

The LSR button was then paired with the rigid thermoplastic molding and the CNC-machined backing in a matched assembly. Dimensional control on the LSR side was verified so the ±0.03 mm sealing and engagement features mated to the rigid and machined parts without rework, and first-article inspection confirmed the elastic recovery and sealing behavior.

Rapid tooling was used to hit the 7-week window. A soft-steel or aluminum LSR tool with the correct parting-line and venting design was cut and validated, and the cell was released to the 300,000-unit yearly volume once the cure consistency and biocompatibility-related cleanliness were confirmed.

Quality was verified at the material and the part level. The one-to-one metering was confirmed and the cure was checked so every shot was fully vulcanized, and first-article inspection confirmed the ±0.03 mm sealing and engagement features against the rigid and CNC mating parts. Post-cure and cleanliness controls supported the skin-contact requirement.

The cell was set up for repeatable volume. The rapid tool and the locked process parameters let the 300,000-unit yearly run proceed without per-shift tuning, and the documentation gave the medical device OEM the traceability expected for a patient-contact component. A spare tooling strategy kept the program available if wear appeared.

We controlled the chemistry at the source. The metering pumps and the static mixer were qualified and the one-to-one ratio was monitored so every shot cured fully and consistently, and post-cure was applied where needed to drive the part to its final properties and to remove residual volatiles. The result was an LSR button whose elasticity and skin safety were baked in by the process, not inspected in afterward.

The mold was built for the material. Tight parting-line fits, correct venting, and a validated temperature profile gave complete cure without flash, and first-article inspection confirmed the ±0.03 mm sealing and engagement features against the rigid and CNC mating parts. The rapid tool and the locked cell parameters let the 300,000-unit yearly run proceed without per-shift tuning.

We qualified the button as part of the assembly. The LSR molding, the rigid thermoplastic, and the CNC backing were measured together so the ±0.03 mm features located correctly against each other, and the process was locked to hold that relationship across the 300,000-unit run. The 7-week rapid tooling delivered a cell whose output dropped into the wearable without a fit iteration.

The Result

The program produced elastic, skin-safe LSR buttons held to ±0.03 mm and shipped as part of the complete rigid plus machined assembly. Cure consistency was verified across the run, giving the medical device OEM a wearable actuator that met patient-contact requirements without post-mold bonding.

The 7-week rapid-tooling schedule was met and the cell sustained 300,000 units per year. The LSR button sealed cleanly against the rigid backing, holding the electrode interface free of ingress across the production volume.

First-pass yield met the program target, and the LSR button sealed cleanly against the rigid backing without post-mold bonding, removing an assembly and a failure mode from the wearable actuator. The elastic, skin-safe part met the patient-contact requirement across the production volume.

The 7-week rapid-tooling schedule delivered a cell the medical device OEM could carry into series production with confidence, and the documented process and material controls supported the regulatory evidence needed for the device. The validated approach gave a baseline for follow-on wearable button programs.

The LSR button met the wearable requirement without a bonded assembly. Molded platinum-cure and paired with the rigid thermoplastic and the CNC backing, it sealed cleanly against the rigid member and gave the elastic, skin-safe actuation the ECG monitor needed, all while holding ±0.03 mm on the features that mattered. The 300,000-unit yearly volume ran from a single validated cell.

The 7-week rapid-tooling window was held and the medical device OEM received a part with the material controls and traceability a patient-contact component demands. The documented process supported the regulatory evidence for the device, and the validated approach gave a baseline for follow-on wearable button programs using the same LSR discipline.

The ECG button performed as an assembly component, sealing against the rigid backing and actuating the wearable as designed across 300,000 units per year. By holding the ±0.03 mm features relative to the mating parts, it let the medical device OEM build the monitor without rework, and the documented LSR process supported the regulatory evidence the device required.