CNC Machining & Injection Molding — DFM/Moldflow Support, CMM Inspection, Prototype to Production Solutions.
A practical troubleshooting matrix for 20 common molding defects—symptom patterns, ranked root causes, quick checks, and do-first process moves. Includes downloadable templates.
Utilize the symptom + location pattern to instantly access ranked root causes for 20+ common defects.
Follow the Do First → Do Next parameter hierarchy to avoid random tuning and stabilize processes quickly.
Maintain consistent quality with built-in SOP checklists and process window recording templates.
| Symptom | Location | Ranked Causes | Process Fixes (Do First → Next) | Material Fixes (ABS/PC/PA) | Trade-offs |
|---|---|---|---|---|---|
| Short Shot | End-of-fill / Thin walls | 1. Inadequate V/P point 2. Melt/Mold too cold 3. Blocked vents |
Safe ↑ Injection Speed Risky ↑ Melt Temp (Actual) |
PA/PC: Check for nozzle freeze-off; verify drying. | Higher speed may cause Burn Marks. |
| Sink Marks | Over Ribs / Boss Roots | 1. Early gate freeze 2. Low pack pressure 3. Thick sections |
Safe ↑ Pack Pressure/Time Safe ↓ Melt Temp |
ABS: Sensitive to rib-to-wall ratio (>50%). | Overpacking may cause Sticking/Flash. |
| Splay | Near Gate / Flow path | 1. Moisture (#1 cause) 2. Shear heating 3. Degradation |
Safe Check Dryer (↓ Dew point) Risky ↓ Injection Speed |
PC/PA: Hydrolysis risk; dry to <0.02%. | Lower speed may cause Weld Lines. |
Splay is often moisture, but air traps and degradation look similar. Check location and odor immediately.
Polycarbonate undergoes hydrolysis if moisture is >0.02%. Excessive residence time breaks molecular chains.
Polyamide (Nylon) shrinks heavily. Filled grades warp due to fiber alignment, even when parts look "full".
ABS is highly sensitive to flow front instability. Venting cleanliness is the #1 driver for cosmetic scrap.
What are injection molding verification tests? They are standardized, low-cost diagnostic procedures—such as short-shot studies and weight plateau tests—used to isolate root causes like air traps or gate freeze time without expensive mold rework or material changes.
The fastest way to visualize the flow front, identify last-to-fill areas, and pinpoint where air traps form.
Used to scientifically confirm Gate Freeze Time—the point after which additional packing has no effect.
A simple destructive test to distinguish between vacuum voids, moisture bubbles, and delamination.
Essential for diagnosing warpage, dimensional drift, and cooling imbalances across the mold surface.
High-resolution, shop-floor ready SOP. Optimized for standard A4/Letter printing to be placed directly on injection molding machines.
Download PDF MatrixClean, embeddable HTML/CSS code for your internal engineering wiki, LMS, or technical documentation portal.
Copy HTML SnippetTab-Separated Values format. Perfect for importing into Excel or Google Sheets to create custom shop-floor defect logs.
Download TSV LogCommon defects include short shots, flash, sink marks, and internal voids. Surface issues like splay (silver streaks), burn marks (dieseling), and weld lines are also frequent. Dimensional instability, such as warpage or bowing, occurs due to non-uniform cooling or residual stresses during the molding cycle.
Verify the dryer’s dew point is below -40°C and ensure resin moisture content is under 0.02%. Reduce residence time by minimizing barrel hold time. If splay persists, decrease initial injection speed to reduce shear heating and confirm that vents are clean to eliminate trapped gas.
Perform a weight plateau test by increasing pack/hold time in one-second increments while keeping other parameters constant. Weigh the parts on a precision scale (0.01g). The gate is frozen when the part weight stops increasing, ensuring the cavity is fully packed and isolated from the runner.
Excessive pack pressure forces more material into the cavity, creating high residual internal stresses near the gate. This overpacking leads to uneven shrinkage gradients across the part. Upon cooling, these stresses manifest as structural cracking or directional warpage as polymer chains attempt to reach a lower energy state.
Poor venting primarily causes burn marks (dieseling) at the end-of-fill as trapped air compresses and ignites. It also triggers air traps, resulting in shiny spots or surface haze. Furthermore, back-pressure from trapped air can cause short shots or localized splay during high-speed injection phases.
Perform a cross-section cut: a vacuum void appears as a smooth, shiny-walled hole caused by internal shrinkage. Conversely, moisture bubbles or gas traps appear as spongy, ragged, or clustered holes. Moisture often correlates with an acrid odor during purging and splay on the part surface.
The Velocity-to-Pressure (V/P) switchover defines when the machine shifts from filling to packing. An early switchover often leads to short shots because the cavity isn't volumetrically full. A late switchover causes a pressure spike that overcomes clamp tonnage, resulting in flash at the parting line.
Warpage is driven by differential shrinkage. Even if fully packed, an imbalance in mold temperature between the core and cavity causes the part to bow toward the hotter side. Additionally, fiber orientation in filled resins (like PA-GF) creates anisotropic shrinkage that pack pressure alone cannot override.
