Sticking (poor mold release) occurs when adhesion between molten plastic and mold surface during cavity filling, cooling, and ejection becomes greater than product's internal cohesive forces or ejection force provided by ejector mechanism, preventing product from being smoothly released from mold. This can manifest as stretching, cracking, or partial product residue remaining in mold cavity. This imbalance is essentially due to an imbalance of forces acting at multiple interfaces: van der Waals forces, chemical bonding, or mechanical interlocking (such as material shrinkage and mold clamping during cooling) exist between mold surface and material. During ejection, ejection force (from ejector pin and stress relief) is insufficient to overcome these adhesion forces. Microscopic observation reveals tensile deformation (broken fibers) at mold interface or scratches on mold surface (caused by forced demolding).

Mold sticking is result of a synergistic failure in mold design, process parameters, material properties, and ejection system. It can be categorized into five major, overlapping causes:
- Poor mold surface condition (the most direct contributing factor): High surface roughness of mold cavity/runner (Ra > 0.8μm), presence of microscopic grooves or pits (which provide mechanical engagement points), residual mold release agent/foreign matter (which creates an additional bonding layer), and insufficient taper (draft angle < 0.5°, increasing demolding resistance).
- Out-of-control process parameters (a key driver): Excessive holding pressure (> 70% of injection pressure, resulting in excessive material compaction and mold clamping), insufficient cooling time (parts are not fully solidified, shrinkage and clamping forces are not released), excessive injection speed (melt impacts mold, and uneven shrinkage after cooling exacerbates sticking), and low melt temperature (poor material flowability, resulting in a tighter mold fit during filling).
- Ejector system design flaws (visible triggers): Insufficient ejector pin count (less than 0.5 pins per square centimeter, resulting in localized stress concentration), rough ejector pin surface (Ra > 0.8μm, increasing friction coefficient), excessive ejection speed (impact causing product deformation and cracking), and lack of secondary ejection (internal stress in thick-walled products is not released, causing the entire product to cling to mold).
- Material shrinkage and flow properties (hidden risks): High-shrinkage materials (e.g., PP shrinkage of 1.5%-2.5% and PA66 shrinkage of 1.8%-2.2%) result in high mold clamping forces during cooling; low-flow materials (e.g., PC MI = 6-8g/10min) adhere tightly to mold during filling, resulting in high demolding resistance; excessive proportion of recycled material (reduced molecular weight, shrinkage fluctuations of ±0.3%, and unstable clamping forces).
- Equipment deterioration (long-term hidden dangers): Fluctuations in clamping force (causing slight mold deformation, localized gap variations, and increased sticking), platen parallelism errors (>0.03mm/m, resulting in uneven ejector force), and insufficient cylinder thrust (pressure decay during ejection, failing to overcome sticking).

 

Based on cost of repairing mold sticking, its impact on product integrity, and probability of recurrence, we recommend systematically addressing issue according to following priorities:

Core Logic: Microscopic defects on mold surface serve as "physical anchor points" for sticking. Reducing surface roughness, removing foreign matter, and optimizing taper can directly reduce source of sticking.
2.1.1 Precise Location and Treatment of Mold Surface Defects
- Surface Roughness Testing: Use a profilometer to measure Ra value of key areas of cavity (such as ribs, bone areas, and deep cavities), with a target of ≤0.4μm (≤0.2μm for precision parts). For areas with Ra >0.8μm, polish with diamond paste (1μm → 0.2μm) in a step-by-step process to eliminate grooves and pits (mechanical keying points).
- Foreign Matter Removal and Surface Activation: Avoid using steel brushes for cleaning; instead, use a copper brush and anhydrous ethanol for wiping. (Use a dedicated degreaser to ultrasonically clean mold release agent residues.) For stubborn stains (such as carbonized materials), use a plasma cleaner to remove organic contaminants and reduce surface energy.
- Draft Angle Optimization: For vertical surfaces or deep cavities (depth > 10mm), increase outer draft angle (from 0.5° to 1°-1.5°) and inner undercut angle (0.3° to 0.8°) to reduce demolding friction (theoretical demolding force is inversely proportional to tanθ; for every 0.5° increase in θ, friction decreases by 15%).
Operational Details:
- After polishing, mold should be purged with compressed air (pressure 0.3MPa) to ensure that no polishing dust remains (to avoid secondary contamination).
- During mass production, wipe cavity with alcohol every two hours (focusing on runner corners and areas near gate) to prevent release agent buildup and formation of a sticking layer.
Case: On a home appliance housing (ABS), sticking was concentrated at the base of rib. Profilometer testing showed Ra = 1.2μm (design requirement ≤ 0.6μm), and draft angle was only 0.3°. After polishing to Ra = 0.3μm and adding a 0.5° draft angle at rib root, sticking rate dropped from 30% to below 2%.

Core Principle: By reducing material holding force and optimizing shrinkage, adhesion resistance during demolding is reduced, quickly improving sticking defects.
2.2.1 Coordinated Optimization of Holding Pressure and Cooling Parameters
- Holding Pressure and Time: Adopt a "low pressure, short holding" strategy: Holding pressure = injection pressure × 50%-60% (e.g., injection pressure 100MPa, holding pressure 50-60MPa) to avoid over-compaction of material; holding time = part wall thickness × 1.5s/mm (e.g., 2mm wall thickness, holding pressure 3s) to prevent excessive mold holding force during cooling and shrinkage.
- Melt and Mold Temperature Control:
- Melt temperature: Increase melt temperature appropriately within material's flow range (e.g., from 200℃ to 220℃ for PP, from 260℃ to 280℃ for PA6) to reduce melt viscosity (viscosity reduced from 2000 Pa·s to 1500 Pa·s will reduce mold adhesion by 20%).
- Mold temperature: Reduce mold temperature to below material's heat distortion temperature (e.g., from 60℃ to 50℃ for ABS, from 80℃ to 70℃ for PC) to accelerate surface curing and reduce shrinkage and sticking.
- Extended Cooling Time: Cooling time in thick-walled areas should be controlled separately (e.g., by zoning mold cooling circuit) to ensure that center temperature of part drops below material's glass transition temperature (e.g., from 50℃ to 30℃ for PP), fully release any sticking forces.
Operational Details:
- For multi-cavity molds, balance cooling time of each cavity (using a mold temperature controller for zoning) to prevent sticking due to insufficient cooling in individual cavities.
- After cooling time is extended, ejection parameters need to be adjusted simultaneously (such as adding an ejector cushion to prevent product deformation).
Case study: A certain automotive interior panel (PP) had a 25% sticking rate. Original holding pressure was 70 MPa (injection pressure 100 MPa), holding time was 5 seconds (wall thickness 2 mm x 2.5 seconds/mm = 5 seconds), and mold temperature was 60℃. After adjusting holding pressure to 60 MPa, holding time to 3 seconds, and lowering mold temperature to 50℃, sticking rate dropped to 3%.

Core Logic: Ejection is final step in demolding. By increasing ejection force, reducing frictional resistance, and optimizing stress distribution, adhesive forces can be overcome to achieve demolding.
2.3.1 Ejector Pin Layout and Surface Treatment
- Ejector Pin Quantity and Distribution: Increase number of ejector pins (e.g., increase from 4 to 6) based on principle of "≥1 ejector pin per 50 cm² of product area" to reduce stress on each pin (ejection stress reduced from 15 MPa to <10 MPa). Prioritize placement of ejector pins in thick-walled areas and at the base of ribs (where adhesion is greatest).
- Ejector Pin Surface Treatment: For ejector pins φ2-5 mm, use hard chrome plating (thickness 10-15 μm, Ra ≤ 0.4 μm) or TD treatment (carbide layer thickness 5-8 μm, friction coefficient reduced to <0.1). For ejector pins φ>5 mm, use nitriding (hardness HV 900-1100).
- Ejector speed and stroke: Segmented control: 0.5-1mm/s in low-speed range (contacting product) and 2-3mm/s in high-speed range (releasing mold) to prevent cracking caused by impact. A "secondary ejection" setting (initial ejection of 2mm, a 0.5s pause, and a secondary ejection of remaining stroke) is configured to relieve internal shrinkage stress in product.
Operational Details:
- Ejector Pin Lubrication Cycle: Lubricate ejector pins with WD-40 every 100 molds (focusing on lubricating gap between pin and ejector hole). Clean any residual release agent on ejector pin surface with alcohol at the end of each shift.
- Ejector Force Calibration: Use a pressure sensor to monitor ejector pin force to ensure that maximum ejection force exceeds product adhesion force (theoretical calculation: adhesion force = mold surface energy × contact area + shrinkage clamping force).
Case: A mobile phone midframe (PC) experienced cracking around ejector pins during mold adhesion. Original ejector pins were 3mm in diameter (unchrome plated, Ra = 1.0μm), with an ejection speed of 4mm/s and only four pins. Replaced with 4mm chrome-plated ejector pins (Ra = 0.3μm), increasing number of ejector pins to six, reduced ejection speed to 2mm/s, and added a secondary ejector. Tear rate dropped from 15% to 0.1%.

Core Logic: Material shrinkage characteristics and equipment performance are long-term risks for mold sticking, requiring establishment of a standardized control process.
2.4.1 Material Shrinkage Matching and Modification
- Material Selection: Prioritize low-shrinkage grades (e.g., for PP, choose random copolymer PP with a shrinkage of 1.2%-1.8% instead of homopolymer PP with a shrinkage of 2.0%-2.5%). For high-shrinkage materials (e.g., PA6), add 3%-5% glass fiber (reducing shrinkage to 0.8%-1.2%).
- Recycled Material Control: Recycled material proportion ≤ 15% (to prevent shrinkage fluctuations caused by molecular weight loss) and must be dried separately (80℃ for 4 hours) to prevent moisture from affecting material flow and shrinkage uniformity.
2.4.2 Equipment Preventive Maintenance
- Clamping Force and Platen Parallelism: Clamping force must be ≥ theoretical value × 1.2 (theoretical clamping force = projected area × material pressure) to prevent mold deformation; platen parallelism must be adjusted to ≤ 0.02 mm/m (checked with a laser alignment tool) to ensure uniform ejector force.
- Ejector Mechanism Calibration: Regularly check ejector hole clearance (≤ 0.01 mm); any deviations require reaming and repair; perform cylinder thrust testing (ejector pressure loss during the holding phase is < 5%) to ensure stable ejector force.
Case Study: A precision gear (PA66+GF30) experienced frequent mold sticking. Investigation revealed material shrinkage fluctuations (due to a 20% recycled content and a decrease in molecular weight), and injection molding machine platen parallelism was 0.04 mm/m out of tolerance. After increasing recycled content to 10% and calibrating platen, sticking rate dropped from 20% to 1%.

 

- Symptom: The entire part adhered to mold, preventing it from releasing during ejection. Forced demolding resulted in cracking of ribs, resulting in an 18% scrap rate.
- Troubleshooting Process:
1. Mold Inspection: Cavity Surface Ra = 1.5μm (out of tolerance), Draft Angle Only 0.4° (inadequate), Insufficient Number of Ejector Pins (4 pins/100cm²);
2. Process Inspection: Holding Pressure 80MPa (Injection Pressure 100MPa), Mold Temperature 70℃ (too high), Insufficient Cooling Time (Wall Thickness 2.5mm x 2s/mm = 5s, Actual 3s);
3. Equipment Inspection: Platen Parallelism 0.05mm/m (out of tolerance), Ejector Force Loss 10% (70MPa → 63MPa).
- Solution:
- Mold: Polished cavity to Ra = 0.4μm, increased draft angle to 0.8°, and increased number of ejector pins to 6 per 100cm²;
- Process: Reduced holding pressure to 60MPa, mold temperature to 60℃, and extended cooling time to 7.5s;
- Equipment: Corrected mold plate parallelism to 0.02mm/m, and overhauled hydraulic system (pressure loss reduced to 5%).
- Result: Mold sticking rate was reduced to 0, product was fully demolded, and yield rate increased from 82% to 99.5%.

- Symptom: Mold stuck in logo groove area of product, causing damage to groove sidewalls and resulting in poor appearance. Incidence rate was 12%.
- Troubleshooting Process:
1. Mold Inspection: No ejector pins were present in groove, ejection was solely based on parting surface, draft angle was 0.3° (inadequate), and groove surface Ra = 1.0μm (rough);
2. Process Troubleshooting: Injection speed of 90mm/s (too fast) resulted in material accumulation in groove and high clamping force after cooling;
3. Material Verification: PP recycled content was 25% (shrinkage fluctuation ±0.4%).
- Solution:
- Mold: Added a 2mm chrome-plated ejector pin to the groove, polished surface to Ra = 0.3μm, and increased draft angle to 0.7°;
- Process: Reduced injection speed to 60mm/s, and increased holding time from 4s to 6s (wall thickness 2mm x 3s/mm = 6s);
- Material: Reduced recycled content to 10%, and increased material drying (80℃ x 4h).
Results: Mold sticking rate was reduced from 12% to 0.5%, and mold life was extended by 15%.

Management of mold sticking requires a systematic approach: mold surface is foundation, process adjustment is the key, ejection optimization is guarantee, and material control is supplement. Frontline engineers must master core skills such as mold surface roughness measurement (profilometer), draft angle calculation (tanθ = release force/adhesion force), and ejection stress calibration. They should shift from "forced mold release" to "preventive design + precise parameter control" to achieve a long-term solution to mold sticking.