A molder can adjust temperature, pressure, injection speed and cooling time, and a skilled one will get remarkable results from a difficult part. What no process setting can do is make a thick boss cool at the same rate as the thin wall beside it. That is geometry, and geometry is decided long before the press.<\/p>\n\n\n\n
This guide covers defect prevention at design stage: the five geometry variables that cause most molding problems, the design cause behind each common defect, the rules for ribs and bosses, where design genuinely cannot fix the problem, and a review checklist to run before releasing a part for tooling. For correcting defects on a tool that is already running, our guide to fixing molding defects at the press<\/a> covers the process side.<\/p>\n\n\n\n Sink marks, warpage, voids and weld lines are usually described as process problems because that is where they are discovered. In practice they are typically the visible result of a geometry decision: a wall that changes thickness abruptly, a boss thicker than the wall it sits on, a gate positioned where flow has to travel from thin to thick, or a corner without a radius. Process adjustment can reduce the severity of these, and rarely eliminates them.<\/p>\n\n\n\n This matters commercially because design corrections are nearly free before tooling and expensive afterward. A wall thickness change in CAD costs an afternoon. The same change after hardened steel is cut means welding and re machining.<\/p>\n\n\n\n This is the highest leverage rule in part design and the one most frequently broken. Thick sections cool more slowly than thin ones. As the thick region cools last and shrinks, it pulls material from the surface, producing a sink mark, or it forms an internal void if the skin has already frozen. Where thick and thin sections cool at different rates across the same part, the differential shrinkage pulls the whole part out of shape as warpage.<\/p>\n\n\n\n Keep the nominal wall constant across the part wherever possible. Where a design genuinely needs mass in one region, the correct approach is usually to core out the thick section rather than leave it solid, so the wall remains uniform and the strength comes from geometry instead of bulk.<\/p>\n\n\n\n Where thickness has to change, the change should be gradual rather than abrupt. A step in wall thickness creates a sudden change in cooling rate and a stress concentration at the transition, which shows as sink on the outer surface and as a warpage driver in the part. Blending the transition over a distance rather than stepping it removes most of the effect.<\/p>\n\n\n\n The gate determines the direction plastic flows through the cavity, and flow direction determines where weld lines form, where the part fills last, and how effectively holding pressure can pack out the thick regions. Flow should travel from thick sections toward thin ones, because a gate that feeds a thin region first leaves the thick region unable to receive holding pressure, which guarantees sink and voids there.<\/p>\n\n\n\n Gate position also decides where cosmetic evidence appears. Every gate leaves a vestige, so the location should be on a non cosmetic surface where possible. Weld lines form where two flow fronts meet, typically behind holes and around inserts, and they are both a cosmetic mark and a genuine weak point. Design cannot eliminate weld lines on a part with holes, but design plus gate placement decides whether they land somewhere structurally unimportant.<\/p>\n\n\n\n Draft is the taper applied to walls in the direction the mold opens so the part releases without dragging. Without sufficient draft, ejection scrapes the surface, leaving drag marks, and in worse cases stresses or distorts the part during release.<\/p>\n\n\n\n Draft requirements increase with depth and with surface texture. A deep feature needs more taper than a shallow one, and a textured surface needs considerably more than a smooth one, because the texture itself acts as an undercut. Applying draft late in design is disruptive because it changes mating dimensions, so it is best applied from the beginning.<\/p>\n\n\n\n Sharp internal corners cause three separate problems. They concentrate stress, which is where parts crack in service. They disturb flow, producing an area that fills unevenly. And in the steel they force either a specialty cutter or EDM work, which raises tooling cost.<\/p>\n\n\n\n Generous internal radii solve all three at once. A corner radius that keeps the wall thickness through the corner roughly equal to the adjacent walls prevents the local thick spot that a poorly proportioned corner creates.<\/p>\n\n\n\nMost molding defects are design decisions first<\/strong><\/h2>\n\n\n\n
The five design variables that cause most defects<\/strong><\/h2>\n\n\n\n
1. Uniform wall thickness<\/strong><\/h3>\n\n\n\n
2. Wall transitions<\/strong><\/h3>\n\n\n\n
3. Gate location<\/strong><\/h3>\n\n\n\n
4. Draft<\/strong><\/h3>\n\n\n\n
5. Corner radii<\/strong><\/h3>\n\n\n\n
Defect by defect: the design cause and the design fix<\/strong><\/h2>\n\n\n\n