Injection Molding Parameter Effects
What exactly changes in the process and the part when you increase a parameter? Effect tables for six basic settings — know the result before you make an adjustment.
How to read it: The tables show the tendency when the parameter is increased. When it is decreased, the effects work in the opposite direction. Always change one parameter and wait a few cycles.
Increasing Melt Temperature
Improves flowability and lowers the pressure requirement; in return, thermal stress and cycle time increase.
Effect on the Process
Shrinkageincreases or decreases depending on the part type▲▼ variable
Flowabilitythe effect is much stronger with amorphous plastics▲ increases
Injection pressure requirement▼ decreases
Achievable flow path length▲ increases
Thermal stress on the materialrisk of damage when combined with long residence▲ increases
Holding time▲ increases
Cooling timethe effect is usually negligible▲ increases
Effect on Part Quality
Weld line visibilitythe notch is smoothed out more▼ decreases
Degree of orientation▼ decreases
Degree of crystallization▲ increases
⚠️ Melt temperature must always be measured by injecting "into the open" (an air shot) and using a needle thermometer — the panel display does not give the real melt temperature.
Increasing Mold Wall Temperature
The strongest parameter for surface quality and dimensional stability; the price is cycle time (every 10 °C ≈ 20% longer cooling).
Effect on the Process
Mold shrinkage▲ increases
Post-shrinkagethe part deforms less in the field▼ decreases
Achievable flow path length▲ increases
Holding time▲ increases
Cooling timelengthens by about 20% for every 10 °C▲ increases
Effect on Part Quality
Dimensions▼ decreases
Tolerance fluctuation▼ decreases
Weld line visibility▼ decreases
Surface replicationa glossy surface becomes glossier, a textured surface more matte▲ increases
Degree of orientation▼ decreases
Crystallinity▲ increases
Internal stress▼ decreases
⚠️ Lowering the mold temperature to shorten the cooling time leads to unnoticed losses in internal properties: post-shrinkage and orientation later appear in the part as changes in shape and dimensions.
Increasing Holding Pressure
Suppresses shrinkage and sink marks; in return, the clamping force requirement, the risk of flash and ejection problems increase.
Effect on the Process
Shrinkage▼ decreases
Clamping force requirement▲ increases
Ejection problemsthin-walled/ribbed parts cannot release from the wall▲ increases
Mold deformation▲ increases
Effect on Part Quality
Dimensions▲ increases
Weight▲ increases
Sink marks▼ decreases
Void formation▼ decreases
Weld line strength▲ increases
Surface replication▲ increases
Flash▲ increases
Degree of orientation▲ increases
⚠️ The effective holding time (gate freeze point) is determined by weighing parts produced with increasing times: when the weight stops increasing, that time has been reached.
Increasing Injection Speed
Makes filling easier and eliminates some surface defects; in return, it magnifies shear, venting and burning problems.
Effect on the Process
Thermal stress on the materialABS, PMMA and PC are shear-sensitive▲ increases
Shear▲ increases
Venting problems▲ increases
Achievable flow path length▲ increases
Injection time▼ decreases
Effect on Part Quality
Matte spot near the gate▲ increases
Jetting▲ increases
Air streaks / air hooks▲ increases
Flash▲ increases
Glass fiber streaks▼ decreases
Burn marks▲ increases
Diesel effect▲ increases
Surface replication▲ increases
Record groove effect▼ decreases
⚠️ For a homogeneous surface the melt FRONT speed must be constant — and this is achieved not with a constant screw speed but with a "slow-fast-slow" profile.
⚠️ When the injection speed is changed, the switchover point, time monitoring and pressure limitation must be reset.
Increasing Back Pressure
Improves melt homogeneity and color distribution; with long glass fibers it causes a loss of strength.
Effect on the Process
Melt homogeneity▲ increases
Melt temperaturefrictional heat▲ increases
Flowability▲ increases
Glass fiber length▼ decreases
Plasticizing time▲ increases
Plasticizing time fluctuationswith material containing internal lubricants/color batch, excessively high or low values cause fluctuation▲▼ variable
Effect on Part Quality
Color streaks▼ decreases
Strengthfiber damage with long-glass-fiber materials▼ decreases
Air streaksair drawn in is pushed toward the feed▼ decreases
Increasing Screw Speed
Shortens the plasticizing time but impairs homogeneity and increases thermal stress. The material-specific circumferential speed limit must not be exceeded.
Effect on the Process
Melt homogeneitya very short plasticizing time creates inhomogeneity▼ decreases
Melt temperature▲ increases
Flowability▲ increases
Thermal stress on the material▲ increases
Glass fiber length▼ decreases
Unbalanced mold fillingaxial temperature differences in front of the screw▲ increases
Plasticizing time▼ decreases
Effect on Part Quality
Color streaksthe masterbatch mix becomes unbalanced▲ increases
Strength▼ decreases
⚠️ The maximum permissible speed is calculated from the material-specific circumferential speed limit. See the Material Processing Data table for the values.