Views: 0 Author: Site Editor Publish Time: 2026-02-05 Origin: Site
Injection molding is entering a new era of specialization. As demand grows for advanced applications—from medical implants and wearable electronics to sustainable packaging—engineers are turning to high-performance specialty materials. Two of the most transformative are Liquid Silicone Rubber (LSR) and Bioplastics.
These materials offer extraordinary properties but come with unique processing challenges that demand a fundamental rethinking of mold design. A standard thermoplastic mold will fail here. Success requires a deep understanding of material science and a purpose-built approach.
This guide breaks down the essential technical considerations for designing and building molds for these advanced materials.
LSR is a two-component, platinum-cured elastomer that cures with heat. It’s known for its biocompatibility, extreme temperature resistance, flexibility, and clarity.
Very Low Viscosity: It flows like water, filling incredibly fine details but also seeking every gap to create flash.
Heat-Activated Cure: It vulcanizes (cross-links) under heat (~180-220°C) in an irreversible reaction.
High, Isotropic Shrinkage: Typically 2-4%, but uniform in all directions.
1. Cold Runner System is Mandatory
A hot runner would prematurely cure the material. LSR molds require a cold, temperature-controlled runner system.
Valve-Gated Hot/Cold System: Precise needle valves control injection and prevent drool. The manifold is cooled (20-40°C) to keep LSR liquid, while the mold cavity is hot to cure it.
Streamlined Flow: Runners must be perfectly smooth, with no dead ends where material can stagnate and partially cure.
2. Extreme Emphasis on Venting
Trapped air is the enemy of a perfect LSR part.
Aggressive Venting: Multiple, strategically placed vents are needed along the parting line and ejector pins.
Vacuum-Assisted Molding: For complex parts or bubble-free requirements, integrating a mold vacuum system is highly recommended to evacuate air before injection.
Vent Depth: Extremely shallow—typically 0.002 to 0.005 mm—just enough to let air escape but trap the viscous liquid.
3. “Zero-Flash” Parting Line
The low-viscosity material demands a flawless seal.
High Precision Machining: Parting lines require exceptional flatness, parallelism, and finish.
Hardened Surfaces: The parting line area should be hardened to HRC 50-55 to resist wear and deformation from clamp pressure.
Tight Tolerances: The goal is a perfect seal; even micron-level gaps can cause flash.
4. Efficient, Uniform Cooling & Heating
LSR molds have a unique thermal profile: cold runners, hot cavities.
Rapid Cooling Channels: Once cured, the part must be cooled quickly for cycle time efficiency. Water channels should be close to the cavity surface.
Uniform Cavity Temperature: Heater cartridges or hot oil loops must maintain cavity temperature within ±2°C for consistent cure rates and part quality.
5. Simplified Ejection
LSR parts are flexible and have low shrinkage force.
Minimal Ejector Pins: Fewer pins are often needed compared to rigid plastics.
Preference for Large Surfaces: Stripper plates, air blast ejection, or peel-type ejection are often preferred to avoid marks on the soft silicone.
6. Mold Material & Finish
Corrosion-Resistant Steel: Stainless steel (e.g., 420) is preferred for its polishability and resistance.
Mirror Polishing: Cavities require a #A1 diamond polish for easy demolding and optimal part surface quality, especially for optical or medical parts.
Bioplastics like PLA (Polylactic Acid) are derived from renewable resources and are compostable, offering a sustainable alternative.
Narrow Processing Window: Sensitive to heat; degrade quickly if overheated.
High Viscosity & Shear Sensitivity: Low melt flow requires higher pressure but shear heating can cause degradation.
Hydroscopic: Absorbs moisture aggressively from the air, requiring thorough drying before processing.
Low Heat Deflection Temperature: Parts are prone to warping and deformation if not cooled properly.
1. Optimized Gating and Runner Design
Large Runners & Gates: To accommodate high viscosity and reduce shear heating, use runners 20-30% larger in diameter than for conventional plastics like PP or ABS.
Full Hot Runners or Large Sprue Gates: Minimizes material waste (important for costlier bioplastics) and reduces shear history.
Generous Radii: Avoid sharp corners in flow paths to prevent shear-induced material degradation.
2. High-Efficiency, Uniform Cooling
Warpage is the primary defect with brittle bioplastics like PLA.
Enhanced Cooling Capacity: These materials often crystallize slowly and need strong cooling to achieve cycle times.
Temperature Uniformity: Maintain mold temperature within ±5°C across the cavity to prevent differential shrinkage and warping.
Conformal Cooling: For complex parts, 3D-printed conformal cooling channels that follow the part contour are ideal to ensure even cooling.
3. Controlled Venting
Slightly Deeper Vents: Compared to LSR, vents can be deeper, around 0.03-0.05 mm, to allow gases from potential minor degradation to escape.
4. Corrosion Resistance & Maintenance
Protected Surfaces: Lactic acid formed during PLA processing can be mildly corrosive. Use corrosion-resistant steels or apply protective hard chrome or nickel plating to cavities.
Easy-to-Clean Design: Mold structure should allow for complete purging and cleaning to prevent contamination between material batches.
5. Process Parameters Insight
Precise Temperature Control: Use multi-zone barrel and hot runner controls.
Moderate Injection Speeds: Use slower fill speeds to minimize shear heat.
Higher Mold Temperature: Contrary to instinct, a warmer mold (e.g., 40-70°C for PLA) reduces internal stress and improves crystallinity, resulting in less brittle parts.
Despite their differences, LSR and bioplastic molds share some overarching design philosophies:
Material-First Mindset: The mold is designed from the material's properties, not adapted to them.
Precision Thermal Management: Both require exceptional control—LSR over a hot/cold divide, bioplastics over a narrow, uniform range.
Anticipating Defects: Design proactively to combat their signature flaws: flash for LSR, warpage for bioplastics.
Collaboration is Key: Close cooperation between the material supplier, product designer, and mold maker from the project's outset is non-negotiable for success.
Venturing into LSR or bioplastic molding is not merely an exercise in modifying an existing mold design. It represents a specialized discipline that blends polymer chemistry, rheology, and precision engineering.
For LSR, think "seal and control." For bioplastics, think "gentle and uniform." By respecting the unique language of each material and embedding its requirements into the mold's blueprint, engineers can unlock the vast potential of these transformative polymers, creating products that are safer, more sustainable, and more sophisticated than ever before.
