21 Solutions for Injection Molding Defects Part Two

Injection Molding Defect 11: Flow marks

Appearance of flow marks

Deep grooves can be seen in the entire flow direction even to the end of the flow channel. This phenomenon occurs in the case of high-viscosity (poor fluidity) materials and thick-walled products are used. These grooves look like lines on a record. It is very clear on the product made of PC material. But it is larger on the ABS product, and the color is gray and dark.

Physical reason of flow marks

If during the injection process—especially under the condition of low injection speed, the condensation speed of the melt contacting the mold wall is too fast and the flow resistance is too high, distortion will occur at the front end of the fluid. The solidified outer layer material will not completely contact the cavity wall and form a wave shape. These wavy materials will freeze, and will no longer be able to flatten them, even in the pressure-holding stage.

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

(1) The injection speed is too low. Increase the injection speed.

(2) The melt temperature is too low. Increase the injection molding machine barrel temperature and the screw back pressure as well.

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

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

Reasons and improvement measures related to the mold design:

(1) The cross section of the runner gate is too small. Increase the cross section of the gate and shorten the runner.

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

Injection Molding Defect 12: Weld line

Appearance of weld line

In the filling mode, the weld line refers to a line when the front ends of the fluids meet. Especially where the mold has a highly polished surface, the weld line on the product looks like a scratch or a groove, especially on dark or transparent products. The position of the weld line is always in the direction of material flow.

Physical reason of weld line

The place where the weld line is formed is where the trickle of melt diverges and connects together. The most typical is the melt plastic flow around the product central or the product with multiple gates. Where the trickle meets again, the surface will form weld line and stream lines. The larger the central around the melt or the longer the runner between the gates, the more obvious the weld lines formed. The small weld line will not affect the strength of the product.

However, at the place of long distance or of insufficient temperature and pressure , insufficient mold filling will cause obvious grooves. The main reason is that the front end of the fluid is not uniformly fused to produce a weak spot. Spots may occur where pigments are added in the polymer because of the obvious difference in orientation. The number and location of runner gates determine the number and location of weld line. The smaller the angle when the fluid fronts meet, the more obvious the weld line.

In most cases, it is impossible to completely avoid weld line or material flow lines during the injection molding 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 the injection molding machine parameters setting:

(1) The injection speed is too low. Increase the injection speed.

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

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

(4) The holding pressure is too low. Increase the holding pressure and switch to the holding pressure as soon as possible.

Reasons and improvement measures related to the mold design:

(1) Unreasonable runner gate position. Reposition the gate and move it to an invisible place.

(2) There is no air escape hole at the mold cavity. The size of air escape hole should meet the characteristics of the resin.

Injection Molding Defect 13: Water Mark

Appearance of water mark

The water mark is a long silver wire on the surface of the product, and its opening direction is along the material flow direction. Where the product is not completely filled, the fluid front end is rough.

Physical reason of water mark

Some plastics such as PA, ABS, PMMA, SAN and PBT are easy to absorb water. If the plastic storage conditions are not good, moisture will enter the resin or attach to its surface. When the resin melts, the moisture will turn into steam to form bubbles. During the injection, these bubbles will be exposed on the surface of the fluid front, burst and then produce irregular lines.

Reasons and improvement measures related to processing parameters:

The residual moisture in the particles is too much.

  • Check the storage conditions of the material.
  • Shorten the time the resinsare in the hopper.
  • Provide sufficient pre-drying for the materialsbefore feeding into the barrel.

Injection Molding Defect 14: Uneven color

Appearance of uneven color

Color unevenness is the difference in the color of the surface of the product. It can occur near and far away from the material head, and occasionally in the sharp-edged material flow area.

Physical reason of uneven color

Uneven color is caused by uneven distribution of pigments, especially when adding color through masterbatch, toner or liquid color material.

When the temperature is lower than the recommended processing temperature, the masterbatch or pigment cannot be completely homogenized. When the molding temperature is too high, or the remaining time in the barrel is too long, it is easy to cause thermal degradation of the pigment or plastic, resulting in uneven color.

When the material is plasticized or homogenized at the correct temperature, if the injection is too fast through the cross section of the material head, frictional heat may be generated. And that may cause pigment degradation and color change.

Generally, when using color masterbatch, ensure the compatibility of the chemical and physical properties of the resin to be colored by the pigment and its solution.

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

(1) The materials are not evenly mixed. Reduce the screw speed. And increasethe barrel temperature and the screw back pressure.

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

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

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

Reasons and improvement measures related to the design:

(1) The screw stroke is too long. Use a screw and barrel with a larger diameter.

(2) The residence time of the molten material in the barrel is short. Use a screw and barrel with a larger diameter.

(3) Screw L: D ratio is too low. Use a screw and barrel with a relatively large diameter.

(4) The screw compression ratio is low. Use a high compression ratio screw.

(5) There is no shearing section and mixing section in the screw. Apply the screw with shearing section and (or) mixing section.

Injection Molding Defect 15: Burn marks

Appearance of burn marks

The very dark streaks of silver and light brown showed on the product surface.

Physical reason of burn marks

injection-molding-defects--burn-marks

The burnt dark lines are caused by excessive thermal degradation of the melted plastic. The light brown dull streaks are due to oxidation or decomposition of the melt plastic. Silver streaks are generally caused by friction in the screw, check ring, nozzle, narrow cross-section or sharp-edged area in the product.

Generally speaking, the plastic will be severely degraded or decomposed during the time when the machine is stopped and the barrel is still on heating.

If streaks are found only near the material head, the reason is not only insufficient optimization of the runner temperature control, but also related to the nozzle of the injection molding machine.

Even if the temperature of the molten material is slightly high, with the relatively long residual time in the barrel, that will also cause the mechanical properties of the product to decrease.

Under the effect of the chain reaction of degradation caused by the thermal movement of the molecules, the fluidity of the melt will increase. So that the mold overflow ( the flash ) will inevitably occur. Be especially careful with complex molds.

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

(1) The melt temperature is too high. Reduce the barrel temperature.

(2) The temperature of the hot runner is too high. Check the temperature of the hot runner and reduce it.

(3) The residual time of the molten plastic material in the barrel is too long. Use a barrel of small diameter.

(4) The injection speed is too high. Reduce the injection speed: use multi-stage injection: fast-slow.

Injection Molding Defect 16: Glass fiber silver streaks

Appearance of glass fiber silver streaks

The surface of the plastic molded product with glass fiber has various defects: dark, rough. And some part may appear obvious features such as metal bright spots, especially the convex part of the flow area, near the joint line where the fluid meets again.

Physical reason of glass fiber silver streaks

If the injection temperature and the mold temperature are too low, the glass fiber-containing material tends to condense too quickly on the mold surface. So the glass fiber will never be embedded in the melt again. When the fronts of the two streams meet, the orientation of the glass fiber is in the direction of each stream. And that will cause irregular surface materials at the intersection, resulting in the formation of weld seams or stream lines.

These phenomena are more obvious when the melt in the barrel is not completely mixed. For example, if the screw stroke is too long, it will cause the melt unevenly mixed.

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

(1) The injection speed is too low. Increase the injection speed and consider using multi-stage injection: first slow-then fast.

(2) Mold temperature is too low. Increase mold temperature.

(3) The melt plastic temperature is too low. Increase the barrel temperature and the screw back pressure.

(4) High fluctuation of the melt plastic temperature, such as uneven melt. Then, increase the screw back pressure and reduce the screw speed. Or use a ring screw and barrel set to shorten the stroke.

Injection Molding Defect 17: Flash

Appearance of flash

There are thin flashes around the recess, along the parting line or on the sealing surface of the mold.

Physical reason of flash

In most cases, the flash occurs because the clamping force of the machine is not enough during the injection and pressure holding process, and the mold cannot be locked and sealed along the parting line.

If there is a place in the cavity where the pressure is high, the deformation of the mold here may cause the flash. Under the conditions of high molding temperature and injection speed, the melt plastic can still flow fully at the end of the material. In this case the mold is not locked, it will produce edge flash.

If the flash is only found at a certain point on the mold, it means that the mold itself is defective: the mold is not completely sealed here.

The some part’s flash is produced due to the mold defect. While the expansion to the whole products is due to insufficient clamping force.

Attention! In order to avoid the flash, you should be cautious when increasing the clamping force. Because excessive clamping force will easily damage the mold. It is recommended to carefully confirm the real cause of the flash. Especially before using a multi-cavity mold, it is good to prepare some mold analysis data. So that it can provide correct answers to all questions.

Reasons and improvement measures related to injection molding parameters setting:

(1) Insufficient clamping force. So increase the clamping force.

(2) The injection speed is too fast. Reduce the injection speed: use multi-stage injection: fast-slow.

(3) The holding pressure switch is late. Switch the holding pressure earlier.

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

(5) The mold wall temperature is too high. Reduce the mold wall temperature.

(6) The holding pressure is too high. Reduce the holding pressure.

Reasons and improvement measures related to the mold design:

(1) Insufficient mold strength. Increase mold strength.

(2) The mold is insufficiently sealed at the parting line or flange. Redesign the mold.

Injection Molding Defect 18: Sink marks

Appearance of sink marks

There are sink marks in the material accumulation area on the surface of the plastic part. Shrinkage mainly occurs where the wall thickness of the plastic part is thick or where the wall thickness changes.

Physical reason of sink marks

When the product cools down, shrinkage (volume reduction) occurs. At this time, the outer tight mold wall freezes first, forming an internal stress in the center of the product. If the stress is too high, it will cause plastic deformation of the outer layer of plastic. In other words, the outer layer will sink inward. If the shrinkage occurs and the deformation of the outer wall is not stable (because it has not been cooled), the holding pressure does not replenish the melt into the mold in time, and a settlement will form between the mold wall and the outer layer of the solidified product.

These settlements are usually seen as contractions. If the product has a thick section, such shrinkage may also occur after ejection. This is because there is still heat inside of the product, it will pass through the outer layer and produce heating effect on the outer layer. The tensile stress generated inside of the product causes the hot outer layer to settle inward, causing shrinkage.

injection-molding-defect-sink-marks

Reasons and improvement measures related to injection molding parameters setting:

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

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

(3) The mold wall temperature is too high. Reduce the mold wall temperature.

(4) The temperature of the melt is too high. Reduce the temperature of the melt and lower the temperature of the barrel.

 

Injection Molding Defect 19: Insufficient injection

Appearance of insufficient injection

The cavity is not completely filled, mainly in places far away from the material head or thin-walled area.

Physical reason of insufficient injection

The injection pressure and/or injection speed of the melt plastic is too low. The melt plastic is cooled during its runs to the furthest end of the mold. It usually occurs when injecting highly viscous materials under the conditions of low melt temperature and mold temperature. Besides, it also occurs when high pressure injection is required but the holding pressure setting is disproportionately low.

In fact, when requiring high injection pressure, the holding pressure should also be increased proportionally. Normally, the holding pressure should be about 50% of the injection pressure. But if using a high injection pressure, the holding pressure should be 70% to 80%.

If the injection is found to be insufficient near the material head, it can be explained that the fluid front is blocked at these points, and the thicker areas are filled first. In this way, after the cavity is almost filled, the melt at the thin wall has condensed and a small amount of it flow in the center of the fluid, resulting in insufficient injection.

Reasons and improvement measures related to injection molding parameters setting:

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

(2) The injection speed is too low. Increase the injection speed.

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

(4) The holding pressure switch is too early. Delay the switch from injection to holding pressure.

(5) The melt temperature is too low. Increase the barrel temperature and the screw back pressure.

(6) The holding time is too short. Extend the holding time.

Reasons and improvement measures related to the mold design:

(1) The cross section of the runner/material head is too small. Increase the cross section of the runner/material head.

(2) Insufficient mold air exhaust. Improve mold air exhaust.

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

(4) The thickness of the thin wall is not enough. Increase the thickness of this section.

Injection Molding Defect 20: Warpage

Appearance of warpage

The shape of the product is rotated or twisted after the ejection or a period of time later. The typical performance is that the flat part of the product has undulations, and the straight edges are bent or twisted inward or outward.

Physical reason of warpage

Because of the product characteristic-the frozen molecular chain undergoes internal displacement under the action of stress. During ejection, depending on the shape of the product, the stress inside of the plastic product will often cause different degrees of deformation. The internal stress causes uneven shrinkage of the product, displacement of small particles, unbalanced cooling in the particles or excessive pressure in the particles. Especially products made of partially crystalline materials, such as PE, PP, and POM, are more likely to shrink and warp than non-crystalline materials such as PS, ABS, PMMA and PC.

Reasons and improvement measures related to injection molding parameters setting:

(1) The pressure inside of the mold is too high. Reduce the holding pressure and advance the holding pressure switch.

(2) Mold temperature is too low. Increase mold temperature.

(3) Fluid front, viscosity is too low. Increase injection speed.

(4) The melt temperature is too low. Increase the barrel temperature and the screw back pressure.

Reasons and improvement measures related to the mold design:

(1) Mold temperature is unstable. Provide a mold with cooling/heating controller equipment.

(2) Irregular cross-section thickness. Redesign the shape and size of the product according to the characteristics of the resin.

Injection Molding Defect 21: Ejector marks

Appearance of ejector marks

On the side of the product facing the nozzle, that is, where the ejector pin is located on the other side of the product, stress whitening and stress rise occurs.

Physical reason of ejector marks

If the necessary ejection force is too high or the surface of the ejector pin is relatively small, the surface pressure here will be very high, and deformation will eventually cause the ejection part to become white.

injection-molding-defect-ejector-marks

Reasons and improvement measures related to injection molding parameters setting:

(1) The holding pressure is too high. Reduce the holding pressure.

(2) The holding time is too long. Shorten the holding time.

(3) The holding pressure time switch is too late. Advance the holding pressure switch.

(4) The cooling time is too short. Extend the cooling time.

Reasons and improvement measures related to the mold design:

(1) The draft angle is not enough. Select the right draft angle according to the specifications.

(2) Rough surface in the ejection direction. Polish the mold in the demolding direction.

(3) A vacuum is formed on the ejection side. Fill the core with air.

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