Anyone working in injection molding has surely encountered this frustrating issue: the same injection molding machine, the same mold, the same raw material — yet during production, product dimensions sometimes pass, sometimes run large, and sometimes run small. Most people's first reaction is to shift the blame: aging machine, poor raw material batch, insufficient mold precision. Few look inward at the underlying logic of process control. Applying ordinary plastic part molding logic to high-precision parts will only result in batch defects and delivery delays.

I. Uneven Mold Temperature Distribution

Many factories use a series design for their cooling water channels, where cooling water flows sequentially through cavities 1, 2, and 3. The inlet water temperature is low, while the outlet water temperature at the end rises significantly, creating an inherent temperature difference across all cavities.

As production continues, the water temperature difference keeps widening: qualified production requires cavity temperature differences to be controlled within 3°C, while series water channels often exhibit differences of 5°C, 10°C, or more. With inconsistent plastic cooling shrinkage, dimensions naturally fluctuate up and down.

In addition, unreasonable layout of mold water channel openings and varying distances between water channels and cavities can also exacerbate mold temperature imbalance.

II. Incorrect Molding Parameter Setting Logic

Experienced machine operators making ordinary housings are accustomed to relying on time-based switchover to holding pressure, which has a high fault tolerance and shows no obvious problems.

However, precision plastic parts are extremely sensitive to pressure and melt filling. Relying solely on time to control holding pressure means the melt fill volume and compression volume fluctuate with each shot, and the accumulated positive/negative dimensional deviation can reach 0.1–0.2 mm.

Many people, upon finding dimensions too small, simply increase holding pressure — which treats the symptom rather than the root cause: the problem is dimensional instability, not simply undersized parts. Blindly adjusting holding pressure will only worsen the fluctuation.

III. Long-Term Neglect of Cooling System Maintenance

The cooling system is an equipment critical point that is极易 overlooked. Cooling towers and mold water channels that go uncleaned for a long time accumulate scale and sludge on the inner pipe walls, drastically reducing the heat exchange capacity of the cooling water.

Under the same cooling time, unstable mold heat dissipation causes the mold temperature to fluctuate up and down, and the plastic shrinkage rate changes continuously — directly leading to batch dimensional deviations. This is a widespread problem commonly found in small and medium-sized injection molding factories.

IV. Raw Material Batch Flowability Differences

The same grade of plastic from different batches, different manufacturers, and different storage periods inherently has variations in melt flow index and melt flowability.

When incoming materials on the shop floor are not tested for melt flow index, old and new materials are mixed arbitrarily, or raw material brands are swapped without adjustment, the melt filling resistance differs from shot to shot, the filling weight becomes unstable, and ultimately product dimensions fluctuate continuously.

V. Injection Speed Exceeding the Stable Range

This is the core critical point most easily overlooked: every type of plastic and every mold has a stable speed range.

Adjusting speed purely by experience makes it easy to fall into the sensitive range, resulting in product dimensions that are good one moment and bad the next.