# Injection Molding Defects, Causes and Fixes Reference > Source: https://netray.co/tools/injection-molding-defects-causes-and-fixes-reference > Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems. > Section: Free Tools ## Short answer A searchable troubleshooting table for common molded part defects, with the usual causes and the first adjustments to try. ## Category quality ## Industry Plastics and Injection Molding ## Audience - Process technician - Molding engineer - Quality engineer ## Tool **Kind:** worksheet **Mode:** reference **Columns:** #### Defect **Key:** defect **Type:** text #### What you see **Key:** look **Type:** text #### Typical causes **Key:** causes **Type:** text #### First fixes to try **Key:** fixes **Type:** text #### Group **Key:** group **Type:** text **Rows:** **Defect:** Flash **Look:** Thin excess plastic at parting line, shutoffs or ejector pins **Causes:** Clamp force too low for projected area, worn or damaged parting surfaces, excess injection pressure or pack, viscosity too low, vents too deep **Fixes:** Reduce pack and hold pressure, lower melt temperature, check transfer position, verify clamp tonnage, inspect and repair parting line, check vent depth **Group:** Fill **Defect:** Short shot **Look:** Cavity or feature not completely filled **Causes:** Insufficient shot size, low melt or mold temperature, restricted gate or runner, blocked vents, check ring leakage, low injection speed or pressure **Fixes:** Increase shot size or transfer volume, raise melt and mold temperature, raise injection speed, clean vents, check check ring and nozzle **Group:** Fill **Defect:** Sink marks **Look:** Local depression on the surface opposite a rib, boss or thick section **Causes:** Insufficient pack or hold time, gate freezes early, thick section, high melt temperature, low cushion **Fixes:** Increase pack pressure and hold time, enlarge gate, lower melt temperature, keep cushion constant, review rib to wall ratio **Group:** Pack **Defect:** Voids (vacuum bubbles) **Look:** Internal cavities inside thick sections, often seen when the part is cut **Causes:** Skin freezes before the core shrinks, short hold time, gate too small, melt too hot **Fixes:** Increase hold time and pressure, enlarge gate, reduce melt temperature, core out the thick section in design **Group:** Pack **Defect:** Splay (silver streaks) **Look:** Silver or white streaks fanning out from the gate in the flow direction **Causes:** Moisture in resin, excessive melt temperature or residence time, trapped air, wrong or contaminated material **Fixes:** Dry resin to supplier specification and check dew point, lower melt temperature and screw speed, reduce residence time, check material lot **Group:** Material **Defect:** Burn marks **Look:** Black or brown discoloration, usually at the last area to fill **Causes:** Trapped air compressed and heated (dieseling), blocked vents, injection speed too high, melt too hot **Fixes:** Clean or deepen vents, slow the end of fill with velocity profiling, reduce melt temperature, add venting at the burn location **Group:** Fill **Defect:** Weld lines (knit lines) **Look:** Visible line or weak seam where two flow fronts meet **Causes:** Low melt or mold temperature, low injection speed, poor venting at the meeting point, gate placement **Fixes:** Raise melt and mold temperature, raise injection speed, add vents at the weld, review gate location with flow simulation **Group:** Fill **Defect:** Flow lines **Look:** Wavy lines or rings on the surface near the gate **Causes:** Low melt or mold temperature, slow injection speed, thin gate or runner **Fixes:** Raise melt and mold temperature, raise injection speed, enlarge gate **Group:** Fill **Defect:** Jetting **Look:** Snake-like pattern starting at the gate **Causes:** Melt shoots into the cavity without touching a wall, small gate with high speed, gate not aimed at a wall or pin **Fixes:** Slow initial injection speed using a profile, enlarge gate, redirect the gate so melt hits a wall, use a submarine or tab gate **Group:** Fill **Defect:** Warpage **Look:** Part bowed, twisted or dimensionally out of shape after ejection **Causes:** Uneven cooling, differential shrinkage, high residual stress, over-packing near the gate, fiber orientation **Fixes:** Balance mold temperatures between halves, increase cooling time, reduce pack pressure, use fixture cooling, review wall thickness uniformity **Group:** Cooling **Defect:** Delamination **Look:** Surface layers peel off like flakes **Causes:** Contamination with a dissimilar polymer, moisture, excessive shear, low melt temperature **Fixes:** Purge the barrel thoroughly, verify material identity, dry resin, raise melt temperature, check regrind contamination **Group:** Material **Defect:** Black specks **Look:** Dark specks in the part, often in natural or light colors **Causes:** Degraded resin from long residence time, hot spots in barrel or hot runner, dirty screw, contaminated regrind **Fixes:** Purge, reduce barrel temperature and residence time, clean hot runner, inspect screw and barrel for wear and dead spots **Group:** Material **Defect:** Gate blush (halo) **Look:** Dull or glossy ring around the gate **Causes:** Excess shear at the gate, injection speed too high through a small gate **Fixes:** Reduce injection speed at gate, enlarge gate, increase melt and mold temperature **Group:** Fill **Defect:** Ejector pin marks **Look:** Circular marks or stress whitening on the ejected side **Causes:** Part not cool enough or still sticking, over-packing, high ejection speed, small pin area **Fixes:** Increase cooling time, reduce pack, polish and add draft, use larger or sleeve ejectors, slow ejection **Group:** Ejection **Defect:** Drag marks **Look:** Scratches parallel to the direction of mold opening on walls **Causes:** Insufficient draft, rough or damaged cavity surface, undercut, part held by vacuum **Fixes:** Add draft, polish the cavity, remove undercut, add air assist at ejection **Group:** Ejection **Defect:** Part sticking in cavity **Look:** Part does not release or tears on ejection **Causes:** Over-packing, insufficient draft, hot mold, damaged surface, vacuum effect **Fixes:** Reduce pack, increase cooling, add draft, polish, apply compliant mold release only if the process allows **Group:** Ejection **Defect:** Sprue sticking **Look:** Sprue stays in the stationary half when the mold opens **Causes:** Sprue bushing damaged or poorly polished, nozzle misalignment, insufficient sprue puller, too short cooling **Fixes:** Polish bushing, check nozzle seat and alignment, add or modify the sprue puller, increase cooling time **Group:** Ejection **Defect:** Brittle parts **Look:** Parts crack or snap easily **Causes:** Degraded resin or molecular weight loss, excess regrind, moisture in hydrolytic resins, low melt temperature, weld line stress **Fixes:** Dry resin, limit regrind percentage, reduce residence time, raise melt temperature, review part design stress points **Group:** Material **Defect:** Cold slug marks **Look:** Blemish or rough patch near the gate from a cold material slug **Causes:** Cold material at nozzle tip, nozzle temperature too low, no cold slug well **Fixes:** Increase nozzle temperature, add a cold slug well to the runner, use suck-back carefully **Group:** Fill **Defect:** Color streaks **Look:** Uneven color or streaks in the part **Causes:** Poor mixing of masterbatch, wrong let-down ratio, low back pressure, low screw speed, contamination **Fixes:** Increase back pressure, use a mixing screw or mixing nozzle, check let-down ratio and feeder calibration **Group:** Material **Defect:** Dimensional variation shot to shot **Look:** Part weight or dimension varies between shots **Causes:** Inconsistent cushion, worn check ring, unstable temperature, inconsistent resin feed, unstable hydraulic pressure **Fixes:** Check cushion repeatability, replace check ring, stabilize temperature control, monitor part weight with SPC **Group:** Process **Notes:** Causes are listed as possibilities, not a diagnosis. Change one variable at a time, record the change in your process sheet and verify with a documented sample before continuing. Resin and tool specifics always override general guidance. ## Content **Intro:** When a molded part comes off the press wrong, the first question is which defect it is and what usually causes it. This table lists common injection molding defects with typical causes and first adjustments, and you can search and export it for your shop. **Sections:** #### How to use this table **Body:** Search by symptom or defect name, then read the typical causes and the first fixes to try. Start with the easiest, lowest-risk change, usually a process setting such as melt temperature, hold time or injection speed, before touching the tool. Change one variable at a time and wait for the process to stabilize for several shots before judging the result. Write the change down on the process sheet so the next shift can see what was tried. If the defect only appears at one cavity, suspect the tool rather than the process. **Bullets:** - Search the Group column to narrow to fill, pack, cooling, ejection, material or process causes - Treat the cause list as hypotheses, then confirm by measuring or inspecting - Never raise clamp tonnage beyond the press rating to cure flash #### Fill, pack and cool: where the defect comes from **Body:** Most defects trace back to one phase of the cycle. Fill problems such as short shots, burn marks and weld lines respond to injection speed, melt temperature and venting. Pack problems such as sink and voids respond to pack pressure, hold time and gate size. Cooling problems such as warpage depend on mold temperature balance and cooling time. Ejection problems come from draft, polish and part temperature. Knowing the phase tells you which setting to try first and keeps you from chasing a pack problem with a fill adjustment. #### Material and process drift **Body:** A defect that appears suddenly with no setup change often points to the material: moisture in a hygroscopic resin, a new lot, too much regrind or a color masterbatch change. Record resin lot, dryer settings and regrind percentage with every defect entry. Capturing the same information in your ERP or MES lets you correlate scrap with lots and tools later. If defects drift slowly over a run, look at worn check rings, plugged vents and cooling channel scale before changing process windows. #### Common mistakes **Body:** The most frequent mistakes are changing several settings at once, raising melt temperature to fix everything, and compensating for a worn tool with higher pressure. High melt temperature trades one defect for another, such as flash, splay or black specks. Also avoid treating a defect as a one-off. If it recurs, log it with cause and corrective action, and use a Pareto view to decide which problem to fix permanently. **Cta:** If you want scrap codes, resin lots and tool numbers from your ERP or MES analyzed together to find recurring molding defects, Netray can help you set that up on your own data. **Faq:** #### What is the most common cause of flash? **Answer:** Flash usually comes from clamp force that is too low for the projected area, worn parting surfaces, or over-packing. Check that the mold sits flat, inspect the parting line, and reduce pack pressure and melt temperature before suspecting the press itself. #### How do I tell moisture splay from trapped air? **Answer:** Moisture splay is usually silver streaks that fan out from the gate and vary with resin drying. Trapped air tends to occur at the end of fill or at weld lines and often shows burning. Drying the resin and checking dew point separates the two quickly. #### Should I increase melt temperature to fix most defects? **Answer:** No. Higher melt temperature helps short shots and weld lines but can cause flash, splay, black specks and longer cooling. Change one variable at a time and stay inside the resin supplier's processing range. #### Does this table replace a flow simulation? **Answer:** No. It is a quick troubleshooting guide. Gate location, wall thickness and cooling layout problems are best addressed in design with flow and cooling simulation, and by a qualified molding engineer. ## Search terms injection molding defects and solutions, injection molding troubleshooting guide, flash short shot sink marks causes, splay silver streaks causes, weld line causes and fixes, plastic part warpage causes, injection molding defect chart, molding troubleshooting table excel --- Talk to an engineer: https://netray.co/contact?intent=erp_ai | Machine catalog: https://netray.co/agent/catalog.json