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In the mold industry, success on the first trial shot is the ideal scenario, but it is rarely achieved in practice. Based on industry experience, an injection mold of medium complexity requires an average of 2 to 4 rounds of trial shots and modifications before a qualified part is produced. Simple molds may need only 1–2 rounds, while complex appearance parts or precision components often require 5 or more rounds.

What is the typical first-shot success rate?

We define "first-shot success" as the ability to produce samples that fully conform to the drawing immediately after the mold is mounted on the machine, without any modification. This success rate typically falls between 30% and 50%. In other words, at least half of all molds will reveal issues during the very first trial shot.

Analysis of Main Causes

The three primary causes of trial shot failure are, in order: inaccurate shrinkage rate > uneven cooling > ejection deformation.

1. Inaccurate Shrinkage Rate (approximately 40%)

This is the most common cause. Every plastic material has a specific shrinkage rate (ranging from 0.3% to 2.5%), but there is often a discrepancy between the theoretical shrinkage rate used during mold design and the actual shrinkage that occurs after molding.

Symptoms: Part dimensions are either too large or too small, assembly features do not align, and flatness exceeds tolerance.

Root causes: Variations in material batches, or mismatches between actual molding conditions (pressure, temperature, holding time) and design assumptions.

2. Uneven Cooling (approximately 30%)

Uneven temperature distribution across the mold causes different areas of the part to cool at different rates.

Symptoms: Warpage, distortion, concentrated internal stress. In severe cases, the part may twist like a potato chip.

Root causes: Poorly designed cooling channels (too far from the cavity or unevenly distributed), or differences in the thermal conductivity of mold steel.

3. Ejection Deformation (approximately 15%)

The part experiences uneven force at the moment of ejection, or the ejection occurs while the part temperature is still too high.

Symptoms: Ejector pin marks (white spots or cracks), local indentations, especially on slender ribs or thin-walled areas.

Root causes: Improper ejector pin placement, insufficient number of pins, excessive ejection speed, or insufficient cooling time.

Other Causes (approximately 15% combined):

Gas trapping: Inadequate venting design, leading to trapped gas that becomes compressed and burns the part edges.

Unbalanced filling: In multi-cavity molds, cavities fill at different rates.

Flash: Insufficient clamping force or worn parting line surfaces.

Part sticking: Insufficient draft angle or overly rough cavity surface.

How to Improve First-Shot Success Rate?

Use CAE software to simulate filling, cooling, and warpage during the design phase, optimizing gate locations in advance. Conduct thorough pre-trial inspections to verify that vents and cooling channels are clear. Additionally, standardize process parameters—for example, by establishing a reference process table for commonly used materials to reduce blind adjustments during trial shots.

Summary

An average of 2–4 modifications before achieving a qualified part is the reality of the industry. For mold manufacturers, improving the first-shot success rate is not just a matter of technical capability—it is also key to cost control. Each additional trial shot incurs more material costs, machine occupancy, and delivery risk.

Instead of "guessing the problem" on the trial floor, spend an extra day on mold flow analysis during the design phase. One successful first shot saves more money than ten mold repairs.