When melt is injected into a cavity with a large volume under high-speed and high-pressure conditions, it is very easy to cause melt fracture. At this time, surface of melt appears transverse fracture, fracture area is roughly mixed on the surface of plastic part to form a paste spot. Especially when a small amount of melt is directly injected into cavity that is prone to be too large, melt fracture will be more serious, and paste spot will be larger.
Nature of melt fracture is caused by elastic behavior of polymer melt. When melt flows in barrel, melt near barrel is rubbed by barrel wall, stress is greater, flow velocity of melt is lower. Once melt is injected from nozzle, stress on tube wall disappears, melt flow rate in the middle of barrel is extremely high, melt at barrel wall is carried and accelerated by melt at the center. Since flow of melt is relatively continuous, flow velocity of inner and outer melts will be rearranged, tending to an average speed.
In this process, melt will undergo a sharp stress change and strain will occur. Because of extremely fast injection speed, stress received is extremely large, which is far greater than strain capacity of melt, leading to melt fracture.
If melt encounters a sudden shape change in runner, such as diameter shrinkage, expansion, and a dead angle, melt stays and circulates at dead angle. Force is different from normal melt, and shear deformation is larger. When it is mixed into normal flow material and injected, due to inconsistency of deformation recovery of the two, it cannot be bridged. If disparity is large, fracture and rupture will occur, its manifestation is also melt rupture.

 

This is also an important reason for scorching and paste spots on the surface of plastic part, especially size of injection speed has a great influence on it. When flow material is slowly injected into cavity, flow state of melt is laminar flow; when injection speed rises to a certain value, flow state gradually becomes turbulent.
Under normal circumstances, surface of plastic part formed by laminar flow is relatively bright and smooth. Plastic part formed under turbulent flow condition is not only prone to smearing on the surface, but also prone to pores inside plastic part. Therefore, injection speed cannot be too high, and flow material should be controlled to fill mold under laminar flow.
If temperature of molten material is too high, it is easy to cause molten material to decompose and coke, resulting in a paste spot on the surface of plastic part. Generally, screw rotation of injection molding machine should be less than 90r/min, and back pressure should be less than 2mpa, so as to avoid excessive frictional heat in barrel.
If excessive frictional heat is generated during molding process due to too long rotation time when screw is retracted, it can be overcome by appropriately increasing screw speed, extending molding cycle, reducing screw back pressure, increasing temperature of barrel feeding section, and using poor lubricity materials.
During injection process, too much melt flow back along screw groove and resin retention at check ring will cause melt to depolymerize and decompose. In this regard, a resin with a higher viscosity should be selected, injection pressure should be appropriately reduced, and an injection molding machine with a relatively large diameter should be used instead. Non-return rings commonly used in injection molding machines are more likely to cause retention and discoloration. When decomposed and discolored molten material is injected into cavity, it will form a brown or black focus. In this regard, screw system centered on nozzle should be cleaned regularly.

If mold vent is blocked by release agent and solidified material precipitated from raw material, mold vent setting is not enough or position is incorrect, and filling speed is too fast, air from mold is too late to be discharged by adiabatic compression and high temperature gas will cause resin to decompose and coke. In this regard, obstruction should be removed, clamping force should be reduced, poor exhaust of mold should be improved.
Determination of form and position of mold gate is also very important, flow state of melt and exhaust performance of mold should be fully considered in design.
In addition, amount of release agent should not be too much, and surface of cavity should maintain a high degree of smoothness.

If content of moisture and volatile matter in raw material is too high, melt index is too large, excessive use of lubricant will cause scorching and sticking spots.
In this regard, hopper dryers or other pre-drying methods should be used to process raw materials, switch to resins with a smaller melt index and reduce amount of lubricants.