Key Points for Injection Molding and Mold Design of Polyurethane Sealing Rings

Polyurethane (PU) features outstanding wear resistance, oil resistance and high elasticity, extensively used for hydraulic and pneumatic sealing rings. Dimensional accuracy and surface integrity directly determine sealing performance. Improper mold design or unreasonable forming parameters easily trigger bubbles, sink marks, flash and tearing failures. Combined with characteristics of annular sealing products, this article sorts out critical standards for mold structure and injection technology.

1. Core Design Principles for Polyurethane Sealing Ring Molds

Sealing rings are rotary thin-wall annular components. Mold design focuses on cavity precision, parting surface sealing, venting and balanced ejection. Select suitable mold steel for cavity manufacturing and polish surfaces to mirror finish to avoid scratches that cause leakage. Arrange parting lines on outer diameter non-sealing areas to prevent flash on main working surfaces. Small O-rings usually adopt vertical parting, while large rectangular seals use horizontal parting. Control parting clearance strictly to avoid circumferential flash.

For gating systems, thin gates or pinpoint gates distributed evenly around the ring are preferred. Balanced feeding ensures uniform melt flow and prevents concentrated weld lines that weaken sealing strength. Design short, thick runners to shorten melt flow path and avoid premature crosslinking of polyurethane. Venting carries critical importance. PU releases small molecule gas during forming; set fine vent slots at weld positions and cavity terminals. Control vent depth to exhaust gas without overflowing melt.

Arrange circular cooling channels evenly around cavities to eliminate elliptical deformation caused by uneven circumferential shrinkage. Avoid single-point ejection which squeezes annular parts. Plate ejection delivers uniform ejection force to reduce distortion and tearing. Place cooling channels close to cavities to minimize temperature fluctuation inside mold cavities.

2. Process Control in Polyurethane Sealing Ring Injection Molding

Raw material pretreatment is essential. Polyurethane absorbs water rapidly, and moisture generates carbon dioxide under heat, creating internal bubbles. Dry raw materials thoroughly and store pellets in sealed dry environment. Control melt temperature properly: low temperature reduces fluidity and weakens weld strength; overheating accelerates crosslinking, shortens injection cycle and makes finished parts brittle.

Adopt medium-low injection speed. High velocity creates intense shear heat and leads to local material degradation and air entrapment. Maintain stable injection pressure: insufficient pressure reduces product density and elastic recovery; excessive pressure produces flash and raises residual stress. Avoid prolonged high holding pressure, as fast curing of PU generates large internal stress and long-term deformation after molding.

Stable mold temperature is required. Low mold temperature causes dull surfaces and loose internal structure; high mold temperature increases forming shrinkage and brings unstable dimensions. Adjust cooling time according to wall thickness. Start ejection only after complete curing. Post-curing after ejection promotes full crosslinking, stabilizes dimensions and lowers permanent compression set.

3. Optimization Solutions for Typical Forming Defects

Internal bubbles and pinholes mainly stem from insufficient drying and poor venting. Reduce injection speed and adjust gate layout to improve flow conditions. Cracks along weld lines require relocating gates away from sealing surfaces, raising local melt temperature and optimizing venting. Elliptical deformation and unstable dimensions result from uneven cooling and unbalanced ejection. Optimize circular cooling layout and switch to plate ejection to lower residual stress. Continuous flash demands tighter parting clearance and moderate reduction of injection and holding pressure.

4. Trial Mold and Mass Production Optimization

Check mold coaxiality during trial runs. Coaxial deviation creates uneven wall thickness and sealing failure under load. Unify raw material batches and production ambient humidity, since different batches change shrinkage rate. Install replaceable inserts at high-wear mold areas to lower maintenance expenses.

Summary

For polyurethane sealing ring production, mold design centers on reasonable parting layout, balanced feeding, uniform circular cooling, sufficient venting and smooth integral ejection. Forming technology emphasizes raw material dehumidification, stable temperature range, limited shear intensity and complete curing cycle. Structural mold defects can hardly be fully compensated by parameter adjustment. Conduct mold inspection on venting, cavity precision and ejection system before parameter fine-tuning. Matching mold structure and forming technology produces high-quality PU seals with low permanent deformation, minimal bubbles and weak weld lines, satisfying long-term sealing requirements of hydraulic and pneumatic equipment.