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Analysis of the Problems and Causes of Injection Mould

2022-08-23 Hits: 8 views

Cause Analysis of Cracking in Injection Molding Products
MINGYU MOLD company is a professional injection mold manufacturer in China. With more than 25 years experience on Automobile mold , home appliance mold, packing mold, electrical parts mold and precision mold ,etc. The following is our analysis of the problems and causes of injection molds
Cracking, including filamentary crack, micro-crack, top white, cracking on the surface of the part, or trauma crisis caused by mold sticking of the part and mold sticking of the runner, can be divided into demoulding cracking and application cracking according to the cracking time. There are mainly the following reasons:
. Processing:
(1) Excessive processing pressure, too fast speed, more filling, too long injection and pressure holding time will cause excessive internal stress and cracking.
(2) Adjust the mold opening speed and pressure to prevent the rapid and strong drawing parts from causing demolding cracking.
(3) Increase the mold temperature appropriately to make the product easy to demould, and decrease the material temperature appropriately to prevent decomposition.
(4) Prevent cracking due to lower mechanical strength caused by weld line and plastic degradation.
(5) Appropriate use of release agent, pay attention to often eliminate the 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 forming, and the generation of cracks can be reduced.
2. Mould:
(1) The ejection should be balanced, such as the number and cross-sectional area of ejector pins should be enough, the draft angle should be enough, and the cavity surface should be smooth enough, so as to prevent the ejection residual stress from concentrating and cracking due to external forces.
(2) The structure of the part should not be too thin, and the transition part should adopt arc transition as far as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Use metal inserts as little as possible, so as to prevent the internal stress from increasing due to different shrinkage rates of inserts and products.
(4) Appropriate demoulding air inlet should be set for deep-bottom parts to prevent the formation of vacuum negative pressure.
(5) The main runner is large enough to demould the gate material before it solidifies, so that demoulding is easy.
(6) The joint between the main runner bushing and the nozzle should prevent the product from sticking to the fixed mold due to the drag of the cold hard material.
3. Materials:
(1) The recycled material content is too high, resulting in low strength of the finished product.
(2) The humidity is too high, which causes some plastics to react with water vapor, reducing the strength and causing ejection cracking.
(3) If the material itself is not suitable for the environment being processed or the quality is poor, it will crack if it is polluted.
4. Machine:
The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small, the plasticizing capacity will not be fully mixed and it will become brittle. If it is too large, it will degrade.
Cause Analysis of Bubbles in Injection Molded Products
Bubbles (vacuum bubbles) are very thin and belong to vacuum bubbles. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubble is due to insufficient filling of plastic or low pressure. Under the rapid cooling of the die, the fuel at the corner of the die cavity pulls, resulting in volume loss.
Solution:
(1) Increase the injection energy: pressure, speed, time and material quantity, and increase the back pressure to make the mold full.
(2) Increase the material temperature and flow smoothly. Lower the material temperature, reduce the shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at the part where the vacuum bubble is formed.
(3) Set the gate in the thick part of the part, improve the flow condition of the nozzle, runner and gate, and reduce the pressure consumption.
(4) Improve the exhaust condition of the die.
Analysis of color stripe, color line and color pattern of injection molding products
This kind of defect is mainly caused by the common problems of plastic products colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye-paste coloring in color stability, color purity and color migration, the distributivity, that is, the mixing uniformity of color particles in diluted plastics is relatively poor, and the finished products naturally have 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 its temperature is close to or slightly higher than that of the melting section, so that the color masterbatch can be melted as soon as possible when it enters the melting section, thus promoting uniform mixing with dilution and increasing the chance of liquid mixing.
(2) When the screw speed is constant, increasing the back pressure can improve the melt temperature and shearing action in the barrel.
(3) Modify the mold, especially the gating system. If the gate is too wide and the molten material passes, the turbulence effect is poor and the temperature rise is not high, so it is uneven, and the ribbon mold cavity should be narrowed.
Cause Analysis of Shrinkage Depression of Injection Molded Products
In the process of injection molding, shrinkage and depression of products is a common phenomenon. The main reasons for this are:
1. machine:
(1) The nozzle hole is too big, which causes the melt to flow back and shrink; when it is too small, the resistance is too large, and the amount of material is insufficient, which leads to shrinkage.
(2) If the clamping force is insufficient, the flash will shrink. Check whether there is any problem with the clamping system.
(3) If the plasticizing amount is insufficient, the machine with large plasticizing amount should be selected, and whether the screw and the barrel are worn or not should be checked.
2. Mould:
(1) The product design should make the wall thickness uniform and ensure the consistent shrinkage.
(4) Appropriate demoulding air inlet should be set for deep-bottom parts to prevent the formation of vacuum negative pressure.
(5) The main runner is large enough to demould the gate material before it solidifies, so that demoulding is easy.
(6) The joint between the main runner bushing and the nozzle should prevent the product from sticking to the fixed mold due to the drag of the cold hard material.
3. Materials:
(1) The recycled material content is too high, resulting in low strength of the finished product.
(2) The humidity is too high, which causes some plastics to react with water vapor, reducing the strength and causing ejection cracking.
(3) If the material itself is not suitable for the environment being processed or the quality is poor, it will crack if it is polluted.
4. Machine:
The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small, the plasticizing capacity will not be fully mixed and it will become brittle. If it is too large, it will degrade.
Cause Analysis of Bubbles in Injection Molded Products
Bubbles (vacuum bubbles) are very thin and belong to vacuum bubbles. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubble is due to insufficient filling of plastic or low pressure. Under the rapid cooling of the die, the fuel at the corner of the die cavity pulls, resulting in volume loss.
Solution:
(1) Increase the injection energy: pressure, speed, time and material quantity, and increase the back pressure to make the mold full.
(2) Increase the material temperature and flow smoothly. Lower the material temperature, reduce the shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at the part where the vacuum bubble is formed.
(3) Set the gate in the thick part of the part, improve the flow condition of the nozzle, runner and gate, and reduce the pressure consumption.
(4) Improve the exhaust condition of the die.
Analysis of color stripe, color line and color pattern of injection molding products
This kind of defect is mainly caused by the common problems of plastic products colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye-paste coloring in color stability, color purity and color migration, the distributivity, that is, the mixing uniformity of color particles in diluted plastics is relatively poor, and the finished products naturally have 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 its temperature is close to or slightly higher than that of the melting section, so that the color masterbatch can be melted as soon as possible when it enters the melting section, thus promoting uniform mixing with dilution and increasing the chance of liquid mixing.
(2) When the screw speed is constant, increasing the back pressure can improve the melt temperature and shearing action in the barrel.
(3) Modify the mold, especially the gating system. If the gate is too wide and the molten material passes, the turbulence effect is poor and the temperature rise is not high, so it is uneven, and the ribbon mold cavity should be narrowed.
Cause Analysis of Shrinkage Depression of Injection Molded Products
In the process of injection molding, shrinkage and depression of products is a common phenomenon. The main reasons for this are:
1. machine:
(1) The nozzle hole is too big, which causes the melt to flow back and shrink; when it is too small, the resistance is too large, and the amount of material is insufficient, which leads to shrinkage.
(2) If the clamping force is insufficient, the flash will shrink. Check whether there is any problem with the clamping system.
(3) If the plasticizing amount is insufficient, the machine with large plasticizing amount should be selected, and whether the screw and the barrel are worn or not should be checked.
2. Mould:
(1) The product design should make the wall thickness uniform and ensure the 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 smooth and the resistance should not be too large. For example, the size of the main runner, branch runner and gate should be appropriate, the smoothness should be enough, and the transition zone should be circular arc transition.
(4) For thin parts, the temperature should be increased to ensure smooth materials, and for thick-walled parts, the mold temperature should be reduced.
(5) The gate should be opened symmetrically, as far as possible in the thick-walled part of the workpiece, and the volume of the cold material well should be increased.
3. Plastic:
Compared with amorphous plastics, crystalline plastics suffer from shrinkage. When processing, it is necessary to appropriately increase the amount of materials, or add additives in plastics to speed up crystallization and reduce shrinkage and depression.
4. Processing:
(1) The barrel temperature is too high, and the volume changes greatly, especially the front furnace temperature. For plastics with poor fluidity, the temperature should be appropriately increased to ensure smoothness.
(2) Too low injection pressure, speed and back pressure, too short injection time, resulting in insufficient material quantity or density and shrinkage pressure, too high speed and back pressure, and too long time leading to flash and shrinkage.
(3) The feeding amount means that the injection pressure is consumed when the buffer pad is too large, and the feeding amount is insufficient when it is too small.
(4) For the parts that don’t require precision, after the injection pressure keeping is completed, the outer layer is basically condensed and hardened, while the sandwich part is still soft and can be ejected, the mold should be removed as soon as possible, and then it should be cooled slowly in the air or hot water, which can make the shrinkage depression gentle and less conspicuous without affecting the use.
Cause analysis of color and gloss defects of injection products
Under normal circumstances, the gloss of the surface of injection molded parts is mainly determined by the type of plastic, colorant and the smoothness of the mold surface. However, some other reasons often cause defects such as the surface color and gloss of products, and the dark surface. The causes and solutions are analyzed as follows:
(1) poor finish of the mold, rust on the surface of the cavity, and poor exhaust of the mold.
(2) The gating system of the mold is defective, so the cold material well should be increased, and the runner, polished main runner, shunt runner and gate should be increased.
(3) The material temperature and mold temperature are low, and local heating of the gate can be used when necessary.
(4) Processing pressure is too low, speed is too slow, injection time is insufficient, back pressure is insufficient, resulting in poor compactness and dark surface.
(5) Plastic should be fully plasticized, but the degradation of materials should be prevented. It should be stable when heated and fully cooled, especially for thick walls.
(6) Prevent cold material from entering the finished product, and if necessary, use self-locking spring or lower the nozzle temperature.
(7) Too much recycled materials are used, plastics or colorants are of poor quality, mixed with water vapor or other impurities, and lubricants used are of poor quality.
(8) The clamping force should be sufficient.

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