Spraying, also called spray marks and spray saliva, refers to the 65 thread-like flow lines that appear at the gate of the product, mostly appearing when the mold is a side gate. When the plastic melt flows at high speed through the nozzle, runner and gate, and other narrow areas, then suddenly enters the open, relatively wide area, the molten material will bend forward along the flow direction like a snake, and cool quickly after contact with the mold surface.
Since this part of the material does not blend well with the resin that subsequently enters the cavity, it creates a distinct jet stream pattern on the product. Under certain conditions, the resin is ejected from the nozzle at a relatively low temperature at the beginning of the stage. Before contacting the cavity surface, the resin becomes very viscous, thus creating a snake-like flow. Next, as the higher temperature melt continues to enter the cavity, the initial material is squeezed deeper into the mold, thus leaving the aforementioned jet flow marks. As shown in the figure
Nozzle salivation fault analysis and elimination methods
1) Improper operation of process conditions.
The causes and treatment of the production methods are as follows.
A nozzle at the local temperature is too high. Should be appropriate to reduce the nozzle temperature.
B melt temperature is too high.
Should be appropriate to reduce the barrel temperature or shorten the molding cycle, as well as the nozzle set in the filter network.
C The residual pressure in the barrel is too high.
The injection pressure should be lowered and the residual pressure time should be reduced, and the injection time should be shortened.
D The nozzle hole is too large. Should be replaced with a small aperture nozzle, or use spring needle valve nozzle and inverted slope nozzle.
2) Raw material moisture does not meet the use requirements.
Molding raw material moisture content is too high, which will also cause nozzle salivation. In this regard, raw materials should be pre-dried or use hopper dryer.
(3) Hot runner mold design is not reasonable.
In the hot runner mold, to prevent nozzle salivation, a device that can release the residual stress in the collector cavity should be set.