Diagnose the problem, fix the root cause. Even with the best intentions, manufactured parts can exhibit flaws. This comprehensive guide helps you identify common defects across all major processes, understand their underlying causes, and apply targeted solutions to prevent them in future designs and production runs.
Core Philosophy: A Defect is a Design or Process Conversation
Every flaw tells a story about a mismatch between the design intent and the manufacturing reality. Learning this language is key to reliability.
Flaws arising from the subtractive cutting process, tooling, or setup.
- Chatter & Vibration Marks: Visible wavy patterns on the surface.
* Cause: Lack of rigidity, incorrect speeds/feeds, tool wear.
* Fix: Increase tool or part support, adjust parameters, use a sharper tool. (See: CNC Fundamentals for parameter guides)
- Burrs & Sharp Edges: Unwanted raised material on part edges.
* Cause: Tool pushing material instead of shearing it; ductile materials.
* Fix: Optimize feed rate, use sharper tools, specify deburring as a required Post-Process.
- Tool Marks / Poor Surface Finish: Unintended roughness or visible stepovers.
* Cause: Incorrect stepover, worn tool, or improper finishing pass.
* Fix: Reduce stepover distance, use a new tool, add a finishing operation. (See: Surface Finish (Ra) Guide)
- Geometric Inaccuracy: Parts not matching CAD dimensions.
* Cause: Tool deflection, thermal expansion, or machine calibration.
* Fix: Use more rigid tools, manage cutting fluid temperature, work within standard Machining Tolerances.
Flaws unique to layer-by-layer deposition or fusion of material.
- Warping & Layer Separation: Corners lifting or layers splitting.
* Cause: Uneven thermal contraction, poor bed adhesion.
* Fix: Improve bed adhesion (heat, glue), use a heated chamber, redesign with uniform Wall Thickness.
- Stringing & Oozing: Thin plastic hairs between model parts.
* Cause: Uncontrolled filament flow during non-print moves.
* Fix: Optimize retraction settings, lower printing temperature.
- Under-Extrusion & Gaps: Missing layers or weak, porous structure.
* Cause: Clogged nozzle, low filament flow, incorrect filament diameter setting.
* Fix: Clean nozzle, calibrate extruder steps, verify filament settings.
- Support-Related Scars & Damage: Rough surfaces where supports were attached.
* Cause: Supports fused too strongly to the model.
* Fix: Optimize support Z-distance, use soluble supports, or redesign to be Support-Free.
Flaws occurring during the filling, packing, or cooling of plastic in a mold.
- Sink Marks: Localized depressions on thick sections.
* Cause: Insufficient packing pressure, cooling too fast in thick areas.
* Fix: Increase pack pressure, redesign for uniform Wall Thickness, use thinner ribs.
- Warpage: Twisted or bent part after ejection.
* Cause: Uneven cooling and internal stresses.
* Fix: Optimize cooling channel design, adjust mold temperatures, balance material flow. (See: Mold Design Basics)
- Short Shot: Incompletely filled mold cavity.
* Cause: Insufficient material, low temperature, or blocked flow.
* Fix: Increase shot size, raise melt/mold temperature, add/relocate vents.
- Flash: Thin excess plastic on mold parting lines or ejector pins.
* Cause: Excessive injection pressure, worn mold, or clamp force too low.
* Fix: Reduce injection pressure, repair mold, increase clamp tonnage.
Flaws arising from cutting, bending, or forming sheet stock.
- Springback: Bent angle not holding its intended degree after release.
* Cause: Material elasticity.
* Fix: Over-bend to compensate (using material-specific K-Factor data).
- Cracking on the Bend: Material fracture at the outer bend radius.
* Cause: Bend radius too tight for the material, grain direction parallel to bend.
* Fix: Increase bend radius, reorient part on sheet grain, use more ductile material.
- Burrs on Cut Edges: Rough, sharp edges from laser or plasma cutting.
* Cause: Cutting parameters not optimized for material thickness.
* Fix: Optimize power/speed settings, specify secondary deburring as a Post-Process.
- Distortion & Denting: Unwanted deformation in flat sections.
* Cause: Internal stresses from cutting heat or improper handling.
* Fix: Use stress-relieving cutting techniques, implement careful handling procedures.
Flaws that can appear across multiple processes or during finishing.
- Poor Adhesion of Coatings (Paint, Powder Coat): Peeling or flaking.
* Cause: Contaminated or improperly prepared base surface.
* Fix: Implement proper cleaning (degreasing, abrasive blasting) and use correct primers.
- Uneven Anodizing or Plating: Streaky, blotchy, or inconsistent color/finish.
* Cause:** Uneven current distribution, contaminated electrolyte, or poor surface preparation.
* Fix:** Ensure proper racking, maintain bath chemistry, and thoroughly clean parts before processing. (See: Anodizing Guide)
- Scratches & Tool Marks: Visible surface damage from handling or machining.
* Cause: Abrasive contact during production or post-processing.
* Fix: Implement protective handling (soft jaws, gloves), optimize final machining steps.
A systematic approach to solving any manufacturing quality issue.
Encountered a Defect? Follow This Path:
- Identify & Categorize: Use the sections above to match the flaw to its most likely process family.
- Analyze the Root Cause: Review the "Cause" and consider your specific design and process parameters.
- Implement the Fix: Apply the suggested solution, which often involves a design tweak or process adjustment.
- Prevent Recurrence: Use the resources in Section 6 to build a more robust design and quality process.
Prevention is the most powerful cost-saving strategy. A deep understanding of defects informs better Basic Design choices and is integral to effective Cost Reduction.