Every molded part that comes out wrong is telling you something specific. Sink marks point somewhere different from warpage. Flash points somewhere different from short shots. Learning to read the defect saves considerable time, because the wrong correction often makes the problem worse.<\/p>\n\n\n\n
Most defects have both a process cause and a design cause. The process cause can be adjusted at the machine within minutes. The design cause requires a tool modification or a part change, which is far more expensive. Understanding which you are dealing with determines whether you are looking at an afternoon of parameter adjustment or a tooling discussion.<\/p>\n\n\n\n
This guide covers the eight defects that account for most molding problems, what causes each, and what to do about it.<\/p>\n\n\n\n
1. Sink Marks<\/strong><\/p>\n\n\n\n What you see:<\/strong> localised depressions on the part surface, typically opposite a thick section such as a rib, boss or wall intersection.<\/p>\n\n\n\n Why it happens:<\/strong> the outer skin solidifies first, then the thicker material inside cools and contracts, pulling the surface inward. Thicker sections cool more slowly and shrink more, so the surface above them collapses.<\/p>\n\n\n\n Process fixes:<\/strong> increase holding pressure and holding time, lower melt temperature, lower mold temperature on the affected side, increase cooling time.<\/p>\n\n\n\n Design fixes:<\/strong> this is fundamentally a wall thickness problem. Reduce rib thickness to around 50 to 60 percent of the adjoining wall. Core out thick sections rather than leaving solid material. Avoid sharp changes in section thickness, transitioning gradually instead.<\/p>\n\n\n\n Which to reach for:<\/strong> if sinks are mild and appear only in cosmetic areas, process adjustment often resolves them. If they are pronounced and appear consistently at thick sections, the design is the cause and process adjustment will only reduce, not eliminate, them.<\/p>\n\n\n\n 2. Warpage<\/strong><\/p>\n\n\n\n What you see:<\/strong> the part distorts from its intended shape, bowing, twisting or having flat surfaces that are no longer flat.<\/p>\n\n\n\n Why it happens:<\/strong> different areas of the part shrink by different amounts or at different rates, and the resulting internal stresses pull the part out of shape. Uneven wall thickness, uneven cooling, and fibre orientation in filled materials are the usual culprits.<\/p>\n\n\n\n Process fixes:<\/strong> balance mold temperature between the two halves, extend cooling time, reduce injection speed, reduce packing pressure if overpacking is causing stress.<\/p>\n\n\n\n Design fixes:<\/strong> uniform wall thickness is the single most effective measure. Add ribs to stiffen large flat areas, which resist distortion better than plain panels. For filled materials, consider gate position, since fibres orient along the flow direction and shrink differently along and across that direction.<\/p>\n\n\n\n Tooling fixes:<\/strong> improve cooling channel layout so both halves cool evenly. Uneven cooling is a common and often overlooked cause.<\/p>\n\n\n\n Warpage is among the harder defects to resolve after tooling exists, which is why wall thickness uniformity deserves attention during design review rather than after T1.<\/p>\n\n\n\n 3. Flash<\/strong><\/p>\n\n\n\n What you see:<\/strong> thin fins of excess plastic at the parting line, around ejector pins or at slide interfaces.<\/p>\n\n\n\n Why it happens:<\/strong> material escapes where the mold halves meet, either because clamp force is insufficient to hold them closed against injection pressure, or because the mating surfaces are worn or damaged.<\/p>\n\n\n\n Process fixes:<\/strong> increase clamp force, reduce injection pressure or speed, reduce melt temperature, check that the machine tonnage is adequate for the projected area of the part.<\/p>\n\n\n\n Tooling fixes:<\/strong> inspect and repair parting line surfaces, check for debris preventing full closure, check vent depths since over-deep vents flash.<\/p>\n\n\n\n Which to reach for:<\/strong> if flash appeared suddenly on a tool that previously ran clean, suspect process settings or debris. If it has appeared gradually over many cycles, suspect wear on the parting line, which needs tool maintenance.<\/p>\n\n\n\n 4. Short Shots<\/strong><\/p>\n\n\n\n What you see:<\/strong> the part is incomplete, with material failing to reach the extremities of the cavity.<\/p>\n\n\n\n Why it happens:<\/strong> insufficient material entering the cavity, or material freezing before the cavity fills. Causes include inadequate shot size, low injection pressure or speed, low melt temperature, blocked gates, or trapped air with nowhere to escape.<\/p>\n\n\n\n Process fixes:<\/strong> increase shot size, injection pressure, injection speed or melt temperature. Check that the nozzle and gates are clear.<\/p>\n\n\n\n Design fixes:<\/strong> thin walls, particularly long thin flow paths, restrict filling. Increasing wall thickness in the affected area or shortening the flow length helps.<\/p>\n\n\n\n Tooling fixes:<\/strong> improve venting. Air trapped at the end of fill prevents material reaching it, and inadequate venting is a common cause that gets misdiagnosed as a pressure problem. Reposition or enlarge gates where flow length is excessive.<\/p>\n\n\n\n 5. Weld Lines<\/strong><\/p>\n\n\n\n What you see:<\/strong> a visible line where two flow fronts met, often around a hole or where flow split around an obstruction.<\/p>\n\n\n\n Why it happens:<\/strong> melt fronts meeting after flowing around an obstacle do not fully re-fuse, particularly if they have cooled. The result is a visible line and a genuine reduction in strength at that location.<\/p>\n\n\n\n Process fixes:<\/strong> increase melt temperature, increase mold temperature, increase injection speed so fronts meet while still hot.<\/p>\n\n\n\n Design fixes:<\/strong> reposition holes or obstructions if possible. Where a weld line is unavoidable, design so it falls in a non-critical, non-cosmetic area rather than across a loaded feature.<\/p>\n\n\n\n Tooling fixes:<\/strong> change gate location to alter how the melt fronts meet. Add overflow wells to move the weld line off the visible part surface.<\/p>\n\n\n\n Weld lines matter beyond appearance. Because strength is genuinely reduced there, a weld line crossing a loaded feature is a structural concern rather than a cosmetic one.<\/p>\n\n\n\n 6. Burn Marks<\/strong><\/p>\n\n\n\n What you see:<\/strong> dark or black discolouration, usually at the end of fill or in a corner.<\/p>\n\n\n\n Why it happens:<\/strong> air trapped in the cavity is compressed by the incoming melt, which heats it enough to scorch the plastic. This is the diesel effect. It can also result from excessive melt temperature or residence time causing degradation.<\/p>\n\n\n\n Process fixes:<\/strong> reduce injection speed, particularly at the end of fill. Reduce melt temperature. Reduce screw residence time.<\/p>\n\n\n\n Tooling fixes:<\/strong> add or deepen vents where burning occurs, since burn marks are usually a venting problem and consistently appear at the same location, which points directly at where venting is inadequate.<\/p>\n\n\n\n 7. Flow Lines<\/strong><\/p>\n\n\n\n What you see:<\/strong> wavy patterns, streaks or lines on the surface, often concentric around the gate.<\/p>\n\n\n\n Why it happens:<\/strong> the melt cools unevenly as it fills, or flows at inconsistent speed, leaving a record of the filling process in the surface.<\/p>\n\n\n\n Process fixes:<\/strong> increase injection speed, increase melt and mold temperature, increase holding pressure.<\/p>\n\n\n\n Tooling fixes:<\/strong> enlarge or reposition the gate, since a small gate causes rapid cooling as material passes through it. Round sharp transitions in the flow path.<\/p>\n\n\n\n Design fixes:<\/strong> avoid abrupt changes in wall thickness, which disturb flow and show as surface marks.<\/p>\n\n\n\n 8. Voids and Bubbles<\/strong><\/p>\n\n\n\n What you see:<\/strong> enclosed hollow spaces within the part, visible in transparent materials or discovered on sectioning.<\/p>\n\n\n\n Why it happens:<\/strong> two distinct causes that need distinguishing. Vacuum voids form as thick sections cool and shrink internally, pulling material apart. Gas bubbles form from moisture in the resin or from trapped air.<\/p>\n\n\n\n Process fixes:<\/strong> for vacuum voids, increase holding pressure and time, reduce melt temperature. For gas bubbles, dry the resin properly, since moisture is the most common cause and many resins are hygroscopic.<\/p>\n\n\n\n Design fixes:<\/strong> for vacuum voids, reduce section thickness and core out heavy areas, which is the same fundamental fix as sink marks.<\/p>\n\n\n\n Diagnosing which:<\/strong> vacuum voids appear in thick sections and are usually irregular. Gas bubbles can appear anywhere and tend to be spherical. If bubbles appear across the part regardless of thickness, suspect moisture and check drying before adjusting anything else.<\/p>\n\n\n\n Quick Reference<\/strong><\/p>\n\n\n\n The Pattern Worth Noticing<\/strong><\/p>\n\n\n\n Look at the design fixes across these defects. Sink marks, warpage and vacuum voids all trace back to the same root cause: non-uniform wall thickness. Weld lines, burn marks and short shots all relate to how the cavity fills.<\/p>\n\n\n\n That means a relatively small number of design principles prevent most molding problems. Uniform wall thickness, ribs at 50 to 60 percent of adjoining wall, generous radii at transitions, adequate draft, and gate positions chosen with flow in mind will eliminate the majority of defects before a tool is cut.<\/p>\n\n\n\nDefect<\/strong><\/td> Most common cause<\/strong><\/td> First thing to try<\/strong><\/td><\/tr> \u30b7\u30f3\u30af\u30de\u30fc\u30af<\/td> Thick sections<\/td> Increase hold pressure, then review rib thickness<\/td><\/tr> Warpage<\/td> Uneven wall or cooling<\/td> Balance mold temperatures<\/td><\/tr> \u30d5\u30e9\u30c3\u30b7\u30e5<\/td> Insufficient clamp or worn parting line<\/td> Increase clamp force, inspect parting line<\/td><\/tr> Short shot<\/td> Inadequate fill or venting<\/td> Increase pressure and speed, check vents<\/td><\/tr> Weld lines<\/td> Flow fronts meeting cool<\/td> Raise melt and mold temperature<\/td><\/tr> Burn marks<\/td> Trapped air<\/td> Reduce injection speed, add venting<\/td><\/tr> Flow lines<\/td> Uneven filling<\/td> Increase injection speed and temperature<\/td><\/tr> Voids<\/td> Thick sections or moisture<\/td> Dry resin, then increase hold pressure<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n