Last week, while debugging a car dashboard mold (PP+EPDM, projected area 1200cm²), client called late at night: "Product has a 0.6mm flash, weld lines are appalling, and there are bubbles in thick-walled areas!"  Arriving at scene and seeing defective products piled on pallet, I deeply realized that mold trial is not a "fix-it-where-it's-broken" firefighting game, but rather requires establishing a diagnostic mindset of "symptom-root cause-systematic solution." Today, using this mold as an example, I will break down in-depth diagnosis and solutions for four major mold trial problems–teaching you how to transform from "passive firefighting" to "proactive prevention," and from "experience-based mold adjustment" to "systematic solutions."

 

Problem Phenomenon: 0.6mm flash on parting line, causing direct assembly interference
During initial mold trial, seeing 0.6mm flash squeezed out of parting line, my new apprentice immediately suggested "increasing clamping force." But I knew this was a typical "treating symptom, not cause" approach–essence of flashing is a double failure of excessive mold cavity pressure and insufficient mold matching accuracy.
Three-Layer Logic of In-Depth Diagnosis:
- First Layer: Mold Cavity Pressure Calculation: Theoretical value of mold cavity pressure for PP material is 6-8MPa, but actual measurement reached 12MPa (over 40% higher). Where did pressure come from? Injection pressure of 10MPa was too high, coupled with an excessively long holding pressure time (15 seconds), causing melt to be continuously pushed.
- Second Layer: Mold Matching Accuracy: Actual flatness of parting line was 0.05mm (standard ≤0.02mm), and slider gap was 0.03mm (standard ≤0.01mm). This is equivalent to mold's "door crack" not being properly closed, so even with high pressure, leakage is inevitable.
- Third Layer: Process parameter matching–a clamping force of 150T seems sufficient, but due to poor mold fit accuracy, actual effective clamping force is reduced by 30%, equivalent to "a loose latch, more locking won't help."
Root Cause Solution and Practical Details:
1. Mold Repair – Precision Machining of Mating Surfaces
- Parting surface was precision machined on a lathe, reducing flatness from 0.05mm to 0.015mm (feels like a mirror to touch);
- Slider gap adjustment: originally 0.03mm, adjusted to 0.008mm with shims (repeatedly verified with a feeler gauge). Key technique: After mold repair, an "empty mold closing test" must be performed. Use a flashlight to illuminate parting surface; it's only considered qualified if there is no light leakage.
2. Process Optimization – Pressure and Time Dual Control
- Injection pressure reduced from 10MPa to 7MPa (mold cavity pressure controlled within 8MPa safety limit);
- Holding pressure time reduced from 15 seconds to 8 seconds (to avoid melt backflow). Key judgment: Longer holding pressure time is not always better; continuing to apply pressure after melt solidifies will only cause flashing.
3. Equipment Calibration – Precise Clamping Force Matching
- Clamping force reduced from 150T to 120T (actual effective clamping force still has a 20% surplus);
- Check clamping force curve after each mold closing, controlling fluctuations within ±5%.
Effect Verification: Flashing reduced from 0.6mm to 0.05mm (undetectable by touch), with no recurrence after 100 consecutive production cycles.

Problem Phenomenon: Obvious weld lines on appearance surface, impact strength reduced by 20%
Weld lines on appearance surface of this instrument panel looked like "centipedes" crawling on the surface, leading to direct rejection by customer. Traditional approach is "increase temperature + increase pressure," but this carries risk of material degradation. Essence of weld lines is superimposed effect of unbalanced melt flow and differences in crystallization orientation. In-depth Diagnosis - Three-Layer Logic:
- First Layer: Flow path analysis revealed uneven product wall thickness of 2-4mm. When melt flows from thin-walled area to thick-walled area, a "fountain effect" occurs. Leading edge of melt cools down, while high-temperature melt from rear impacts it, resulting in weak interfacial bonding.
- Second Layer: Crystalline material characteristics. PP crystallizes during flow, and molecular chains at weld line are misaligned, similar to sewing two pieces of fabric with different orientations, resulting in inherently insufficient strength.
- Third Layer: Mold venting defects. Vent groove at corresponding position of weld line is only 0.01mm deep (standard ≥0.02mm). Residual gas leads to surface "air pockets," further weakening weld.
Root Cause Solution and Practical Details:
1. Mold Modification – Allowing melt to "meet gently"
- Deepen vent groove: From 0.01mm to 0.025mm, adding a φ0.5mm vent hole every 50mm;
- Relocate gate position: From edge to center, allowing melt to flow symmetrically and avoiding "one side fast, one side slow." Key technique: Vent groove should be "deeper rather than shallower," but not too deep (>0.03mm will cause material leakage), 0.025mm is optimal.
2. Process Adjustment – Controlling Crystallization Orientation
- Mold temperature from 80℃ to 90℃ (optimal temperature range for PP crystallization, reducing molecular chain orientation differences);
- Segmented injection speed: 70% fast filling for thin-walled area in the first stage, and 50% slow injection for thick-walled area in second stage, avoiding impact. Key judgment: Observe area near gate; absence of "jetting marks" indicates a matching speed.
3. Material Optimization – Reducing Crystallization Differences
- Add 0.2% external lubricant (zinc stearate) to reduce melt fracture;
- Raw material drying: 80℃ * 4 hours, reducing moisture content from 0.1% to 0.03% (moisture can cause local hydrolysis, leading to more uneven crystallization).
Effect Verification: Weld line strength increased by 30%, appearance defects were essentially eliminated, and impact strength returned to standard value.

 

Problem Phenomenon: Dense and unevenly distributed bubbles in thick-walled areas.
Thick-walled areas of product are full of bubbles, like a "honeycomb," and are hollow inside after cutting. Essence of bubbles is a combination of material gas content, process shear, and uneven cooling.
In-depth Diagnosis - Three-Layer Logic:
- First Layer: Material Gas Content - PP raw material was not sufficiently dried, with a moisture content of 0.2% (standard ≤0.05%). During melting, moisture vaporized, forming bubble nuclei.
- Second Layer: Process Shear Heating - Injection speed of 90% was too high, resulting in a local shear rate exceeding 10000 s⁻¹, causing material decomposition and gas generation (a slight acidic smell was detected).
- Third Layer: Uneven Cooling - Mold cooling channel layout was unbalanced, leading to slow cooling in thick-walled areas, giving bubbles time to grow and preventing them from being expelled before solidification.
Solutions and Practical Details:
1. Material Pre-treatment - Controlling Gas at Source
- Raw material drying upgrade: Dehumidifying dryer at 80℃ for 4 hours, reducing moisture content to 0.03%;
- Add 0.1% antioxidant (hindered phenol type) to inhibit high-temperature decomposition and gas generation. Practical tip: Dried raw materials must be "sealed and stored," as exposure to air for 2 hours will cause moisture content to rebound.
2. Process Optimization - Reducing Shear and Temperature
- Reduce injection speed from 90% to 70%, reducing shear heating;
- Reduce barrel temperature from 230℃ to 210℃ to avoid material decomposition (observe melt color, keeping it transparent without yellow spots). Key point: Higher barrel temperature is not always better; PP starts to decompose above 230℃, producing small molecular gases.
3. Mold Improvement - Optimizing Cooling and Bubble Removal
- Add cooling channels to thick-walled areas: φ8mm copper pipes, 20mm spacing, directly to product center;
- Reduce gate size: Φ4mm → Φ3mm, reducing shear area. Practical tip: Cooling channels should be "designed close to the wall," only 5mm from product surface, to ensure rapid cooling. Effect Verification: Bubble rate reduced from 15% to 0.5%, product density is uniform, and ultrasonic testing shows no internal defects.

Problem Phenomenon: Planarity deviation of 0.3mm, uneven gap between product and frame during assembly.
Product bends like a "banana" after cooling, with a planarity of 0.3mm (standard ≤ 0.1mm). Essence of deformation is a chain reaction of uneven cooling, shrinkage differences, and stress concentration.
In-depth Diagnostic Three-Layer Logic:
- First Layer: Asymmetrical cooling. Mold cooling channel design is unbalanced, with 4 φ10mm water pipes on the left side and only 2 on the right, resulting in a temperature difference of 15℃ between left and right sides, leading to inconsistent shrinkage.
- Second Layer: Shrinkage difference. Product wall thickness is 2-6mm. Thin-walled area shrinks by 1.2%, while thick-walled area shrinks by 2.5%, a difference of double shrinkage amount, inevitably causing warping.
- Third Layer: Stress concentration. Ejection position is unreasonable, with only central ejection. Stress at four corners cannot be released, resulting in rebound deformation after demolding.
Root Cause Solution and Practical Details:
1. Mold Optimization – Balancing Cooling and Stress
- Symmetrize cooling channels: Add 2 φ10mm water pipes to the right side, resulting in 4 pipes on each side;
- Modify ejection system: Add one ejector pin to each of four corners to disperse ejection force and avoid stress concentration. Key technique: Ejector pin diameter should be ≥ φ3mm; too small and it will easily bend, leading to unbalanced ejection.
2. Process Control – Uniform Cooling and Pressure Reduction
- Increase cooling time from 20 seconds to 35 seconds to ensure sufficient solidification of thick-walled area;
- Reduce holding pressure from 7MPa to 5MPa to reduce shrinkage pressure and internal stress. Key judgment: Product should not deform after ejection; this indicates proper cooling.
3. Post-processing Enhancement – Releasing Residual Stress
- Add an annealing process: 80℃ * 2 hours to slowly release internal stress;
- Adopt "segmented cooling": Divide mold into 3 temperature-controlled segments, allowing thick-walled area to cool slowly and thin-walled area to cool quickly. Effect Verification: Flatness improved from 0.3mm to 0.08mm (meeting standard), and deformation rate was <0.5% after 50 consecutive production cycles.

 

Establishing a "Problem Tree" Diagnostic Model
Problem Phenomenon → Direct Cause → Root Cause → Systemic Solution
Taking flashing problem as an example:
- Phenomenon: 0.6mm flashing on parting line
- Direct Cause: Excessive injection pressure (10MPa)
- Root Cause: Insufficient mold matching accuracy + improper process parameters
- Systemic Solution: Mold repair + parameter adjustment + equipment calibration
Key Mindset Shift:
- Don't just treat symptoms, find root cause of problem;
- Don't rely on "experience-based parameter adjustment," use "data to speak";
- Don't settle for "one-time solutions," aim for "systematic root cause elimination."
Developing a "Mold Trial Quality Checklist"
Before end of each mold trial, following items must be checked:
1. Mold Health: Parting line flatness, slider clearance, ejection system flexibility;
2. Material Consistency: Raw material moisture content, batch stability, drying effect;
3. Process Rationality: Temperature curve, pressure-time matching, speed segmentation;
4. Equipment Status: Clamping force stability, temperature control accuracy, injection repeatability.

Three Don'ts:
- Don't treat only symptoms: Flashing is not just a clamping force problem; it may be a combination of mold accuracy and process parameter failures;
- Don't adjust parameters blindly: Parameter adjustments must have theoretical support, not rely on "feel" and "experience";
- Don't ignore small problems: A 0.1mm flash can escalate into a major customer return issue.
Three Musts:
- Must establish a problem archive: Phenomenon, root cause, and solution of each problem must be recorded in detail;
- Must conduct comparative verification: Effect must be verified before and after adjustments using equipment such as CMM and universal testing machines;
- Must standardize process: Similar problems should have standard operating procedures so that new employees can quickly get started. Finally: Transformation from "mold adjustment technician" to "quality engineer"
After systematic optimization, this instrument panel mold achieved following results:
- Defect rate reduced from 25% to 1.5% (approaching zero defects);
- Production efficiency increased by 30% (cycle time reduced from 45 seconds to 30 seconds);
- Mold life extended by 50% (improved fit and reduced wear after maintenance).
The highest level of mold trial and adjustment is to establish a quality system based on "prevention first, systematic root cause analysis"—predicting problems before they occur and eliminating defects at their inception. This is not just "mold adjustment," this is "manufacturing engineering."
Front-line mantra (memorize this, and quality problems won't recur):
Check mold fit first for flashing, pressure parameters must match;
Check flow path for weld lines, consider exhaust temperature;
Dry material first for bubbles, control shear rate;
Cooling must be uniform for deformation, stress release must be thorough;
Establish a system for problem diagnosis, systematic solutions prevent recurrence;
Mold trial is not just for sampling, quality control is goal.