Dimensional Variation
Causes, Solutions & Prevention in Injection Molding
Dimensional instability comes from inconsistent shrinkage process instability or tool wear. Plastics use DIN 16742 not DIN 7168 metal tolerances.
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🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Shot weight variation inconsistent pack dimensional scatter
- Holding pressure variation
- Cooling water temperature fluctuation
- Mold temperature drift
🔲 Mold
- Shrinkage calculated incorrectly averaged min max should test with flat plate mold
- Cavity imbalance in multi-cavity mold
- Cooling system imbalance differential shrinkage
🧪 Material
- MFI drift between lots shrinkage drift
- Additive change talc in PP changes shrinkage
- High regrind ratio lower MW more shrinkage
- Moisture hydrolysis lower MW more shrinkage
🗂 Mold Design
- Shrinkage calculation in mold is incorrect
- Runner and cold slug well problems in mold
- Gate location / flow length ratio is incorrect
- Sprue and runner cross-section area and shape are incorrect.
- Cooling system faulty
- Dimensional differences between cavities in multi-cavity mold
🔧 Mold Condition
- Mold parting line shift
📐 Part Design
- Tolerancing is incorrect
- Corner designs at 90° (no radius)
- Mold-dependent and independent dimensional tolerances not specified in drawing
📋 General
- Wrong tolerance system DIN 7168 metal used instead of DIN 16742 plastic
- Sharp corners in part shear stress dimensional change
- Screw position repeatability shot weight variation
✅Solutions & Corrective Actions
⚙️ Process
- Stabilize shot weight fix check valve barrel heaters thermocouple contacts
- Monitor holding pressure and cooling water temperature
- Use SPC on shot weight and key dimensions
🔲 Mold
- Test shrinkage on flat plate mold for exact value not average
- Balance multi-cavity runner system
- Fix cooling circuit imbalance
- Polish parting line by procedure only 11-step protocol
🧪 Material
- Materials with high MFI have greater shrinkage. Lower MFI grades can be tried.
- If additive use is decided after mold construction, part dimensions tend to come out larger. For example, adding talc to PP makes it behave like PS — shrinkage decreases and parts come out longer.
🗂 Mold Design
- In a flat plate mold, first measure the mold dimension. Then inject the material and measure the part. The difference precisely determines the shrinkage rate.
- Incorrect runner diameter and cross-section design can cause dimensional variation. Runner cross-section must be full round. Missing cold slug wells can also reduce polymer flow into the cavity. Runner cross-section and cold slug well design must be done correctly.
- The maximum flow length at minimum wall thickness must be known and design should be made accordingly
- Runner cross-section size must be calculated correctly based on part weight and must be full round. In hot runner systems, correct sizing must be selected based on material type and part weight.
- As a principle, mold surface temperature must be uniform at every point. If not achieved, differential shrinkage will cause dimensional errors.
- If cavity balance is incorrect, part dimensions vary depending on injection speed.
🔧 Mold Condition
- Incorrect pressure during mold polishing causes mold dimensions to grow after certain use. Polishing should be done on a bench in the mold shop, not on the machine, to apply correct pressure.
📐 Part Design
- DIN 16742 tolerance system is used for plastics. If tolerances are not given according to mold-dependent and independent shrinkage values per material type, different dimensions result. Designers sometimes use DIN 7168 which is a metalworking tolerance.
- Sharp-cornered plastic part designs create shear stress and internal stress. Minimum 0.2 mm radius should be given in plastic part design.
- If the part designer does not account for these tolerances in the drawing, the mold maker machines directly on CNC. These dimensions are given in DIN 16901.
📋 General
- Apply DIN 16742 tolerances not DIN 7168
- Get MFI certificate for each incoming lot
- Reduce regrind ratio
🛠Step-by-Step Troubleshooting
Check Process Parameters
Stabilize shot weight fix check valve barrel heaters thermocouple contacts
Inspect the Mold
Test shrinkage on flat plate mold for exact value not average
Verify Material Condition
Materials with high MFI have greater shrinkage. Lower MFI grades can be tried.
❓Frequently Asked Questions
What causes dimensional variation in injection molding?
Dimensional Variation is caused by several factors including: Shot weight variation inconsistent pack dimensional scatter; Holding pressure variation; Shrinkage calculated incorrectly averaged min max should test with flat plate mold; Cavity imbalance in multi-cavity mold; MFI drift between lots shrinkage drift; Additive change talc in PP changes shrinkage.
How do you fix dimensional variation in injection molding?
Start by checking key parameters: Shot weight (g), Cooling time, Mold temperature, Shrinkage compensation. Stabilize shot weight fix check valve barrel heaters thermocouple contacts
Which plastics are most affected by dimensional variation?
Common materials include: PA6, PA66, POM, PP, PBT. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent dimensional variation in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Shot weight (g), Cooling time; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between dimensional variation and similar defects?
Part dimensions vary batch to batch or within a batch. Part does not fit assembly. Dimensions shift over time. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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