Views: 0 Author: Site Editor Publish Time: 2026-04-09 Origin: Site
In injection molding, few things are more frustrating than this: you’re using the same machine, the same mold, and the same process settings—yet your parts come out with different dimensions from batch to batch, or even from cavity to cavity.
This problem is called dimensional instability.
Simply put: the parts are too big when they should be small, too small when they should be big, or inconsistent when they should be identical.
Today, we’ll break down the six major causes of dimensional instability and provide practical solutions for each.
Dimensional instability means that under the same injection molding machine and processing conditions, the dimensions of molded parts vary between production batches or between different cavities in the same mold.
The root causes generally fall into six categories:
Inconsistent process conditions or improper operation
Poor material selection or handling
Mold defects
Equipment malfunctions
Inconsistent measurement methods or conditions
Environmental factors
Let’s go through each one.
This is the most common cause on the production floor. Temperature, pressure, and timing must be strictly controlled according to the process specifications. The molding cycle must be consistent from shot to shot—no arbitrary changes.
Injection pressure too low
Hold/pack time too short
Mold temperature too low or uneven
Barrel or nozzle temperature too high
Insufficient part cooling
Generally, higher injection pressure and speed, longer fill and hold times, and higher melt/mold temperatures help overcome dimensional instability.
Problem | Solution |
|---|---|
Part dimensions larger than required | Lower injection pressure and melt temperature; raise mold temperature; shorten fill time; reduce gate cross-section (this increases shrinkage) |
Part dimensions smaller than required | Apply opposite measures (higher pressure/temperature, longer hold time, etc.) |
Important note: Changes in ambient temperature also affect dimensions. Adjust your process settings when seasonal temperature shifts occur.
The shrinkage rate of your material has a direct impact on dimensional precision. Even with a high-precision machine and mold, if the material has a high shrinkage rate, you won’t achieve tight tolerances. The higher the shrinkage, the harder it is to control dimensions.
Material with too wide a shrinkage range
Inconsistent pellet size
Poor drying (especially for hygroscopic materials like PA, PET)
Uneven mixing of virgin and regrind material
Batch-to-batch variation in material properties
Semi-crystalline resins (PP, PE, PA, POM) have higher shrinkage rates and wider shrinkage ranges than amorphous resins (ABS, PC, PS).
For semi-crystalline materials: higher crystallinity = more shrinkage; smaller spherulites = less shrinkage and better impact strength.
When selecting a material, ensure its shrinkage range is narrower than the required dimensional tolerance. Also, verify that the material is properly dried, batch consistency is maintained, and regrind is mixed uniformly.
The design and manufacturing precision of your mold set the upper limit on dimensional accuracy.
Insufficient mold rigidity: High cavity pressure deforms the mold, causing dimensional variation.
Worn guide components: Excessive clearance between guide pins and bushings reduces positioning accuracy.
Cavity wear: Materials with hard fillers or glass fibers gradually erode cavity surfaces.
Multi-cavity imbalance: Differences in cavity dimensions, runner sizes, or gate geometry lead to inconsistent filling.
Single-cavity mold with thickness variation → usually caused by mold mounting errors or poor alignment between cavity and core. For high-precision parts, don’t rely only on guide pins; add additional positioning devices.
Multi-cavity mold with thickness variation → error starts small but grows during continuous production, mainly due to accumulated tolerance differences. This is especially common with hot runner molds.
Design molds with adequate strength and rigidity
Maintain tight machining tolerances
Use wear-resistant cavity materials with surface hardening (heat treatment or cold hardening)
For high-precision parts, avoid multi-cavity molds if possible. If necessary, add auxiliary precision features—but expect higher tooling costs.
When making a mold, it’s common practice to machine the cavity slightly smaller than required and the core slightly larger, leaving room for adjustments.
If the molded hole’s inner diameter is much smaller than the outer diameter: make the core pin larger (shrinkage around holes is greater and occurs toward the hole center).
If the inner diameter is close to the outer diameter: the core pin can be made slightly smaller.
Use a floating core design, ensuring the core and cavity are concentric. Also, consider a dual cooling circuit with minimal temperature difference between circuits to control wall thickness variation.
The machine itself can be the source of dimensional instability.
Insufficient plasticizing capacity
Unstable feeding system
Inconsistent screw rotation speed
Non-return valve (check ring) leakage → melt flows back during injection
Temperature control system failure (burned-out thermocouple, broken heater band, etc.)
Inspect each system methodically. The non-return valve is often overlooked but very common—check it first. Repair or replace components as needed.
This is a frequently overlooked cause—the measurement itself can create the illusion of dimensional instability.
Temperature: Plastic’s coefficient of thermal expansion is about 10 times that of metal. The same part measured at 20°C vs. 30°C can differ by 0.05–0.1 mm.
Time: Parts continue to shrink significantly for 10 hours after ejection and only stabilize after approximately 24 hours.
Method: Variations in measurement points, contact force, or datum selection produce inconsistent readings.
Use standard-specified methods and temperature conditions for all measurements
Allow parts to fully cool and stabilize before measuring (recommend 24 hours after ejection)
Create a standardized measurement work instruction and ensure all operators follow it
While mentioned earlier, these deserve emphasis:
Seasonal temperature changes affect mold temperature baselines and the workload on temperature controllers.
Humidity variation impacts dimensions of hygroscopic materials (PA, PET, etc.).
Vibration from nearby equipment (presses, compressors) can affect mold closing precision or measurement readings.
When facing dimensional instability, follow this priority order:
Is the molding cycle consistent from shot to shot?
Is hold/pack pressure and time adequate?
Is mold temperature uniform?
Is the material’s shrinkage range suitable for the required tolerance?
Is the material properly dried? Is the batch consistent?
Are mold guide components worn?
Are all cavities in a multi-cavity mold producing identical parts?
Is the non-return valve leaking?
Is temperature control functioning correctly?
Are measurement temperature, time, and method standardized?
Have parts been allowed to stabilize for 24 hours before measurement?
Dimensional instability is a multi-factor problem, but most cases can be solved by following a logical sequence:
Check process first – cycle consistency, hold pressure, mold temperature
Check material – shrinkage range, drying, batch consistency
Check mold – rigidity, guiding, wear, multi-cavity balance
Check equipment – non-return valve, temperature control, feeding system
Don’t forget measurement – standardized conditions, adequate cooling time
The two most overlooked factors are process consistency (especially cycle-to-cycle variation) and measurement standardization (temperature and cooling time). Pay extra attention to these.
We hope this article helps you quickly identify the root cause of dimensional instability and reduce trial-and-error time on the shop floor. If you have specific cases or questions, feel free to reach out.
