[Science Stickers] Comprehensive analysis of the causes of injection defects

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

Cracking, including filamentous cracks, microcracks, whitening, cracking, and trauma crisis caused by the sticking of the workpiece and the runner sticking on the surface of the part, is divided into demoulding cracking and application cracking according to the cracking time. The main reasons are as follows:

1. Processing aspects:

(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 speed and pressure of mold opening to prevent mold release and cracking caused by rapid forced drawing.

(3) Properly increase the mold temperature to make the part easy to demould, and appropriately lower the material temperature to prevent decomposition.

(4) Prevent cracking due to weld marks and degradation of plastics, resulting in lower mechanical strength.

(5) Appropriate use of mold release agents, and pay attention to frequently removing substances such as aerosol attached to the mold surface.

(6) The residual stress of the part can be eliminated by annealing and heat treatment immediately after forming to reduce the generation of cracks.

2. Mold aspect:

(1) The ejection should be balanced, such as the number of ejector pins, the cross-sectional area should be sufficient, the demolding slope should be sufficient, and the cavity surface should be smooth enough, so as to prevent the ejection residual stress from being concentrated and cracking due to external force.

(2) The structure of the part should not be too thin, and the transition part should use 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 workpiece.

(4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent the formation of vacuum negative pressure.

(5) The main runner is sufficient to allow the gate material to be demolded when it is cured in the future, so that it is easy to demold.

(6) The sprue bushing and the nozzle should be prevented from being dragged by the chilled material to make the parts stick to the fixed mold.

3. Material aspect:

(1) The content of recycled materials is too high, causing the strength of the parts to be too low.

(2) Excessive humidity causes some plastics to chemically react with water vapor, reducing the strength and causing ejection cracking.

(3) The material itself is not suitable for the processing environment or the quality is not good, and it will cause cracking if it is polluted.

4. On the machine side: the plasticizing capacity of the injection molding machine should be appropriate. If it is too small, it will become brittle due to insufficient plasticization. If it is too large, it will degrade.

2. Analysis of the causes of bubbles in injection molded products

The gas in the bubble (vacuum bubble) is very thin and belongs to the 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 plastic injection or low pressure. Under the action of the rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in volume loss.

Solution:

(1) Increase the injection energy: pressure, speed, time and material volume, and increase the back pressure to make the filling full.

(2) Increase material temperature and flow smoothly. Lower the material temperature to reduce 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 workpiece to improve the flow conditions of the nozzle, runner and gate, and reduce the consumption of pressure.

(4) Improve mold exhaust.

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

The deformation, bending, and distortion of injection molded products are mainly due to the fact that the shrinkage rate in the flow direction is greater than that in the vertical direction during plastic molding, which makes the parts shrink and warp due to different shrinkage rates. There is a large internal stress remaining inside the part that causes warpage, which are all manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, the mold design determines the warpage tendency of the part. It is very difficult to suppress 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. Mould aspect:

(1) The thickness and quality of the parts should be uniform.

(2) The design of the cooling system should make the temperature of each part of the mold cavity uniform, and the pouring system should make the material flow symmetrical to avoid warping due to different flow directions and shrinkage rates, and appropriately thicken the runners and mainstreams of the difficult-to-form parts Road, try to eliminate the density difference, pressure difference, and temperature difference in the cavity.

(3) The transition zone and corners of the thickness of the part should be smooth enough and have good mold release. For example, increase the mold release margin, improve the polishing of the mold surface, and maintain the balance of the ejection system.

(4) Good exhaust.

(5) Increase the wall thickness of the part or increase the direction of anti-warping, and strengthen the anti-warping ability of the part by reinforcing ribs.

(6) The strength of the material used in the mold is insufficient.

2. Plastic aspect:

Crystalline plastics have more chances of warping deformation than amorphous plastics. In addition, crystalline plastics can use the crystallization process of crystallinity to decrease with the increase of cooling rate and shrinkage rate to correct the warpage.

3. Processing aspects:

(1) The injection pressure is too high, the holding time is too long, and the melt temperature is too low and the speed is too fast, which will cause the internal stress to increase and warp deformation.

(2) The mold temperature is too high and the cooling time is too short, which will cause the part to be ejected due to overheating during demolding.

(3) Reduce the screw speed and back pressure to reduce the density while keeping the minimum filling amount to limit the generation of internal stress.

(4) If necessary, the parts that are prone to warping and deformation can be soft-shaped or demolded and then returned.

Fourth, the analysis of the color line of the color bar of the injection molded product

This kind of defect is mainly caused by the common problem of plastic parts colored by masterbatch, although masterbatch coloring is better than dry powder coloring and dyeing paste in terms of color stability, color purity and color migration. Coloring, but the distribution, that is, the degree of uniform mixing of the colored particles in the diluted plastic is relatively poor, and the finished product naturally has a regional color difference. The main solution:

(1) Increase the temperature of the feeding section, especially the temperature at the back end of the feeding section, to make the temperature close to or slightly higher than the temperature of the melting section, so that the masterbatch will melt as soon as possible when entering the melting section, promote uniform mixing with dilution, and increase the chance of liquid mixing.

(2) When the screw speed is constant, increasing the back pressure can increase the temperature of the melt in the barrel and the shearing effect.

(3) Modify the mold, especially the pouring system, if the gate is too wide, the turbulence effect is poor when the molten material passes, and the temperature rise is not high, so it is not uniform. The ribbon mold cavity should be narrowed.

Five, analysis of the reasons for the shrinkage of injection molded products 

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

1. For the machine:

(1) The nozzle hole is too large to cause the melt to flow back and shrink, and the resistance is too small and the amount of material is insufficient to shrink.

(2) If the clamping force is insufficient, the flash will also shrink. Check whether there is any problem with the clamping system.

(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 barrel are worn.

2. Mould aspect:

(1) The design of the part should make the wall thickness uniform and ensure uniform shrinkage.

(2) The cooling and heating system of the mold must ensure that the temperature of each part is consistent.

(3) The gating system must be unobstructed and the resistance must not be too large. For example, the size of the main runner, runner, and gate must be appropriate, the smoothness must be sufficient, and the transition zone must be arc transitioned.

(4) The temperature should be increased for thin parts to ensure smooth material, and the mold temperature should be lowered for thick-walled parts.

(5) The gate should be opened symmetrically, and should be opened in the thick wall part of the product as much as possible, and the volume of the cold slug well should be increased.

3. Plastic aspect:

Crystalline plastics are more harmful than non-crystalline plastics, so the amount of material should be appropriately increased during processing, or a replacement agent should be added to the plastic to speed up crystallization and reduce shrinkage.

4. Processing aspects:

(1) The temperature of the barrel is too high and the volume changes greatly, especially the forehearth temperature. For plastics with poor fluidity, the temperature should be appropriately increased to ensure smoothness.

(2) The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the material volume or density is insufficient and the contraction pressure, the speed, the back pressure is too large, and the time is too long to cause flashing and contraction.

(3) The feeding amount means that when the cushion is too large, the injection pressure will be consumed, and if it is too small, the amount of material will be insufficient.

(4) For parts that do not require precision, after the injection and holding pressure, the outer layer is basically condensed and hardened, but the sandwich part is still soft and can be ejected, and the parts are ejected early and allowed to cool slowly in air or hot water. , Can make the contraction depression gentle and not so conspicuous without affecting the use.

Sixth, analysis of the reasons for the transparent defects of injection molded products

Melt spots, silver streaks, cracked polystyrene, plexiglass transparent parts, sometimes you can see some shimmering silky silver streaks through light. These silver streaks are also called sparkles or cracks. This is because the stress is generated in the vertical direction of the tensile stress, and the use-right polymer molecules undergo a heavy flow orientation, which shows the difference in the folding rate from the unoriented part.

Solution:

(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 section by section, and increase the mold temperature appropriately.

(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 exhaust condition of the runner and cavity.

(6) Clean up possible blockages in the nozzle, runner and gate.

(7) Shorten the molding cycle. After demolding, annealing can be used to eliminate silver streaks: for polystyrene, keep it at 78°C for 15 minutes, or at 50°C for 1 hour, for polycarbonate, heat it to 160°C or more and keep it for several minutes. .

Seven, analysis of the reasons for the uneven color of injection molded products

The main reasons and solutions for the uneven color of injection molded products are as follows:

(1) Poor diffusion of the colorant, which often causes patterns to appear near the gate.

(2) The thermal stability of plastics or colorants is poor. To stabilize the color of the parts, the production conditions must be strictly fixed, especially the material temperature, material volume and production cycle.

(3) For crystalline plastics, try to make the cooling rate of each part of the part consistent. For parts with large wall thickness differences, colorants can be used to mask 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 impact on the filling of the plastic, causing some parts of the part to produce color difference, which should be modified if necessary.

8. Analysis of the causes of color and gloss defects of injection molded products 

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

(1) The mold finish is poor, the cavity surface has rust, etc., and the mold exhaust is poor.

(2) The gating system of the mold is defective, the cold slug well should be enlarged, the runner, polished main runner, runner and gate should be enlarged.

(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 dark surface.

(5) Plastics must be fully plasticized, but to prevent degradation of the material, stable heating and adequate cooling, especially thick-walled ones.

(6) Prevent cold material from entering the part, use self-locking spring or lower nozzle temperature when necessary.

(7) Too much recycled materials are used, plastics or colorants are of poor quality, water vapor or other impurities are mixed, and lubricants used are of poor quality.

(8) The clamping force must be sufficient.

Nine, analysis of the causes of silver streaks in injection molded products 

The silver streak of injection molded products includes surface bubbles and internal pores. The main reason for the defects is the interference of gas (mainly water vapor, decomposition gas, solvent gas, and air). The specific reasons are as follows:

1. For the machine:

(1) The barrel and screw are worn out, or there are dead angles of material flow in the rubber head or rubber ring, which will be decomposed by long-term heating.

(2) The heating system is out of control, causing the temperature to be too high and decomposing. Check whether there are any problems with the heating elements such as thermocouples and heating coils. Improper screw design can cause problems or easily bring in air.

2. Mould:

(1) Poor exhaust.

(2) The friction resistance of the runner, gate and cavity in the mold is large, causing local overheating and decomposition.

(3) Unbalanced gate and cavity distribution and unreasonable cooling system will cause unbalanced heating and local overheating or blocking of air passages.

(4) The cooling passage leaks water into the cavity.

3. Plastic aspect:

(1) The plastic has high humidity, the proportion of added recycled materials is too large or contains harmful scraps (the scraps are easily decomposed), the plastic should be fully dried and the scraps removed.

(2) To 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, stabilizer, etc. added to the plastic is too much or the mixing is uneven, or the plastic itself contains volatile solvents. The mixed plastic will also decompose when the degree of heating is difficult to balance.

(4) The plastic is contaminated and mixed with other plastics.

4. Processing aspects:

(1) Setting temperature, pressure, speed, back pressure, melt glue motor speed is too high to cause decomposition, or pressure, speed is too low, injection time, pressure holding is insufficient, back pressure is too low, because the high pressure and density cannot be obtained If the gas cannot be melted and the silver streak appears, the appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.

(2) Low back pressure and fast speed make it easy for air to enter the barrel and enter the mold with the melt. When the cycle is too long, the melt will decompose in the barrel after being heated for too long.

(3) Insufficient amount of material, too large cushioning pad, too low material temperature or too low mold temperature will affect the material flow and molding pressure, and promote the formation of bubbles.

10. Analysis of the reasons why plastic products have welded seams 

When the molten plastic meets the insert hole, the area where the flow rate is inconsistent, and the area where the filling flow is interrupted in the cavity, when it meets in the form of multiple strands, a linear weld seam is produced because it cannot be completely fused. In addition, the sprue injection molding will also produce welded joints, and the strength and other properties of the welded joints are poor. The main reasons are as follows:

1. Processing aspects:

(1) The injection pressure and speed are too low, the barrel temperature, and the mold temperature are too low, causing the molten material entering the mold to cool prematurely and the weld seam appears.

(2) When the injection pressure and speed are too high, jetting will occur and weld seams will appear.

(3) The speed should be increased, and the back pressure should be increased to decrease the viscosity of the plastic and increase the density.

(4) Plastics should be dried well, and recycled materials should be used less. If the amount of release agent is too much or the quality is not good, welded joints will appear.

(5) Reduce the clamping force to facilitate exhaust.

2. Mould aspect:

(1) If there are too many gates in the same cavity, the gates should be reduced or set symmetrically, or set as close as possible to the weld joint.

(2) Poor exhaust at the welded joints, an exhaust system should be installed.

(3) The runner is too large, the size of the pouring system is improper, and the gate is opened to avoid the flow of melt around the hole of the insert, or use the insert as little as possible.

(4) If the wall thickness changes too large or the wall thickness is too thin, the wall thickness of the part should be made uniform.

(5) When necessary, a fusion well should be opened at the welding seam to make the welding seam separate from the workpiece.

3. Plastic aspect:

(1) Lubricants and stabilizers should be appropriately added to plastics with poor fluidity or heat sensitivity.

(2) Plastics contain a lot of impurities. If necessary, replace with good quality plastics.

11. Analysis of the causes of shock lines in injection molded products 

On the surface near the gate of rigid plastic parts such as PS, dense corrugations are formed with the gate as the center, sometimes called shock lines. The reason is that when the melt viscosity is too large and the mold is filled in the form of stagnant flow, the material at the front end will quickly condense and shrink as soon as it touches the surface of the cavity, and the later melt expands and shrinks the cold material to continue the process. The continuous alternation causes the material flow to form surface vibrations as it advances.

Solution:

(1) Increase the temperature of the barrel, especially the temperature of the nozzle, and also increase the temperature of the mold.

(2) Increase the injection pressure and speed to quickly fill the mold cavity.

(3) Improve the size of runners and gates to prevent excessive resistance.

(4) The mold exhaust should be good, and a sufficiently large cold slug well should be set up.

(5) Do not design the parts to be too thin.

12. Analysis of the causes of swelling and blistering of injection molded products 

Some plastic parts swelling or blistering soon on the back of the metal insert or in particularly thick parts after being molded and released from the mold. This is because the plastic that has not been completely cooled and hardened releases gas and expands under the action of the internal pressure penalty.

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 the holding pressure and time.

4. Improve the situation that the wall surface of the part is too thick or the thickness changes greatly.

This article comes from the Internet, only for learning and communication, no commercial purpose.

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