The Real Challenges of Silicone Overmolding: Preventing Delamination and Flashing

Silicone overmolding (also known as insert molding) is a highly specialized process where liquid silicone rubber (LSR) or high-consistency rubber (HCR) is molded directly onto a substrate, such as a metal component, printed circuit board (PCB), or rigid plastic part. This process creates a seamless, watertight, and highly durable multi-material component used in everything from medical surgical tools to automotive sensor housings.

However, silicone overmolding is notoriously difficult to perfect. Unlike traditional plastic-on-plastic overmolding, silicone does not naturally adhere to most substrates. When product developers transition from prototyping to mass production with an inexperienced manufacturer, they often face catastrophic failure rates. At Reemane, we solve these engineering bottlenecks daily. Here is a deep dive into the real challenges of silicone overmolding and how we engineer solutions to guarantee long-term performance.

1. The Adhesion Challenge: Why Delamination Occurs

The most common and devastating failure in silicone overmolding is delamination (the silicone peeling away from the rigid insert). Silicone’s unique chemical structure prevents it from forming a natural bond with materials like stainless steel, aluminum, or standard polycarbonates (PC).

To achieve a destructive bond—where the silicone will tear before it separates from the substrate—we employ two primary strategies:

• Chemical Bonding via Primers: Before molding, the inserts must be meticulously coated with a specialized primer. This primer acts as a chemical bridge, bonding with the substrate on one side and cross-linking with the silicone during the curing process. The application must be flawless; if the primer layer is too thick, it turns into a brittle powder and fails. If it is too thin, the bond is weak.
• Mechanical Interlocking: Whenever possible, our Design for Manufacturing (DFM) team recommends adding undercuts, through-holes, or knurling to the rigid insert. This allows the silicone to flow through and lock the parts together physically, providing a fail-safe backup to the chemical bond.

2. Temperature Conflicts: When Curing Melts Your Insert

Silicone requires high temperatures to vulcanize (cure). Standard compression molding for HCR typically operates between 160°C and 200°C. If your rigid insert is made of a standard thermoplastic like ABS or standard PC, the heat of the mold will cause the insert to warp, soften, or completely melt before the silicone cures.

How do we solve this thermal conflict?
First, we analyze the substrate material. If you are using plastic inserts, we strongly recommend engineering-grade, high-temperature plastics like PA66 (Nylon), PEEK, or PPS. Second, if changing the plastic is not an option, we can utilize specialized Low-Temperature Cure Liquid Silicone Rubber (LSR). These advanced formulations can cure at temperatures as low as 100°C to 120°C, safely encapsulating temperature-sensitive plastics or delicate electronic components without causing thermal damage.

3. Insert Tolerance and Excessive Flashing

In overmolding, the silicone mold is machined to precisely hold the rigid insert in place. But what happens if your metal or plastic inserts have inconsistent dimensions?

If the insert is slightly too large, closing the heavy steel mold will crush and damage the component. If the insert is slightly too small (even by 0.05mm), the highly pressurized, fluid-like silicone will leak into the microscopic gaps. This creates aggressive “flash” over areas of the insert that were supposed to remain clean (such as electrical contact pins or aesthetic surfaces).

At Reemane, we mandate strict incoming quality control (IQC) on all substrates. Furthermore, our tooling engineers design molds with “crush ribs” or spring-loaded shut-offs. These dynamic tooling features absorb minor dimensional variations in the inserts, ensuring a tight seal that prevents flashing without damaging the core component.

4. The Invisible Enemy: Surface Contamination

Even with the perfect mold and the best primer, an overmolded part will fail if the insert is dirty. Microscopic oils from human skin, machine coolant residue from CNC processing, or trace amounts of mold release agents on plastic parts will instantly destroy the chemical bond.

Reliable overmolding requires a clinical approach to cleanliness. Inserts must undergo rigorous cleaning protocols—often involving ultrasonic solvent baths or plasma surface treatments—immediately before the primer is applied. Operators must wear lint-free gloves, and the staging time between priming and molding must be strictly controlled to prevent atmospheric contamination.