Today, we continue our journey of exploration into the common issues that happen in plastic production. This time we will take some time to get how to avoid jetting and air lines! Want to know more about other common issues? Check out our blog post about weld lines issues.
What is Jetting?
In the process of injection molding, if the melt is injected too fast at the gate, the surface of the parts (in front of the side gate) will produce a serpentine spray line. This is commonly called jetting.
Most of the jetting is found when the gate type of the mould is side gate. When the plastic melt flows through the nozzle, runner, gate, and other narrow areas at high speed, it suddenly enters the open and relatively wide area of the mold where the melt material will bend forward along the flow direction like a snake. The molten plastic will then cool down quickly after contact with the mold surface. Because this part of the material cannot have a good fusion with the subsequent resin into the cavity, the result is a clear spray on the product. Under certain conditions, the melt in the early stages come in at a relatively low temperature when expelled from the nozzle so, before the contact surface of the cavity, the melt viscosity is very high. As the lower temperature resin flows in like a snake, the as the higher temperature of subsequent melt comes into the cavity unceasingly, it causes the initial melt extrusion to be deeper in the mold leaving a jetting form.
The reason and improvement method of injection pattern are shown in the table:
Improper placement of gate（Direct injection into the cavity）
Change gate position（Move to the corner）
The material temperature or mold temperature is too high
Reduce the material temperature and mould temperature
Injection speed is too fast（in-gate）
Reduce the injection speed
The gate is too small or improper（side gate）
Change the bigger gate
Plastic is too mobile（FMI is too high）
Change the plastic with low mobile
What are Air Lines?
This is not about flying the friendly skies, but there is a gate involved. In the process of injection molding, if the gate is too small and the injection speed is too fast, the melt flows violently and pulls air into the melt. This will leave air marks in the gate position, turning points, and step positions of the plastic parts. It's easiest to see the air lines at the gate like with materials like ABS, PC, PPO and other injectable plastics. The picture of a red plastic part here shows this condition:
The reasons and improved methods of air lines are as follows
The melt temperature is too high or the mold temperature is too low
Reduce the material temperature (to prevent decomposition) or increase the mold temper
The gate is too small or improperly positioned
Increase gate size or change gate position
The injection speed of the air lines is too fast
Multistage injection of adhesive to slow down the injection speed of corresponding parts
Excessive length or fineness of flow passage (melt is easy to cool)
Shorten or enlarge runner size
Product step/corner position without arc transition
Product steps/corner and arc
Bad mold exhaust (trapped air)
Improve the mold exhaust effect
The coolant hole in the flow channel is too small or insufficient
Increase or enlarge the cold hole
Raw materials are not fully dried or decomposed by overheating
Fully dry the raw materials and prevent them from overheating and decomposition
The viscosity of plastic is large and the fluidity is poor
Switch to more liquid plastic
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