21 Solutions for Injection Molding Defects Part One

Injection Molding Defect 1: There is a dark halo near the runner gate

Appearance of dark halo

There is a discernible ring around the runner gate. If a center gate is used, it is a center circle. And it’s a concentric circle if the side gate is used. This is because the ring size is small and looks dim. Such a phenomenon occurs when processing high viscosity (low fluidity) materials, such as PC, PMMA and ABS.

Physical reason of dark halo

If the injection speed is too high, the melt flow rate is too fast and the viscosity is high, and the surface material near the runner gate is easily dislocated and infiltrated. These dislocations will show a dim halo on the outer layer.

Near the runner gate, the flow speed is extremely high, and then gradually decreases. As the injection speed becomes constant, the front end of the flow body expands into a gradually widening circle. At the same time, in order to obtain a low fluid flow velocity near the runner gate, multi-stage injection speed must be used. For example: slow-faster-fast. The purpose is to obtain a uniform melt forward flow rate throughout the filling cycle.

It is usually thought that the halo is caused by the dislocation of the melt during the pressure holding stage. In fact, with the plastic melt flow forwarding the melt is moved into the interior of the product during the pressure holding stage.

Reasons and improvement measures of machine parameters setting:

(1) The flow rate is too high. Use multi-stage injection speed: slow-fast-fast.

(2) The melt plastic’s temperature is too low. Increase the barrel temperature and the screw back-pressure.

(3) The mold wall’s temperature is too low. Increase the mold wall temperature.

Reasons and improvement measures related to the mold design:

(1) The gate and the product form an acute angle. Make the gate and the product form an arc.

(2) The gate diameter is too small. Increase the gate diameter

(3) Gate position error. Reposition the runner gate.

Injection Molding Defect 2: The sharp edge material flow area has a dark area

Appearance of the dark area

After molding, the surface of the product is very good, until sharp edges. After sharp edges, the surface appears dark and rough.

Physical reason of dark area

If the injection speed is too fast, that is, the flow rate is too high, especially for high-viscosity (poor fluidity) melts, the surface layer is likely to shift and infiltrate behind the slope and sharp edges. These displaced outer layers of cold material appear as dark areas and rough surfaces.

Reasons and improvement measures of the injection molding machine parameters setting:

  • The fluid front end is too fast.
  • Adopt multi-stage injectionspeed: fast-slow, and reduce the injection speed before the fluid front reaches the sharp edge.

Reasons and improvement measures related to the mold design:

Acute angle transition in mold provides smooth transition.

Injection Molding Defect 3: Uneven surface gloss

Appearance of the uneven gloss

Although the mold has a uniform surface material, the product surface is still dull and uneven in gloss.

Physical reason of uneven gloss

The product surface produced by injection molding machine is more or less a replica of the surface of the mold. The surface roughness depends on the thermoplastic material itself, its viscosity and injection molding parameters such as injection speed, holding pressure and mold temperature. Therefore, for the imitation of the surface roughness, the product surface will appear dull, darker or smooth.

Theoretically, when the pitted or eroded mold surface has been accurately imitated, the light projected on the product surface will be diffusely reflected. Therefore, dark areas will appear on the surface. For the surface with less precise imitation, the diffuse reflection phenomenon will be controlled and the product surface will have a good gloss effect.

Reasons and improvement measures related to processing parameters:

(1) The pressure-hold is too low. Increase the pressure-hold.

(2) The pressure-holding time is too short. Increase the pressure-holding time.

(3) The mold wall’s temperature is too low. Increase the mold wall’s temperature.

(4) The melt plastic’s temperature is too low. Increase the melt plastic’s temperature.

Reasons and improvement measures related to the mold design:

(1) The mold wall’s difference is too large to provide a more uniform die wall.

(2) Too much material accumulation or too large edge size. Avoid excessive material accumulation or too large edge size.

(3) Poor air-exhaust at the material flow line. Improve the air-exhaust of the mold at the material flow line.

Injection Molding Defect 4: Voids

Appearance of the voids

The voids inside the product are in the form of round or elongated bubbles. The voids can only be seen from the outside of the transparent products. It can not measured from the outside of opaque products. Voids tend to occur in products with relatively thick walls and in the thickest places.

Physical reason of voids

When bubbles are generated in the product, they are often considered to be bubbles. In fact, it’s the air in the mold is engulfed by the melt plastic flowing into the mold cavity. Another explanation is that the water vapor and bubbles in the injection unit barrel will try to get into the inside of the product. Therefore, there are many causes for the generation of these “bubbles”.

At the beginning, the produced product will form a hard outer skin. And it will develop faster or slower depending on the degree of mold cooling down. However, in the thick-walled area, the central part continues to remain viscous for a long time. The outer skin is strong enough to resist any stress contraction. As a result, the molten material inside is stretched out, forming a void in the center part of the product that is still plastic.

Reasons and improvement measures related to machine parameters setting:

(1) The holding pressure is too low. Increase the holding pressure.

(2) The holding time is too short. Increase the holding time.

(3) The mold wall temperature is too low. Increase the mold wall temperature.

(4) The melt temperature is too high. Reduce the melt temperature.

Reasons and improvement measures related to the mold design:

(1) The runner gate is too small. Increase the gate size and shorten the runner.

(2) The nozzle hole is too small. Increase the nozzle hole.

(3) The gate is at the thin-walled area. Change the gate to the thick-walled area.

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Injection Molding Defect 5: Bubbles

Appearance of the bubbles

There are many bubbles on the surface and inside of the product. They are mainly near the runner gate, in the middle of the runner and away from the material head. They are not only in the place where the product wall is thick. Bubbles have different sizes and different shapes.

Physical reason of bubbles

Bubbles mainly occur in heat-sensitive plastic that must be processed at high temperatures. If the necessary molding temperature is too high, the plastic resin will be decomposed through molecular splitting. Besides, the melt will be in danger of thermal degradation. And bubbles will be easily generated during the molding process.

If the cycle time is long, it may usually because the too long residual time of the plastic in the barrel and underutilization of the screw stroke. It may also be because the melt plastic in the barrel is overheated.

Reasons and improvement measures related to processing parameters:

(1) The melt temperature is too high. Reduce the barrel temperature, screw back pressure and screw speed.

(2) The residual time of the molten material in the barrel is too long. Use the barrel of a smaller diameter.

Reasons and improvement measures related to the design:

Wrong screw applies, which is with unreasonable screw geometry. Change to the low compression screw.

Injection Molding Defect 6: White spot

Appearance of the white spot

There are unmelted particles near the material head. It is impossible for thin-walled products to obtain a smooth surface.

Physical reason of white spot

Due to the short cycle time of the thin-walled products, it must be plasticized at a high screw speed to shorten the residual time of the melt plastic in the screw barrel.

When it comes to the production of thin-walled products, usually including PE and PP, the machine operator tries to reduce the melt temperature in order to shorten the later cooling time. Then, the particles that are not completely melted will be injected into the mold cavity.

Reasons and improvement measures related to injection molding machine parameters setting:

(1) The melt temperature is too low. Increase the barrel temperature.

(2) The screw speed is too high. Reduce the screw speed.

(3) Screw back pressure is too low. Increase screw back pressure.

(4) The cycle time is short, that is, the residual time of the molten material in the barrel is short. Extend the cycle time.

Reasons and improvement measures related to the design:

Wrong screw applied, which is with unreasonable screw geometry. Select the screw with appropriate geometry (including metering shear zone).

Injection Molding Defect 7: Gray and black markings

Appearance of the gray and black markings

Gray-black markings may occur near the gate, in the middle of the runner and away from the gate. It can only be seen in transparent parts, and often seen in the products made of PMMA, PC and PS materials.

Physical reason of gray and black markings

If the metering process starts too early and the air entrapped by the plastic resin in the screw feeding area does not overflow the feeding port, the air will be squeezed into the melting zone. We think that the pressure in the feeding zone is too low to force the air behind. The air squeezed into the molten plastic in the barrel will cause gray and black markings in the product.

Just like what happens in a compression ignition diesel engine, the coking phenomenon caused by the air squeezed into the barrel is sometimes called the “diesel effect.”

The coking phenomenon can be explained as follows: At the place where the molten material and the extruded bubbles meet, high temperature is generated due to compression, and the oxygen in the air causes the molten material to fracture through oxidation.

Process debugging should start at the melting stage when plastic is at the middle of the feeding area. At this zone where the melt pressure is already high, forcing the air move backwards and overflow from the material feeding gate.

Reasons and improvement measures related to processing parameters:

(1) Screw back pressure is too low. Increase injection molding machine screw back pressure.

(2) The barrel temperature in the feeding area is too high. Reduce the temperature of the barrel in the feeding area.

(3) The screw speed is too fast. Reduce the screw speed.

(4) The cycle time is short, that is, the residual time of the molten plastic in the barrel is short. Extend the cycle time.

Reasons and improvement measures related to the mold design:

Wrong screw applied which is with unreasonable geometry. Choose a screw with a long feeding section and a deep groove in the feeding section.

Injection Molding Defect 8: There are black or silver streaks near the material head

Appearance of the silver or black streaks

Silver or black streaks appear on the surface of the product, centered on the gate or a point nearby. If using low-viscosity (high fluidity) materials and high molding temperature, the streaks are mostly black. And if using high-viscosity (low fluidity) materials, the streaks are mostly silver-white.

Physical reason of silver or black streaks

This is another kind of bubble that is squeezed in and compressed. If the screw pressure drops too high (screw retracts), or the pressure drop too fast, and the melt in front of the screw tip will be released too much, which will generate negative pressure in the melt. When the temperature of the melt is too high, it will easily form the bubbles in the melting plastic.

These bubbles will be compressed again in the subsequent injection stage, resulting in the formation of black lines in the product, which will eventually become the “diesel effect”.

If the gate is a central gate, the streaks will radiate outward from the material head. In the case of the mold of hot runner, the streaks will only appear after a certain section of the runner. Because the material in the hot runner does not contain any bubbles, so the material will not produce burnt marks. Only the molten material at the front of the barrel will produce burnt marks.

If it is a low-viscosity melt, its streaks are darker and larger than that of a high-viscosity material. Because the former is prone to vacuum and voids during the screw pressure reduction process.

Reasons and improvement measures related to injection molding machine parameters setting:

(1) The screw pressure drop is too high. Reduce the screw pressure drop amplitude.

(2) The screw pressure drop rate is too high. Reduce the screw pressure drop rate.

(3) The melt temperature is too high. Reduce the barrel temperature, the screw back pressure, and the screw rotation speed.

Defect 9: Radiation streaks

Appearance of the radiation streaks

Injected from the mold runner gate, a grayish-dark melt stream was immediately enveloped by the molten material injected after it slightly touched the mold wall. This defect may be partially or completely hidden inside the product.

Physical reason of radiation streaks

Radiation streaks often occur in the direction where the front end of the fluid stops forwarding after the melt plastic enters the mold cavity. It often occurs in a mold with a large size cavity, where the melt flow does not directly touch the mold wall or encounter obstacles. After passing through the gate, some of the hot melt plastic contacts the relatively cold cavity surface and then cools down. And it cannot be tightly combined with the subsequent melt together during the filling process.

In addition to the obvious surface defects, radial streaks are accompanied by non-uniformity. And the melt is frozen and stretched. Then residual stress and cold strain are generated, these factors all affect the product quality.

In most cases, it is unlikely to be improved only by adjusting the molding parameters. And it can be avoided only by improving the gate position and geometry size.

Reasons and improvement measures related to injection molding machine parameters setting:

(1) The injection speed is too fast. Reduce the injection speed

(2) Not just use one stage injection speed. Adopt the multi-stage injection speed: slow-fast.

(3) The melt temperature is too low. Increase the barrel temperature (for heat-sensitive materials only in the metering area). Increase screw back pressure.

Reasons and improvement measures related to the mold design:

(1) The transition between the gate and the mold wall is not good. Then, provide the arc transition.

(2) The gate is too small. Increase the gate size.

(3) The gate is located in the center of the section thickness. Reposition the gate and use barrier injection.

Injection Molding Defect 10: Cold feed head

Appearance of the cold feed head

This refers to a piece of cold material stuck on the surface near the material head. The cold head will cause marks on the surface of the product, and severely reduce the mechanical properties of the product.

Physical reason of cold feed head

The molten material can be cooled when it is near the machine nozzle or hot runner, and a cold head is often produced.

Since the molten material injected first always gathers near the gate, defects will occur in this area. It is caused by unreasonable temperature control around the machine nozzle or hot runner nozzle.

Reasons and improvement measures related to injection molding machine parameters setting:

(1) The temperature of the hot runner is too low. Increase the temperature of the hot runner.

(2) The nozzle temperature is too low. Measure the nozzle temperature, increase the nozzle temperature, and reduce the nozzle contact area.

Reasons and improvement measures related to the mold design:

(1) The nozzle is too small. Increase the nozzle cross section.

(2) The geometric size of the runner gate is unreasonable. Change the geometric size of the gate to leave the cold head in the channel.

(3) The geometric size of the hot runner is unreasonable. Change the geometric size of the hot runner nozzle.

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