Solutions to the Shrinkage Problem of Medical Silicone Injection-Molded Products

In the field of precision medical injection molding, silicone products are widely used in critical components such as catheters, seals, and implants due to their excellent biocompatibility, high-temperature resistance, and good elasticity. However, the shrinkage problem has always been a core challenge restricting product yield. This article systematically analyzes the causes of silicone injection molding shrinkage and provides solutions from three dimensions: material characteristics, process optimization, and mold design, combined with the characteristics of the medical industry.

I. Root Causes of Shrinkage in Medical Silicone

1. Material Characteristics Determine Shrinkage Patterns
   Silicone is a thermosetting elastomer, and its shrinkage rate (usually 1.2%-3%) is significantly affected by the cross-linking density, curing agent type, and addition ratio. For example, silicone with added nano-silica can reduce the shrinkage rate to below 1.5%, while high-hardness silicone (e.g., Shore A 70 and above) has a shrinkage rate about 0.5% lower than that of soft silicone (Shore A 30-50) due to its higher cross-linking degree. Medical-grade silicone needs to pass biocompatibility certifications such as USP Class VI or ISO 10993, and the composition of its additives must be strictly controlled, which further limits the means of adjusting the shrinkage rate.
2. Imbalance in Process Parameters Leads to Chain Reactions
   • Curing Temperature and Time: Every 10°C increase in curing temperature raises the shrinkage rate by 0.2%-0.3%. For example, a two-stage curing process (160°C pre-curing + 180°C post-curing) for a pacemaker housing reduces the shrinkage rate from 2.8% to 1.9%.
   • Injection Pressure and Speed: Insufficient pressure results in incomplete filling, while excessive pressure causes flash. In the production of medical catheters, a “graded injection” strategy (initial pressure of 80 MPa for rapid filling and final pressure of 50 MPa for pressure maintenance) can reduce the shrinkage rate by 0.3%.
   • Pressure Maintenance Stage Control: Insufficient pressure maintenance time (e.g., < 5 seconds) can lead to vacuum voids inside the product, while excessive pressure maintenance (e.g., > 15 seconds) may cause mold core displacement. A mold for implantable capsules reduces the shrinkage rate fluctuation range from ±0.5% to ±0.2% by extending the pressure maintenance time to 8 seconds and precisely controlling the mold temperature (170°C ± 2°C) with a mold temperature controller.
3. Key Details in Mold Design
   • Gating System Optimization: Medical-grade silicone products usually have a wall thickness of ≤ 2 mm and require a “pin-point gate + fan-shaped runner” design. For example, a mold for a minimally invasive surgical instrument handle reduces the filling time by 30% and the shrinkage rate by 0.4% by increasing the gate diameter from 0.8 mm to 1.2 mm and the runner diameter to 2.5 mm.
   • Cooling System Layout: For thick-walled components (e.g., orthopedic implant bases), a “conformal cooling water circuit” design improves cooling efficiency by 40%. A mold for a hip joint prosthesis reduces the mold temperature uniformity from ±5°C to ±1.5°C and the standard deviation of the shrinkage rate from 0.3% to 0.1% by embedding copper cooling pins in the thick-walled area.
   • Venting Structure Innovation: Silicone releases a small amount of gas during curing, and poor venting can lead to bubbles. A mold for medical catheters eliminates over 90% of surface air marks by using a “vacuum venting system” (vacuum degree of -0.08 MPa) combined with venting slots 0.02 mm deep.

II. Specialized Solutions for Medical Scenarios

1. Process Control for High-Precision Medical Products
   • Multi-stage Pressure Maintenance Strategy: For micro-structured products such as cardiac stents, a “three-stage pressure maintenance” (initial pressure of 120 MPa for 3 seconds for rapid shrinkage compensation, medium pressure of 80 MPa for 5 seconds for pressure maintenance, and final pressure of 50 MPa for 2 seconds for stress release) can control the shrinkage rate fluctuation within ±0.1%.
   • Dynamic Mold Temperature Regulation: For temperature-sensitive silicone (e.g., liquid silicone rubber (LSR)), a “mold temperature zoning control technology” (180°C at the gate area, 175°C at the main runner area, and 170°C at the cavity area) reduces shrinkage differences caused by temperature gradients. A mold for a nerve stimulation electrode reduces the product warpage from 0.5 mm to 0.1 mm with this technology.
2. Mold Improvements for Complex-Structured Products
   • Core-Back Design Application: For medical containers with a wall thickness of > 3 mm, a “honeycomb core-back structure” (individual core-back holes with a diameter of 1.5 mm and a spacing of 5 mm) reduces the shrinkage rate from 2.5% to 1.8% while ensuring structural strength.
   • Ejection Mechanism Innovation: For silicone products prone to sticking to the mold (e.g., respirator mask seals), a “pneumatic ejection + nitrogen-assisted demolding” system improves ejection force uniformity by 60% and reduces demolding damage rate from 15% to below 2%.
3. Material Modification Technology Breakthroughs
   • Nano-filler Modification: Silicone filled with 2% fumed silica can reduce the shrinkage rate to 1.2% while increasing the tensile strength by 30%, meeting the mechanical property requirements of orthopedic implants.
   • Blending Modification Technology: Blending silicone with thermoplastic polyurethane (TPU) in a 7:3 ratio to form a thermoplastic elastomer (TPE) reduces the shrinkage rate to 0.8% and enables the production of soft-hard combined structures through overmolding.

III. Typical Case Studies

Case 1: Shrinkage Problem of the Seal Ring of an Insulin Injection Pen from a Certain Brand

• Problem: The initial shrinkage rate was 2.8%, resulting in an assembly clearance exceeding the tolerance (allowable value ≤ 0.1 mm).
• Solutions:
  1. Mold optimization: Increase the number of gates from 2 to 4 and the gate diameter from 0.6 mm to 1.0 mm.
  2. Process adjustment: Reduce the curing temperature from 175°C to 170°C and extend the pressure maintenance time from 6 seconds to 10 seconds.
  3. Material upgrade: Switch to silicone filled with 3% nano-calcium carbonate, reducing the shrinkage rate to 1.5%.
• Effect: The assembly clearance is stabilized at 0.05 mm, and the yield increases from 72% to 95%.

Case 2: Shrinkage Deformation of the Joint Bearing of a Minimally Invasive Surgical Robot

• Problem: The shrinkage rate of the bearing inner ring was uneven (maximum difference of 0.6%), resulting in excessive rotational resistance.
• Solutions:
  1. Mold improvement: Use “conformal cooling + mold temperature zoning control” to improve mold temperature uniformity from ±4°C to ±1°C.
  2. Process innovation: Implement a “dynamic curing process” (initial pressure of 100 MPa, medium pressure of 80 MPa, and final pressure of 50 MPa).
  3. Structural optimization: Reduce the wall thickness of the bearing inner ring from 4 mm to 3.5 mm and add a 0.5 mm core-back slot.
• Effect: The standard deviation of the shrinkage rate is reduced to 0.15%, and the fluctuation range of the rotational resistance is narrowed from ±0.5 N·m to ±0.1 N·m.

IV. Future Technological Trends

As the medical industry demands micron-level precision, the following technologies will be key to solving the shrinkage problem:

1. Upgrade of Mold Flow Analysis Software: AI-based simulation algorithms can predict shrinkage rate distribution in advance, shortening the mold design optimization cycle by 40%.
2. Intelligent Temperature Control System: Infrared thermal imaging is used to monitor mold temperature in real time, combined with PID control algorithms to achieve mold temperature fluctuations of < ±0.5°C.
3. 4D Printing Mold Technology: Molds made of shape memory alloys can dynamically adjust runner sizes to adapt to the shrinkage characteristics of different batches of materials.

The control of shrinkage in medical silicone injection molding is a comprehensive battle involving materials science, fluid mechanics, and thermodynamics. Through millimeter-level adjustments in process parameters, millimeter-level optimizations in mold design, and micron-level innovations in material modification, extreme precision of medical products can be achieved while ensuring biosafety.