In injection molding production, mold sticking (poor demolding) is a "killer of efficiency"—it leads to product damage, mold damage, and even forced shutdowns for cleaning, with single mold repair time potentially exceeding 30 minutes. Its essence is that adhesion force between melt and mold surface is greater than ejection separation force, preventing product from being successfully released during demolding. This article, from four major modules—mold sticking mechanism, five-dimensional root cause analysis, layered solutions, and zero-mold-sticking control—combined with case studies from automotive and electronics industries, teaches you how to systematically solve this "high-frequency problem."

 

Core contradiction of mold sticking is that interfacial bonding force between melt and mold core/cavity after cooling and solidification exceeds effective separation force of ejection system. Underlying logic needs to be analyzed from five dimensions: material surface energy, process cooling rate, mold surface treatment, ejection system design, and release agent usage.
1. Material Characteristics: "Inherent Adhesion" of Surface Energy and Polarity
Molecular structure and surface characteristics of material directly determine risk of mold sticking:
High surface energy materials (such as PC, PMMA, PA): Strong molecular polarity (PC surface energy 45 mN/m), high interfacial affinity with mold steel (surface energy approximately 50 mN/m), easily "welded" (e.g., a mobile phone frame made of PC material, with a demolding damage area of 20%).
Low lubricity materials (such as POM, PP): High hardness after melt cooling (POM Rockwell hardness M80), large friction coefficient with the mold (μ > 0.3), easily stuck during ejection (a home appliance gear made of POM material, demolding force > 500N, far exceeding ejection system capacity).
Material degradation: High-temperature decomposition produces low-molecular-weight substances (such as HCl from PVC decomposition), forming an "adhesive layer" at interface (a wire casing made of PVC material, mold sticking rate increases by 30% with production time).
2. Process Parameters: "Timing Mismatch" in Cooling and Ejection
Process settings determine "timing" and "force" of separation force:
Excessive holding pressure: When pressure is >100MPa, melt is excessively compressed, and bonding force with mold interface increases (e.g., ABS holding pressure 12s → 15% mold sticking rate, 8s → 5%).
Insufficient cooling time: Product is ejected before it is fully solidified (e.g., ejection when PC gate temperature is >70℃), leading to "stringing and mold sticking" due to melt softening (a mobile phone case made of PC material, 8s cooling → 20% mold sticking rate, 12s cooling → 2%).
Improper ejection speed/position: Excessive ejection speed (>60%) leads to product tearing (residual burrs); too slow speed (<30%) leads to secondary shrinkage of melt (tighter mold sticking).
3. Mold Design: "Acquired Defects" in Surface Treatment and Structure
Mold condition is "main cause" of mold sticking, common problems include:
Unpolished core/cavity: Surface roughness Ra > 0.8μm (e.g., unpolished steel mold), microscopic pits adsorb melt (a mold with Ra 1.2μm → 30% mold sticking rate, polished to Ra 0.2μm → 5%).
Insufficient draft angle: Angle < 0.5° (standard ≥ 1°), increasing friction during ejection (e.g., a thin-walled part with a draft angle of 0.3° → demolding force > 800N, 1° → 300N).
Blocked vent grooves: Residual gas forms an "air cushion," hindering separation (e.g., vent grooves at the bottom of a deep cavity are blocked by mold release agent, increasing mold sticking rate by 20%).
4. Equipment Capabilities: "Basic Guarantee" of Ejection System and Temperature
Ejection accuracy of injection molding machine affects consistency of mold sticking:
Ejection pressure fluctuation: Unstable hydraulic system pressure (fluctuation > 5%), resulting in fluctuating ejection force (e.g., a device with ejection pressure of 30±1.5 tons → 10% mold sticking rate, stabilized to 30±0.3 tons → 3%).
Poor mold parallelism: Parallelism between moving and stationary molds is >0.05mm, causing product to tilt during ejection, resulting in insufficient local force (e.g., one end of a housing sticks to mold after ejection, while the other end has no residue).
5. Release Agent Usage: "Mismatch" between coating and process
Improper selection or application of release agent can exacerbate mold sticking:
Coating too thin/too thick: Too thin (<0.001mm) fails to isolate interface; too thick (>0.005mm) leads to coating peeling off, leaving "white frost" residue (for a certain part, release agent application amount of 0.003mm → 5% mold sticking rate, 0.002mm → 1%).
Solvent residue: Strong solvents (such as acetone) are not completely volatilized, dissolving mold surface coating (after cleaning mold with acetone for a certain PC part, mold sticking rate increased from 2% to 15%).

Mold sticking is easily categorized as a "mold problem," requiring a five-step troubleshooting process combining location, morphology, and process:
Step 1: Observe location and morphology of mold sticking (5-minute quick assessment)
Large-area tearing: Mostly due to unpolished molds (Ra > 0.8μm) or insufficient draft angle (<0.5°).
Localized point-like mold sticking: May be due to clogged vent grooves (gas residue) or uneven release agent application.
Periodic mold sticking (occurring every 5-10 cycles): Often due to ejection pressure fluctuations (unstable hydraulic system) or material degradation (periodic decomposition).
Step 2: Verify material surface energy and degradation (5-minute test)
Measure surface energy: When surface energy of PC/PMMA is >40mN/m, risk of mold sticking is high; consider using low surface energy materials (such as POM with a surface energy of 20mN/m).
Check for degradation: Observe whether there is yellowing/black spots at mold sticking area (material decomposition), measure barrel temperature (e.g., PVC material temperature >190℃ is prone to decomposition, needs to be reduced to 180℃).
Material Comparison: Switching to a lower polarity material (e.g., PC to PP, surface energy from 45→30 mN/m), if mold sticking decreases, material problem is ruled out.
Step 3: Adjust Process Cooling and Ejection (10 minutes of trial and error)
Shorten holding pressure time: Holding pressure time from 12s→8s (33% reduction), for example, mold sticking rate of a certain ABS part decreased from 15% to 5%.
Extend cooling time: PC gate cooling time from 8s→12s (ensuring temperature <60℃), eliminating tearing and mold sticking.
Optimize ejection parameters: Ejection speed from 70%→50% (reducing tearing), ejection pressure from 30 tons→35 tons (increasing separation force), mold sticking rate of a certain gear decreased from 20% to 2%.
Step 4: Check Mold Surface and Structure (30 minutes - 1 hour)
Measure surface roughness: Measure core/cavity with a roughness meter (Ra > 0.8 μm requires polishing, target Ra ≤ 0.2 μm).
Check draft angle: Measure angle with an angle gauge (<0.5° requires repair, target ≥1°).
Clean vent grooves: Use compressed air + copper brush to clear vent grooves (depth 0.02mm, width 5mm) to avoid gas residue.
Step 5: Confirm Equipment and Release Agent (10 minutes of testing)
Calibrate ejection pressure: Pressure fluctuation controlled within ±1% (e.g., a certain equipment's ejection pressure is 30±0.3 tons, and mold sticking rate is stable).
Check release agent spraying: Measure coating thickness with a thickness gauge (target 0.001-0.002mm) to avoid being too thin/too thick.

 

If main cause is process parameters:
Optimize cooling gradient: Increase local cooling in thick-walled areas (water temperature 15℃, 10℃ lower than main body), accelerating solidification and separation (e.g., mold sticking rate of automotive trim panels decreased from 1.5% to 0.3%).
Adopting "Soft Ejection": Using a servo ejection mechanism (adjustable speed) to avoid tearing caused by hard ejection (e.g., precision gear ejection force reduced from 800N to 300N).
If main cause is a mold problem:
Polishing core/cavity: Polishing with diamond paste to Ra ≤ 0.2μm (e.g., after polishing, mold sticking rate of a certain mold decreased from 30% to 5%).
Increasing draft angle: Thin-walled parts' draft angle increased from 0.3° to 1°, reducing demolding force from 800N to 300N.
If main cause is a material problem:
Adding external lubricants: Adding 0.2% silicone powder to PC (reducing surface energy to 35mN/m), reducing mold sticking rate by 70%.
Replacing with low-polarity materials: Replacing PA with PP, reducing surface energy from 45 to 30mN/m, reducing the mold sticking rate by 80%.

1. Design and Mold Trial Stage: Embedding "Anti-Mold Sticking Genes"
Mold surface treatment: Hard chrome plating (Ra ≤ 0.1μm) or PVD coating (e.g., TiN, reducing friction coefficient to μ < 0.1) on core/cavity.
Mold trial verification standard: Continuous production of 50 molds, mold sticking rate ≤ 1% (demolding force < 200N detected by a tensile testing machine), optimize mold/process if exceeding tolerance.
2. Production Stage: Dynamic Monitoring and Mold Maintenance
Establishing a "Mold Sticking-Process" database: For each product, record "process window without mold sticking" (e.g., cooling 12±1s, ejection pressure 35±0.5 tons), and quickly adjust when abnormalities occur.
Regular mold maintenance: Check surface roughness every 50,000 molds (Ra ≤ 0.2μm), and repolish every 100,000 molds.
3. Demolding Agent Standardization: "Zero Defect" in Spraying and Selection
Automatic spraying system: Robot spraying, thickness controlled at 0.001-0.002mm (error < ±0.0005mm).
Solvent Control: Use weak solvents (such as isopropanol), with a volatilization time of <5 seconds, to avoid residue.

Essence of mold sticking is a mechanical imbalance where "interfacial adhesion > separation force." Troubleshooting requires a five-dimensional approach, considering materials (surface energy/degradation), process (cooling/ejection), mold (surface/taper), equipment (ejection pressure), and release agent (coating/solvent). Prioritize process optimization (low cost, quick results), then modify mold (to solve structural problems), and finally standardize use of release agents (to avoid interfacial contamination). The key to long-term zero mold sticking is "predictive design + dynamic monitoring + standardized operation," ultimately achieving "zero mold damage and zero production downtime."
Core Mantra: First observe shape of mold damage, materials and mold are main causes; polishing and taper must be precise, cooling and ejection must be balanced; release agent should be thin and even, and data should be archived to prevent recurrence.