The main reasons are as follows:
(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 workpiece can be eliminated by annealing and heat treatment immediately after forming to reduce the generation of cracks.
(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 to prevent the 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 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 to prevent internal stress from increasing due to the difference in shrinkage between inserts and parts.
(4) Appropriate demolding air inlets should be provided for deep bottom parts to prevent the formation of vacuum negative pressure.
(5) The main runner is sufficient to allow the gate material to be released when it is cured in the future, so that it is easy to release.
(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.
(1) The content of recycled materials is too high, resulting in too low strength of the parts.
(2) Excessive humidity causes some plastics to react chemically with 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 poor, and it will cause cracking if it is polluted.
4. For machines:
The plasticizing capacity of the injection molding machine should be appropriate. If it is too small, it will become brittle due to insufficient plasticization, and it will degrade when it is too large.
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 the vacuum bubble is due to insufficient plastic injection or low pressure. Under the action of rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in volume loss.
(1) Increase injection energy: pressure, speed, time and material volume, and increase back pressure to make 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.
Analysis of Causes of Warpage and Deformation of Injection Molded Products
The deformation, bending, and twisting 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 causes the parts to shrink and warp due to different shrinkage rates. The large internal stress remaining inside the part 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) 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 a balance in 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.
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, 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, so that the part is overheated during demolding and ejects deformation.
(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.
Analysis of color lines of injection molded products
This kind of defect is mainly caused by the common problem of plastic parts colored by masterbatch. Although the coloring of masterbatch 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 regional color differences. 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 liquid mixing opportunities.
(2) When the screw speed is constant, increasing the back pressure can increase the melt temperature and shearing effect in the barrel.
(3) Modify the mold, especially the gating 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.
Analysis of the causes of shrinkage and depression 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) If the nozzle hole is too large, the melt will flow back and shrink, and if the resistance is too small, the amount of material will be insufficient and 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, use a machine with a large amount of plasticization to check whether the screw and barrel are worn.
(1) The design of the part should make the wall thickness uniform and ensure uniform shrinkage.