LSR overmolding for medical device components

Liquid Silicone Rubber (LSR) overmolding is a critical process for creating multi-material medical parts that combine rigid substrates with soft, biocompatible seals, grips, or diaphragms. This technique improves device ergonomics, reduces assembly steps, and ensures reliable performance under sterilization cycles. World Strong Rubber (WSR) leverages precise LSR injection molding expertise to deliver ISO‑13485‑compliant overmolded components for diagnostics, drug delivery, and surgical instruments. All medical components are produced under our medical silicone molding program with ISO 13485 quality management.

Key Design Parameters for LSR Overmolding

Designing LSR overmolds for medical applications requires a careful balance between material softness, substrate adhesion, and geometric fidelity. Engineers must consider the coefficient of thermal expansion (CTE) mismatch between silicone and the insert material; excessive mismatch can induce residual stresses that lead to delamination after repeated sterilization cycles. Selecting a substrate with a CTE close to that of the chosen LSR grade minimizes these stresses and improves long‑term seal integrity.

Surface preparation of the insert is equally vital. Plasma treatment or the application of a silane‑based primer creates reactive sites that promote covalent bonding between the silicone and the substrate. Without adequate surface activation, bond strengths can fall below the 1.5 MPa threshold required for many Class II and III devices, increasing the risk of seal failure under pressure or flexural loads. Designers should also incorporate venting and overflow channels in the mold to accommodate LSR’s low viscosity, preventing flash while ensuring complete cavity fill.

LSR Overmolding vs. Traditional Two‑Shot Molding

When comparing LSR overmolding to conventional two‑shot thermoplastic processes, the primary advantage lies in material versatility. Silicone can be bonded to a wide range of substrates—including stainless steel, titanium, PEEK, polycarbonate, and even certain elastomers—without requiring a chemical compatibilizer. This flexibility enables designers to combine the mechanical strength of metals or high‑performance polymers with the sealing and tactile benefits of silicone in a single part.

Another distinguishing factor is the compression set behavior of LSR. Silicone exhibits exceptionally low compression set (typically <5 % after 24 h at 70 °C), which means seals maintain their shape and sealing force over long‑term use. Thermoplastic elastomers used in two‑shot molding often show higher compression set, leading to gradual seal degradation and potential leakage in fluid‑handling devices. Additionally, LSR’s inherent resistance to microbial growth and its ability to withstand repeated gamma‑sterilization cycles make it preferable for implantable and reusable surgical tools.

Step‑by‑Step LSR Overmolding Process at WSR

The success of an LSR overmold hinges on meticulous control of each process stage, beginning with substrate preparation. At WSR, inserts undergo a multi‑step cleaning protocol that includes ultrasonic degreasing, isopropyl alcohol rinse, and a low‑pressure plasma treatment to increase surface energy. This treatment raises the contact angle of water on the surface from >90° to <20°, indicating a highly hydrophilic surface that promotes robust silicone adhesion.

Following preparation, inserts are placed using a six‑axis robotic system equipped with vision guidance. The vision system verifies positional accuracy to within ±0.02 mm and checks for orientation errors before the mold closes. This level of precision eliminates the need for manual alignment and reduces cycle‑to‑cycle variation, which is critical for maintaining tight tolerances on features such as fluid channels or mating surfaces.

1. Substrate Preparation – Clean, plasma‑treated, or primed metal/plastic inserts to achieve >90 % bond strength.
2. Insert Placement – Precise robotic positioning within the mold cavity, maintaining ±0.02 mm positional tolerance.
3. LSR Injection – Metered A/B components mixed at 1:1 ratio, injected at 1000 bar, 175 °C mold temperature, using precision LSR injection molding equipment.
4. Cure & Demold – Controlled cure time (typically 15‑25 seconds) followed by ejection with <0.1 mm flash.
5. Post‑Processing & Inspection – Visual inspection, durometer testing (Shore A), and dimensional CMM verification per our quality control protocol.

Design for Manufacturability (DFM) Tips for LSR Overmolds

Applying DFM principles early in the design phase can prevent costly redesigns and ensure a smooth transition from prototype to production. One key recommendation is to incorporate generous draft angles (≥2°) on all vertical walls that interface with the silicone. Adequate draft facilitates easy part ejection and reduces the risk of tearing the thin silicone flash during demold, especially when overmolding delicate features such as micro‑channels or thin‑walled grips.

Another important consideration is the placement of gates and vents. LSR’s low viscosity allows it to flow into fine details, but it can also trap air if vents are insufficient. Designers should locate vents at the highest points of the cavity and use porous venting inserts or micro‑vent channels to evacuate trapped gas. Proper venting not only eliminates voids and burns but also ensures consistent durometer across the part, which is essential for predictable sealing performance.

1. Include ≥2° draft on all silicone‑facing walls to aid ejection.
2. Position gates away from critical sealing surfaces to avoid weld lines.
3. Use radius fillets (≥0.5 mm) at corners to reduce stress concentrations.
4. Design uniform wall thickness where possible to minimize differential shrinkage.
5. Incorporate overflow wells to capture excess material and prevent flash.

Partner with World Strong Rubber to accelerate your medical device development with proven LSR overmolding capabilities. Our team supports material selection, design for manufacturability, and full validation documentation. Request a quote today to discuss your project requirements.

Related: Custom LSR mold making · WSR certifications · Medical silicone products