Shenzhen Alu Rapid Prototype Precision Co., Ltd.
Industry News
Splay appears as silver or whitish streaks that fan across the surface of an injection-molded part, often in the flow direction. The defect can come from moisture, trapped air, volatile contamination, or thermal degradation. Because these mechanisms can look similar, changing random machine settings often wastes time and may hide the real cause.
A disciplined investigation separates material handling, plasticating, filling, venting, and mold-surface effects. The goal is not only to produce one acceptable shot, but to establish a stable process that prevents the streaks from returning.
Confirm That the Defect Is Splay
Start with clear photographs and a marked part. Record where the streak begins, its direction, whether it repeats in the same cavity and location, and whether it appears on every cycle or intermittently. Compare the pattern with weld lines, flow marks, glass-fiber streaking, gate blush, jetting, and scratches.
Moisture splay often looks like fine silver streaks distributed along the flow path. Air-related streaks may originate near gates, ribs, bosses, sudden thickness changes, or end-of-fill regions. Degradation can add discoloration, black specks, odor, or inconsistent material viscosity. These are clues, not final proof.
Preserve the Original Process Evidence
Before adjusting the press, save the process setup and collect several consecutive parts. Record resin grade, lot, colorant and regrind percentage, dryer conditions, hopper residence time, barrel profile, screw speed, back pressure, decompression, recovery time, cushion, injection velocity profile, transfer position, peak pressure, fill time, and cycle time.
Note any recent change in material lot, dryer maintenance, color, mold cleaning, hot-runner setting, shutdown procedure, or production schedule. An intermittent defect frequently follows a change that was not captured in the setup sheet.
Check Moisture With Data
Many engineering thermoplastics are hygroscopic and must be dried to a resin-specific residual moisture limit. Moisture can turn to vapor in the melt and create surface streaks. For some polymers, hydrolysis also reduces molecular weight and mechanical performance, so a visually acceptable part is not proof that wet resin caused no damage.
Use the current material supplier's processing data for drying temperature, time, dew point, airflow, and maximum allowable moisture. Do not apply one drying recipe to every polymer. Covestro's guidance, for example, shows that permissible residual moisture can be very low for its engineering thermoplastics and recommends appropriate analytical methods when the limit cannot be measured reliably by a simple weight-loss test.
Verify the complete drying system:
Actual pellet temperature, not only the controller setpoint
Dry-air dew point and airflow under production load
Material depth and residence time in the hopper
Return-air filters and desiccant regeneration
Leaks, open lids, damaged hoses, and incorrect valve positions
Moisture pickup during conveying and at the machine hopper
Wet regrind, color concentrate, or additives
A successful drying trial should use material whose moisture has been measured and whose handling remains controlled from dryer to feed throat.
Separate Moisture From Volatile Contamination
If properly dried virgin resin eliminates the defect, add colorant and regrind back one variable at a time. Contamination can come from incompatible resin, excessive mold release, cleaning solvent, oil, grease, degraded purge material, wet pigment, or an additive carrier that is unsuitable for the base polymer.
Inspect material storage and changeover practices. Confirm labels, lot segregation, grinder cleanliness, sealed containers, and the approved regrind limit. A small amount of incompatible material may stretch into silver streaks without producing an obvious color change.
Do not simply increase the drying temperature. Excess heat or time can degrade some resins, cause pellets to soften or bridge, and damage additives. Stay within the supplier's processing window.
Evaluate Thermal Degradation
Polymer can degrade when melt temperature or residence time is too high. Compare the actual melt temperature with the resin supplier's recommendation; barrel setpoints alone do not show shear heating or the temperature of material leaving the nozzle.
Review machine shot size relative to barrel capacity, screw recovery time, screw speed, back pressure, nozzle restriction, hot-runner temperature, and idle periods. A small shot in a large barrel or long interruption can leave material hot for too long. Excessive shear through a restrictive gate, nozzle, or check ring can create local overheating.
Run a controlled purge and inspect for discoloration, gas, black specks, or odor. If the defect is worse after a production stop and improves after purging, residence time or dead spots deserve attention. Follow the resin and equipment suppliers' safe purge and shutdown procedures.
Check Air Entrapment and Venting
Air must leave the cavity as the melt advances. If it is trapped and compressed, it can create streaks, burns, short shots, or weak areas. Use short shots to study the flow front and identify where separate fronts meet or where air is driven into a dead end.
Inspect vents for contamination, damage, or insufficient land and depth. Check parting-line vents, ejector clearance, inserts, slides, and vacuum systems. Confirm that a recent mold polish, repair, or flash correction did not close an effective vent path.
Injection speed is not a universal fix. A slower profile may allow air to escape in one geometry, while a faster profile may prevent hesitation and unstable flow in another. Change velocity by position and document fill time so the result can be repeated.
Review Gate and Flow Geometry
A small or abrupt gate can generate excessive shear and a high-velocity jet. Sudden changes in wall thickness, sharp internal corners, ribs, and obstructed flow paths can split the flow and trap air. Identify whether the streak consistently begins at the gate or after a specific feature.
Check the gate for damage, mismatch, freeze-off, contamination, and actual dimensions. For hot runners, compare nozzle and manifold temperatures and look for dead spots or material leakage. A gate change should be evaluated with filling analysis and mold-trial data because it can also affect pressure, packing, weld lines, warpage, and appearance.
Use a Controlled Troubleshooting Matrix
Change one factor at a time when possible and record the response. A practical sequence is:
Document the defect and baseline process.
Run verified dry virgin resin without optional additives.
Confirm actual melt temperature and residence-time risk.
Study short shots and vent condition.
Evaluate injection velocity by flow-front position.
Add colorant and approved regrind back separately.
Repeat the best condition across multiple cycles and material lots.
A single clean part is not enough. Confirm stability after normal interruptions, hopper refills, shift changes, and startup, because those conditions often expose moisture or residence-time problems.
Verify Part Performance After the Cosmetic Fix
If moisture or degradation was suspected, test the properties that matter for the application. Impact strength, elongation, stress-crack resistance, weld-line strength, flammability performance, and long-term durability may change even when the surface looks acceptable.
Retain samples and process data from the failed and corrected conditions. For critical parts, compare material testing or functional test results rather than approving the change by appearance alone.
Splay Troubleshooting Checklist
Photograph and map the streak pattern by cavity
Save the baseline machine and hot-runner settings
Measure resin moisture against the grade-specific limit
Verify dryer temperature, dew point, airflow, and residence time
Isolate virgin resin, colorant, regrind, and contamination risks
Measure actual melt temperature and review residence time
Inspect vents and study the flow with short shots
Check gate condition and high-shear restrictions
Validate the correction over a representative production run
Confirm mechanical performance when degradation is possible
Aluprototype can support injection mold trials, defect analysis, process documentation, dimensional inspection, and functional testing. Send the part drawing, resin grade, defect photographs, cavity number, process sheet, and recent change history so the investigation can begin with evidence.
Technical references: Covestro, Drying for High-Quality Injection-Molded Parts; Covestro Apec Processing and Drying Guidance; DuPont injection molding and failure-analysis resources.