1. Appearance
There is a discernible ring around sprue - a center circle if a center gate is used, and a concentric circle if a side gate is used. This is because ring size is small and looks like a 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 flows too fast and viscosity is high, part of surface material near sprue is easily dislocated and infiltrated. These dislocations will appear as halos on outer layer.
Near sprue, flow speed is particularly high, then gradually decreases. As injection speed becomes constant, front end of flow body expands into a gradually widening circle. At the same time, in order to obtain a low fluid front flow velocity near material head, multi-stage injection must be used, such as: slow-faster-faster. Purpose is to obtain a uniform melt front flow velocity throughout mold filling cycle.

 

It is usually believed that halo is caused by dislocation of melt during pressure holding stage. In fact, role of front flow effect is to move melt into interior of product during pressure holding stage.
Causes and improvement measures related to processing parameters:
1. Flow rate is too high. Use multi-stage injection: slow-faster-faster
2. Melt temperature is too low. Increase barrel temperature and screw back pressure
3. Mold wall temperature is too low. Increase mold wall temperature
Causes and improvement measures related to design:
1. Gate and product form an acute angle. There is an arc between gate and product
2. Gate diameter is too small. Increase gate diameter
3. Gate position is wrong. Reposition gate

1. Appearance: Surface of product after molding is very good until sharp edge. After sharp edge, surface appears dark and rough.
Physical reasons
If injection speed is too fast, that is, flow rate is too high, especially for high viscosity (poor fluidity) melts, surface layer is prone to shift and infiltration behind bevel and sharp edge. These displaced outer cold materials appear as dark areas and rough surfaces.
Causes and improvement measures related to processing parameters:
1. Front speed of fluid is too fast
Use multi-stage injection: fast-slow, reduce injection speed before front of fluid reaches sharp edge
Causes and improvement measures related to design:
1. Sharp angle transition in mold provides smooth transition

1. Appearance
Although mold has a uniform surface material, surface of product still appears gray and uneven gloss.
Physical reasons
Surface of product produced by injection molding is more or less a replica of mold surface. Surface roughness depends on thermoplastic material itself, its viscosity, speed setting, and molding parameters such as injection speed, holding pressure and mold temperature. Therefore, due to imitation surface roughness, surface of product will appear gray, darker or smooth.
In theory, when pitted or eroded mold surface has been accurately imitated, light projected onto the surface of product will be diffusely reflected. Therefore, dark areas will appear on the surface. For surfaces with less precise imitation, diffuse reflection will be controlled and product surface will have a good gloss effect

 

Reasons and improvement measures related to processing parameters:
1. Holding pressure is too low. Increase holding pressure
2. Holding time is too short. Increase holding time
3. Mold wall temperature is too low. Increase mold wall temperature
4. Melt temperature is too low. Increase melt temperature
Reasons and improvement measures related to design:
1. Mold wall section is too different. Provide a more uniform mold wall section
2. Too much material accumulation or too large edge size. Avoid excessive material accumulation or too large edge size
3. Poor exhaust at material flow line. Improve exhaust of mold at material flow line

1. Appearance
Void inside product is in the form of a round or elongated bubble. Only transparent products can see void inside from outside; opaque products cannot be measured from outside. Voids often occur in products with relatively thick walls and are in the thickest place.
Physical reasons
When bubbles are generated in product, they are often considered to be bubbles, which are air in mold that is entrained by molten material flowing into mold cavity. Another explanation is that moisture and bubbles in barrel will find ways to enter interior of product. Therefore, there are many reasons for generation of such "bubbles".
At the beginning, product will form a hard outer skin, and it will develop faster or slower depending on degree of mold cooling. However, in thick wall area, center part continues to remain sticky for a long time. Outer skin is strong enough to resist any stress shrinkage. As a result, melt inside is stretched outward, and a gap is formed in still plastic center of product

 

Reasons and improvement measures related to processing parameters:
1. Holding pressure is too low. Increase holding pressure
2. Holding time is too short. Increase holding time
3. Mold wall temperature is too low. Increase mold wall temperature
4. Melt temperature is too high. Reduce melt temperature
Reasons and improvement measures related to design:
1. Gate cross section is too small. Increase gate cross section and shorten runner
2. Nozzle hole is too small. Increase nozzle hole
3. Gate is opened in thin-walled area. Gate is opened in thick-walled area

1. Appearance
There are many bubbles on the surface and inside of product—mainly near head. In the middle of runner and away from head—not only in thick part of product wall. Bubbles have different sizes and different shapes.
Physical reasons
Bubbles mainly occur in heat-sensitive materials that must be processed at high temperatures. If required molding temperature is too high, material will decompose through molecular splitting, melt will be in danger of thermal degradation, and bubbles will be easily generated during molding process.
If cycle time is long, it may usually be due to too long residual time and insufficient stroke utilization. It may also be because melt in barrel is overheated.
Causes and improvement measures related to processing parameters:
1. Melt temperature is too high. Reduce barrel temperature, screw back pressure and screw speed.
2. Melt stays in barrel for too long. Use a smaller barrel diameter.
Causes and improvement measures related to design are shown in the table below:
1. Unreasonable screw geometry. Use a low-compression screw.

1. Appearance. There are unmelted particles near material head. It is impossible to obtain a smooth surface for thin-walled products.
Physical reasons
Since production molding cycle of thin-walled products is short, it is necessary to plasticize at a very high screw speed to shorten residual time of melt in screw barrel. When encountering production of thin-walled products, usually including PE and PP, molder will try to reduce melt temperature to shorten cooling time, and incompletely melted particles will be injected into mold.
Reasons and improvement measures related to processing parameters:
1. Melt temperature is too low. Increase barrel temperature
2. Screw speed is too high. Reduce screw speed
3. Screw back pressure is too low. Increase screw back pressure
4. Cycle time is short, that is, melt stays in barrel for a short time. Extend cycle time
Reasons and improvement measures related to design:
1. Unreasonable screw geometry. Select a screw with appropriate geometry (including metering shear zone)

1. Appearance
Grey or black clouding may occur near gate, in the middle of runner and away from gate. It can only be seen in transparent parts, often products made of PMMA, PC and PS materials have this phenomenon.
Physical reasons
If metering process starts too early, air entrained by pellets in screw feed zone does not overflow feed port, and air will be squeezed into melt. However, pressure in feed zone is too low to move air to back. Air squeezed into melt in barrel will cause gray and black streaks in product.
Just like what happens in a compression-ignition diesel engine, coking phenomenon caused by air squeezed into barrel is sometimes called "diesel effect".
Coking phenomenon explains high temperature generated at intersection of melt and squeezed air bubbles due to compression, and oxygen in air causes melt to break through oxidation.
Process debugging should start melting process in the middle of feed zone, where melt pressure is already high, forcing air between pellets to move backward and overflow feed port.
Causes and improvement measures related to processing parameters:

1. Screw back pressure is too low. Increase screw back pressure

2. Barrel temperature in feeding zone is too high. Reduce barrel temperature in feeding zone
3. Screw speed is too fast. Reduce screw speed
4. Cycle time is short, that is, molten material has a short residual time in barrel. Extend cycle time
Causes and improvement measures related to design:
1. Unreasonable screw geometry. Use a screw with a long feeding section and a deep screw groove in feeding section

1. Appearance
Silver or black lines appear on the surface of product radiating outward from gate or a point nearby. If low-viscosity (high-flowability) materials and high molding temperatures are used, lines are mostly black. If high-viscosity (low-flowability) materials are used, lines are mostly silver-white.
Physical reasons
This is caused by another type of bubble that is squeezed in and compressed. If screw pressure reduction is too high (screw retraction), pressure reduction speed is too fast, and molten material in front of screw head is released too much, negative pressure will be generated in molten material. When temperature of molten material is too high, bubbles will easily form in molten material.
These bubbles will be compressed again in subsequent injection stage, resulting in formation of black lines in product, which eventually becomes "diesel effect".
If gate is a central gate, lines will radiate outward from material head. In the case of hot runner injection, lines will only appear after a certain section of runner, because material in hot runner does not contain any bubbles, so material will not produce burnt marks. Only molten material at barrel head will produce burnt marks.

 

If it is a low-viscosity molten material, lines are grayer and larger than those of high-viscosity materials, because the former is prone to vacuum and gaps during screw pressure reduction process.
Causes and improvement measures related to processing parameters:
1. Screw pressure reduction is too high. Reduce screw pressure reduction amplitude
2. Screw pressure reduction rate is too high. Reduce screw pressure reduction rate
3. Melt temperature is too high. Reduce barrel temperature, reduce screw back pressure, and reduce screw speed

1. Appearance
A grayish-black melt is ejected from gate and is immediately covered by subsequently injected melt after slightly touching mold wall. This defect may be partially or completely hidden inside product.
Physical reasons
Jetting often occurs in direction where front end of fluid stops developing when melt enters mold cavity. It often occurs in molds with large cavities, where melt does not directly contact mold wall or encounter obstacles. After passing through gate, some hot melt contacts relatively cold mold cavity surface and cools down, cannot be tightly combined with subsequent melt during filling process.
In addition to obvious surface defects, radial lines are accompanied by unevenness, and melt produces frozen stretching, residual stress and cold strain, all of which affect product quality.
In most cases, it is unlikely to be improved by adjusting molding parameters alone. Only by improving gate position and geometric shape and size can it be avoided.
Causes and improvement measures related to processing parameters:
1. Injection speed is too fast. Reduce injection speed
2. Injection speed is single-stage. Use multi-stage injection speed: slow-fast
3. Melt temperature is too low. Increase barrel temperature (for heat-sensitive materials, only in metering area). Increase low screw back pressure
Causes and improvement measures related to design:
1. Transition between gate and mold wall is not good. Provide an arc transition
2. Gate is too small. Increase gate
3. Gate is located at the center of section thickness. Reposition gate and use barrier injection

1. Appearance
This refers to a piece of cold material stuck or stuck on the surface near slug. Cold slugs can cause marks on the surface of product, and in severe cases, they can reduce mechanical properties of product.
Physical reasons
Cold slugs are often produced when melt can cool near machine nozzle or hot runner. Since melt injected first always gathers near gate, defects will occur in this area. It is caused by unreasonable temperature control around machine nozzle or hot runner nozzle.
Reasons and improvement measures related to processing parameters:
1. Hot runner temperature is too low. Increase hot runner temperature.
2. Nozzle temperature is too low. Measure nozzle temperature, increase nozzle temperature, and reduce nozzle contact area.
Reasons and improvement measures related to design:
1. Nozzle cross section is too small. Increase nozzle cross section.
2. Gate geometry is unreasonable. Change gate geometry to keep cold slug in channel.
3. Hot runner geometry is unreasonable. Change hot runner nozzle geometry.

Gramophone rippie
1. Appearance
Deep grooves can be seen in the entire material flow direction and even at the end of runner. This phenomenon occurs when using high-viscosity (poor fluidity) materials and thick-walled products. These grooves look like texture on a record. They are very clear on products made of PC materials, but larger and gray on ABS products.
Physical reasons
If melt contacting mold surface solidifies too quickly and flow resistance is too high during injection process, especially at low injection speeds, distortion will occur at the front of fluid. Solidified outer layer of material will not completely contact cavity wall and form waves. These wavy materials will freeze, and holding pressure will no longer be able to flatten them.
Causes and improvement measures related to processing parameters:
1. Injection speed is too low. Increase injection speed
2. Melt temperature is too low. Increase barrel temperature and screw back pressure
3. Mold surface temperature is too low. Increase mold temperature
4. Holding pressure is too low. Increase holding pressure
Causes and improvement measures related to design:
1. Gate cross section is too small. Increase gate cross section and shorten runner
2. Nozzle hole is too small. Increase nozzle hole

Appearance
In mold filling method, weld line is line where front ends of various fluids meet. Especially where mold has a highly polished surface, weld line on product looks like a scratch or a groove, especially on dark or transparent products. Location of weld line is always in direction of material flow.
Physical reasons
Weld line is formed where thin streams of molten material fork and connect together again, most typically melt flow around core or product using multiple gates. Where thin streams meet again, surface will form a weld line and material flow line. The larger core around molten material or the longer flow channel between gates, the more obvious weld line formed. Small weld lines will not affect strength of product.
However, where process is very long or temperature and pressure are insufficient, mold filling will not be fully filled and will cause obvious grooves. Main reason is that front ends of fluids are not evenly fused to produce weak light spots. Spots may be produced where pigments are added to polymer because there are obvious differences in orientation. Number and location of gates determine number and location of weld lines. The smaller angle at which front ends of fluids meet, the more obvious weld line.
In most cases, it is impossible to completely avoid weld seams or material flow lines during process debugging. What can be done is to reduce their brightness or move them to an inconspicuous or completely invisible place

 

Reasons and improvement measures related to processing parameters:
1. Injection speed is too low. Increase injection speed
2. Melt temperature is too low. Increase barrel temperature
3. Mold surface temperature is too low. Increase mold temperature
4. Holding pressure is too low. Increase holding pressure and switch holding pressure as soon as possible
Reasons and improvement measures related to design:
1. Unreasonable gate position. Reposition gate and move it to an invisible place
2. No vent hole in material flow channel. Size of vent hole should be consistent with characteristics of material

Appearance
Moisture streaks are long silver threads on the surface of product, and opening direction of moisture streaks is along material flow direction. Where product is not fully filled, front end of fluid is rough.
Physical reasons
Some plastics such as PA, ABS, PMMA, SAN and PBT are easy to absorb water. If plastic is not stored properly, moisture will enter pellets or attach to surface. When pellets melt, moisture will turn into steam to form bubbles. During injection, these bubbles will be exposed to the surface of fluid front, burst and then produce irregular lines
Reasons and improvement measures related to processing parameters:
1. Residual moisture in pellets is too high
Check storage conditions of pellets, shorten time pellets are in hopper, and provide sufficient pre-drying for material.

Appearance
Color unevenness is color of surface of product is different, which can appear near and far from head, and occasionally in sharp edge of material flow area.
Physical reasons
Color unevenness is caused by uneven distribution of pigments, especially when adding color through masterbatch, color powder or liquid colorant.
When temperature is lower than recommended processing temperature, masterbatch or colorant cannot be completely homogenized. When molding temperature is too high or residual time in barrel is too long, it is also easy to cause thermal degradation of pigment or plastic, resulting in color unevenness.
When material is plasticized or homogenized at correct temperature, if it is injected too quickly through cross section of head, friction heat may be generated, causing degradation of the pigment and change of color.
Usually when using masterbatch, compatibility of pigment and its dissolving liquid with resin to be colored in terms of chemical and physical properties should be ensured.
Causes and improvement measures related to processing parameters:
1. Material is not mixed evenly. Reduce screw speed; increase barrel temperature and increase screw back pressure
2. Melt temperature is too low. Increase barrel temperature and increase screw back pressure
3. Screw back pressure is too low. Increase screw back pressure
4. Screw speed is too high. Reduce screw speed
Causes and improvement measures related to design:
1. Screw stroke is too long. Use a barrel with a larger diameter or a larger aspect ratio
2. Melt stays in barrel for a short time. Use a barrel with a larger diameter or a larger aspect ratio
3. Screw L:D is too low. Use a barrel with a larger aspect ratio
4. Screw compression ratio is low. Use a high compression ratio screw
5. There is no shearing section and mixing section. Provide a shearing section and (or) mixing section

Appearance: Surface of product shows very dark stripes of silver and light brown.
Physical reasons
Scorched dark lines are caused by excessive thermal degradation of melt. Light brown dark lines are caused by oxidation or decomposition of melt. Silver lines are generally caused by friction between screw, check ring, nozzle, sprue, narrow cross-section or sharp edge area in product.
Generally speaking, plastic will be severely degraded or decomposed during time when machine stops and barrel continues to heat.
If streaks are only found near sprue, reason is not only insufficient optimization of hot runner temperature control, but also related to nozzle of machine.
Even if temperature of melt is slightly high, relatively long residual time of melt in barrel will also lead to a decrease in mechanical properties of product.
Due to degradation chain reaction caused by molecular thermal motion, fluidity of melt will increase, so that mold will inevitably overflow. Be especially careful with complex molds.
Causes and improvement measures related to processing parameters:
1. Melt temperature is too high. Reduce barrel temperature
2. Hot runner temperature is too high. Check hot runner temperature and reduce it
3. Melt remains in barrel for too long. Use a small diameter barrel
4. Injection speed is too high. Reduce injection speed: use multi-stage injection: fast-slow

Appearance
Surface of plastic molded product with glass fiber added has various defects: gray, rough, and some metal bright spots appear, which are very obvious features, especially in convex part of material flow area, near joint line where fluid meets again.
Physical reasons
If injection temperature is too low and mold temperature is too low, material containing glass fiber tends to condense too quickly on mold surface, and glass fiber will never be embedded in melt again. When the two stream fronts meet, orientation of glass fiber is in direction of each stream, which will cause irregular surface material at intersection, resulting in formation of joints or material flow lines.

 

These phenomena are more obvious when melt in barrel is not completely mixed, for example, screw stroke is too long, resulting in injection of unevenly mixed melt.
Reasons and improvement measures related to processing parameters:
1. Injection speed is too low. Increase injection speed: consider multi-stage injection: slow first-fast later
2. Mold temperature is too low. Increase mold temperature
3. Melt temperature is too low. Increase barrel temperature and screw back pressure
4. Melt temperature changes are high, such as uneven melt
Increase screw back pressure; reduce screw speed; use a longer barrel to shorten stroke