For further reading, please refer to Solutions and Practical Guide to Shrinkage Problems in Injection Molding Machines Under 800T.

Glue and material overflow (abbreviated as flash) in injection molded products refers to excess flakes, filaments, or nodules formed at the edges or joints of product due to insufficient contact stress between mold closing surfaces during cavity filling process. This occurs when molten plastic overflows from parting surface, slider gaps, ejector pin holes, insert seams, other locations due to insufficient contact stress between mold closing surfaces. This occurs when melt pressure (P) exceeds actual contact stress (σ) between mold closing surfaces. That is, when P > σ, melt breaks through mold gap (δ), resulting in flash (formula: δ = P/σ, where δ is mold gap (in mm)). Microscopically, flash is composed of interwoven, continuous melt fibers, typically 0.02-0.5mm thick, but exceeding 1mm in severe cases. Surface is often characterized by "burrs" or "flashing."

Flash is result of multi-dimensional imbalances among mold precision, process parameters, material properties, and equipment status. It can be categorized into four major, interdependent causes:
- Insufficient mold precision (the most fundamental cause): Excessive parting surface flatness (>0.02mm/m²), excessive clearance between slider and lifter (>0.01mm), wear of ejector pin (diameter expansion of 0.03mm or more), and loose insert joints (clearance >0.015mm), which directly provide a channel for melt to overflow.
- Out-of-control process parameters (key drivers): Excessive injection pressure (>120 MPa), cumulative holding pressure (total pressure >150 MPa), insufficient clamping force (actual clamping force < projected area * material pressure), and excessive injection speed (melt instantly impacts mold, causing gaps to expand).
- Excessive material fluidity (hidden risk): High MI materials (e.g., PP MI = 35 g/10 min, PA6 MI = 40 g/10 min) have low flow resistance and easily penetrate tiny gaps. Low-viscosity modified materials (e.g., PC/ABS alloy with 20% compatibilizer added) experience a 30% decrease in viscosity and a significant increase in flashing.
- Equipment deterioration (long-term risk): Wear of clamping mechanism (crank arm connecting rod clearance >0.5 mm), excessive platen parallelism (>0.03 mm/m), and aging of cylinder seal (clamping force reduction of 10%-15%), resulting in an inability to maintain stable actual clamping force.

 

Based on cost of repairing flash issues, their impact on production efficiency, and probability of recurrence, we recommend systematically addressing them according to following priorities:

Core Logic: Mold gaps are "physical pathway" for flash. Improving mold accuracy can directly cut off melt overflow path, and most issues can be quickly resolved through mold repair.
2.1.1 Accurately Measuring and Repairing Gap in Key Mold Areas
- Parting Surface Inspection: Scan parting surface using a flatness gauge (0.001mm accuracy) and mark concave/convex areas (e.g., a localized concave area of 0.03mm). Observe contact area using a blue lead test (applying red lead powder to close mold) (target >90%). Areas without contact are considered gap points.
- Slider/Bevel Fit: Use a feeler gauge to check clearance between slider and guide rail (standard ≤ 0.01mm). If it exceeds tolerance (e.g., 0.02mm), slider bevel or guide rail guide block needs to be repaired (grind high point with an oilstone and then polish to Ra ≤ 0.8μm).
- Ejector Hole and Insert Clearance: If ejector hole wears out, causing diameter to expand (e.g., from φ3mm to φ3.03mm), it needs to be reamed (to IT7 precision). Insert joint should be repaired with argon arc welding and then re-milled to ensure a clearance of < 0.01mm.
Operation Details:
- After repairing parting surface, it needs to be ground entirely using a grinder (feed rate 0.01mm/pass) to ensure a flatness of ≤ 0.01mm/m².
- Apply high-temperature grease (such as molybdenum disulfide) to slider/bevel mating surface to reduce dynamic clearance expansion caused by sliding friction.
Case: Flash on a mobile phone case (PC+ABS) was concentrated at the edge of parting surface. Flatness testing revealed a localized depression of 0.04mm. By grinding parting surface to a flatness of 0.008mm and using Landan testing to increase contact area to 95%, flash problem was eliminated immediately and has not recurred in subsequent production runs.

Core Principle: Reduce melt pressure and optimize clamping force matching to minimize melt's potential to break through mold gap.
2.2.1 Coordinated Optimization of Injection and Clamping Parameters
- Injection Pressure and Holding Pressure: A "step-down" strategy was employed, gradually reducing injection pressure from an initial 100MPa to a holding pressure of 60MPa (for thick-walled products) to prevent high-pressure shock from expanding mold gap. Holding time was shortened to 1s/mm of product wall thickness * 1s (e.g., for a 2mm wall thickness, a 2s holding time) to reduce continuous pressure buildup.
- Precisely match clamping force: Calculate theoretical clamping force = product projected area (cm²) * material pressure (MPa) * safety factor (1.2). For example, for an ABS product with a projected area of 500 cm² (material pressure 80 MPa), theoretical clamping force = 500 * 80 * 1.2 = 48,000 kN (approximately 480 tons). Ensure actual clamping force of injection molding machine is ≥ 500 tons (allowing for a 20% margin).
- Injection speed control: Use a low filling speed (30-50 mm/s) to avoid instantaneous melt impact on mold. (High filling speeds can cause gaps to temporarily open, shrinking after cooling but leaving residual material.)
Operational Details:
- For multi-cavity molds, balance filling pressure across all cavities (monitored by pressure sensors) to prevent individual cavities from flashing due to excessive pressure.
- Adjust clamping force gradually (by 50 tons each time), monitoring flash changes to prevent mold deformation caused by overpressure.
Case: A certain automotive interior panel (PP) was experiencing severe flash. Original injection pressure was 110 MPa, and clamping force was 450 tons (projected area 520 cm², theoretically requiring 500 tons). Adjusting injection pressure to 80 MPa and increasing clamping force to 520 tons reduced flash from batch scrapping (80%) to sporadic flash (<2%).

Core Logic: Deteriorating equipment precision can cause fluctuations in clamping force. Even if mold is qualified, flash may still occur, requiring regular maintenance.
2.3.1 Maintaining Parallelism of Clamping Mechanism and Platen
- Clamping Mechanism Inspection: Check clearance of crankshaft connecting rod (measured with a feeler gauge, standard ≤0.2mm). If it exceeds tolerance, connecting rod bushing should be replaced. Cylinder seals that are aged (leakage or reduced clamping force) should be completely replaced (recommended maintenance every 2000 hours of production).
- Platen parallelism correction: Use a laser alignment tool to check parallelism of movable and fixed platens (standard ≤ 0.02mm/m). If deviations occur, adjust platen leveling bolts (0.01mm increments) and verify with a dial indicator.
Operational Details:
- After maintenance on clamping mechanism, perform 50 dry runs to test clamping force stability (fluctuation <5%).
- After platen parallelism correction, recalibrate mold mounting reference surface to prevent localized insufficient clamping force due to platen tilt.
Case Study: Frequent flashing of a home appliance casing (ABS). Investigation revealed a 0.6mm gap between clamping mechanism's crankshaft connecting rod and a 0.04mm/m deviation in platen parallelism. After replacing connecting rod bushing and correcting platen, clamping force fluctuation decreased from 15% to 3%, completely resolving flashing issue.

Core Logic: Material flowability and process-related auxiliary measures can further reduce flashing, especially for highly fluid materials.
2.4.1 Material Flow Control
- Material Selection: Prioritize low- to medium-MI grades (e.g., for PP, choose MI = 20-25g/10min, instead of MI = 35g/10min), or add 1%-2% nano-montmorillonite (reducing melt flow by 30%).
- Drying and Mixing: Hygroscopic materials (PA6) should be thoroughly dried (120℃ for 4 hours) to prevent moisture evaporation and melt expansion (volume increase of 10%-15%, exacerbating flashing). Masterbatch/regrind addition ratio should be ≤ 15% to prevent localized viscosity reduction due to uneven dispersion.
2.4.2 Auxiliary Process Optimization
- Mold Temperature Control: Increase mold temperature (e.g., from 50℃ to 70℃ for ABS) to reduce melt cooling shrinkage (from 1.2% to 0.8%) and minimize secondary gap opening caused by shrinkage.
- Ejector Synchronization: When ejecting multiple ejectors, ensure consistent ejection speeds (deviation <5%) to avoid uneven local force on product and resulting in flash residue.
Case Study: Flash occurred on a precision gear (PA66+GF30). Original material used was a MI of 40g/10min and mold temperature was 60℃. After switching to a material with a MI of 25g/10min and raising mold temperature to 80℃, flash rate dropped from 12% to 1%.

 

- Symptom: Continuous flash (0.05mm thick) occurred at parting line, causing assembly jamming.
- Troubleshooting Process:
1. Mold Inspection: Parting surface flatness was 0.03mm/m² (out of tolerance), and Landan test showed only 75% contact area.
2. Process Adjustment: Injection pressure was reduced from 100 MPa to 80 MPa, and clamping force was increased from 450 tons to 500 tons. Flashing was reduced but not eliminated.
3. Equipment Inspection: Clamping mechanism's crankshaft connecting rod clearance was 0.4 mm (out of tolerance), causing clamping force fluctuations.
- Solution:
- Mold: Grind parting surface to a flatness of 0.008 mm, increasing blue pigment contact area to 92%.
- Equipment: Replace connecting rod bushing and calibrate platen parallelism to 0.015 mm/m.
- Result: Flashing was completely eliminated, with no recurrence in subsequent production of 2,000 molds. Yield rate increased from 88% to 99.2%.

- Symptom: Radial flash (5-8 mm in length) occurred at slider joint of product, resulting in a 20% scrap rate during mass production.
- Troubleshooting Process:
1. Mold Inspection: Clearance between slider and guide rail was 0.02mm (out of tolerance), and clearance at insert joint was 0.018mm.
2. Process Inspection: Injection speed was 80mm/s (too fast), causing melt impact gaps.
3. Material Verification: PP MI = 35g/10min (excessive fluidity).
- Solution:
- Mold: Slider bevel was repaired to reduce clearance to 0.008mm; insert joint was repaired by argon arc welding and then milled to 0.005mm.
- Process: Injection speed was reduced to 40mm/s, and holding pressure was reduced from 90MPa to 60MPa.
- Material: A PP grade with MI = 25g/10min was replaced.
- Result: Scrap rate was reduced to 0.5%, and slider life was extended from 3,000 to 10,000 molds.

Product flash management must adhere to systematic approach of "mold precision is fundamental, process control is key, equipment assurance is bottom line, and material optimization is supplementary." Frontline engineers must master core skills such as mold gap measurement (flatness tester/feeler gauge), clamping force calculation (projected area x material pressure), and equipment status diagnosis (bend arm clearance/platen parallelism). This allows them to shift from "mold repair and leak prevention" to "preventive cost control" to achieve a long-term, complete solution to flash.