Appearance A identifiable ring appears around sprue—a central circle if using a center gate, and concentric circles if using a side gate. This is because ring size is small, appearing as a dark halo. This phenomenon mainly occurs when processing high-viscosity (low-flow) materials such as PC, PMMA, and ABS.
Physical Reasons
If injection speed is too high, melt flow rate is too fast and viscosity is high, making it easy for some material near sprue to be displaced and seep in. These displacements will appear as dark halos on outer layer. Near sprue, flow velocity is particularly high, then gradually decreases, and as injection speed becomes constant, front of fluid expands into a gradually widening circle. Simultaneously, to obtain a low fluid front velocity near sprue, multi-stage injection must be used, for example: slow—faster—faster. Goal is to achieve a uniform melt front flow velocity throughout the entire mold filling cycle. It's commonly believed that haloing is caused by melt misalignment during holding pressure stage. In reality, front flow effect moves melt into interior of part during holding pressure stage.

 

Causes and improvements related to processing parameters:
* Flow rate too high: Use multi-stage injection: slow-faster-faster
* Melt temperature too low: Increase barrel temperature, increase screw back pressure
* Mold wall temperature too low: Increase mold wall temperature
Causes and improvements related to design:
* Gate at an acute angle to part: Create an arc between gate and part
* Gate diameter too small: Increase gate diameter
* Incorrect gate position: Reposition gate

Molded part surface is very good until sharp edges. After sharp edges, dark areas and roughness appear on the surface.
If injection speed is too fast, i.e., flow rate is too high, especially for highly viscous (poor flowability) melts, surface layer is prone to displacement, seepage behind bevels and sharp edges. These displaced outer cold materials manifest as dark areas and rough surfaces.
To address issue of excessively high fluid front speed, employ multi-stage injection: fast-slow, reducing injection speed before fluid front reaches sharp edge.
Provide smooth transitions for sharp angles within mold.

Even with a uniform mold surface material, product surface still appears dull and uneven in gloss.
Surface of an injection-molded product is a replica of mold surface. Gloss/roughness of product surface is result of combined effects of material properties, molding parameters (injection speed, holding pressure, mold temperature), and mold surface's ability to replicate surface. For surfaces with less precise replication, diffuse reflection can be controlled, resulting in a better gloss effect on product surface.
Insufficient holding pressure: Increase holding pressure.
Insufficient holding time: Increase holding time.
Insufficient mold wall temperature: Increase mold wall temperature.
Insufficient melt temperature: Increase melt temperature.
Excessive difference in mold wall cross-section: Provide a more uniform mold wall cross-section.
Poor venting at material flow line: Improve venting at material flow line.

Voids inside product manifest as round or elongated bubbles. Only transparent products can show these voids from outside; opaque products cannot be measured from outside. Voids often occur in relatively thick-walled products, especially at the thickest point.
When bubbles are generated inside product, they are often thought to be air trapped inside mold by molten material flowing into mold cavity. Another explanation is that moisture and air bubbles in barrel try to enter interior of product. Therefore, formation of such "bubbles" has multiple causes. Initially, produced product forms a hard outer layer, which develops inward at varying speeds depending on degree of mold cooling. However, in thick-walled areas, central portion remains viscous for a longer period. Outer skin has sufficient strength to resist any stress shrinkage. As a result, molten material inside is stretched outwards, creating voids in still plastic central portion within product.
Holding pressure too low: Increase holding pressure
Holding time too short: Increase holding time
Mold wall temperature too low: Increase mold wall temperature
Melte temperature too high: Decrease melt temperature
Gate cross-section too small: Increase gate cross-section, shorten runner
Nozzle orifice too small: Increase nozzle orifice
Gate in thin-walled area: Gate in thick-walled area

Many gas bubbles are present on the surface and inside product—primarily near sprue. They also appear in runner and away from sprue—not just in thick-walled areas. Bubbles come in various sizes and shapes.
Gas bubbles primarily occur in heat-sensitive materials that must be processed at high temperatures. If required molding temperature is too high, material decomposes through molecular splitting, posing a risk of thermal degradation of molten material, making it prone to bubble formation during molding. Long cycle times are usually caused by excessively long residual time and insufficient stroke utilization. It could also be due to overheating of molten material in barrel. Moisture in raw material (containing moisture) can also cause bubbles.
Important issues
Molten material temperature too high: Reduce barrel temperature, screw back pressure, and screw speed.

 

Excessive residual time of molten material in barrel: Use a smaller barrel diameter.
Moist raw material: Thoroughly dry raw material.
Inappropriate screw geometry: Use a low-compression screw.

Appearance: Unmelted particles are present near sprue. A smooth surface is impossible to achieve for thin-walled products.
Due to short molding cycle of thin-walled products, high screw speeds are necessary for plasticizing to shorten residual time of molten material in screw and barrel. When producing thin-walled products, typically including PE and PP, mold makers often try to lower molten material temperature to shorten cooling time, resulting in unmelted particles being injected into mold.
Melt temperature too low: Increase barrel temperature.
Screw speed too high: Decrease screw speed.
Screw back pressure too low: Increase screw back pressure.
Short cycle time (i.e., short residual time of molten material in barrel): Extend cycle time.
Inappropriate screw geometry: Select a screw with an appropriate geometry (including metering shear zone).

Grey or black clouding can occur near gate, in the middle of runner, and in parts far from gate. It is only visible in transparent parts, often occurs in products made of PMMA, PC, and PS materials.
If metering process starts too early, air trapped in particles in screw feeding zone does not overflow feed port and is forced into molten material. However, pressure in feeding zone is too low to move air to the back. Air forced into molten material in barrel causes grey or black clouding in product. Similar to what happens in a compression-ignition diesel engine, coking phenomenon caused by air forced into barrel is sometimes called "diesel effect." Coking can be explained by high temperature generated at junction of molten material and forced-in air bubbles due to compression, while oxygen in the air causes molten material to break apart through oxidation. Process adjustments should begin melting process in the middle of feeding zone, where molten material pressure is already high, forcing air between particles to move backward and overflow gate.
Feeding zone barrel temperature too high: Reduce feeding zone barrel temperature.
Screw back pressure too low: Increase screw back pressure.
Screw speed too high: Reduce screw speed.
Inappropriate screw geometry: Select a screw with a long feeding section and a deep feeding channel.

Silver or black streaks radiating outward from gate or a nearby point appear on the surface of product. If a low-viscosity (high-flow) material and a high molding temperature are used, texture will be grayish-black; if a high-viscosity (low-flow) material is used, texture will mostly be silvery-white.
This is due to another type of air bubble that is squeezed and compressed. If screw pressure drop is too high (screw retraction) and pressure drop rate is too fast, too much molten material is released in front of screw head, creating negative pressure within molten material. At high molten material temperatures, this easily leads to formation of air bubbles. These bubbles are compressed again in subsequent injection stages, causing black texture to form within product, ultimately creating a "diesel engine effect." If gate is a center gate, texture will radiate outwards from sprue head. In hot runner injection, texture will only appear after a certain section of runner, because material in hot runner does not contain any air bubbles, and therefore will not show signs of burning. Only molten material at barrel head will show signs of burning. If molten material is low-viscosity, texture will be darker and larger than that of high-viscosity materials because the former is more prone to creating vacuum and voids during screw pressure drop.
Screw pressure drop too high: Reduce screw pressure drop amplitude.
Screw pressure drop rate too high: Reduce screw pressure drop rate.
Molten material temperature too high: Lower barrel temperature, reduce screw back pressure, reduce screw speed.

A dark gray stream of molten material ejected from gate is immediately enveloped by subsequently injected molten material after only slight contact with mold wall. This defect may be partially or completely hidden inside part.
Jetting often occurs in direction where fluid front stops developing after entering mold cavity. It frequently occurs in molds with large cavities where molten material does not directly contact mold wall or encounter any obstacles. After passing through gate, some hot molten material cools after contacting relatively cool mold cavity surface and cannot bond tightly with subsequent molten material during mold filling. Besides obvious surface defects, jetting is accompanied by inhomogeneity, frozen stretching of molten material, residual stress, and cold strain, all of which affect product quality. In most cases, improvements are unlikely to be achieved simply by adjusting molding parameters; only improvements to gate location and geometry can prevent this.
Injection speed too fast: Reduce injection speed.
Single-stage injection speed: Use multi-stage injection speed: slow-fast.
Melt temperature too low: Increase barrel temperature (for heat-sensitive materials, only in metering zone). Increase screw back pressure.
Poor transition between gate and mold wall: Provide a rounded transition.
Gate too small: Increase gate size.
Gate located at the center of cross-sectional thickness: Reposition gate, use barrier injection.

This refers to a piece of cold slug stuck or adhering to surface near slug head. Cold slugs can cause marks on product surface and, in severe cases, reduce product's mechanical properties.
Cold slugs often occur when melt can be cooled near machine nozzle or hot runner. Since the first injected melt always accumulates near gate, defects occur in this area. This is caused by inadequate temperature control around machine nozzle or hot runner nozzle.

 

Hot runner temperature too low: Increase hot runner temperature.
Nozzle temperature too low: Measure nozzle temperature, increase nozzle temperature, reduce nozzle contact area.
Nozzle cross-section too small: Increase nozzle cross-section.
Gate geometry inappropriate: Change gate geometry to leave cold slug head in channel.
Hot runner geometry inappropriate: Change hot runner nozzle geometry.

Deep grooves are visible throughout flow direction, even to end of runner. This phenomenon occurs when using high-viscosity (poor flowability) materials and thick-walled products; these grooves resemble texture on a record. They are very clear on PC products, but larger and darker on ABS products.
If, during injection—especially at low injection speeds—melt in contact with mold surface solidifies too quickly, resulting in high flow resistance, distortion occurs at fluid front. Solidified outer layer material does not fully contact mold cavity wall, forming a wavy shape. This wavy material freezes, and holding pressure can no longer smooth it out.
Injection speed too low: Increase injection speed.
Melt temperature too low: Increase barrel temperature, increase screw back pressure.
Mold surface temperature too low: Increase mold temperature.
Holding pressure too low: Increase holding pressure.
Gate cross-section too small: Increase gate cross-section, shorten runner.
Nose orifice too small: Increase nozzle orifice size.
For further reading, please refer to Analysis of 21 Typical Injection Molding Defects (Part 2).