Melt cracking leads to scorching
When the melt is injected into a cavity with a large volume under high pressure and high pressure conditions, the melt fracture phenomenon is easy to occur. At this time, the transverse fracture occurs on the surface of the melt, and the fracture area is coarsely mixed with the surface of the plastic part to form a green spot. Especially when a small amount of molten material is directly injected into the cavity which is easily too large, the melt fracture is more serious and the larger the presented spot is.
The nature of the melt fracture is due to the elastic behavior of the polymer melt. When the melt flows in the barrel, the melt near the barrel is subjected to friction by the barrel wall, the stress is greater, and the melt flow rate is higher. When the molten material is injected from the nozzle, the stress acting on the tube wall disappears, and the flow rate of the melt in the middle of the barrel is extremely high. The melt at the barrel wall is accelerated by the melt at the center and is accelerated due to the flow of the melt. Relatively continuous, the flow velocity of the inside and outside of the melt will rearrange, tending to the average speed.
During this process, the molten material will undergo sudden stress changes and will produce strain. Because the injection speed is extremely fast, the stress received is extremely large, far greater than the strain capacity of the melt, resulting in melt fracture.
If the melt encounters abrupt shape changes in the flow path, such as diameter shrinkage, expansion and dead angle, etc., the melt stays in the dead corner and circulates. It is different from the normal force of the frit, and the shear deformation is larger. Mixed into the normal flow material, when the injection, because the deformation of the two recovery is inconsistent, can not be bridged, if the disparity is very large, then the rupture fracture occurs, its manifestation is also the melt fracture.
Improper control of molding conditions leads to scorching
This is also an important reason that causes burnt and stain on the surface of plastic parts. In particular, the size of the injection speed has a great influence on it. When the flow material is injected into the cavity at a slow speed, the flow state of the molten material is laminar; when the injection speed When it rises to a certain value, the flow state gradually becomes turbulent.
Under normal circumstances, the surface of the plastic parts formed by the laminar flow is relatively bright and smooth. The plastic parts formed under the turbulent flow conditions are not only susceptible to surface smearing, but also tend to produce air holes inside the plastic parts. Therefore, the injection speed should not be too high and the flow material should be too high. Controlling filling in laminar flow.
If the temperature of the frit is too high, the frit will decompose and decompose easily, resulting in the surface of the plastic parts. Generally, the screw speed of the injection molding machine should be less than 90r/min, and the back pressure should be less than 2mpa. This can avoid excessive friction in the cylinder. heat.
If excess heat of friction is generated during the molding process due to the excessive rotation time of the screw when it is retracted, the screw rotation speed can be appropriately increased, the molding cycle can be prolonged, the back pressure of the screw can be reduced, the temperature of the feed section of the cylinder can be increased, and raw materials with poor lubricity can be used. Method to overcome.
During the injection process, too much molten material flows back along the screw groove and resin stays at the check ring. This will lead to depolymerization of the melt. This should be done using a resin with a high viscosity, and the injection pressure should be appropriately reduced. Large injection molding machines. The commonly used anti-backup rings of injection molding machines are more likely to cause retention, discoloration and decomposition. When the discolored melted material is injected into the cavity, brown or black spots are formed. In this regard, the nozzles should be cleaned regularly. Center of the screw system.
Mold failure leads to scorching
If the mold vent is blocked by the release agent and solidified material precipitated, the mold exhaust is not set or the position is incorrect, and the mold filling speed is too fast, the mold will not be able to exhaust the air in an adiabatic compression and the high temperature gas will decompose the resin to decompose In this regard, the obstruction should be removed to reduce the mold clamping force and improve the mold exhaust.
The determination of the form and position of the mold gate is also very important. In the design, the flow state of the melt and the exhaust performance of the mold should be fully considered.
In addition, the amount of release agent can not be too much, the cavity surface to maintain a high degree of finish.
Raw materials do not meet the requirements and cause burnt
If the content of water and volatile matter in the raw material is too high, the melt index is too large, and excessive use of the lubricant can cause scorch and blemishes.
In this regard, hopper dryers or other pre-drying methods should be used to treat the raw materials, replacing the resin with a lower melt index and reducing the amount of lubricant.