I. Defect 1: Sink Marks — The Nemesis of Surface Dents
1. Phenomenon Description
Dents or depressions appear on the surface opposite thick sections (e.g., behind ribs or boss walls). This severely affects texture and aesthetics, commonly seen in crystalline materials like PC, ABS, and PA.
2. Root Cause Analysis
Insufficient pack/hold pressure or uneven cooling causes the molten coreto shrink inward as it cools, pulling the outer skin inward.
3. Mold Modification Solutions
Reduce Wall Thickness (Most Effective): Modify the mold core/cavityto thin down ribs and bosses. A general rule isto limit rib thickness to 50%-70% of the adjacent wall thickness. This is the fundamental fix.
Increase Localized Cooling: Install Beryllium Copper (Be-Cu) Inserts in the core where sink marks appear. Be-Cu’s high thermal conductivity accelerates cooling in the thick section.
Overflow Wells: Cut “Volcano” overflow wells on the cavity surface correspondingto the sink area. This forces shrinkage to occur inside the waste well, not on the product surface.
II. Defect 2: Weld Lines — Eliminating Visible “Split Lines”
1. Phenomenon Description
When two or more flow fronts meet, they failto fuse completely due to premature cooling, creating a visible line (often silverish). This is a stress concentration point and a cosmetic flaw.
2. Root Cause Analysis
Low melt-front temperature upon meeting, excessive flow-join angles, or trapped air.
3. Mold Modification Solutions
Add Venting (Crucial): Use EDM (Electrical Discharge Machining)to cut Venting Grooves (depth: 0.02mm–0.05mm) on the cavity/core surface where weld lines are predicted. This allows trapped airto escape.
Reposition Gates: Modify the runner systemto move the gate location so weld lines form on non-appearance surfaces or structural hidden areas.
Increase Mold Temperature: Modify the heating system by adding Heater Cartridges near the weld zone to raise local mold temperature and delay melt-front freezing.
III. Defect 3: Flash / Burrs — Eliminating Overflow and Snagging
1. Phenomenon Description
Thin, unwanted plastic overflows occur at parting lines, slider interfaces, or ejector pin gaps, causing assembly interference or sharp edges.
2. Root Cause Analysis
Insufficient clamp force, poor mold fit, or excessive injection pressure.
3. Mold Modification Solutions
Refit Parting Surfaces (Fit & Finish): Perform CNC remachining or manual “Blue Matching” on the mold parting line to ensure a zero-gap shut-off.
Hardened Inserts: Replace soft steel at slider/cam interfaces with SKD61 or S136 hardened steel. This prevents wear and gap enlargement after long production runs.
Reduce Vent Depth: If flash is caused by overly deep vents, regrind the vent slots, reducing depth by 0.02mm–0.03mm.
IV. Defect 4: Flow Marks / Waviness — Achieving High-Gloss, Seamless Surfaces
1. Phenomenon Description
Snake-skin or wavy patterns appear on the surface, resembling wood grain. Common on large flat panels or appearance faces.
2. Root Cause Analysis
Slow injection speed combined with low mold temperature causes the leading edgeof the meltto cool and solidify, and subsequent flow pushes against it.
3. Mold Modification Solutions
Enlarge Gate Size: If using a pinpoint or submarine gate, increase the gate diameterto reduce shear rate and allow hotter materialto flow smoothly.
Add Cold Slug Wells: Cut Cold Slug Wells at the endsof runners. This catches the initially cooled slug, preventing itfrom entering the cavity and causing flow marks.
Modify Runner Layout: Adjust runners to use a Fan Gate or Film Gate insteadof a single point gate, promoting laminar (smooth) flow.
V. Defect 5: Silver Streaks / Splay Marks — Solving Surface Bubbles and Silver Lines
1. Phenomenon Description
Silver-white streaks or bubbles appear on the surface, usually aligned with the flow direction. Often caused by moisture or gas decomposition.
2. Root Cause Analysis
Moisture in material causing hydrolysis, or gas decomposition in the barrel due to high heat.
3. Mold Modification Solutions
Improve Ventilation (Primary): This is the mold’s key contribution. Add Vent Pins or Gas-permeable Steel Inserts in deep cores or blind spots where air is trapped.
Adjust Gate Position: Avoid directing the gate straight at core pins or inserts (which act as heat sinks). Modifyto an Overlap Gate to reduce air entrapment.
Streamline Flow Paths: Increase draft angles and polish flow paths within the moldto reduce resistance and prevent gas compression.
VI. Conclusion: From “Mold Repair” to “DFM Prevention”
Mold modification is a remedial measure; true cost reduction lies in the design phase.
Every mold repair (Re-cutting, Re-polishing, Insert changes) involves downtime losses and high labor costs. Therefore, a DFM (Design for Manufacturability) analysis must be performed before cutting steel:
Wall Thickness Uniformity: Strictly adhereto the 3mm–4mm uniform thickness rule.
Draft Angles: Reserve at least 1°–2° on appearance surfaces to prevent drag marks.
Radius Transitions: Apply R-angle (Radius) treatment to all internal corners to prevent stress concentration and air traps.
If you are experiencing frequent mold repairs and appearance defects, send us your 3D drawings. Our engineering team will provide a free assessment report including mold modification proposals and DFM optimization suggestions to help you pass T0 trials on the first shot.