12 kinds of defects and solutions of Injection Mold

1. The Analysis of shrinkage and depression for Injection mold Plastic products.

The shrinkage and depression of the plastic products will be encountered In the process of injection mold.  The main reasons can be analyzed from the following points.

*Injection Mold machine aspect

1. Please check if the nozzle hole is the right size.

2. Please check if there is any problem with the clamping system.

3. Please check whether the screw and barrel are worn.

*Injection Mold aspect

1. The design of the injection mold parts should make the wall thickness uniform to ensure consistent shrinkage.

2. The cooling and heating system of the Injection Mold should ensure that the temperature of each part is consistent.

3. The pouring column system should be smooth and the resistance should not be too large, such as the main channel, the runner, and the size of the gate should be appropriate to ensure that the smoothness is sufficient, and the transition area should be smooth and excessive.

4. The temperature should be increased for thin parts to ensure a smooth flow of material, 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.

*Plastic Materials aspect

Crystalline plastics shrink more than non-crystalline plastics. 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 depressions.

*Processing

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

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 the flash to shrink.

3. The feeding amount means that the injection pressure will be consumed when the buffer pad is too large. After a few hours, the feeding amount will be insufficient.

4. For parts that do not require precision, after the injection and pressure keeping are completed, the outer layer is condensed and hardened, 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 so that the It cools slowly in the air or hot water to make shrinkage dents smoother and less noticeable without affecting use.

2. The Analysis of the reasons for the vibration pattern of plastic parts.

The reason is that the viscosity of the solution is too large and the mold is filled in the form of stagnation. As soon as the front part contacts the surface of the cavity, it condenses and shrinks quickly, and the molten material expands the shrunk cold material and continues to move forward.

*The Solutions are as follows

1. Increase the temperature of the Barrel, Nozzle, and Mould.

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

3. Improve runner and gate size to prevent excessive resistance.

4. The exhaust of the mold should be great and set the large enough cold.

5. The mold components aren’t designed too thin.

6. Increase the temperature of the Barrel, Nozzle, and Mould.

7. Increase the injection pressure and speed to quickly fill the mold cavity.

8. Improve runner and gate size to prevent excessive resistance.

9. The exhaust of the mold should be great and set large enough cold.

10. The mold components aren’t designed too thin.

3. The reasons analyze for swelling of Injection Mold parts

After some plastic parts are molded and remolded, swelling or blisters appear on the back of the metal insert or in a particularly thick part soon. This is because the plastic that is not completely cooled and hardened releases gas and expands under the action of the internal pressure penalty.

*The Solutions are as follows

1. Effective cooling, lower injection mold temperature, extend mold opening time, reduce material drying and processing temperature.

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.

4. The cracked reasons for Injection parts

*Processing aspect

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

2. Adjust the mold opening speed and pressure to prevent molding cracking caused by rapidly forced drawing parts.

3. Properly increase the injection mold temperature to make the part easy to de-mold, and appropriately lower the material temperature to prevent decomposition.

4. Prevent cracking due to weld lines, and plastic degradation caused by lower mechanical strength.

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

6. The residual stress of the workpiece can be treated by annealing heat after forming to eliminate the internal stress and reduce the generation of cracks.

*Injection Mold aspect

1. The ejection should be balanced, such as the number of ejectors, the cross-sectional area, and the drafting slope should be sufficient, and the cavity surface should be sufficiently smooth, to prevent cracking due to the concentration of residual stress in the ejection caused by an 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 sharp corners and chamfers causing stress concentration.

3. Minimize the use of metal inserts to prevent internal stress concentration caused by different shrinkage rates of inserts and parts.

4. For deep-bottomed parts, an appropriate de-molding channel should be set to prevent the formation of vacuum negative pressure.

5. The main channel is enough to make the gate material injection mold when it has time to cure, so it is easy to de-mold.

6. The joint between the sprue bushing and the nozzle should prevent the hard material from being dragged and make the part stick to the fixed die.

*Materials aspect

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.

*Injection Mold Machine aspect

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

5. The reasons for analysis of bubbles for Injection parts

If it is found that there are bubbles in the product when the mold is opened, this is a bubble interference problem. The reason is that due to insufficient plastic injection or low pressure, under the cooling effect of the mold, the fuel at the corner of the cavity is held in place. result in volume loss.

*The Solutions are as follows

1. Increase the Injection energy, press, speed, time, quantity, and back pressure, to make the filling mold plump.

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

3. Setting the gate in the thick part of the part to improve the flow conditions of nozzles, runners, and gates, and reduce the consumption of pressing services.

4. Improving the situation of mold exhaust.

6. Warpage of injection mold parts

The main reason for the deformation, bending, and twisting of plastic products is that the shrinkage rate of the plastic in the flow direction is larger than that in the vertical direction during molding, which makes the parts warp due to different shrinkage rates. Warpage is caused by large internal stress remaining inside the part. Therefore, the design of the mold determines the warpage tendency.

*Injection Mold aspect

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 warpage caused by different flow directions and shrinkage rates, try to eliminate the density difference, pressure difference, and temperature difference in the injection mold cavity.

3. The transition area and corners of the thickness of the part should be smooth enough and have good demolding properties, such as increasing the de-molding 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 the anti-warping, and the anti-warping ability of the part is enhanced by the reinforcing ribs.

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

*The Plastic aspect

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

*Processing aspect

1.If the injection pressure is too high, the holding time is too long, the melt temperature is too low and the speed is too fast, the internal stress will increase and warp deformation will occur.

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

3.Reducing the screw speed and back pressure while keeping the charge to a minimum reduces the density to limit the generation of internal stress.

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

7. Analysis of color stripe, color line, and color pattern of  Injection mold parts

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 color particles are relatively poor in the degree of mixing evenly when diluting the plastic, and the finished product naturally has regional color differences.

*The Solutions are as follows

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.

8. The reason for analysis of Transparent Defects in Injection Molding Parts

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.

*The Solutions are as follows

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 injection 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 conditions of the runner and cavity.

6. Clean up the 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.

9. The reason analysis of color uniform in Injection mold parts

1. Poor colorant diffusion, which tends to create patterns 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 part consistent. For parts with large differences in wall thickness, 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.

10. The reason for analysis of Color and Gloss Defect

Under normal situations, the gloss on the surface of injection mold parts is mainly determined by the type of plastic, colorant, and 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 for this and the solutions are as follows.

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

2. The gating system of the injection mold is defective, and the cold slug well should be enlarged, and the runner, polished main runner, runner, and 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 a 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 the cold material from entering the part, and use a self-locking spring or reduce the 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.

11. The reason for analysis of Silver Streaks in Injection Mold Parts

Silver streaks on injection mold 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, and air). The specific reasons are analyzed as follows.

*Injection Mold 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 decomposing. 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.

*Injection Mold aspect

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 an unreasonable cooling system will cause unbalanced heating and local overheating or block air passages.

4. The cooling passage leaks water into the cavity.

*Plastic aspect

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 the colorant, the colorant should also be dried, and it is best to install a dryer on the machine.

3. The number 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.

*The processing aspect

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 and the holding pressure are not sufficient, and the back pressure is too low, the density cannot be melted because the high pressure cannot be obtained. If silver streaks appear in the gas, appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.

2. Low back pressure and fast rotation speed make it easy for the air to enter the barrel and enter the mold with the molten material. When the cycle is too long, the molten material is heated in the barrel for too long 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 injection mold pressure of the material, and promote the formation of bubbles.

12. Analysis of the causes of welded seams in injection mold parts

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 the follows

*Processing aspect

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 runner 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 few inserts 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 to separate the fusion seam from the workpiece.

*Plastic Aspect

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

2. Plastics contain many impurities, and if necessary, replace plastics with good quality.