Interpret the causes of injection mold problems​

 Analysis of the causes of cracking of injection molded products
  , including filamentous cracks, micro-cracks, top whitening, cracking on the surface of the part, and trauma crises caused by the mold sticking of the part and the flow channel. According to the cracking time, demoulding cracking and application cracking are divided . Mainly due to the following reasons:
  
1. Processing:
  (1) Too much processing pressure, too fast speed, too much filling, injection, and too long pressure holding time will cause excessive internal stress and cracking.
  (2) Adjust the mold opening speed and pressure to prevent demolding and cracking caused by rapid forced drawing of parts.
  (3) Increase the mold temperature appropriately to make the parts easy to demould, and lower the material temperature appropriately to prevent decomposition.
  (4) Prevent cracking due to lower mechanical strength due to welding marks and plastic degradation.
  (5) Use release agents appropriately and pay attention to frequently removing aerosol and other substances attached to the mold surface.
  (6) The residual stress of the part can be eliminated by annealing heat treatment immediately after molding to reduce the generation of cracks.
  
2. Mold aspects:
  (1) Ejection must be balanced. For example, the number of ejector pins and cross-sectional area must be sufficient, the draft angle must be sufficient, and the cavity surface must be smooth enough to prevent cracking due to the concentration of residual stress in ejection caused by external forces.
  (2) The structure of the parts should not be too thin, and arc transitions should be used as much as possible in the transition parts to avoid stress concentration caused by sharp corners and chamfers.
  (3) Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between the insert and the finished product.
  (4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent the formation of vacuum negative pressure.
  (5) The sprue is large enough to allow the gate material to be demoulded before it has time to solidify, making it easy to demould.
  (6) The connection between the main flow bushing and the nozzle should prevent the cold hard material from being dragged and causing the part to stick to the fixed mold.
 
3． In terms of materials:
  (1) The content of recycled materials is too high, resulting in low strength of the parts.
  (2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing their strength and causing ejection cracking.
  (3) The material itself is not suitable for the environment being processed or is of poor quality. Contamination will cause cracking.
  
4. Regarding the machine:
  The plasticizing capacity of the injection molding machine must be appropriate. If it is too small, the plasticization will be insufficient and it will not be fully mixed and become brittle. If it is too large, it will degrade.
  Analysis of the Causes of Bubbles in Injection Molded Products
  The gas in bubbles (vacuum bubbles) is very thin and belongs to vacuum bubbles. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. Vacuum bubbles are formed due to insufficient filling of the plastic or low pressure. Under the rapid cooling of the mold, the fuel in contact with the cavity is pulled, resulting in volume loss.
  Solution:
  (1) Increase the injection energy: pressure, speed, time and material amount, and increase the back pressure to make the mold full.
  (2) Increase the material temperature and smooth the flow. Lower the material temperature to reduce shrinkage, and increase the mold temperature appropriately, especially the local mold temperature where the vacuum bubble is formed.
  (3) Set the gate in the thick part of the part to improve the flow conditions of the nozzle, runner and gate, and reduce the consumption of pressure.
  (4) Improve the mold exhaust condition.

Analysis of the Causes of Warpage Deformation of Injection Molded
  Products The occurrence of deformation, bending, and twisting of injection molded products is mainly due to the fact that the shrinkage rate in the flow direction is greater than that in the vertical direction during plastic molding, causing the parts to warp due to different shrinkage rates in each direction, and due to the injection molding process. During mold filling, it is inevitable that large internal stress will remain inside the part, causing warping. These are all manifestations of deformation caused by high stress orientation. So fundamentally speaking, mold design determines the warping tendency of parts. It is very difficult to suppress this tendency by changing molding conditions. The final solution to the problem must start with mold design and improvement. This phenomenon is mainly caused by the following aspects:  

1. Mold aspects:
  (1) The thickness and quality of the parts must be uniform.
  (2) The cooling system should be designed to make the temperature of all parts of the mold cavity uniform. The pouring system should make the material flow symmetrical to avoid warping due to different flow directions and shrinkage rates. Appropriately thicken the shunt channels and main flow of the difficult-to-form parts. Road, try to eliminate the density difference, pressure difference and temperature difference in the cavity.
  (3) The transition area and corners of the thickness of the part must be smooth enough and have good demoulding properties, such as increasing the demoulding margin, improving the polishing of the mold surface, and the ejection system must be balanced.
  (4) The exhaust should be good.
  (5) Increase the wall thickness of the part or increase the anti-warping direction, and use reinforcing ribs to enhance the anti-warping ability of the part.
  (6) The material used in the mold has insufficient strength.
  
2. In terms of plastics:
  Crystalline plastics have more chances of warping deformation than amorphous plastics. In addition, crystalline plastics can correct warping deformation by using the crystallization process that decreases as the cooling rate increases and the shrinkage rate decreases.
  
3． Processing:
  (1) The injection pressure is too high, the holding time is too long, the melt temperature is too low and the speed is too fast, which will cause increased internal stress and warping deformation.
  (2) The mold temperature is too high and the cooling time is too short, causing the parts to be overheated during demoulding and cause ejection deformation.
  (3) Reduce the screw speed and back pressure to reduce the density while maintaining a lower minimum charging amount to limit the generation of internal stress.
  (4) If necessary, parts that are prone to warping and deformation can be soft-shaped or de-molded after demoulding.
  Analysis of color bars, color lines, and flowers in injection molded products.
  The occurrence of this defect is mainly a common problem in plastic parts colored with masterbatch. Although masterbatch coloring has important aspects such as color stability, color purity, and color migration. In all aspects, it is better than dry powder coloring and dye paste coloring, but the distribution, that is, the uniformity of mixing of color particles in diluted plastics is relatively poor, and the finished products naturally have regional color differences.

Main solutions:

  (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 color masterbatch can melt as soon as possible when entering the melting section, promote uniform mixing with dilution, and increase the opportunity for liquid mixing.
  (2) When the screw speed is constant, increasing the back pressure will increase the melt temperature and shearing effect in the barrel.
  (3) Modify the mold, especially the pouring system. If the gate is too wide, the turbulence effect is poor when the melt passes through, and the temperature rise is not high, so it is uneven and the ribbon mold cavity should be narrowed.

  Analysis of the causes of shrinkage and dents in injection molded products
  During the injection molding process, shrinkage and dents in products are a relatively common phenomenon. The main reasons for this situation are:
  
1. Regarding the machine:
  (1) If the nozzle hole is too large, the melt will flow back and cause shrinkage; if it is too small, the resistance will be high and the amount of material will be insufficient, causing shrinkage.
  (2) Insufficient clamping force will cause shrinkage of flash edges. Check whether there is any problem with the clamping system.
  (3) If the plasticizing amount is insufficient, a machine with a large plasticizing amount should be selected, and the screw and barrel should be checked for wear.
  
2. Mold aspects:
  (1) Part design should make the wall thickness uniform and ensure consistent shrinkage.
  (2) The cooling and heating system of the mold must ensure that the temperature of each part is consistent.
  (3) The pouring system must be smooth and the resistance must not be too large. For example, the size of the main channel, runner, and gate must be appropriate, the smoothness must be sufficient, and the transition area must be an arc transition.
  (4) For thin parts, the temperature should be increased to ensure smooth material flow, and for thick-walled parts, the mold temperature should be lowered.
  (5) The gate should be opened symmetrically, try to open it in the thick-walled part of the part, and the cold slug well volume should be increased.
  
3． In terms of plastics:
  Crystalline plastics shrink more than amorphous plastics. During processing, the amount of material should be appropriately increased, or a replacement agent should be added to the plastic to speed up crystallization and reduce shrinkage dents.
  
4. Processing:
  (1) The temperature of the barrel is too high and the volume changes greatly, especially the temperature of the front furnace. For plastics 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, resulting in insufficient material volume or density and shrinkage pressure, speed, and back pressure that are too high, and the time is too long, causing flash and shrinkage.
  (3) When the amount of material added is too large, the injection pressure will be consumed; if it is too small, the amount of material will be insufficient.
  (4) For parts that do not require precision, after the injection and pressure-holding is completed, the outer layer is basically condensed and hardened but the sandwich part is still soft and can be ejected. Eject the mold as soon as possible and let it cool slowly in air or hot water. , can make shrinkage dents gentle and less conspicuous without affecting use.

  Analysis of the causes of transparent defects in injection molded products:
  Melting spots, silver streaks, cracked polystyrene, and plexiglass transparent parts. Sometimes some sparkling filament-like silver streaks can be seen through the light. These silver streaks are also called flash spots or cracks. This is due to the stress generated in the vertical direction of the tensile stress. The polymer molecules undergo heavy flow orientation and the folding rate difference between the non-oriented parts is reflected.
  Solution:
  (1) Eliminate the interference of gas and other impurities, and fully dry the plastic.
  (2) Reduce the material temperature, adjust the barrel temperature step by step, and increase the mold temperature appropriately.
  (3) Increase injection pressure and reduce injection speed.
  (4) Increase or decrease the pre-molding back pressure and reduce the screw speed.
  (5) Improve the exhaust conditions of the runner and cavity.
  (6) Clean possible blockages in the nozzle, runner and gate.
  (7) Shorten the molding cycle. After demoulding, annealing can be used to eliminate silver streaks: for polystyrene, hold at 78°C for 15 minutes, or at 50°C for 1 hour. For polycarbonate, heat to above 160°C for several minutes. .
  Analysis of the causes of uneven color of injection molded products
  The main causes and solutions of uneven color of injection molded products are as follows:
  (1) Poor diffusion of colorant, which often causes patterns to appear near the gate.
  (2) Plastics or colorants have poor thermal stability. To stabilize the color tone of parts, production conditions must be strictly fixed, especially material temperature, material quantity and production cycle.
  (3) For crystalline plastics, try to keep the cooling rate of all parts of the part consistent. For parts with large wall thickness differences, colorants can be used to mask the color difference. For parts with more uniform wall thicknesses, the material temperature and mold temperature must be fixed. .
  (4) The shape and gate form and position of the part have an impact on the plastic filling situation, causing color differences in some parts of the part, and must be modified if necessary.
  Analysis of the Causes of Color and Gloss Defects of Injection Molded Products
  Under normal circumstances, the gloss of the surface of injection molded products is mainly determined by the type of plastic, colorant and the smoothness of the mold surface. However, defects such as surface color and gloss defects, surface darkness, etc. are often caused by some other reasons. The causes and solutions are analyzed as follows:
  (1) The mold finish is poor, there are rust stains on the surface of the cavity, etc., and the mold exhaust is poor.
  (2) The pouring system of the mold is defective. The cold slug well should be enlarged, the runner should be enlarged, the main channel should be polished, the runner and the gate should be polished.
  (3) The material temperature and mold temperature are low. If necessary, local heating 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 must be fully plasticized, but the degradation of the material must be prevented, the heating must be stable, and the cooling must be sufficient, especially for thick walls.
  (6) To prevent cold material from entering the workpiece, use a self-locking spring or reduce the nozzle temperature if necessary.
  (7) Too much recycled material is used, the plastic or colorant is of poor quality, mixed with water vapor or other impurities, and the lubricant used is of poor quality.
  (8) The clamping force must be sufficient.
  
Analysis of the causes of silver streaks in injection molded products
  Silver streaks in injection molded products include surface 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:
  1. Machine aspect:
  (1) The barrel and screw are worn or there is a dead corner in the material flow in the rubber head and rubber ring, which will decompose due to long-term heating.
  (2) The heating system is out of control, causing the temperature to be too high and decomposing. Check whether there are problems with heating elements such as thermocouples and heating coils. Improper screw design may cause loosening or easily introduce air.
  2. Mold aspects:
  (1) Poor exhaust.
  (2) The friction resistance of the runners, gates, and cavities in the mold is large, causing local overheating and decomposition.
  (3) Unbalanced distribution of gates and mold cavities, and unreasonable cooling systems will cause unbalanced heating, resulting in local overheating or blocking of air channels.
  (4) Water leaks from the cooling passage and enters the mold cavity.
  3． In terms of plastics:
  (1) Plastics have high humidity, add too much recycled material or contain harmful debris (the debris is easily decomposed), the plastic should be fully dried and the debris should be eliminated.
  (2) If moisture is absorbed 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 excessive or unevenly mixed, or the plastic itself contains volatile solvents. Decomposition can also occur when mixed plastics are heated to a difficult balance.
  (4) The plastic is contaminated and mixed with other plastics.
  4. In terms of processing:
  (1) If the temperature, pressure, speed, back pressure, and melt motor speed are set too high, decomposition may occur, or if the pressure and speed are too low, the injection time, holding pressure are insufficient, or the back pressure is too low, failure to obtain High pressure and insufficient density cannot melt the gas and cause silver streaks. 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 can easily cause air to enter the barrel and enter the mold with the molten material. If the cycle is too long, the melt will be heated in the barrel for too long and decompose.
  (3) Insufficient amount of material, too large feeding buffer, too low material temperature or too low mold temperature will affect the flow of material and molding pressure, and promote the generation of bubbles.
  
Analysis of the reasons why plastic products have weld seams.
  When molten plastic merges in multiple strands in the mold cavity due to encountering insert holes, areas with incoherent flow rates, and areas with interrupted filling material flow, linear seams will occur because they cannot be completely fused. Welded seams. In addition, welding seams will also be generated when gate injection mold filling occurs, and the strength and other properties of the welding seams are very poor. The main reasons are analyzed as follows:
  1. Processing:
  (1) The injection pressure and speed are too low, and the barrel temperature and mold temperature are too low, causing the melt entering the mold to cool prematurely and cause welding seams.
  (2) When the injection pressure and speed are too high, spraying will occur and welding seams will appear.
  (3) The rotation speed should be increased and the back pressure should be increased to reduce the viscosity of the plastic and increase the density.
  (4) Plastics should be dried well and recycled materials should be used sparingly. Welding seams will occur if too much release agent is used or if the quality is poor.
  (5) Reduce the clamping force to facilitate exhaust.
  2. Mold aspects:
  (1) If there are too many gates in the same cavity, the gates should be reduced or set symmetrically, or set as close to the weld seam as possible.
  (2) If the exhaust at the weld joint is poor, an exhaust system should be installed.
  (3) The sprue is too large and the gating system is improperly sized. The gate should be opened to avoid melt flow around the holes of the insert, 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 part should be made uniform.
  (5) If necessary, a fusion well should be opened at the weld seam to separate the weld seam from the workpiece.
  3． Plastics:
  (1) Lubricants and stabilizers should be added appropriately to plastics with poor fluidity or heat sensitivity.
  (2) Plastic contains a lot of impurities. If necessary, replace it with better quality plastic.
  Analysis of the causes of chatter marks in injection molded products:
  On the surface of rigid plastic parts such as PS near the gate, dense ripples are formed centered on the gate, sometimes called chatter marks. The reason is that when the melt viscosity is too high and the mold is filled in the form of stagnation, the front material quickly condenses and shrinks as soon as it contacts the surface of the cavity, and the subsequent melt expands the shrunk cold material and continues the process. The continuous alternation causes the material flow to form surface vibration patterns as it advances.
  Solution:
  (1) Increase the temperature of the barrel, especially the nozzle, and the mold temperature.
  (2) Increase the injection pressure and speed to quickly fill the mold cavity.
  (3) Improve the size of the flow channel and gate to prevent excessive resistance.
  (4) The mold must be well vented and a large enough cold slug well must be installed.
  (5) Do not design the parts too thin.

Analysis of the Causes of Swelling and Bubbling in Injection Molded Products
  Some plastic parts will soon have swelling or bubbling on the back of the metal insert or in particularly thick parts after being molded and demoulded. This is caused by the expansion of gas released by the plastic that is not completely cooled and hardened under the action of internal pressure.
  Solutions:
  1. Effective cooling. Reduce the mold temperature, extend the mold opening time, and reduce the drying and processing temperature of the material.
  2. Reduce mold filling speed, shorten molding cycle, and reduce flow resistance.
  3． Increase holding pressure and time.
  4. Improve the situation where the wall surface of the part is too thick or the thickness changes greatly.