1、 Cause analysis of injection products cracking
Cracking, including surface filiform crack, micro crack, top white, cracking and damage crisis caused by die sticking and runner die sticking, can be divided into demoulding cracking and application cracking according to cracking time. The main reasons are as follows:
1）. Processing:
(1) If the processing pressure is too high, the speed is too fast, the more the material is filled, and the injection and pressure holding time is too long, the internal stress will be too large and the cracks will occur.
(2) Adjust the mold opening speed and pressure to prevent demoulding cracking caused by rapid drawing.
(3) Adjust the mold temperature properly to make the parts easy to demould, and adjust the material temperature properly to prevent decomposition.
(4) Prevent cracking due to low mechanical strength due to weld lines and plastic degradation.
(5) Appropriate use of mold release agent, pay attention to often eliminate the mold surface attached to the aerosol and other substances.
(6) The residual stress of parts can be eliminated by annealing immediately after forming to reduce the generation of cracks.
2）. Mold:
(1) The ejection should be balanced, such as the number and cross-sectional area of ejector bars should be enough, the demoulding slope should be enough, and the cavity surface should be smooth enough, so as to prevent cracking due to residual stress concentration caused by external force.
(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) metal inserts should be used as little as possible to prevent the increase of internal stress caused by different shrinkage rates of inserts and parts.
(4) For deep bottom parts, appropriate demoulding air inlet duct should be set to prevent the formation of vacuum negative pressure.
(5) The sprue is enough to demould before the gate material can be solidified, which is easy to demould.
(6) The connection between the sprue bushing and the nozzle should prevent the dragging of the cold and hard material from making the parts stick to the fixed die.
3）. Materials:
(1) The recycled material content is too high, resulting in the low strength of the parts.
(2) Too high humidity causes chemical reaction between some plastics and water vapor, which reduces the strength and causes ejection cracking.
(3) The material itself is not suitable for the processing environment or the quality is not good, and the pollution will cause cracking.
4）. Machines:
The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small, it will become brittle due to insufficient plasticizing and incomplete mixing. If the plasticizing capacity is too large, it will degrade.
2、 Cause analysis of bubbles in injection molded products
The gas of bubble (vacuum bubble) is very thin and belongs to vacuum bubble. Generally speaking, if the bubble has been found at the moment of mold opening, it belongs to the problem of gas interference. The formation of vacuum bubble is due to insufficient plastic filling or low pressure. Under the rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in volume loss.
terms of settlement:
(1) Improve the injection energy: pressure, speed, time and material quantity, and increase the back pressure to make the filling full.
(2) Increase the material temperature, smooth flow. Reduce the material temperature, reduce the shrinkage, and properly increase the mold temperature, especially the local mold temperature of the vacuum bubble.
(3) The gate is set in the thick part of the part to improve the flow condition of nozzle, runner and gate, and reduce the consumption of pressure.
(4) Improve the mold exhaust condition.
3、 Analysis of warpage of injection molded parts
The deformation, bending and distortion of injection molded parts are mainly caused by the higher shrinkage in the flow direction than in the vertical direction, which makes the parts warped due to different shrinkage in all directions. Moreover, the warping is caused by the large residual internal stress in the parts during injection molding. All these are the manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, die design determines the warpage tendency of parts. It is very difficult to restrain this tendency by changing the forming conditions. The final solution to the problem must start from die design and improvement. This phenomenon is mainly caused by the following aspects:
1）. Mold:
(1) The thickness and quality of the parts should be uniform.
(2) The cooling system should be designed to make the temperature of each part of the mold cavity uniform, the gating system should make the material flow symmetrical, avoid warping due to different flow direction and shrinkage rate, appropriately thicken the porthole and main channel of the difficult forming part, and try to eliminate the density difference, pressure difference and temperature difference in the mold cavity.
(3) The transition zone and corner of the part thickness should be smooth enough and have good demoulding performance, such as increasing the demoulding redundancy, improving the polishing of the die surface, and keeping the balance of the ejection system.
(4) Exhaust well.
(5) By increasing the thickness of the part or increasing the anti warping direction, the anti warping ability of the part is enhanced by stiffeners.
(6) The strength of the material used in the mould is insufficient.
2）. Plastics:
In addition, the crystallinity of crystalline plastics decreases with the increase of cooling rate and the shrinkage decreases to correct the warpage.
3）. Processing:
(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 warpage will occur.
(2) The mold temperature is too high and the cooling time is too short, which makes the parts overheat and lead to ejection deformation.
(3) In order to limit the generation of internal stress, the screw speed and back pressure are reduced to reduce the density while keeping the minimum charge.
(4) If necessary, the parts which are easy to warp and deform can be set or demoulded, and then the rice can be retreated.
4、 Analysis of color stripe, color line and color pattern of injection molded products
Although the color stability, color purity and color migration of color masterbatch are better than those of dry powder and dye paste, the distribution of Color Masterbatch, that is, the mixing uniformity of color masterbatch in diluting plastic is relatively poor, and the finished products naturally have regional color differences.
The main solutions are as follows:
(1) Increase the temperature of the feeding section, especially the temperature of 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 it enters the melting section, promote the uniform mixing with dilution, and increase the chance of liquid mixing.
(2) When the screw speed is fixed, increasing the back pressure can improve the melt temperature and shear effect in the barrel.
(3) Modify the mold, especially the gating system. If the gate is too wide, the turbulence effect is poor and the temperature rise is not high when the molten material passes through, so the color band cavity is uneven, which should be narrowed.
5、 Cause analysis of shrinkage depression of injection molded products
In the process of injection molding, shrinkage depression is a common phenomenon. The main reasons are as follows
1）. Machine:
(1) When the nozzle hole is too large, the melt will flow back and shrink. When the nozzle hole is too small, the resistance is large and the material quantity is insufficient.
(2) Insufficient clamping force will cause the flash to shrink, so it is necessary to check if there is any problem in the clamping system.
(3) If the plasticizing amount is insufficient, the machine with large plasticizing amount should be selected to check whether the screw and barrel are worn.
2）. Mold:
(1) The design of parts should make the wall thickness uniform and ensure the same shrinkage.
(2) The cooling and heating system of the mould should ensure the same temperature of each part.
(3) The gating system should be smooth, and the resistance should not be too large. For example, the size of main flow channel, shunt channel and gate should be appropriate, the finish should be enough, and the transition zone should be arc transition.
(4) For thin parts, the temperature should be increased to ensure smooth material flow, and for thick wall parts, the mold temperature should be reduced.
(5) The gate should be opened symmetrically, as far as possible in the thick wall of the parts, and the volume of the cold well should be increased
3）. Plastics:
The shrinkage of crystalline plastics is more harmful than that of non crystalline plastics. It is necessary to increase the amount of material or add a change agent in the plastics to speed up the crystallization and reduce the shrinkage depression.
4）. Processing:
(1) The barrel temperature is too high, 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 smooth operation.
(2) Injection pressure, speed, back pressure too low, injection time too short, so that the amount of material or density is insufficient and shrinkage, pressure, speed, back pressure too large, too long cause flash and shrinkage.
(3) When the cushion is too large, the injection pressure will be consumed. When the cushion is too small, the injection pressure will be insufficient.
(4) For parts that do not require precision, after the injection and pressure maintaining, the outer layer is basically condensed and hardened, and the sandwich part is soft and can be ejected, the mold should be removed as soon as possible to allow it to cool slowly in air or hot water, so that the shrinkage depression can be smooth and not so conspicuous, and the use will not be affected.
6、 Cause analysis of transparent defects in injection molded products
The transparent parts of melting spot, craze, crack, polystyrene and plexiglass can sometimes see some glistening sliver like filaments through the light. These crazes are also called bright spots or cracks. This is due to the stress in the vertical direction of the tensile stress, and the difference of the percentage of the polymer molecules with and without flow orientation.
resolvent:
(1) Eliminate the interference of gas and other impurities, and fully dry the plastic.
(2) Reduce the temperature of the material, adjust the temperature of the barrel by sections, and increase the mold temperature appropriately.
(3) Increase injection pressure and decrease injection speed.
(4) Increase or decrease the back pressure of pre molding, reduce the screw speed.
(5) Improve the exhaust condition of runner and cavity.
(6) Clean the nozzle, runner and gate of possible blockage.
(7) After demoulding, the crazing can be eliminated by annealing: polystyrene can be kept at 78 ℃ for 15 minutes, or at 50 ℃ for 1 hour, and polycarbonate can be heated to 160 ℃ for several minutes.
7、 Cause analysis 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 makes the pattern near the gate.
(2) The thermal stability of plastics or colorants is poor. In order to stabilize the color tone of products, the production conditions must be strictly fixed, especially the material temperature, material quantity and production cycle.
(3) For crystalline plastics, the cooling rate of each part of the parts should be the same as far as possible. For the parts with large wall thickness difference, colorant can be used to mask the color difference. For the parts with uniform wall thickness, the material temperature and mold temperature should be fixed.
(4) It is necessary to modify the position and filling form of the plastic part.
8、 Cause analysis of color and luster defects of injection molded products
Under normal circumstances, the surface gloss of injection molded parts is mainly determined by the type of plastic, colorant and surface finish of the mold. But often because of some other reasons, the surface color and luster defects of products, surface dark and other defects. The causes and solutions are as follows:
(1) Poor finish of the mold, rust on the surface of the cavity, poor exhaust of the mold.
(2) The pouring system of the mould is defective, so it is necessary to increase the cooling well, flow channel, polishing main flow channel, shunt channel and gate.
(3) The material temperature and mold temperature are low, if necessary, the local heating method of gate can be used.
(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) The plastics should be fully plasticized, but the degradation of materials should be prevented, the heating should be stable, and the cooling should be sufficient, especially for thick wall plastics.
(6) To prevent the cold material from entering the parts, use the self-locking spring or reduce the nozzle temperature when necessary.
(7) Too much recycled materials are used, the quality of plastics or colorants is poor, water vapor or other impurities are mixed, and the quality of lubricants is poor.
(8) The clamping force should be enough.
9、 Cause analysis of craze in injection molded products
There are craze in injection molding products, including surface bubbles and internal pores. The main cause of defects is the interference of gas (mainly water vapor, decomposition gas, solvent gas and air). The specific reasons are as follows:
1）. Machine:
(1) The barrel and screw are worn, or there is a dead angle of material flow when they pass through the rubber head and rubber ring, which will decompose when they are heated for a long time.
(2) If the heating system is out of control and the temperature is too high, it is necessary to check if there is any problem with the heating elements such as thermocouple and heating coil. Screw design is improper, resulting in a solution or easy to bring in air.
2）. Mold:
(1) Poor exhaust.
(2) The friction resistance of runner, gate and cavity in the mold is large, which causes local overheating and decomposition.
(3) Unbalanced distribution of gate and cavity and unreasonable cooling system will result in unbalanced heating and local overheating or blocking of air passage.
(4) The cooling path leaks into the cavity.
3）. Plastics:
(1) If the humidity of plastics is high, the proportion of recycled materials is too much or there are harmful scraps (the scraps are easy to decompose), 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. It is better to install a dryer on the machine.
(3) The amount of lubricants and stabilizers added to plastics is too much or mixed unevenly, or the plastics have volatile solvents. When the heating degree of mixed plastics is difficult to take into account, it will decompose.
(4) The plastic is contaminated and mixed with other plastics.
4）. Processing:
(1) When setting temperature, pressure, speed, back pressure, too high speed of glue melting motor causes decomposition, or when the pressure and speed are too low, the injection time, pressure holding is not sufficient, and the back pressure is too low, due to the lack of high pressure and insufficient density, it is impossible to melt the gas, resulting in craze. Appropriate temperature, pressure, speed and time should be set, and multi-stage injection speed should be adopted
(2) Low back pressure and fast rotating speed make air enter the barrel easily. As the molten material enters the mold, the molten material will decompose when it is heated too long in the barrel when the cycle is too long.
(3) Insufficient material quantity, too large feeding buffer, too low material temperature or too low mold temperature all affect the material flow and forming pressure, and promote the formation of bubbles.
10、 Analysis on the cause of fusion joint in plastic products
When the molten plastics meet in the cavity in the form of multiple strands due to the hole in the insert, the area with discontinuous flow velocity and the area with interrupted filling material flow, the linear fusion joint will be produced due to incomplete fusion. In addition, in the event of gate injection mold filling, there will be fusion joint, and the strength of fusion joint is very poor. The main reasons are as follows
1）. Processing:
(1) Too low injection pressure and speed, too low barrel temperature and mold temperature lead to premature cooling of molten material into the mold, resulting in weld seam.
(2) When the injection pressure and speed are too high, there will be spray and fusion joint.
(3) It is necessary to increase the rotational speed and back pressure to decrease the viscosity and increase the density of plastics.
(4) Plastic should be dried well, recycled materials should be used less, too much release agent or poor quality will also appear fusion joint.
(5) Reduce the clamping force, easy to exhaust.
2. Mold:
(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 joint as possible.
(2) Exhaust system should be set at the seam of poor fusion.
(3) The runner is too large, the size of the gating system is improper, the gate should be opened to avoid the melt flowing around the hole of the insert, or the insert should be used as little 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, the fusion well should be set at the fusion joint to make the fusion joint separate from the parts.
3. Plastics:
(1) Lubricants and stabilizers should be added to plastics with poor fluidity or heat sensitivity.
(2) There are many impurities in plastics. If necessary, replace them with good quality plastics.
11、 Analysis of the causes of the chatter marks in injection molded products
PS and other rigid plastic parts in its gate near the surface, with the gate as the center of the formation of dense waviness, sometimes known as chatter. The reason is that when the melt viscosity is too high and the mold is filled in the form of stagnant flow, the front-end material will condense and contract as soon as it contacts the surface of the mold cavity, and the later melt will expand, and the contracted cold material will continue to move forward. The continuous alternation of the process makes the material flow form surface chatter.
resolvent:
(1) In order to increase the barrel temperature, especially the nozzle temperature, the mold temperature should also be increased.
(2) Increase the injection pressure and speed to make it fill the cavity quickly.
(3) Improve the size of runner and gate to prevent excessive resistance.
(4) Mold exhaust to be good, to set up a large enough cold well.
(5) Don't design the parts too thin.
12、 Cause analysis of swelling and bubbling of injection products
After demoulding, some plastic parts swell or blister on the back of the metal insert or in the very thick part. This is due to the expansion of the gas released from the partially cooled and hardened plastic under the action of internal pressure penalty.
Solutions:
1. Effective cooling. Reduce the mold temperature, extend the mold opening time, reduce the drying and processing temperature of the material.
2. Reduce the filling speed, forming cycle and flow resistance.
3. Increase the holding pressure and time.
4. Improve the condition that the wall of the part is too thick or the thickness changes greatly.
