How to Solve Leakage and Poor Overmolding Adhesion in Medical Injection Molded Products

In the medical injection molding industry, leakage and poor overmolding adhesion are two of the most common and frustrating quality issues. For products like infusion devices, syringes, micropump systems, and other medical devices, these problems directly impact patient safety and are zero-tolerance defects.

Today, we'll provide a comprehensive analysis of how to solve these issues from three dimensions: root cause analysis → systematic solutions → validation methods.

Part 1: First, Understand the Nature of the Problem

1.1 Root Causes of Leakage

Leakage is essentially a seal failure issue. In injection molded medical products, seal failures typically stem from:

• Interfacial leakage: Poor bonding between two materials (rigid substrate + soft overmold), creating microscopic leakage paths

• Structural leakage: Inherent design flaws that prevent effective sealing

• Assembly leakage: Tolerance stack-up in multi-component assemblies compromising seal integrity

• Material-related leakage: Insufficient chemical or physical bonding at the material interface

1.2 Root Causes of Poor Overmolding Adhesion

Poor overmolding adhesion is fundamentally an insufficient interfacial bonding issue. Common causes include:

• Material incompatibility: Chemical structure mismatch between the rigid substrate and soft overmold material

• Interfacial contamination: Substrate surface contaminated with mold release agents, oils, or dust

• Improper process parameters: Temperature, pressure, or timing not optimized for bonding

• Design flaws: Lack of mechanical interlocking features, relying solely on chemical adhesion

Part 2: Systematic Solutions

2.1 Design Level: Solving Problems at the Source

(1) Adopt Multi-Component Integrated Design

Case Study Reference: Trelleborg's solution for medical micropump systems

The traditional design used a three-piece construction: "plastic piston + two silicone O-rings," which presented several problems:

• Poor mold quality caused piston eccentricity, creating leakage paths

• Automated O-ring assembly reliability couldn't be 100% guaranteed

• Tolerance stack-up between piston, O-rings, and grooves increased friction

• Contact surfaces and materials weren't optimized for friction performance

Solution: Integrate the three-piece assembly into a single multi-component LSR (Liquid Silicone Rubber) component

Design features:

• Compression inner seal: Withstands pressure from both sides

• Deflecting outer seal: Reduces contact friction, pressure-activated sealing

• Inner and outer guiding structures: Ensures assembly concentricity

• Chemical bonding between LSR and PA piston: Achieved through careful material selection

• Integrated flow channels: Facilitates LSR injection molding

Result: Completely eliminated leakage and friction issues while simplifying the supply chain and assembly steps, reducing overall costs

(2) Add Mechanical Interlocking Features

For overmolded products, never rely solely on chemical bonding. Consider these design features:

• Physical grooves/holes: Design recesses, holes, or undercuts in the rigid substrate for the soft material to fill, creating mechanical interlock

• Stepped transitions: Design stepped structures at the bonding interface to increase surface area and peel resistance

• Surface texturing: Use EDM (Electrical Discharge Machining) on the mold to create micro-roughness on the rigid substrate surface

(3) Optimize Seal Structure Design

For leakage issues, consider these approaches:

• Multiple sealing elements: Design two or three sealing rings as backup for each other

• Pressure-activated sealing: Use system pressure to enhance sealing effectiveness

• Anti-pullout features: As seen in patent CN206007602U, "rubber stopper anti-slip teeth" prevent stopper displacement during needle insertion

2.2 Material Selection: Compatibility is Key

(1) Ensure Chemical Compatibility

Successful overmolding (whether two-shot or insert molding) requires compatibility between the two materials.

Common compatible material combinations:

• PC + TPE (specific grades)

• ABS + TPE (choose bondable TPE grades)

• PA + LSR (can form chemical bonds)

Special note: TPE materials have complex formulations, and traditional solvent bonding methods are often ineffective

(2) Specialized Solutions for Difficult-to-Bond Materials

If you must use difficult-to-bond materials (such as COC, PEBA, PP, PE, etc.), consider:

Option A: Specialized Adhesives

• Companies like Dymax offer medical-grade adhesive solutions for difficult-to-bond substrates

• Consider joint design, sterilization method impact, and failure analysis

Option B: Medical-Grade Bonding Agents

• Products like MXBON 22507M are specifically designed for medical TPE materials

• Features: No solvent treatment needed, strong bonding, environmentally friendly, solvent-free, fast curing, low odor, low whitening

• Certification: ISO 10993-5 biocompatibility certified

• Applications: Medical tubing, syringes, medical packaging, etc.

(3) Avoid Mold Release Agents

Mold release agents are the #1 enemy of overmolding adhesion. If absolutely necessary, use weldable/bondable grades or thoroughly clean surfaces before secondary molding.

2.3 Process Control: Parameter Optimization and Validation

(1) Common Overmolding Issues and Solutions

(2) Critical Process Parameters

Temperature:

• Rigid substrate surface temperature: Should reach above the softening temperature of the overmold material for true molecular bonding

• Mold temperature: Higher mold temperatures extend melt flow time and enhance bonding

Pressure and Speed:

• Injection speed: Too fast may erode the substrate surface; too slow may cause incomplete filling

• Hold pressure: Adequate hold pressure compensates for shrinkage and enhances interfacial bonding

Timing:

• Delay time: Minimize the interval between substrate molding and overmolding to prevent surface contamination or oxidation

• Cooling time: Ensure adequate cooling before ejection to prevent stress-induced interfacial separation

(3) Process Validation and Risk Management

For medical devices that undergo final sterilization, process validation must follow ISO 11607-2 standards.

Validation requirements include:

• Molding process validation

• Sealing process validation

• Assembly process validation

• Risk management application

Validation methods:

• Finite Element Analysis (FEA): Predict product performance under application conditions

• Design for Manufacturing (DFM) analysis: Includes material flow simulation to ensure manufacturability

• Prototype mold testing: Produce test samples using prototype tooling to validate designs

Part 3: Specialized Solutions for Leakage Issues

3.1 Seal Structure Optimization

3.2 Anti-Pullout Design

For products requiring needle penetration (like infusion containers), refer to patent CN206007602U:

Problem: Rubber stoppers tend to dislodge from inner/outer covers during needle insertion, potentially falling partially or completely into the liquid, causing leakage and contamination

Solution:

• Add upper stopper anti-slip teeth on the top surface of the stopper cavity

• And/or add lower stopper anti-slip teeth on the bottom surface

• These anti-slip teeth increase the connection between stopper and covers, preventing dislodgement during puncture

Part 4: Testing and Validation Methods

4.1 Leak Testing

4.2 Adhesion Strength Testing

4.3 Accelerated Aging and Sterilization Validation

Medical products must verify post-sterilization performance retention:

• Sterilization method impact: EO sterilization, gamma radiation, electron beam, steam autoclave—each affects material adhesion differently

• Accelerated aging: Simulates performance changes over product lifecycle

• Real-time aging: Complementary validation to accelerated aging

Part 5: Practical Checklist

Before finalizing product design and committing to molds, run through this checklist:

Design Checklist

• Have you considered integrated multi-component design to reduce assembly steps?

• Does the overmolding interface include mechanical interlocking features (grooves, holes, undercuts)?

• Is the seal structure designed with multiple redundant elements?

• Are anti-pullout features (like anti-slip teeth) incorporated?

• Has FEA analysis been performed to validate design feasibility?

Material Checklist

• Are the rigid and soft materials chemically compatible?

• Have you selected medical-grade materials with biocompatibility certification?

• Have you avoided mold release agents? If necessary, is there a cleaning protocol?

• For difficult-to-bond materials, do you have a specialized adhesive solution?

Process Checklist

• Have process parameters (temperature, pressure, speed) been optimized through DOE?

• Has DFM analysis and material flow simulation been performed?

• Is there a prototype mold testing plan?

• Does process validation comply with ISO 11607-2 requirements?

Testing & Validation Checklist

• Is the leak test method defined? Are acceptance criteria?

• Is adhesion strength testing included in quality control plans?

• Is post-sterilization performance planned?

• Is accelerated aging testing designed?

Summary: Systematic Approach to Solving Leakage and Adhesion Problems

Solving leakage and overmolding problems in medical products requires systems thinking:

• Design is fundamental: Integrated design, mechanical interlocking, multiple seals—solve problems at the source

• Material selection is the foundation: Prioritize compatibility; use specialized adhesives when necessary

• Process control is critical: Parameter optimization, process control, thorough validation

As the Trelleborg case demonstrates, early collaboration and joint development with suppliers often leads to better solutions at lower costs than traditional designs.