In mold filling, a weld line is a line where fluid fronts meet. Especially where mold has a highly polished surface, weld line on part resembles a scratch or a groove, particularly noticeable on dark-colored or transparent parts. Weld line is always located in direction of material flow.
Weld lines form where thin streams of molten material branch and then reconnect, most typically around core or in parts using multiple gates. Where thin streams meet again, a weld line and a flow line will form on the surface. The larger core around molten material or the longer runner between gates, the more noticeable weld line will be. Small weld lines do not affect strength of part. However, in areas with long flow paths or insufficient temperature and pressure, incomplete filling can cause noticeable grooves. Main reason is that fluid fronts are not uniformly fused, resulting in weak bright spots. Spots may appear where pigments are added to polymer due to significant differences in orientation. Number and location of gates determine number and location of weld lines. The smaller angle at which fluid fronts meet, the more noticeable weld lines. In most cases, process adjustments cannot completely eliminate weld lines or runout lines. The best approach is to reduce their visibility or move them to inconspicuous or completely invisible locations.
Molten material temperature too low: Increase barrel temperature.
Holding pressure too low: Increase holding pressure and switch to holding pressure as early as possible.
Gate location inappropriate: Reposition gate and move it to an invisible location.
No vents at runner: Vent size should conform to material properties.

 

Moisture streaks are long silver streaks on the surface of product, with opening direction along flow direction. Fluid front is rough where product is not fully filled.
Some plastics, such as PA, ABS, PMMA, SAN, and PBT, are prone to absorbing water. If plastic is stored under poor conditions, moisture can enter particles or adhere to surface. When granules melt, moisture turns into vapor, forming bubbles. During injection, these bubbles are exposed to surface of fluid front, burst, and create irregular streaks.
Excessive residual moisture in the granules: Check granule storage conditions, shorten granule time in hopper, and provide sufficient pre-drying for material.

Appearance: Color streaks are inconsistencies in color of finished product surface. They can occur near and far from sprue, and occasionally in flow area at sharp edges.
Color streaks are caused by uneven pigment distribution, especially when adding colorant via masterbatch, powder, or liquid pigments. Masterbatch or pigments cannot be completely homogenized when temperature is below recommended processing temperature. Excessively high molding temperatures or prolonged barrel dwell time can also easily cause thermal degradation of pigments or plastics, leading to color streaks. When material is plasticized or homogenized at correct temperature, injecting too quickly through sprue cross-section can generate frictional heat, causing pigment degradation and color changes. When using masterbatches, it is essential to ensure chemical and physical compatibility of pigment and its solvent with resin to be colored.
Uneven material mixing: Reduce screw speed; increase barrel temperature, increase screw back pressure.
Screw speed too high: Reduce screw speed.
Screw stroke too long: Use a barrel with a larger diameter or length-to-diameter ratio.
Short residence time of molten material in barrel: Use a barrel with a larger diameter or length-to-diameter ratio.
Low screw L:D ratio: Use a barrel with a larger length-to-diameter ratio.
Low screw compression ratio: Use a high compression ratio screw.
No shearing and mixing sections: Provide shearing and/or mixing sections.

Appearance: Very dark streaks of silver and light brown appear on the surface of product.
Charred streaks are caused by excessive thermal degradation of molten material. Light brown streaks are caused by oxidation or decomposition of molten material. Silver streaks are generally caused by friction from screw, check ring, nozzle, sprue, narrow cross-sections or sharp edges within product. Generally, plastics undergo severe degradation or decomposition while machine is stopped but barrel continues to heat. If streaks are only observed near sprue, cause is not solely due to insufficient hot runner temperature control optimization, but also related to machine's nozzles. Even a slightly high melt temperature, or a relatively long residual time of molten material in barrel, can lead to a decrease in mechanical properties of finished product. Under chain reaction of degradation caused by molecular thermal motion, fluidity of molten material increases, inevitably causing mold overflow. Particular caution is needed for complex molds.
Melt temperature too high. Lower barrel temperature.
Hot runner temperature too high. Check hot runner temperature and lower it.
Melt residue time in barrel too long. Use a smaller diameter barrel.
Injection speed too high. Reduce injection speed: Use multi-stage injection: fast-slow.

Plastic molded parts with added glass fibers exhibit various surface defects: dullness, roughness, and some with obvious metallic highlights, especially in raised areas of flow path and near junction line where fluids rejoin.
If injection temperature and mold temperature are too low, glass fiber-containing material often solidifies too quickly on mold surface, and glass fiber will no longer be embedded in melt. When two flow leaders meet, glass fiber is oriented in direction of each stream, resulting in irregular surface material at intersection, resulting in seams or flow lines. These phenomena are more pronounced when melt is not fully mixed in barrel, such as when screw stroke is too long, causing unevenly mixed melt to be injected.
Injection speed too low: Increase injection speed; consider multi-stage injection: slow first, then fast.
Mold temperature too low: Increase mold temperature.
Melt temperature too low: Increase barrel temperature, increase screw back pressure.
High melt temperature variation, such as uneven melt flow: Increase screw back pressure; decrease screw speed; use a longer barrel to shorten stroke.

Appearance: Thin flash appears around the recess, along parting line, or on mold sealing surface.
In most cases, flash occurs because machine's clamping force is insufficient during injection and holding pressure, failing to lock and seal mold along parting line. If there is a high pressure area in mold cavity, mold deformation at that location can cause flash. Under high molding temperatures and injection speeds, melt can still flow sufficiently at the end of runner; if mold is not locked, flash will occur. If flash is only found at a single point on mold, it indicates a defect in mold itself: mold is not completely sealed at that point. Typical flash situations: Localized flash is due to mold defects, while flash extending to the entire surrounding area is due to insufficient clamping force. Caution is advised! Increasing clamping force should be done with caution to avoid flash, as excessive clamping force can damage mold. Correct approach is to carefully identify true cause of flash. Especially before using multi-cavity molds, preparing some mold analysis data is a good practice, as this can provide accurate answers to all problems.
Insufficient clamping force: Increase clamping force
Injection speed too fast: Reduce injection speed; use multi-stage injection: fast-slow
Late holding pressure switch: Switch holding pressure earlier
Mold wall temperature too high: Reduce mold wall temperature
Holding pressure too high: Reduce holding pressure
Insufficient mold strength: Increase mold strength
Inadequate sealing at the parting line or protruding edges: Redesign mold

Sink marks are indentations in areas of material buildup on the surface of plastic part. Shrinkage mainly occurs in areas of thick wall or where wall thickness changes.
When product cools, shrinkage (volume reduction, shrinkage) occurs. At this time, outer layer, closest to mold wall, freezes first, creating internal stress in the center of product. If stress is too high, it will cause plastic deformation of outer layer; in other words, outer layer will cave inwards. If, during shrinkage and before outer wall deformation has stabilized (because it hasn't cooled completely), holding pressure doesn't replenish molten material into mold, settling will occur between mold wall and solidified outer layer of product. This settling is usually considered shrinkage. If product has a thick cross-section, such shrinkage may also occur after demolding. This is because there is still heat inside, which passes through outer layer and heats it. Tensile stress generated within product causes heated outer layer to sink inwards, resulting in shrinkage.
Insufficient holding pressure: Increase holding pressure.
Insufficient holding time: Extend holding time.
Insufficient melt temperature: Reduce melt temperature and barrel temperature.
Insufficient sprue cross-section: Increase sprue cross-section.
Insufficient sprue length: Shorten sprue length.
Insufficient nozzle orifice diameter: Increase nozzle orifice diameter.
Excessive material buildup: Avoid material buildup.
Inappropriate wall/rib cross-section: Provide a more reasonable wall/rib cross-section ratio.

 

Appearance: Mold cavity is not completely filled, mainly occurring far from sprue or thin-walled areas.
Physical Causes: Injection pressure and/or injection speed of melt is too low, causing melt to cool as it travels towards end of flow. This usually occurs when injecting high-viscosity materials under low melt and mold temperatures. It can also occur when high-pressure injection is required but holding pressure setting is disproportionately low. In practice, when high injection pressure is required, holding pressure should also be increased proportionally: normally, holding pressure should be about 50% of injection pressure, but if a high injection pressure is used, holding pressure should be 70%~80%. If incomplete injection is observed near sprue, it can be explained as follows: fluid front is blocked at these points, and thicker areas are filled first. Thus, after mold cavity is almost filled, molten material at thinner walls has solidified, and there is only a small amount of flow in fluid center, resulting in insufficient injection.
Injection pressure too low: Increase injection pressure
Injection speed too low: Increase injection speed
Premature holding pressure switch: Delay switch from injection to holding pressure
Small runner/sprue cross-section too small: Increase cross-section of runner/sprue
Insufficient mold venting: Improve mold venting
Insufficient thickness at thin walls: Increase cross-sectional thickness

Shape of part rotates or twists after demolding or shortly thereafter. Typically, flat parts of part have undulations, and straight edges bend or twist inwards or outwards.
Due to its properties, frozen molecular chains in molded product undergo internal displacement under stress. During demolding, stress often causes varying degrees of deformation depending on product shape. Internal stress leads to uneven shrinkage, displacement of small particles, uneven cooling within particles, or excessive pressure within particles. Products made from partially crystalline materials, such as PE, PP, and POM, are particularly prone to wall shrinkage and warping compared to amorphous materials like PS, ABS, PMMA, and PC.
Excessive in-mold pressure: Reduce holding pressure or advance holding pressure switch. Insufficient mold temperature: Increase mold temperature.
Fluid front, low viscosity: Increase injection speed.
Unstable mold temperature: Provide a mold with balanced cooling/heating.
Irregular cross-sectional thickness: Redesign product shape and dimensions according to resin properties.

Appearance: Stress whitening and increased stress are observed on nozzle-facing side of product, where ejector pin is located on ejector side of mold.
If required demolding force is too high or ejector pin surface is relatively small, surface pressure at this point will be very high, causing deformation and ultimately resulting in whitening of ejection area.
Holding pressure time too long: Shorten holding pressure time.
Holding pressure switching too late: Advance holding pressure switching.
Cooling time too short: Extend cooling time.
Insufficient draft angle: Select appropriate draft angle according to specifications.
Rough surface in demolding direction: Polish mold in demolding direction.
Vacuum formation on ejection side: Install an air valve inside core.