Cause analysis and explanation of crack in injection molded parts
Cracking includes filamentous cracks, microcracks, top white, cracking on the surface of the parts, or the trauma crisis caused by the sticking of the parts and the runner. According to the cracking time, the cracking can be divided into demoulding cracking and application cracking.The main reasons are as follows:
1.Processing:
(1) If the processing pressure is too high, the speed is too fast, the more materials are filled, and the injection and pressure holding time are too long, the internal stress will be too large and crack.
(2) Adjust the opening speed and pressure to prevent demoulding cracking caused by rapid and strong drawing of parts.
(3) Properly adjust the mold temperature to make the parts easy to demould, and properly adjust the material temperature to prevent decomposition.
(4) To prevent the weld line, plastic degradation caused by low mechanical strength and cracking.
(5) Proper use of release agent, pay attention to often eliminate the mold surface attached to the mist and other substances.
(6) The residual stress of parts can be eliminated by annealing immediately after forming to reduce the cracks.
2.Mold aspect:
(1) The ejection should be balanced. For example, the number and cross-sectional area of ejector rods should be sufficient, the demoulding slope should be sufficient, 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 chamfering.
(3) Try to use less metal inserts to prevent the increase of internal stress caused by different shrinkage rates between inserts and products.
(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 the sprue before curing, so it is easy to demould.
(6) When the sprue bushing is connected with the nozzle, the cold and hard material should be prevented from dragging and sticking the workpiece to the fixed die.
3.Materials:
(1) The content of recycled material is too high, resulting in the low strength of parts.
(2) The humidity is too high, causing some plastics to react with water vapor, reducing the strength and cracking.
(3) The material itself is not suitable for the environment being processed or the quality is poor, and it will cause cracking if it is polluted.
4.Machine side:
The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small, the plasticization will not be fully mixed and become brittle, and if it is too large, it will degrade.
Cause analysis of bubbles in injection molded parts
The gas of bubble (vacuum bubble) is very thin and belongs to 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 die, the fuel pulling at the corner with the cavity results in volume loss.
5.terms of settlement:
(1) Increase injection energy: pressure, speed, time and material quantity, and increase back pressure to make mold filling plump.
(2) Increase the material temperature and flow smoothly.Reduce the material temperature, reduce the shrinkage, and increase the mold temperature appropriately, especially the local mold temperature of the vacuum bubble forming part.
(3) The gate is set in the thick part of the part to improve the flow condition of the nozzle, runner and gate, and reduce the consumption of pressing service.
(4) Improve the exhaust condition of die.
Cause analysis of warpage of injection molded parts
The deformation, bending and distortion of injection molded parts are mainly due to the higher shrinkage rate in the flow direction than that in the vertical direction, which makes the parts warped due to different shrinkage rates in each direction. Moreover, due to the large residual internal stress in the parts during injection filling, the warpage is caused by high stress orientation.Therefore, fundamentally speaking, the mold design determines the warpage tendency of the parts. It is very difficult to restrain this tendency by changing the molding conditions. The final solution to the problem must start with the mold design and improvement.This phenomenon is mainly caused by the following aspects:
1.Mold aspect:
(1) The thickness and quality of the products should be uniform.
(2) The design of the cooling system should make the temperature of each part of the mold cavity uniform, the gating system should make the material flow symmetrical, avoid warpage caused by different flow direction and shrinkage rate, appropriately thicken the shunt channel and main channel of the difficult 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 workpiece thickness should be smooth enough and have good demoulding performance. For example, increase the stripping redundancy, improve the polishing of the die surface, and keep the ejection system balanced.
(4) Exhaust well.
(5) By increasing the wall thickness or increasing the anti warping direction, the anti warping ability of the part can be enhanced by reinforcing rib.
(6) The material used in the mould is not strong enough.
2.For Plastics:
In addition, the crystallized plastics can use the crystallization process that the crystallinity decreases with the increase of cooling rate and the shrinkage rate decreases to correct the warpage deformation.
3.Processing:
(1) If the injection pressure is too high, the holding time is too long, the melting temperature is too low and the speed is too fast, the internal stress will increase and the warpage will appear.
(2) The mold temperature is too high and the cooling time is too short, so that the parts are overheated and the ejection deformation occurs.
(3) The internal stress is limited by reducing the screw speed and back pressure and reducing the density while keeping the minimum charge.
(4) If necessary, soft setting or demoulding can be carried out for the parts easy to warp and deform.
Analysis of color stripe, line and flower of injection molding products
This defect is mainly caused by the color masterbatch coloring of plastic parts. Although the color masterbatch coloring is better than dry powder coloring and dye paste coloring in terms of color stability, color quality purity and color migration, the distributive property, that is, the degree of mixing uniformity of color granules 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 back 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 melts as soon as possible when it enters the melting section, promotes the uniform mixing with dilution, and increases the chances of liquid mixing.
(2) When the screw speed is constant, the melt temperature and shear effect in the barrel can be improved by increasing the back pressure.
(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 melt passes through. Therefore, the color belt mold cavity should be narrowed.
Cause analysis of shrinkage depression of injection molded parts
In the process of injection molding, shrinkage depression is a common phenomenon.The main reasons for this are as follows:
1.Machine side:
(1) If the nozzle hole is too large, it will cause the melting material to return and shrink. If it is too small, the resistance will be large and the material quantity will be insufficient.
(2) If the clamping force is insufficient, the flash will shrink, so check whether there is any problem with the mold locking 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 aspect:
(1) The wall thickness should be uniform and the shrinkage should be consistent.
(2) The cooling and heating system of the mould 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, distributor and gate should be appropriate, the finish should be sufficient, and the transition area should be circular.
(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 set symmetrically, and should be set in the thick wall part of the workpiece as much as possible, and the volume of cold material well should be increased.
3.For Plastics:
The shrinkage time of crystalline plastics is worse than that of non crystalline plastics. It is necessary to increase the amount of materials or add additives in the plastics to accelerate the crystallization and reduce the shrinkage depression.
4.Processing:
(1) If the temperature of the barrel is too high and the volume changes greatly, especially the temperature of the forehearth, the temperature of the plastic with poor fluidity should be properly raised to ensure smooth operation.
(2) If the injection pressure, speed and back pressure are too low and the injection time is too short, the shrinkage pressure, speed and back pressure are too large and the time is too long, resulting in shrinkage due to flash.
(3) When the cushion is too large, the injection pressure will be consumed. If the cushion is too small, the injection pressure will be insufficient.
(4) For the 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. If the parts are allowed to cool slowly in air or hot water, the shrinkage depression will be gentle and less obvious, and the use will not be affected.
Cause analysis of transparent defects in injection molded parts
The transparent products of melting spot, crazing, cracked polystyrene and plexiglass can sometimes be seen through the light.These crazes are also called bright spots or cracks.This is due to the stress in the vertical direction of tensile stress. The polymer molecules of the right of use have heavy flow orientation, and the difference of the yield between the polymer and the non oriented part is shown.
resolvent:
(1) Eliminate the interference of gas and other impurities, and dry the plastic sufficiently.
(2) Reduce the material temperature, adjust the barrel temperature in sections, and increase the mold temperature appropriately.
(3) Increase the injection pressure and decrease the injection speed.
(4) Increase or decrease the back pressure of pre molding and reduce the screw speed.
(5) Improve the exhaust condition of runner and cavity.
(6) Clean the nozzle, runner and gate for possible blockage.
(7) After demoulding, annealing method can be used to eliminate Craze: polystyrene at 78 ℃ for 15 minutes, or 50 ℃ for 1 hour, for polycarbonate, heated to above 160 ℃ for several minutes.
Cause analysis of uneven color of injection molded parts
The main causes and solutions of uneven color of injection molded products are as follows:
(1) The diffusion of colorant is poor, which often leads to the appearance of patterns near the gate.
(2) The thermal stability of plastics or colorants is poor. In order to stabilize the color 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 product should be consistent as far as possible. For parts with large wall thickness difference, colorants can be used to cover the color difference. For parts with uniform wall thickness, the material temperature and mold temperature should be fixed.
(4) The shape, gate form and position of the part have an influence on the filling of plastic, which causes color difference in some parts of the part, and it should be modified if necessary.
Cause analysis of color and gloss defects of injection molded products
Under normal conditions, the gloss of injection molded parts is mainly determined by the type of plastic, colorant and surface finish.But often also because of some other reasons, the surface color and luster defects, surface dark color and other defects.The causes and solutions are as follows:
(1) The mold has poor finish, rust on the surface of the cavity and poor exhaust.
(2) There are defects in the gating system of the mould, so it is necessary to increase the cooling well, the runner, the polishing sprue, the splitter and the gate.
(3) The material temperature and mold temperature are low, if necessary, the gate local heating method 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) 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 ones.
(6) To prevent cold material from entering the parts, use self-locking spring or reduce the nozzle temperature if necessary.
(7) Too much recycled materials, poor quality of plastics or colorants, water vapor or other impurities, and poor quality of lubricants used.
(8) The clamping force should be enough.
Cause analysis of crazing in injection molded products
Crazing of injection molded products, including surface bubbles and internal pores.The main reason for the defect is the interference of gas (mainly water vapor, decomposition gas, solvent gas and air).The specific reasons are as follows:
1.Machine side:
(1) There is a dead angle of material flow when the barrel or screw is worn or the rubber head and rubber ring pass through, which will decompose when heated for a long time.
(2) If the heating element is out of control, check whether the heating element is out of control.Improper design of screw may cause individual solution or easily bring in air.
2.Mold aspect:
(1) Poor exhaust.
(2) The friction resistance of runner, gate and cavity in the mold is large, which causes local overheating and decomposition.
(3) The unbalanced distribution of gate and cavity and unreasonable cooling system will lead to unbalanced heating and local overheating or blocking of air passage.
(4) The cooling passage leaks into the cavity.
3.For Plastics:
(1) If the humidity of plastics is high, the proportion of recycled materials added is too much or there are harmful chips (the chips are easy to decompose), the plastics should be dried sufficiently and the scraps should be eliminated.
(2) Absorb moisture from the atmosphere or from the colorant, the colorant should also be dried, it is best to install a dryer on the machine.
(3) The amount of lubricant and stabilizer added in plastics is too much or mixed unevenly, or the plastic itself has volatile solvents.When it is difficult to take into account the degree of heating, the mixed plastics will decompose.
(4) The plastic is contaminated and mixed with other plastics.
4.Processing:
(1) When the setting temperature, pressure, speed, back pressure and glue melting motor speed are too high to cause decomposition, or the pressure and speed are too low, the injection time and pressure are not sufficient, and the back pressure is too low, crazing occurs due to the lack of density due to failure to obtain high pressure, so appropriate temperature, pressure, speed and time shall be set and multi-stage injection speed shall be adopted.
(2) Low back pressure and high speed make the air enter the barrel easily. With the melting material entering the mold, when the cycle is too long, the melted material will decompose when heated too long in the barrel.
(3) Insufficient material quantity, too large feeding buffer, too low material temperature or too low mold temperature all affect the flow and molding pressure of the material and promote the formation of bubbles.
Analysis of the causes of weld joint in injection molded parts
When the molten plastics meet the insert hole, the area with discontinuous flow velocity and the area with interrupted filling material flow in the mold cavity, the linear fusion joint will be produced because of the incomplete fusion.In addition, in the case of gate injection filling, the weld seam will also be formed, and the strength of the weld joint is very 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, which causes the melt material entering the mold to cool down prematurely and the fusion joint appears.
(2) When the injection pressure and speed are too high, there will be spray and fusion joint.
(3) The viscosity and density of plastics decrease with the increase of speed and back pressure.
(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 aspect:
(1) If there are too many gates in the same cavity, the gate should be set symmetrically or as close as possible to the weld joint.
(2) The exhaust system should be set up in case of poor exhaust at fusion joint.
(3) If the runner is too large, the size of the gating system is not proper, the gate should be opened to avoid the melt flowing around the insert hole, 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, a fusion well should be set at the fusion joint to separate the fusion joint from the parts.
3.For Plastics:
(1) Lubricants and stabilizers should be added to the plastics with poor fluidity or heat sensitivity.
(2) Plastic contains a lot of impurities, if necessary, to change the quality of plastic.
Analysis on the cause of vibration crack in injection molded parts
PS and other rigid plastic parts in the gate near the surface, to the gate as the center of the formation of dense ripples, sometimes known as jarring.The reason is that when the melt viscosity is too high and the mold is filled in the form of stagnant flow, the material at the front end will condense and contract as soon as it contacts the surface of the cavity, and the later molten material will expand and shrink, and the cold material will continue to move forward. The continuous alternation of the process makes the material flow form surface chatter marks in the process of advancing.
resolvent:
(1) The mold temperature should also be increased to increase the barrel temperature, especially the nozzle temperature.
(2) The injection pressure and speed were increased to fill the cavity quickly.
(3) Improve the size of the gate and prevent the gate from being too large.
(4) The exhaust of mould should be good, and enough cold material well should be set up.
(5) Do not design the parts too thin.
Cause analysis of swelling and bubbling of injection molded parts
Some plastic parts appear swelling or bubbling on the back of the metal insert or in the extremely thick parts after the molding and demoulding.This is due to the expansion of the gas released by the plastic which is not completely cooled and hardened under the action of internal pressure penalty.
Solutions:
1.Effective cooling.Reduce the mold temperature, prolong the mold opening time, reduce the drying and processing temperature of the material.
2.It can reduce the filling speed, forming cycle and flow resistance.
3.Increase the holding pressure and time.
4.Improve the condition that the wall is too thick or the thickness changes greatly.
