Everything you need to know about mold tryout, troubleshooting, and acceptance when working with Chinese mold manufacturers.
Mold tryout (also called tool trial, T0, or sampling) is the process of running a newly manufactured injection mold for the first time on a production injection molding machine. This critical step validates that the mold produces parts within specification before it is approved for production. In Chinese mold manufacturing, the tryout process is typically included in the mold purchase price. Most reputable mold makers will conduct at least one free trial run (T0) at their facility, with subsequent trials charged separately. Foreign buyers should plan to attend or send a representative for the initial tryout, especially for complex or high-precision molds.
Before the mold is mounted on the injection machine, several preparations are essential. Verify mold dimensions match the injection machine platen size and tie-bar spacing. Confirm shot size — the molding machine should have adequate injection volume (30-80% of barrel capacity is recommended). Check cooling water connections including flow rate and temperature control unit (TCU) readiness. Inspect the ejector system for stroke and knockout pattern compatibility. Review gate type and location to ensure balanced filling. Pre-dry the plastic material per manufacturer specifications. Set up process monitoring equipment including pressure sensors and temperature probes.
A standard mold tryout follows these steps:
| Issue | Likely Cause | Solution |
|---|---|---|
| Short shots (incomplete filling) | Insufficient injection pressure, low melt temperature, inadequate venting | Increase pressure/speed, raise barrel temperature, check vents for blockage |
| Flash (excess material at parting line) | Excessive clamp force, worn parting line, high injection pressure | Reduce injection pressure, check mold closure, inspect parting line surface |
| Sink marks | Insufficient hold pressure, uneven cooling, thick wall sections | Increase packing pressure/time, balance cooling channels, adjust wall thickness |
| Weld lines | Material flow fronts meeting at low temperature | Increase melt temperature, raise injection speed, improve venting at weld area |
| Ejection problems (sticking) | Insufficient draft angle, undercuts, rough cavity surface | Polish cavity in draw direction, apply mold release, add ejector pins |
| Warpage | Uneven cooling, differential shrinkage, internal stress | Balance mold temperature, lengthen cooling time, adjust fill pattern |
| Burn marks (black/brown spots) | Trapped air igniting, excessive injection speed | Add venting, reduce injection speed, lower melt temperature |
| Dimensional variation | Inconsistent packing, temperature variation, material batch change | Stabilize process parameters, control mold temperature within ±2°C |
Process Adjustments: Changing temperature, pressure, speed, and timing on the injection molding machine. These are the fastest and cheapest changes and can resolve many quality issues without modifying the mold itself.
Venting Adjustments: Adding or deepening vent channels (typically 0.02-0.05 mm deep) on the parting line and ejector pins. Proper venting is critical for preventing burn marks and short shots.
Gate Adjustments: Gate size, location, and type significantly affect filling behavior. Increasing gate cross-section can improve flow; changing gate location can eliminate weld lines in cosmetic areas.
Cooling Adjustments: Adding or rerouting cooling channels, adjusting water temperature, or using baffles and bubblers to improve cooling uniformity.
Steel Adjustments: Removing steel (cutting) to improve filling or add clearance, or adding steel (welding) to correct dimensions or close gaps. Steel adjustments are more expensive but sometimes necessary.
A mold is typically accepted when: parts fill completely and consistently across all cavities; all critical dimensions are within tolerance (±0.05 mm for precision molds, ±0.10 mm for general molds); no flash, sink marks, weld lines, or other cosmetic defects in Class A surfaces; ejection is reliable with no part sticking; cycle time meets or approaches the target; surface finish matches the specified SPI finish grade (A-1 through D-3); and all safety features are functional.
Professional Chinese mold makers provide a tryout report that typically includes: tryout conditions (machine used, material type, process parameters), part weight measurement and consistency across cavities, critical dimension measurements with actual vs. nominal comparison, photos of molded parts (including any defects observed), recommended process parameters for production, and a list of mold modifications made or required.