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[Popular stickers] comprehensive analysis of the causes of injection defects

Update:11-10-2019
Summary:

1. Analysis of the causes of cracking of injection mold […]

1. Analysis of the causes of cracking of injection molded products

Cracking, including filament cracks on the surface of the part, microcracking, whitening, cracking, and damage due to the sticking of the part, the sticking of the flow path, or the traumatic failure, splitting and cracking according to the cracking time. There are mainly the following reasons:
1. Processing:
(1) If the processing pressure is too large, the speed is too fast, the more filling, the injection and the holding time are too long, the internal stress will be too large and crack.
(2) Adjusting the mold opening speed and pressure to prevent demoulding cracking caused by rapid drawing of the workpiece.
(3) Appropriately increase the mold temperature, make the parts easy to demould, and appropriately reduce the material temperature to prevent decomposition.
(4) Prevention of cracking due to weld line degradation due to plastic degradation caused by plastic degradation.
(5) Appropriate use of the release agent, and attention should be paid to the elimination of substances such as aerosols attached to the mold surface.
(6) The residual stress of the workpiece can be reduced by eliminating the internal stress by annealing heat treatment immediately after molding to reduce the generation of cracks.

2. Mold aspect:
(1) The balance should be balanced. For example, if the number of ejector pins and the cross-sectional area are sufficient, the draft angle should be sufficient, and the cavity surface should be sufficiently smooth to prevent cracking due to external force causing residual stress concentration.
(2) The structure of the workpiece should not be too thin, and the transition part should adopt the arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize the use of metal inserts to prevent the internal stress from increasing due to the difference in shrinkage between the insert and the part.
(4) Appropriate stripping intake passages shall be provided for deep-bottomed parts to prevent vacuum underpressure.
(5) The main channel is large enough to release the gate material in the future when it is cured, so that it is easy to demould.
(6) The main channel bushing and the nozzle joint should prevent the chilling material from being dragged to make the workpiece stick to the fixed mold.
3. Material aspects:
(1) The content of recycled materials is too high, resulting in too low strength of the parts.
(2) The humidity is too large, causing some plastics to react with water vapor, reducing the strength and causing cracking.
(3) The material itself is not suitable for the environment or quality that is being processed, and the pollution will cause cracking.
4. Machine side: The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing is too small, it will not be fully mixed and become brittle. When it is too large, it will degrade.

2.the analysis of the causes of bubbles in injection molding products

The gas of the bubble (vacuum bubble) is very thin and belongs to the vacuum bubble. In general, if a bubble is found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubbles is due to insufficient filling or low pressure. Under the rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in a volume loss.
Solution:
(1) Increase the energy of injection: pressure, speed, time and amount of material, and increase the back pressure to make the filling full.
(2) Increasing the temperature of the material flow smoothly. Lower the material temperature to reduce shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at which the vacuum bubble is formed.
(3) The gate is placed in the thick part of the part to improve the flow of the nozzle, runner and gate, and reduce the consumption of press.
(4) Improve the mold exhaust condition.

3. Analysis of the causes of warpage deformation of injection molded products

The deformation, bending and twisting of injection molded products are mainly caused by the fact that the shrinkage rate of the flow direction during plastic molding is larger than that of the vertical direction, so that the shrinkage ratio of the parts is different and warped, and it is inevitable in the injection molding. There is a large internal stress remaining inside the piece to cause warpage, which is a manifestation of deformation caused by high stress orientation. Therefore, fundamentally speaking, the mold design determines the warping tendency of the part. It is very difficult to suppress the tendency by changing the molding conditions. Finally, the problem must be solved from the mold design and improvement. This phenomenon is mainly caused by the following aspects:
1. Mold aspect:
(1) The thickness and quality of the parts should be uniform.
(2) The design of the cooling system is to make the temperature of each part of the mold cavity uniform. The pouring system should make the flow symmetry avoid the warpage caused by the different flow direction and shrinkage rate, and properly thicken the shunt and main flow which are difficult to form part. Road, try to eliminate the density difference, pressure difference, temperature difference in the cavity.
(3) The transition zone and corner of the workpiece should be sufficiently rounded and have good demolding properties, such as increasing the demoulding margin and improving the polishing of the die face. The ejector system should be balanced.
(4) The exhaust should be good.
(5) Increase the wall thickness of the part or increase the direction of warping resistance, and strengthen the rib to enhance the anti-warping ability of the part.
(6) The strength of the material used in the mold is insufficient.

2. Plastics:
The crystal form has more chance of warpage deformation than the amorphous plastic, and the crystal type plastic can be used to correct the warpage deformation by using a crystallizing process in which the crystallinity decreases as the cooling rate increases and the shrinkage rate becomes smaller.
3. Processing:
(1) The injection pressure is too high, the holding time is too long, and the melt temperature is too low. The speed is too fast, which causes the internal stress to increase and warpage.
(2) The mold temperature is too high, and the cooling time is too short, so that the parts during demolding are overheated and the ejection deformation occurs.
(3) Reducing the screw speed and the back pressure reduction density while maintaining the minimum charge amount to limit the generation of internal stress.
(4) If necessary, the mold can be softly shaped or demoulded after the mold is easily warped and deformed.

4. the color line color flower analysis of injection molding products
The occurrence of such defects is mainly caused by the problems of plastic parts colored with masterbatch, although the color masterbatch is superior to dry powder coloring and dyeing in terms of color stability, color purity and color migration. Coloring, but the distribution, that is, the color particles in the diluted plastic in the degree of uniformity is relatively poor, the finished product naturally has a regional color difference. The main solution:
(1) Increasing 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 is melted as soon as possible when it enters the melting section, promoting uniform mixing with the dilution, and increasing the chance of liquid mixing.
(2) When the screw rotation speed is constant, the back pressure is increased to increase the melt temperature and shearing action in the cylinder.
(3) Modify the mold, especially the pouring system. If the gate is too wide, the turbulence effect is poor when the melt passes, and the temperature rise is not high, so it is not uniform, and the ribbon cavity should be narrowed.

5. Analysis of the causes of shrinkage and depression of injection molded products

During the injection molding process, shrinkage of the product is a common phenomenon. The main reasons for this are:

1. Machine side:
(1) The nozzle hole is too large to cause the melt to recirculate and shrink, and when it is too small, the resistance is insufficient to shrink.
(2) If the clamping force is insufficient, the flash will also shrink. Check whether the clamping system has any problems.
(3) If the amount of plasticization is insufficient, a machine with a large amount of plasticization should be used to check whether the screw and the barrel are worn.
2. Mold aspect:
(1) The design of the parts should be such that the wall thickness is uniform and the shrinkage is consistent.
(2) The cooling and heating system of the mold should ensure that the temperature of each part is consistent.
(3) The pouring system should be ensured to be unobstructed, and the resistance should not be too large. For example, the size of the main channel, the runner, and the gate should be appropriate, the finish should be sufficient, and the transition zone should have a circular transition.
(4) For thin parts, the temperature should be raised to ensure smooth flow, and the mold temperature should be reduced for thick-walled parts.
(5) The gate should be opened symmetrically, as far as possible in the thick part of the part, and the volume of the cold well should be increased.

3. Plastics:
Crystalline plastics shrink than conventional non-crystalline plastics. When processing, the amount of material should be increased appropriately, or a replacement agent should be added to the plastic to accelerate crystallization and reduce shrinkage and depression.
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 properly increased to ensure smoothness.
(2) The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the amount or density is insufficient, and the contraction pressure, speed, back pressure is too large, and the time is too long, causing the flash to shrink.
(3) The feeding amount means that the injection pressure is consumed when the cushion is too large, and when the amount is too small, the amount of the material is insufficient.
(4) For parts that do not require precision, after the injection pressure is maintained, the outer layer is basically condensed and hardened, and the part with the sandwich part is still soft and can be ejected, and the mold is released early, and it is slowly cooled in air or hot water. It can make the shrinkage depression smooth and not conspicuous and does not affect the use.

6. Analysis of the causes of transparent defects in injection molding products
Melting, silver, cracked polystyrene, transparent parts of plexiglass, sometimes through the light can see some sparkling filament-like silver. These silver streaks are also known as spots or cracks. This is because the stress in the vertical direction of the tensile stress is generated, and the weight of the polymer molecule is expressed by the heavy flow orientation and the difference in the folding rate of the unoriented portion.

Solution:
(1) Eliminate interference from gases and other impurities, and dry the plastic sufficiently.
(2) Reduce the temperature of the material, adjust the temperature of the barrel in stages, and appropriately increase the 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 flow path and cavity exhaust conditions.
(6) Clean up possible blockages in the nozzles, runners, and gates.
(7) Shortening the molding cycle. After demolding, the silver grain can be removed by annealing: hold the polystyrene at 78 ° C for 15 minutes, or 50 ° C for 1 hour, and heat the polycarbonate to 160 ° C for several minutes. .

7. Analysis of the causes of uneven color of injection molded products

The main reasons and solutions for the uneven color of injection molded products are as follows:
(1) The colorant is poorly diffused, which tends to cause a pattern near the gate.
(2) The plastic or colorant has poor thermal stability. To stabilize the color tone of the part, it is necessary to strictly fix the production conditions, especially the material temperature, the amount of material and the production cycle.
(3) For crystalline plastics, try to make the cooling speed of each part of the parts consistent. For parts with large difference in wall thickness, coloring agent can be used to mask the chromatic aberration. For the parts with uniform wall thickness, the temperature and mold temperature should be fixed. .
(4) The shape of the part and the form of the gate. The position has an influence on the filling condition of the plastic, so that some parts of the part are subject to chromatic aberration and should be modified if necessary.

8. the reasons for the color and gloss defects of injection molding products

Under normal circumstances, the gloss of the surface of an injection molded part is mainly determined by the type of plastic, the colorant and the finish of the mold surface. However, due to some other reasons, the surface color and gloss defects of the product and the dark color of the surface are often defective. The reasons for this and the solution are as follows:

(1) The mold finish is poor, the surface of the cavity is rusted, and the mold is poorly exhausted.
(2) The casting system of the mold is defective. The cold well should be enlarged, the flow channel should be enlarged, the main flow channel should be polished, the runner and the gate should be polished.
(3) The material temperature and the mold temperature are low, and the local heating method of the gate can be used if necessary.
(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 darkening of the surface.
(5) The plastic should be fully plasticized, but the degradation of the material should be prevented, the heat should be stable, and the cooling should be sufficient, especially thick-walled.
(6) Prevent cold material from entering the part, if necessary, use a self-locking spring or reduce the nozzle temperature.
(7) Excessive recycled materials are used, plastic or colorant is of poor quality, mixed with water vapor or other impurities, and the quality of the lubricant used is poor.
(8) The clamping force should be sufficient.

9. Analysis of the causes of silver streaks in injection molding products
Silver pattern of injection molded products, including surface bubbles and internal pores. The main cause of defects is the interference of gases (mainly water vapor, decomposition gas, solvent gas, air). The specific reasons are as follows:

1. Machine side:
(1) The barrel, the screw wears or the rubber head and the apron have a dead angle of the material, which is decomposed by heat for a long time.
(2) If the heating system is out of control, causing the temperature to be too high and decomposed, check whether there are any problems with the heating elements such as thermocouples and heating coils. The screw is not designed properly, causing a solution or easy to bring in air.
2. Mold aspect:
(1) Poor exhaust.
(2) The frictional resistance of the runners, gates and cavities in the mold is large, causing local overheating and decomposition.
(3) Unbalanced distribution of gates and cavities, unreasonable cooling system will cause heat imbalance and local overheating or obstruction of air passage.
(4) The cooling passage leaks into the cavity.
3. Plastics:
(1) The plastic has a large humidity, and the proportion of added recycled materials is too large or contains harmful scraps (the scraps are easily decomposed), and the plastic should be sufficiently dried and the scraps should be eliminated.
(2) Moisture absorption from the atmosphere or moisture absorption from the colorant, and the coloring agent should also be dried. It is preferable to install a dryer on the machine.
(3) The amount of lubricant, stabilizer, etc. added to the plastic is excessive or unevenly mixed, or the plastic itself has a volatile solvent. Decomposition occurs when the mixed plastic is difficult to cope with heat.
(4) The plastic is contaminated and mixed with other plastics.

4. Processing:
(1) Set temperature, pressure, speed, back pressure, melt motor speed is too high to cause decomposition, or pressure, speed is too low, injection time, insufficient pressure, low back pressure, due to failure to obtain high pressure and density Insufficient gas cannot be melted and silver streaks appear. Appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.
(2) The back pressure is low and the rotation speed is fast, so that the air enters the barrel, and the melt enters the mold. When the period is too long, the melt is decomposed in the barrel due to excessive heat.
(3) Insufficient material, the feeding cushion is too large, the material temperature is too low or the mold temperature is too low, which affects the flow of the material and the molding pressure, and promotes the formation of bubbles.

10. Analysis of the causes of welded joints in plastic products
The molten plastic in the cavity meets in the form of multiple strands due to the intrusion of the insert holes, the inconsistent flow rate, and the region where the filling flow is interrupted, and a linear weld seam is produced because it cannot be completely fused. In addition, in the case of gate injection molding, a welded joint is formed, and the strength at the welded joint is poor. The main reasons are as follows:

1. Processing:
(1) The injection pressure and speed are too low, and the temperature of the barrel and the mold temperature are too low, so that the melt entering the mold is cooled prematurely and a weld seam occurs.
(2) When the injection pressure and speed are too high, a spray will occur and a weld seam will appear.
(3) The rotation speed should be increased, and the back pressure should be increased to decrease the viscosity of the plastic and increase the density.
(4) The plastic should be dried well, and the recycled material should be used less. If the amount of the release agent is too much or the quality is not good, the welded joint will appear.
(5) Reduce the clamping force and facilitate the exhaust.
2. Mold aspect:
(1) If there are too many gates in the same cavity, the gate or symmetrical setting should be reduced, or as close as possible to the fusion joint.
(2) Exhaust system should be opened if the exhaust at the welded joint is poor.
(3) If the sprue is too large and the pouring system is not properly sized, the gate should be opened to avoid the flow of the melt around the hole of the insert or to minimize the use of inserts.
(4) If the wall thickness changes too much, or the wall thickness is too thin, the wall thickness of the part should be uniform.
(5) If necessary, a fusion well shall be opened at the weld seam to disengage the weld joint from the workpiece.
3. Plastics:
(1) Lubricants and stabilizers should be added to plastics with poor fluidity or heat sensitivity.
(2) Plastics contain a lot of impurities, and if necessary, change the quality of the plastic.

11. Analysis of the causes of the shock marks in injection molded products
Rigid plastic parts such as PS form a dense corrugation on the surface near the gate, centered on the gate, sometimes called a shock pattern. The reason is that when the melt viscosity is too large and the mold is filled in a stagnant form, the front end material quickly condenses and shrinks as soon as it contacts the surface of the cavity, and the subsequent melt expands the contracted cold material to continue the process. Constant alternating causes the stream to form a surface grain in the forward direction.
Solution:
(1) Increase the barrel temperature, especially the nozzle temperature, and increase the mold temperature.
(2) Increase the injection pressure and speed to make it quickly fill the model cavity.
(3) Improve the size of the runners and gates to prevent excessive resistance.
(4) The mold exhaust should be good, and a large enough cold well should be set.
(5) Do not design the parts too thin.

12. Analysis of the causes of swelling and bubbling of injection molded products
Some plastic parts quickly swell or swell on the back of the metal insert or in a particularly thick part after the mold is demolded. This is because the plastic that is not completely cooled and hardened is caused by the expansion of the gas under the action of the internal pressure.
solution:
1. Effective cooling. Reduce the mold temperature, extend the mold opening time, and reduce the drying and processing temperature of the material.
2. Reduce the filling speed, reduce the forming cycle and reduce the flow resistance.
3. Increase holding pressure and time.
4. Improve the condition that the wall of the part is too thick or the thickness changes greatly.