Injection molding common defects and causes

Injection molding common defects and causes

1. Analysis of the causes of cracking of injection molded products

 

Cracking, including filamentous cracks, micro-cracks, top white, cracking, and trauma crisis caused by the sticking of parts and runners, according to the cracking time, it is divided into demoulding cracking and application cracking. There are mainly the following reasons:

 

1. Processing:

 

(1) If the processing pressure is too high, the speed is too fast, the more material is charged, and the injection and pressure holding time is too long, it will cause excessive internal stress and cracking.

 

(2) Adjust the speed and pressure of mold opening to prevent mold release and cracking caused by rapid forced drawing.

 

(3) Properly increase the mold temperature to make the part easy to demould, and appropriately lower the material temperature to prevent decomposition.

 

(4) Prevent cracking due to weld line and plastic degradation caused by lower mechanical strength.

 

(5) Appropriate use of mold release agent, pay attention to often eliminate the aerosol and other substances attached to the mold surface.

 

(6) The residual stress of the workpiece can be eliminated by annealing heat treatment immediately after forming to reduce the generation of cracks.

 

2. Mould:

 

(1) The ejection should be balanced. For example, the number of ejector pins and cross-sectional area should be sufficient, the demolding slope should be sufficient, and the cavity surface should be sufficiently smooth, so as to prevent cracking due to the concentration of residual stress in the ejection caused by external force.

 

(2) The structure of the workpiece should not be too thin, and the transition part should use arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.

 

(3) Use as little metal insert as possible to prevent the internal stress from increasing due to the difference in shrinkage between the insert and the part.

 

(4) Appropriate demoulding air inlet channels should be set for deep bottom parts to prevent the formation of vacuum negative pressure.

 

(5) The main channel is enough to make the gate material demould when it has time to cure, so that it is easy to demould.

 

(6) The sprue bushing should be joined to the nozzle to prevent the chilled material from being dragged and the part sticking to the fixed die.

 

3. Materials:

 

(1) The content of recycled materials is too high, resulting in low strength of the parts.

 

(2) The humidity is too high, causing some plastics to chemically react with water vapor, reducing the strength and causing ejection and cracking.

 

(3) The material itself is not suitable for the environment being processed or the quality is not good, and the pollution will cause cracking.

 

4. Machine side:

 

The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small, it will not be fully mixed and become brittle. If it is too large, it will degrade.

 

 

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2. Analysis of the causes of bubbles in injection molded products

 

The gas of the bubble (vacuum bubble) is very thin and belongs to the vacuum bubble. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubbles is due to insufficient filling of plastic or low pressure. Under the rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in volume loss.

 

Solution:

 

(1) Increase the injection energy: pressure, speed, time and material quantity, and increase the back pressure to make the filling full.

 

(2) Increase the material temperature and flow smoothly. Lower the material temperature to reduce shrinkage, and appropriately increase the mold temperature, especially the local mold temperature where the vacuum bubble is formed.

 

(3) Set the gate on the thick part of the part to improve the flow conditions of nozzles, runners and gates, and reduce the consumption of pressing services.

 

(4) Improve the exhaust condition of the mold.

 

3. Analysis of the Causes of Warpage and Deformation of Injection Molded Products

 

The phenomenon of deformation, bending and twisting of injection molded products is mainly due to the fact that the shrinkage rate in the flow direction is larger than that in the vertical direction during plastic molding, which makes the parts warp due to different shrinkage rates in each direction. Warpage is caused by the large internal stress remaining in the part, which are all manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, mold design determines the warpage tendency of parts. It is very difficult to suppress this tendency by changing molding conditions. The ultimate solution to the problem must start with mold design and improvement. This phenomenon is mainly caused by the following aspects:

 

1. Mould:

 

(1) The thickness and quality of the parts should be uniform.

 

(2) The design of the cooling system should make the temperature of each part of the mold cavity uniform, and the gating system should make the material flow symmetrical to avoid warping caused by different flow directions and shrinkage rates. It is necessary to eliminate the density difference, pressure difference and temperature difference in the cavity as much as possible.

 

(3) The transition area and corners of the thickness of the workpiece should be smooth enough, and have good demolding properties, such as increasing the demoulding allowance, improving the polishing of the die surface, and maintaining a balance in the ejection system.

 

(4) The exhaust should be good.

 

(5) Increase the wall thickness of the part or increase the direction of anti-warping, and enhance the anti-warping ability of the part by reinforcing ribs.

 

(6) The strength of the material used in the mold is insufficient.

 

2. On the plastic side:

 

Crystalline plastics have more warpage deformation opportunities than amorphous plastics. In addition, crystalline plastics can use the crystallization process that the crystallinity decreases with the increase of cooling rate and the shrinkage rate becomes smaller to correct warpage deformation.

 

3. Processing:

 

(1) The injection pressure is too high, the pressure holding time is too long, and the melt temperature is too low and the speed is too fast, which will increase the internal stress and cause warping deformation.

 

(2) The temperature of the mold is too high and the cooling time is too short, which makes the part overheated during demolding and ejection deformation occurs.

 

(3) Reduce the screw speed and back pressure to reduce the density to limit the generation of internal stress while maintaining the minimum charge.

 

(4) If necessary, the parts that are easy to warp and deform can be softly shaped or demolded and then removed.

 

 

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4. Analysis of color bar color line and color pattern of injection molding products

 

The occurrence of this defect is mainly a common problem of plastic parts colored with masterbatch, although masterbatch coloring is superior to dry powder coloring and dyeing paste in terms of color stability, color purity and color migration. Coloring, but the distribution, that is, the mixing uniformity of the color particles in the diluted plastic is relatively poor, and the finished product naturally has regional color differences.

 

Main solution:

 

(1) Increase the temperature of the feeding section, especially the temperature at the rear end of the feeding section, so that the temperature is close to or slightly higher than the temperature of the melting section, so that the masterbatch melts as soon as possible when it enters the melting section, promotes uniform mixing with dilution, and increases the chance of liquid mixing.

 

(2) Under the condition of constant screw speed, increasing the back pressure can improve the melt temperature and shearing effect in the barrel.

 

(3) Modify the mold, especially the gating system. If the gate is too wide, when the melt passes through, the turbulent flow effect is poor, and the temperature increase is not high, so it is not uniform, and the ribbon mold cavity should be narrowed.

 

5. Analysis of the causes of shrinkage and depression of injection molded products

 

During the injection molding process, the shrinkage of the product is a relatively common phenomenon. The main reasons for this are:

 

1. Machine side:

 

(1) If the nozzle hole is too large, the molten material will flow back and shrink, and if it is too small, the resistance will be too large and the amount of material will be insufficient.

 

(2) If the clamping force is insufficient, the flash will also shrink, so check whether there is any problem with the clamping system.

 

(3) If the amount of plasticization is insufficient, a machine with a large amount of plasticization should be selected to check whether the screw and barrel are worn.

 

2. Mould:

 

(1) The design of the parts should make the wall thickness uniform and ensure consistent shrinkage.

 

(2) The cooling and heating system of the mold should ensure that the temperature of each part is consistent.

 

(3) The gating system should be unobstructed and the resistance should not be too large. For example, the size of the main channel, the runner and the gate should be appropriate, the smoothness should be sufficient, and the transition area should be arc transition.

 

(4) The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be lowered for thick-walled parts.

 

(5) The gate should be opened symmetrically, try to open it in the thick-walled part of the part, and the volume of the cold well should be increased.

 

3. On the plastic side:

 

Crystalline plastics shrink more than non-crystalline plastics, and the amount of material should be appropriately increased during processing, or a change agent should be added to the plastic to speed up crystallization and reduce shrinkage sag.

 

4. Processing:

 

(1) The temperature of the barrel is too high and the volume changes greatly, especially the temperature of the front furnace. For the plastic with poor fluidity, the temperature should be appropriately increased to ensure smoothness.

 

(2) The injection pressure, speed, and back pressure are too low, and the injection time is too short, so that the material volume or density is insufficient and the shrinkage pressure, speed, and back pressure are too large, and the time is too long, causing flash and shrinkage.

 

(3) The feeding amount, that is, when the buffer pad is too large, consumes the injection pressure, and when it is too small, the feeding amount is insufficient.

 

(4) For parts that do not require precision, after injection and pressure keeping, the outer layer is basically condensed and hardened, and the sandwich part is still soft and can be ejected, and the mold is released as soon as possible, so that it can be slowly cooled in air or hot water. , which can make the shrinkage depression smooth and less conspicuous without affecting the use.

 

 

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6. Cause analysis of transparent defects of injection molded products

 

Fusion spots, silver streaks, cracked polystyrene, transparent parts of plexiglass, and sometimes shimmering filaments of silver streaks can be seen through the light. These silver streaks are also known as sparkles or cracks. This is due to the stress generated in the vertical direction of the tensile stress, and the polymer molecules of the right use have a heavy flow orientation and show a difference in the completion rate of the unoriented part.

 

Solution:

 

(1) Eliminate the interference of gas and other impurities, and fully dry the plastic.

 

(2) Reduce the material temperature, adjust the barrel temperature in stages, and appropriately increase the mold temperature.

 

(3) Increase the injection pressure and reduce the injection speed.

 

(4) Increase or decrease the pre-plastic back pressure and reduce the screw speed.

 

(5) Improve the exhaust condition of the runner and cavity.

 

(6) Clean up possible blockage of nozzles, runners and gates.

 

(7) Shorten the molding cycle. After demolding, annealing can be used to eliminate silver streaks: for polystyrene, keep it at 78 °C for 15 minutes, or at 50 °C for 1 hour, and for polycarbonate, heat it to above 160 °C for several minutes. .

 

7. Analysis of the causes of uneven color of injection molded products

 

The main reasons and solutions for the uneven color of injection molded products are as follows:

 

(1) Poor diffusion of colorants, which often causes patterns to appear near the gate.

 

(2) The thermal stability of plastics or colorants is poor. To stabilize the color tone of the parts, the production conditions must be strictly fixed, especially the material temperature, material volume and production cycle.

 

(3) For crystalline plastics, try to make the cooling rate of each part of the part consistent. For parts with large wall thickness differences, colorants can be used to mask the color difference. For parts with uniform wall thickness, the material temperature and mold temperature should be fixed. .

 

(4) The shape and gate form and position of the part have an impact on the filling of the plastic, causing chromatic aberration in some parts of the part, which should be modified if necessary.

 

 

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8. Cause analysis of color and gloss defects of injection molded products

 

Under normal circumstances, the gloss on the surface of injection molded parts is mainly determined by the type of plastic, colorant and the finish of the mold surface. However, defects such as surface color and gloss defects, and dark surface color of products are often caused by some other reasons.

 

The reasons and solutions for this are as follows:

 

(1) The finish of the mold is poor, the surface of the cavity has rust, etc., and the mold exhaust is not good.

 

(2) The gating system of the mold is defective, the cold slug well should be enlarged, the runner, the polished main runner, the runner and the gate should be enlarged.

 

(3) The material temperature and mold temperature are low, if necessary, the local heating method of the gate can be used.

 

(4) The processing pressure is too low, the speed is too slow, the injection time is insufficient, and the back pressure is insufficient, resulting in poor compactness and dark surface.

 

(5) The plastic should be fully plasticized, but the degradation of the material should be prevented, the heating should be stable, and the cooling should be sufficient, especially for thick-walled ones.

 

(6) Prevent cold material from entering the workpiece, and use self-locking springs or reduce nozzle temperature if necessary.

 

(7) Too many recycled materials are used, the quality of plastics or colorants is poor, mixed with water vapor or other impurities, and the quality of lubricants used is poor.

 

(8) The clamping force should be sufficient.

 

9. Analysis of the reasons for silver streaks in injection molded products

 

Silver streaks on injection molded products, including surface air bubbles and internal pores. The main cause of defects is the interference of gases (mainly water vapor, decomposition gas, solvent gas, air). The specific reasons are analyzed as follows:

 

1. Machine aspect:

 

(1) There is a dead angle in the material flow due to the wear of the barrel and the screw or the rubber head and the rubber ring, which will be decomposed by heating for a long time.

 

(2) The heating system is out of control, causing the temperature to be too high and decomposed. Check whether there are any problems with heating elements such as thermocouples and heating coils. Improper screw design, resulting in a solution or easy to bring in air.

 

2. Mould:

 

(1) Poor exhaust.

 

(2) The friction resistance of the runner, gate and cavity in the mold is large, resulting in local overheating and decomposition.

 

(3) Unbalanced distribution of gates and cavities, and unreasonable cooling system will cause unbalanced heating and local overheating or block air passages.

 

(4) The cooling passage leaks water into the cavity.

 

3. On the plastic side:

 

(1) The humidity of the plastic is high, the proportion of recycled materials is too much or it contains harmful scraps (the scraps are easily decomposed), the plastics should be fully dried and the scraps should be eliminated.

 

(2) To absorb moisture from the atmosphere or from the colorant, the colorant should also be dried, and it is best to install a dryer on the machine.

 

(3) The amount of lubricants, stabilizers, etc. added to the plastic is too much or the mixing is uneven, or the plastic itself has volatile solvents. Decomposition of mixed plastics can also occur when the degree of heating is difficult to take into account.

 

(4) The plastic is contaminated and mixed with other plastics.

 

4. Processing:

 

(1) When the temperature, pressure, speed, back pressure, and melting motor speed are too high to cause decomposition, or the pressure and speed are too low, the injection time, the holding pressure are not sufficient, and the back pressure is too low, the density will increase due to the failure to obtain high pressure. Insufficient to melt the gas and appear silver streaks, should set appropriate temperature, pressure, speed and time and use multi-stage injection speed

 

(2) Low back pressure and fast rotation speed make it easy for air to enter the barrel and enter the mold with the molten material. When the cycle is too long, the molten material is heated for too long in the barrel and decomposes.

 

(3) Insufficient amount of material, too large feeding buffer, too low material temperature or too low mold temperature will affect the flow and molding pressure of the material, and promote the formation of bubbles.

 

 

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10. Analysis of the causes of welded seams in plastic products

 

When the molten plastic meets in the form of multiple strands in the cavity due to encountering insert holes, areas where the flow rate is discontinuous, and areas where the filling material flow is interrupted, a linear weld seam is produced because it cannot be completely fused. In addition, when the gate injection molding occurs, a weld seam will also be formed, and the strength and other properties of the weld seam are poor. The main reasons are as follows:

 

1. Processing:

 

(1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are too low, resulting in the premature cooling of the melt entering the mold and the appearance of weld seams.

 

(2) When the injection pressure and speed are too high, injection will occur and weld seams will appear.

 

(3) The speed should be increased, and the back pressure should be increased to reduce the viscosity of the plastic and increase the density.

 

(4) The plastic should be dry well, and the recycled material should be used less. If the amount of release agent is too much or the quality is not good, weld seams will appear.

 

(5) Reduce the clamping force to facilitate exhaust.

 

2. Mould:

 

(1) There are too many gates in the same cavity, and the gates should be reduced or set symmetrically, or set as close as possible to the weld seam.

 

(2) The exhaust at the weld seam is poor, and an exhaust system should be set up.

 

(3) The sprue is too large, the size of the gating system is improper, and the gate is opened to avoid the flow of the melt around the insert hole, or use as little insert as possible.

 

(4) If the wall thickness changes too much, or the wall thickness is too thin, the wall thickness of the parts should be uniform.

 

(5) If necessary, a fusion well should be opened at the fusion seam so that the fusion seam can be separated from the workpiece.

 

3. On the plastic side:

 

(1) Lubricants and stabilizers should be appropriately added to plastics with poor fluidity or heat sensitivity.

 

(2) Plastics contain a lot of impurities, and if necessary, replace plastics with good quality.

 

11. Analysis of the causes of vibration pattern of injection molded products

 

Rigid plastic parts such as PS form dense corrugations centered on the gate on the surface near the gate, sometimes called vibration pattern. The reason is When the melt viscosity is too large and the mold is filled in the form of stagnation, the material at the front end will condense and shrink as soon as it touches the surface of the cavity, and the subsequent melt expands the shrunk cold material and continues to move forward. The material flow forms a surface shock pattern as it advances.

 

Solution:

 

(1) To increase the barrel temperature, especially the nozzle temperature, the mold temperature should also be increased.

 

(2) Increase the injection pressure and speed to quickly fill the mold cavity.

 

(3) Improve the size of the runner and gate to prevent excessive resistance.

 

(4) The exhaust of the mold should be good, and a large enough cold slug well should be set up.

 

(5) The parts should not be designed too thin.

 

12. Analysis of the causes of swelling and bubbling of injection molded products

 

Some plastic parts show swelling or blisters on the backside of metal inserts or in particularly thick areas soon after mold release. This is caused by the expansion of the released gas under the action of the internal pressure valve of the plastic that has not been completely cooled and hardened.

 

solution:

 

1. effective cooling. Reduce the mold temperature, prolong the mold opening time, and reduce the drying and processing temperature of the material.

 

2. Reduce the filling speed, reduce the forming cycle, and reduce the flow resistance.

 

3. Increase the holding pressure and time.

 

4. Improve the condition that the wall of the workpiece is too thick or the thickness changes greatly


Post time: Jul-30-2022