Five Key Factors Affecting Dimensional Stability in Plastic Injection Molding
Even during injection molding production, dimensional variations can occur even when the same injection molding machine, mold, and material are used. Some parts may meet the required specifications, while others may be oversized or undersized.
When dimensional variations occur, most people instinctively blame machine aging, material batch differences, or inadequate mold precision. However, only a few look deeper into the underlying logic of process control.
Uneven Mold Temperature Distribution
Many factories use a series-connected cooling channel design, where cooling water flows sequentially through Cavity 1, Cavity 2, and Cavity 3. Because the inlet water is cooler while the water temperature rises significantly as it reaches the end of the circuit, temperature differences naturally develop between cavities.
As production continues, this temperature gap can become even more pronounced. For stable production, the temperature difference between cavities should generally be controlled within approximately 3°C. However, series cooling circuits can sometimes result in temperature differences of 5°C or even more than 10°C. This causes the plastic to cool and shrink at different rates, ultimately leading to dimensional variations between parts.
In addition, an improper cooling-channel layout, uneven distances between the channels and cavities, or inconsistent cooling efficiency across different areas of the mold can further aggravate mold temperature imbalance.
Practical Improvement Measures
Measure the actual temperature of every cooling circuit and the surface temperature of each mold cavity at scheduled intervals during every shift. Record and monitor the data systematically, with the temperature difference between cavities strictly controlled within ±3°C.
For multi-cavity molds, parallel cooling circuits are recommended wherever possible to ensure more uniform cooling. Long series-connected cooling layouts should be avoided to minimize temperature differences between cavities and maintain consistent part dimensions.
Incorrect Logic in Molding Parameter Settings
Experienced molding technicians often rely on time-based switching for the holding-pressure stage when producing conventional plastic housings. For less demanding applications, this approach offers a relatively high tolerance for process fluctuations, so potential issues may not be immediately apparent.
However, precision-molded parts are highly sensitive to injection pressure and melt filling behavior. When holding pressure is controlled solely by time, variations in the amount of melt filled and compressed during each cycle can lead to dimensional fluctuations. In some cases, the accumulated dimensional deviation can reach approximately 0.1–0.2 mm.
A common mistake is to simply increase holding pressure whenever the part dimensions are undersized. This addresses the symptom rather than the root cause. The actual problem is dimensional instability, not simply undersized parts. Blindly increasing holding pressure can further amplify process fluctuations and make dimensional consistency even more difficult to control.
Practical Improvement Measures
For precision-molded parts, a position-based switchover to the holding-pressure stage should be adopted as a standard process-control strategy. By accurately controlling the end point of melt filling and stabilizing the compression level within the mold cavity, cycle-to-cycle variations in part dimensions can be minimized at the source.
This approach provides more consistent cavity pressure and filling conditions, reducing dimensional fluctuations caused by variations in melt filling and compression.
Long-Term Neglect of the Cooling System
The cooling system is a critical part of the injection molding process that is often overlooked. When cooling towers and mold cooling channels are not cleaned and maintained regularly, scale, sludge, and other deposits can accumulate along the inner walls of the piping, significantly reducing the heat-transfer efficiency of the cooling water.
Even with the same cooling time, inconsistent heat dissipation can cause mold temperatures to fluctuate from cycle to cycle. As a result, the shrinkage rate of the plastic continues to vary, directly contributing to dimensional deviations across production batches. This is a common process-control issue, particularly in small and medium-sized injection molding facilities.
Practical Improvement Measures
Establish a systematic equipment maintenance log and schedule regular cleaning of cooling towers, filters, and mold cooling channels. For the circulating water system, install descaling and filtration equipment to minimize scale and sediment buildup and maintain stable heat-transfer efficiency.
Consistent cooling performance helps stabilize mold temperature from cycle to cycle, reducing variations in material shrinkage and improving dimensional consistency throughout production.
Batch-to-Batch Variations in Material Flowability
Different batches, suppliers, and storage periods of the same plastic grade can exhibit inherent variations in melt flow rate (MFR) and melt flowability.
When incoming materials are used without MFR testing, or when virgin and recycled materials are mixed arbitrarily or material suppliers are changed without proper process validation, the resistance to melt flow can vary from cycle to cycle. This leads to inconsistent filling weights and ultimately causes continuous dimensional fluctuations in the finished parts.
Practical Improvement Measures
Inspect the melt flow rate (MFR) of incoming materials before storage, and keep virgin and recycled materials separate during use. Whenever the material grade or production batch is changed, re-optimize the complete set of molding process parameters rather than applying the previous settings directly to the new material.
Injection Speed Exceeding the Stable Processing Range
This is one of the most overlooked factors affecting dimensional stability: every plastic material and mold has a specific injection-speed range within which the molding process remains stable.
- When the injection speed remains within the stable range: Minor fluctuations in ambient temperature or hydraulic pressure have little impact on the melt-filling behavior, so part dimensions remain relatively consistent.
- When the injection speed enters a material-sensitive range: Even slight changes in ambient temperature, hydraulic pressure, or mold temperature can alter the melt-filling behavior. This can immediately result in dimensional deviations from one molding cycle to another.
Practical Improvement Measures
Before production, conduct a scientific injection-molding viscosity curve test to accurately identify the stable injection-speed range for the specific material and mold combination.
Once the stable range has been established, maintain the injection speed within this range throughout production and avoid operating within material-sensitive speed zones. This helps stabilize melt-filling behavior and reduce cycle-to-cycle dimensional variations.
