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It means shrinking the volume of polymers during the cooling stage of the polymer forming process. This shrinkage is partly related to the difference in density of polymers in solid and molten states. In the production processes of polymer products, shrinkage occurs mostly in injection molding. When the molded product is cooling, it may show some shrinkage until it reaches temperature and humidity stabilization conditions. In materials such as nylons and steel, this shrinkage is more. If shrinkage in polymer parts is uneven, it creates stresses that may cause cracks later in the use of the part.
Low injection pressure, low cooling time (increasing cooling speed), high melt temperature, high mold temperature, low holding pressure are possible reasons for shrinkage.
This defect usually occurs in the thick parts of the part. The cooling process in the thick parts is slower than in the rest of the parts and this causes a shrinkage phenomenon in the central layers. This state is very similar to a sinkhole in the ground, with the difference that the sinkhole is caused by erosion, but in plastic, it is due to shrinkage.
One of the reasons is improper performance and timing in the cooling process of the plastic injection mold. It can also be caused by the high temperature of the polymer melt. In fact, thick parts cool down more slowly than parts with low thickness. Therefore, this case is usually seen more in thick parts.
Bubbles are air trapped in the piece. Usually this bubble is formed near the surface in its thick part. The formation of these bubbles causes the concentration of stress in that area. As a result, the part becomes brittle.
Vacuum spaces are often caused by uneven freezing between the surface and the internal parts of the part. Also, the low injection pressure causes the air inside the mold to not be fully discharged.
1- Place the material entry gate in the thickest part of the piece. 2- Increase the injection pressure and time.
3- Use materials with low viscosity (more flowable). In this case, it can be ensured that less gases are trapped because the gases are able to escape faster.
4- Make sure that the mold parts are aligned during installation.
This state occurs when two molten streams meet in the injection mold from two different directions.
These lines appear when improper bonding occurs during the solidification process between 2 or more molten streams that meet.
1- Increase the temperature of the mold or molten plastic
2- Increase the injection speed.
3- Adjust the design for the flow pattern in the form of injection in such a way that there is a flow source.
4- Use plastic with low viscosity and melting temperature
This deviation occurs when there is uneven shrinkage in different parts of the part. The result is a twisted, uneven or bent shape that is not intended.
Non-uniform cooling of different parts of the part. Different cooling rates in different parts of the part cause tension in it.
1- Make sure the cooling time is long enough and slow enough to avoid stress in different parts.
2- Design the mold with uniform wall thickness so that the plastic melt flows in one direction
3- Choose plastic materials that are less likely to change shape. Semi-crystalline materials are generally more prone to distortion.
Jetting is a state in which the molten material, due to the injection speed, enters the thickened area from the areas with a small and narrow cross-section, and the polymer melt is not placed on the surface of the mold.
1- Jetting mostly happens when the melting temperature is very low and the viscosity of the material increases. As a result, the melt resists flowing.
2- When the material comes in contact with the walls of the mold, it starts to cool down quickly and the viscosity increases. On the other hand, the materials behind the cooled melt move it forward and this causes there to be spots that we refer to as jetting.
