Why Have Burn Marks in Injection Molding?

When it comes to molding defects, burn marks in injection molding are some of the most common. Why have burn marks in injection molding?

Burn marks in injection molding cause1: Melt fracture

When the melt is injected into the large volume of the cavity at a high speed, high pressure, melt fracture phenomenon is easy to produce, at this time, the melt surface transverse rupture occur, the rupture area of the roughened surface to form inclusions in the plastic paste spot.

In particular, when a small amount of molten material is injected directly into the cavity is easy too large, melt fracture is more serious, the greater presented paste spots.

Melt fracture due to the nature of the elastic behavior of the polymer melt is produced, when the flow of melt in the barrel, the barrel of the melt near the cylinder wall by friction, stress is large, the melt flow rate than small, melt upon injection from the nozzle, the wall stress effect disappears.

The melt flow rate of high central cylinder, the cylinder wall of the frit is carried at the center of the melt is accelerated, because the flow of melt is relatively continuous, the flow rate of the melt inside and outside rearrange, become average speed.