📌 Key Takeaways
- T1 (first trial shot) is not expected to produce production-quality parts — its purpose is to validate mold function and identify issues, not achieve final part dimensions
- The most valuable data from T1 comes from systematic parameter variation, not from running the mold at nominal settings — establish the process window boundaries at T1
- Short shots are more useful than flashed parts at T1 — a short shot shows the fill pattern and reveals air trap locations; a flashed part just shows clamping is insufficient
- Every T1 modification should be documented with before/after photographs and dimensional measurements — a mold trial without documentation is engineering knowledge lost
- T2 acceptance criteria should be agreed in writing before T1 begins — ambiguous acceptance criteria are the leading cause of mold trial disputes between customer and supplier
The injection mold trial process — from T1 first shot through T3 production validation — is where mold engineering theory meets manufacturing reality. Even a perfectly designed and precisely machined mold requires systematic trialing to establish the production process window, identify and resolve any remaining design or machining issues, and generate the dimensional evidence needed for customer acceptance. This guide covers the complete mold trial process, from pre-trial preparation through final approval.
1. Pre-Trial Preparation
A successful mold trial begins before the first shot. Thorough pre-trial preparation prevents common trial failures and ensures the trial time is spent optimizing, not troubleshooting basic setup issues:
- Mold inspection checklist — Verify all cooling connections are made and pressure-tested. Confirm ejector pins retract fully. Check guide pin and bushing fit. Verify all inserts are seated correctly. Inspect vents for correct depth and cleanliness
- Machine selection — Select injection machine with clamping force 1.2× calculated requirement. Verify shot size is 20–80% of machine capacity. Confirm machine has adequate injection speed and pressure capability
- Material preparation — Dry material per supplier specification. Use production-grade resin from the correct lot. Document material lot number and drying parameters for the trial record
- Baseline parameters — Set starting parameters from material data sheet: melt temperature, mold temperature, injection speed (conservative), cooling time (generous). Do not optimize before seeing first shots
- Measurement plan — Prepare measurement plan specifying which dimensions to measure on T1 parts, in what sequence, using which gauges. Pre-mark cavities to track cavity-to-cavity variation
2. T1 — First Trial Objectives & Protocol
T1 is a diagnostic trial, not a production run. The objectives are specific and limited:
| T1 Objective | Method | Acceptable Outcome |
|---|---|---|
| Confirm mold opens and closes without interference | Slow-speed dry cycling before injection | Smooth operation; no binding or damage |
| Confirm ejector system functions | Eject parts at conservative settings | Parts release; no sticking or breakage |
| Confirm cooling system functions | Monitor inlet/outlet temperature delta | Delta <5°C; no leaks |
| Identify gross filling defects | Short shots at 30%, 60%, 90% fill | Fill pattern visible; air traps identified |
| Achieve complete fill | Increase injection speed/pressure to full fill | Parts complete; note any flash or burn marks |
| Preliminary dimensional check | Measure 5 critical dimensions on 3 parts | Document deviation from nominal; no acceptance at T1 |
At T1, do not attempt to optimize. The goal is to understand what the mold is doing, not to make it perfect. Take photographs of every defect. Measure parts even if they look wrong. Document every parameter setting used.
3. T1 Defect Analysis & Modifications
After T1, a systematic defect analysis determines what modifications are needed before T2. Common T1 findings and their solutions:
| Defect | Root Cause Diagnosis | Modification |
|---|---|---|
| Short shot at flow end | Insufficient venting; flow too slow | Add vents at short-shot location; increase injection speed |
| Flash on parting surface | Parting surface damage or insufficient clamp | Stone parting surface; recalculate clamp tonnage |
| Burn marks | Trapped air — vent blocked or missing | Clean existing vents; add vents at burn location |
| Sink marks | Insufficient pack; thick wall section | Increase pack pressure/time; evaluate wall thickness |
| Sticking in cavity | Insufficient draft; rough surface; no air assist | Add draft if possible; polish cavity; add air assist |
| Dimensional oversize | Cavity too large; insufficient shrinkage | Steel-safe — re-machine to remove stock |
| Dimensional undersize | Cavity too small; excessive shrinkage | Weld and re-machine; evaluate material shrinkage rate |
| Weld lines on cosmetic surface | Gate location creating front merger at visible area | Relocate gate; add overflow well; increase mold temp |
4. T2 — Process Optimization & Dimensional Validation
T2 begins after T1 modifications are complete and verified. The T2 objective is to establish the optimized process window and achieve dimensional compliance on critical features:
- Design of Experiments (DOE) — Systematically vary injection speed, pack pressure, mold temperature, and cooling time to establish the proven acceptable range (PAR) of each parameter
- Process window validation — Confirm that parts remain within specification across the edges of the process window. A robust process window means small parameter variations do not cause rejections
- Cavity-to-cavity measurement — For multi-cavity molds, measure 5 parts from each cavity. Maximum acceptable cavity-to-cavity variation: 0.02mm on critical dimensions
- Cosmetic evaluation — Evaluate sink marks, weld lines, gate vestige, and surface finish under agreed lighting conditions (typically D65 illuminant at 500 lux, 45° angle)
- T2 dimensional report — Full measurement of all drawing dimensions on 5 parts per cavity. Report deviations and confirm which dimensions are in tolerance
5. T3 / FAI — Production Validation & Customer Approval
T3 (or First Article Inspection) is the final validation trial, run at the established T2 process parameters to generate the formal acceptance documentation:
- Production run simulation — Run minimum 100 consecutive shots at T2 process settings. Evaluate part consistency, cycle stability, and any process drift
- CMM First Article Inspection — Full 100% dimensional inspection of all drawing dimensions on 5 parts per cavity using calibrated CMM. Results reported against nominal and tolerance
- Visual inspection — 100% visual inspection of trial parts against agreed cosmetic acceptance criteria. Parts graded A (accept), B (review), C (reject)
- Gate vestige measurement — Gate mark height and position documented and confirmed within drawing specification
- T3 documentation package — CMM report, process parameter record, material certificate, 10 sample parts (labeled with cavity number and date), mold trial summary report
Mold acceptance should be based on objective, documented evidence — not visual impression. At BuildMold, every mold trial generates a full trial report with photographs, parameter records, dimensional data, and clearly stated acceptance/rejection status for each drawing requirement. This documentation protects both the mold maker and the customer, and provides the baseline record needed for future production troubleshooting.
Starting a New Mold Project?
BuildMold conducts T1–T3 mold trials on-site with full documentation, CMM FAI reports, and sample parts included with every mold. Contact us to discuss your project.