Charring due to melt fracture
When the melt isinjected into the cavity with large volume at high speed and high pressure, it is easy to produce melt fracture. At this time, the melt surface appears transverse fracture, and the fracture area is rough mixed in the surface of the plastic part to form paste spots. Especially when a small amount of melt is directly injected into the mold cavity which is easy to be too large, the melt fracture is more serious, and the larger the paste spot is.
The essence of melt fracture is due to the elastic behavior of polymer melt. When the melt flows in the barrel, the melt near the barrel is abraded by the wall of the barrel. The stress is large, and the flow speed of the melt is small. Once the melt is injected from the nozzle, the stress acting on the wall of the tube disappears, while the melt flow velocity in the middle of the barrel is very high, and the melt at the barrel wall is carried by the melt at the center As the melt flow is relatively continuous, the internal and external melt flow speed will rearrange to the average speed.
In this process, the melt will have a sharp change in stress, which will produce strain. Because of the extremely fast injection speed, the stress received is particularly large, which is far greater than the strain capacity of the melt, leading to melt fracture.
If there are sudden shape changes in the flow channel, such as diameter shrinkage, expansion and dead angle, the molten material stays and circulates at the dead angle. It is different from the normal molten material in the stress, and the shear deformation is large. When it is mixed with the normal molten material for injection, because the deformation recovery of the two materials is inconsistent, it cannot be bridged. If the difference is large, it will break, and its manifestation will also be It's a melt fracture.
Coking due to improper control of forming conditions
This is also an important reason for scorch and paste spots on the surface of plastic parts, especially the injection speed has a great influence on it. When the flow material is injected into the cavity slowly, the flow state of the melt is laminar; when the injection speed rises to a certain value, the flow state gradually becomes turbulent.
Generally, the surface of the plastic part formed by laminar flow is bright and flat, and the plastic part formed by turbulent flow is not only prone to paste spots on the surface, but also prone to air holes inside the plastic part. Therefore, the injection speed should not be too high, the flow material should be controlled in the laminar flow state.
If the temperature of the melt is too high, it is easy to cause decomposition and coking of the melt, resulting in paste spots on the surface of the plastic parts. In general, the screw speed of injection molding machine should be less than 90r / min, and the back pressure should be less than 2MPa, so as to avoid excessive friction heat of the barrel.
If excessive friction heat is produced due to too long rotation time when the screw is returned in the forming process, it can be overcome by properly increasing the screw speed, prolonging the forming period, reducing the screw back pressure, increasing the temperature of the feeding section of the barrel and using raw materials with poor lubricity.
During the injection process, too much refluxing along the screw groove and resin retention at the check ring will lead to the depolymerization of the melt. Therefore, the resin with high viscosity should be selected, the injection pressure should be reduced properly, and the injection molding machine with large length diameter ratio should be used instead. The check rings commonly used in injection molding machines are easy to cause retention, which makes them decompose and discolor. When the decomposed discolored melted material is injected into the cavity, it will form a brown or black focus. In this regard, the screw system centered on the nozzle should be cleaned regularly.
Burning due to mold failure
If the vent hole of the mold is blocked by the release agent and the solidified substance separated from the raw material, the exhaust setting of the mold is not enough or the position is not correct, and the filling speed is too fast, the high temperature gas produced by the adiabatic compression of the air in the mold which is too late to be discharged will make the resin decompose and coking. In order to solve this problem, it is necessary to remove the obstruction, reduce the closing force and improve the poor exhaust of the mould.
It is also very important to determine the form and position of the mold gate. The flow state of the melt and the exhaust performance of the mold should be fully considered in the design.
In addition, the amount of mold release agent should not be too much, and the surface of the cavity should be kept a high degree of finish.
Scorch caused by raw materials not meeting the requirements
If the content of water and volatile matter in raw materials is too high and the melt index is too large, the excessive use of lubricants will lead to scorch and smudge failure.
In this regard, hopper dryer or other pre drying methods should be used to deal with raw materials, to replace resin with smaller melt index and to reduce the amount of lubricant.
