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In injection molding production, flash is one of the most "stubborn" defects – it not only leads to product scrap and dimensional deviations, but also scratches molds and delays delivery. Its essence is that melt pressure exceeds mold clamping pressure, causing excess material to be "squeezed" out from gaps in mold parting surface, sliders, ejector pins, etc. This article, covering four major modules–flash mechanism, four-dimensional troubleshooting, precise solutions, and long-term error prevention–combined with typical cases from electronics/automotive industries, will teach you how to completely overcome dilemma of "repeated mold repair without addressing root cause."

 

Core contradiction of flash is that melt pressure during filling/holding phase exceeds clamping pressure of mold's mating surfaces. Understanding its underlying logic requires considering four dimensions: material characteristics, process parameters, mold precision, and equipment capabilities.
1. Material Characteristics: "Driving Force" of Viscosity and Shrinkage
Flowability and shrinkage behavior of material directly affect risk of flash:
- Low-viscosity materials (e.g., PP, PE, PA): Good flowability, easily penetrating small gaps. In a trial run of an automotive air duct (PP material), flash appeared on parting surface because PP melt index was as high as 30g/10min (standard ≤ 20g/10min), resulting in a low-viscosity melt that easily penetrated gaps.
- High-shrinkage materials (e.g., POM, PC): Large volume shrinkage during cooling, resulting in higher melt pressure peaks during holding phase. Flash in an electronic connector (POM) was concentrated in ejector pin holes because POM shrinkage rate was 1.5%-3.0% (far exceeding ABS's 0.5%-0.8%), and ejector pin hole gaps were forced open by high-pressure melt during holding.
- Material degradation: Excessive material temperature leads to material decomposition, causing a sudden drop in melt viscosity (e.g., PVC viscosity decreases by more than 50% after decomposition), making it easier to break through mold gaps.
2. Process Parameters: "Assisting Factors" of Pressure and Time
Process settings directly determine "peak" and "duration" of melt pressure:
- Excessive injection pressure/holding pressure: Flashing occurred on parting line of a mobile phone frame (PC+ABS). Analysis revealed that after increasing injection pressure from 100MPa to 130MPa, melt pressure exceeded mold closing pressure (mold clamping force 150 tons, actual requirement 180 tons), causing parting line to be forced open.
- Excessive holding time: The longer holding time, the higher melt pressure is maintained. Flashing occurred in slider gap of a home appliance casing. After extending holding time from 10s to 15s, flashing area increased by 30%—excess pressure continuously "impacted" slider mating surface.
- Excessive injection speed: High-speed injection leads to a surge in instantaneous peak pressure of melt (e.g., from 50% to 90% speed, peak pressure increased from 80MPa to 120MPa). Flashing occurred on a toy car shell; after increasing speed, "jet-like flashing" appeared on parting line, which was alleviated after reducing speed.
3. Mold Accuracy: "Inherent Flaws" of Clearance
Mold processing accuracy directly determines "resistance to extrusion pressure":
- Poor mold surface fit: Foreign objects on parting line (such as iron filings, mold release agent accumulation), uneven grinding (local gap > 0.02mm). Flashing was fixed in the corner of parting line of a car trim panel. After cleaning iron filings and polishing, gap was reduced from 0.03mm to 0.01mm, and flashing disappeared.
- Excessive slider/ejector pin clearance: Clearance between slider and guide rail is > 0.02mm (design standard 0.01-0.015mm). Flashing occurred at corresponding position of slider on a medical equipment casing. Measured clearance was 0.025mm. After grinding slider, it was restored to 0.012mm, and flashing was resolved.
- Ejector pin hole wear: Diameter of ejector pin hole increases after long-term use (e.g., a Φ2mm ejector pin hole wears to Φ2.05mm). A bottle cap mold exhibited flashing around ejector pins. After replacing ejector pins (Φ2.01mm), gap narrowed, and flashing was reduced.
4. Equipment Capabilities: "Hard Constraints" of Clamping Force and Rigidity
Clamping force and platen rigidity of injection molding machine are "last line of defense":
- Insufficient clamping force: Clamping force must meet requirement of "projected area * mold cavity pressure". For a laptop case (projected area 0.8m², mold cavity pressure 80MPa), theoretical clamping force required is 6400 tons, but actual force used is 5000 tons, resulting in large-area flashing on parting line—a clamping force deficit of 22%.
- Platen deformation: Excessive platen parallelism (e.g., dynamic and static platen parallelism > 0.1mm/m) leads to insufficient local clamping force. A large home appliance casing showed "regional" flashing; platen parallelism was measured at 0.15mm/m. After adjusting tie rod bolts, parallelism improved to 0.05mm/m, and concentrated flashing areas disappeared.

 

Flashing is easily confused with trapped air and short shots. A four-step troubleshooting process, considering defect location, morphology, and process conditions, is necessary:
Step 1: Observe location and morphology of flashing (5-minute quick assessment)
- Parting line flashing: Mostly due to insufficient clamping force, poor mold surface contact, or excessive mold cavity pressure (e.g., products with large projected areas).
- Slider/angled pin gap flashing: Flashing is concentrated on the side corresponding to slider, often due to excessive slider clearance or insufficient clamping force to counteract lateral force (e.g., when product has side core pulling).
- Flashing around ejector pins/gates: Mostly due to worn ejector pin holes, oversized gates, or excessive holding pressure (melt overflowing from ejector pin holes during packing).
- Randomly dispersed flashing: May be due to material degradation (low viscosity) or process fluctuations (e.g., pressure fluctuations).
Step 2: Verify Clamping Force and Equipment Status (10-minute check)
- Calculate theoretical clamping force: Clamping force (tons) = Projected area (m²) * Mold cavity pressure (MPa) * Safety factor (1.2-1.5). For a car dashboard (projected area 1.2 m², mold cavity pressure 70 MPa), theoretical clamping force required is 1.2 * 70 * 1.3 = 1092 tons. Actual force used was 900 tons, indicating insufficient clamping force was main reason–flashing issue was resolved after replacing injection molding machine with a 1200-ton machine.
- Check platen parallelism: Measure parallelism of moving and fixed platens using a dial indicator (standard ≤0.05 mm/m). If deviation is excessive, adjust tie rod bolts or replace platen.
- Clean mold parting surface: Check parting surface for metal shavings and oil stains. Clean with compressed air and a copper brush to prevent foreign objects from causing gaps.
Step 3: Adjust Process Parameters (15-minute trial and error)
- Reduce pressure: Prioritize reducing injection pressure and holding pressure (e.g., from 120 MPa → 100 MPa, holding pressure from 100 MPa → 80 MPa). For a mobile phone case with flashing, reducing injection pressure by 20% resulted in melt pressure peak dropping from 110 MPa to 85 MPa (below mold closing pressure of 90 MPa), and flashing disappeared.
- Reduce time: Shorten holding time (e.g., from 15s → 10s) to reduce continuous impact of high-pressure melt on mold. After halving holding time for a home appliance casing, flashing area in slider gap was reduced by 60%.
- Reduce speed: Reduce injection speed (especially at the end of filling) to avoid a surge in pressure peaks. After reducing injection speed of a toy car shell from 90% to 60%, "jet-like flashing" on parting surface became slight burrs.
Step 4: Check Mold Accuracy (30 minutes - 1 hour)
- Measure fitting clearance: Use a feeler gauge to measure slider/guide rail clearance (standard 0.01-0.015 mm) and ejector pin hole clearance (Φ2 mm ejector pin hole clearance ≤0.02 mm). In a precision gear mold, flashing occurred at slider, with a gap of 0.028mm. After grinding, gap was reduced to 0.013mm.
- Check flatness of mold parting surface: Use Prussian blue to inspect parting surface. If there is local non-contact (Prussian blue residue > 20%), it indicates poor fitting, requires repolishing or welding and refitting.
- CAE mold flow analysis: Use Moldflow to simulate mold cavity pressure distribution, visually showing high-pressure areas (e.g., a certain area has a pressure of 100MPa, while mold closing pressure is only 90MPa), guiding mold modification (e.g., adding locking modules locally).

If main cause is process parameters:
- Optimize pressure curve: Adopt "segmented injection + graded holding pressure," such as 80MPa during filling stage, 60MPa (5s) in the first holding pressure stage, and 40MPa (8s) in second stage, to avoid excessively high pressure peaks.
- Reduce mold cavity pressure: Indirectly reduce mold cavity pressure by increasing mold temperature (e.g., from 80℃ to 100℃ for PC, improving melt fluidity and reducing required injection pressure by 15%).
If main cause is a mold problem:
- Adjust fit clearance: When slider clearance is > 0.02mm, grind slider or guide rail; if ejector pin hole is worn, replace new ejector pin or ream hole (e.g., ream a Φ2mm ejector pin hole to Φ2.01mm, with a fit clearance of 0.015mm).
- Optimize mold parting surface design: For large parting surfaces, add "locking modules" (e.g., locally thicken mold plate in flashing area to increase closing pressure); or use a "stepped parting surface" to reduce clearance accumulation.
If main cause is equipment capability:
- Replace/modify equipment: If clamping force is insufficient, upgrade to a larger tonnage injection molding machine; if mold plate is deformed, replace it with a mold plate with higher parallelism (e.g., parallelism ≤ 0.03mm/m).
- Add auxiliary clamping: For large molds, use a "hydraulic clamping auxiliary device" to increase local closing pressure (e.g., install a hydraulic locking module in slider area).

1. Product Design Phase: Avoiding Flash Risk
- Controlling Projection Area: Avoid excessively large projection areas for individual products (e.g., exceeding 80% of injection molding machine's maximum clamping force). If necessary, split mold into multiple cavities to distribute projection area.
- Optimizing Structure to Reduce Lateral Force: Ensure slider core pulling direction is perpendicular to clamping force direction to prevent slider from "spreading" due to lateral force (e.g., changing core pulling angle of a car trim panel slider from 30° to 15° reduced lateral force by 40%).
2. Mold Development Phase: Controlling Gaps from Source
- High-Precision Machining of Mating Surfaces: Mating surface precision should be ≤0.01mm, and slider/guide rail gap should be controlled between 0.01-0.015mm (adjusted according to material viscosity; use a smaller value for low-viscosity materials).
- Reserving Clamping Force Redundancy: Specify "minimum clamping force requirement" during mold design (e.g., projection area * cavity pressure * 1.5) to avoid discovering insufficient clamping force during mold trials.
3. Mold Trial and Verification Phase: Establishing a "Flash Risk Archive"
- Recording Critical Parameters: For each product, record parameters such as "minimum clamping force without flash" and "maximum allowable injection pressure," which can be directly used in subsequent production.
- Flash Sensitivity Testing: During mold trials, deliberately over-adjust parameters (e.g., pressure +10%, clamping force -10%) to observe whether flash occurs and quantify mold/equipment's flash resistance.
4. Production Management Phase: Dynamic Monitoring and Regular Maintenance
- Real-Time Monitoring of Clamping Force: Install clamping force sensors; an alarm is triggered when the clamping force fluctuates by more than 5% (e.g., a certain injection molding machine's clamping force dropped from 1200 tons to 1140 tons, triggering a warning to prevent flash).
- Regular Mold Maintenance: Clean foreign objects from mold mating surfaces weekly, and check slider/ejector pin gaps monthly (measure and record using a feeler gauge) to prevent gaps from gradually increasing due to wear.

 

Essence of flashing is "melt pressure > mold closing pressure." Troubleshooting requires a four-dimensional approach, considering material (viscosity/shrinkage), process (pressure/time), mold (gap/precision), and equipment (clamping force/rigidity). Prioritize adjusting process (low cost, quick results), then repair and modify mold (to address gap issues), and finally upgrade equipment (to address insufficient capacity). The key to long-term error prevention is "design prediction + parameter standardization + dynamic monitoring," ultimately achieving "zero mold flashing and stable production."
Core Mantra: Flashing starts with parting line, insufficient clamping force is the most common cause; try reducing pressure and time, check mold gap; control parting surface during design to avoid lateral issues, and archive data to prevent recurrence.