Target audience: Structural engineers, injection molding engineers, NPIs, SQEs.
Applicable scenarios: T0/T1 trial molding, mass production anomalies, shrinkage /deformation /flash /weld lines /short shot troubleshooting.
Core concept: Machine adjustment isn't about applying pressure, increasing temperature, or delaying based on intuition; it's about understanding what each parameter affects and what side effects it might have.
A real-world case: Last month, during a T0 trial molding of a project, a large-size PP base experienced severe shrinkage. Injection molding plant technician's approach: Apply holding pressure. Shrinkage was corrected. But then a problem arose—
After storage at 70℃, severe deformation occurred; Screw pillars cracked during assembly; The overall weight increased by 3%, exceeding cost limits.
Why? Because holding pressure was excessive; internal stress was locked in, and when released at high temperature, deformation became even more severe. This isn't because technician is unprofessional, but rather because he lacks a systematic framework for understanding these parameters.

Plasticizing → Injection Filling → Pressure Holding and Shrink Compensation → Cooling and Shaping → Mold Opening and Ejection → Post-Shrinkage/Standing Stabilization
Each stage affects final dimensions, appearance, strength, and deformation.

 

Increasing material temperature generally improves flowability, but higher isn't always better.
Suitable situations for increasing material temperature: Short shots; Noticeable weld lines; Difficulty filling at far end; Noticeable flow marks on the surface.
Risks: Increased PP shrinkage; Material decomposition, air bubbles, black spots; Longer cycle times; More pronounced dimensional changes at higher temperatures.
For every 10℃ increase in material temperature, MFR of PP increases by approximately 20-30%. However, this isn't a free lunch—shrinkage also increases simultaneously.

Mold temperature affects surface quality, crystallization, residual stress, and dimensional stability.
For PP: Too low a mold temperature: rapid surface freezing, high residual stress. Too high a mold temperature: longer cycle times, potentially increased shrinkage and dimensional changes. Large temperature difference between moving and fixed molds: prone to warping.
Key points to record for large-size PP parts: Moving mold temperature; Fixed mold temperature; Surface temperature at the moment of demolding; Temperature difference between center and edge of large flat surfaces.
One of the most easily overlooked data points for large-size PP parts: Demolding temperature. If demolding temperature exceeds 60℃, it indicates insufficient cooling time, resulting in a high risk of deformation after high-temperature treatment.

Holding pressure is used to compensate for cooling shrinkage. Core principle is not "the more pressure the better," but rather finding a balance between shrinkage, dimensions, internal stress.
Insufficient holding pressure: Shrinkage; Smaller dimensions; Insufficient holding pressure at far end; Internal voids.
Excessive holding pressure: Gate whitening; Difficult demolding; High internal stress; Deformation due to stress release after high-temperature treatment; Cracking of clips or screw posts.
Recommended pressure holding step: Current pressure holding; Current -10%; Current -20%; Current +10%. Compare initial shrinkage, dimensions, post-high-temperature deformation, drop.
This is lesson from initial case: Higher pressure holding is not always better. It's crucial to consider both post-high-temperature deformation and drop, not just immediate appearance after ejection.

Cooling time determines whether part has sufficient rigidity before ejection.
Insufficient cooling: Continue to deform after ejection; Obvious ejection marks; Large flat surfaces sag; Deformation worsens after high temperature.
However, excessive cooling will sacrifice cycle time, so data-driven judgment is necessary:

Finding "plateau period" is key. If effects of cooling for +10 seconds and +15 seconds are similar, then +10 seconds is optimal solution; extending it further is simply a waste of time.

When encountering deformation/shrinkage, it's not recommended to change many parameters at once.
Fixed materials and measurement methods → Record current parameters → Single-factor experiment → Holding pressure step → Cooling time step → Mold temperature/material temperature window → Retest after high temperature → Lock process card
Golden Rule of Machine Tuning: Change only one parameter at a time. Changing two parameters simultaneously will leave you wondering which one is effective.

Injection molding parameters are not independent knobs. Changing one parameter often alters effect of another.

Therefore, trial molding records cannot simply state "adjusted." Changes in each parameter and their corresponding results must be recorded.

DOE (Design of Experiments) is suitable for distinguishing influence of multiple factors on defects.
For large-sized PP parts deforming after high temperature, a small DOE (Design of Effect) can be performed first:

Record at least for each group: Initial dimensions and flatness; Demolding temperature; Appearance shrinkage; Dimensions after 24 hours; Dimensions after 70℃ × 72 hours; Assembly clearance; High-temperature drop.
Don't just look at "whether it's flat immediately after demolding". The key for large-sized PP parts is their stability after high temperature.

Mass production process cards should not only list a target value, but also allowable values.

For large-size PP parts, it is recommended to add: Exit Temperature Upper Limit; + Moving Mold Temperature Difference Upper Limit; + High-Temperature Sampling Frequency.

Misconception 1: Increasing Holding Pressure for Shrinkage
If shrinkage comes from thick plastic and insufficient cooling, blindly increasing holding pressure may only increase internal stress and deformation after high temperature.
Correct Approach: First, confirm whether shrinkage location corresponds to a thick plastic area. If so, structurally reducing plastic is more effective than adjusting process.
Myth 2: Adding cooling immediately after deformation
Adding cooling can sometimes be effective, but if cooling water channels are uneven, extending cooling time will only lower the overall temperature, not solve localized temperature differences.
Correct approach: First check cooling water channel distribution. Temperature difference between center and outer edge of large surface area should be controlled within 5℃.
Myth 3: Increasing material temperature immediately after a short shot
Increasing material temperature can improve flow, but it may also increase decomposition and shrinkage. Gate, venting, injection speed, and material MFR should also be considered.
Correct approach: First confirm whether short shot location is at the end of flow path. If so, consider whether gate location is unreasonable or runner is not large enough.
Myth 4: Only looking at appearance, not function
A OK appearance immediately after molding does not mean it will be OK after high-temperature assembly. For large parts, dimensions, assembly, high-temperature testing, and drop testing must be considered together.
Correct approach: Trial mold report must include 24-hour dimensional re-measurement + 70℃ high-temperature storage verification.