Dimensional Instability in Injection Molding: Causes and Solutions

1. Inconsistent Molding Conditions or Improper Operation

During injection molding, process parameters such as temperature, pressure, and timing must be strictly controlled according to specifications. The molding cycle for each part must remain consistent and should not be changed arbitrarily.

If the injection pressure is too low, holding time is too short, mold temperature is too low or uneven, or the barrel and nozzle temperature is too high causing insufficient cooling, dimensional instability may occur.

Generally, using higher injection pressure and speed, extending filling and holding times appropriately, and increasing mold and material temperatures can help overcome these issues.

2. Improper Selection of Molding Materials

The shrinkage rate of the material greatly affects dimensional accuracy. Even with precise equipment and molds, materials with large shrinkage rates make accuracy difficult to maintain.

When selecting resins, consider how post-molding shrinkage affects dimensional accuracy. The shrinkage variation should not exceed the part’s tolerance.

Different resins have different shrinkage rates depending on crystallinity. Crystalline and semi-crystalline resins have higher and more variable shrinkage than amorphous resins, leading to greater dimensional fluctuations.

3. Mold Defects

Mold design and manufacturing precision directly affect dimensional accuracy. If a mold lacks rigidity or experiences excessive pressure, deformation can occur, causing instability.

Excessive clearance between guide pins and bushings due to wear or poor manufacturing reduces dimensional accuracy. Hard fillers or glass fiber-reinforced materials can wear the mold cavity. Multi-cavity molds may show filling inconsistencies due to cavity differences, gate or runner misalignment, or unbalanced feed.

Molds should have sufficient strength and rigidity, machining precision should be strictly controlled, and cavity materials should be wear-resistant. Surfaces should ideally be heat-treated or hardened.

For high-precision parts, single-cavity molds are preferred. Multi-cavity molds may require auxiliary devices to ensure precision, increasing cost.

4. Equipment Malfunctions

Insufficient plasticizing capacity, unstable feeding, fluctuating screw speed, malfunctioning stops, hydraulic check valve failure, or temperature control problems can all affect dimensional stability. Once identified, these issues can be corrected with targeted measures.

5. Inconsistent Testing Methods or Conditions

Differences in measurement methods, timing, and temperature can cause large variations in measured dimensions. Temperature has the greatest impact because plastics expand about ten times more than metals.

Part dimensions should be measured using standardized methods and controlled temperature conditions after full cooling. Typically, parts undergo significant dimensional changes within the first 10 hours after demolding and stabilize after 24 hours.