How jetting occur on molding process

Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. The buckled, snake-like jetting stream causes contact points to form between the folds of melt in the jet, creating small-scale "welds". Jetting leads to part weakness, surface blemishes, and a multiplicity of internal defects.China CC Composite Cover Plate Manufacturers
Mainly Caused by Excessive ram speed, Poor gate position ,Lack of melt contact with the mold allows jetting to occur, Inadequate hot runner system design.
Explanation: Excessive injection speed (fill rate) will cause the molten plastic to form jet streams as it is pushed through the gates instead of the more desirable wide "tongue'' of material. These snake-like streams cool independently from the surrounding material and are visible on the molded part surface.China CC Composite Top Plate Manufacturers
Solution: Reducing the injection speed will allow the plastic flow front to stay together and not form the individual streams that cause the jetting patterns on the part surface.
Explanation: When barrel temperatures are too low, the material will not be heated to the proper temperature for adequate flow. The material will push slowly into the mold and the flow front will break up into individual streams as resistance builds up. China CC Composite Fasteners ManufacturersThis will cause jetting patterns. On the other hand, if the barrel temperature is too high, the material is pushed into the mold too quickly. This causes the flow front to split apart as it enters the cavity and the jetting patterns will develop.
Solution: Decrease or increase the barrel temperature accordingly.China CC Composite Heater Manufacturers The material supplier can recommend the best starting point for barrel temperature and it can be adjusted from that point. Make changes in 10 degree F increments and keep the profile so it is heating progressively from back to front.
Explanation: If the nozzle opening (or sprue bushing opening) is too small for the material being molded, the restriction may cause the material to flow too slowly and solidify early.China CC Composite Heat Insulation Bar Manufacturers The flow front may break apart as it travels through the gate (due to sidewall friction) and jetting patterns may develop.
Solution: Increase the nozzle opening. As a general rule, the nozzle opening should never be less than 7/32'' in diameter. The stiffer the material flow, the larger that opening should be. Make sure that the sprue bushing opening diameter matches or is 1/32'' larger than the nozzle opening.