During the injection process, sometimes a little bit of plastic can seep out of the die joint and make a small rib. This is called flashing.
Some examples are pictured below :
How can Flashing be Avoided?
Flashing is not a good phenomenon in injection molding production, and it can cause die joint damage if the flashing can’t be immediately cleaned from the die joint and the clamp. This type of damage will cause more evident flashing in future products. For this reason, it’s very important to look out for flashing marks in injection molding production and remove them before they turn into a persistent problem.
There are many reasons for flashing to occur in injection molding. Some examples include very high injection pressures or terminal injection speeds, and inadequate clamping force.
Below are the some culprits and ways to solve them.
The injection pressure is too high or the injection speed is too fast
Adjust the speed of the injection at the nozzle so as to inject the plastic at the correct rate for the chosen material
Clamping force is too high
Adjust the pressure of the clamps to loosen the grip
The joint face isn’t fit or the accuracy is poor
Repair and check the mold or make the joint face more precise
Mold clamping force isn’t enough (especially true if the part has flashing around edges)
Increase the clamping force
The clamping area of mold is too large
Change the machine to one with larger mold clamping force or the mold design to accommodate a smaller clamping area
Partial flashing due to gate blush
Rebalance the gate either naturally (preferred) or artifically.
Use a harder material for the mold
Pressure drop before the material can set
Conserve the clamping pressure according to cooling temperature
The material of mold is bad or easy to wear and tear.
Choose better mold materials with proper heat treatment
The viscosity of the plastic is too low (like PA or PP).
Change the plastic material with a high viscosity one or adjust with a suitable filler
The material temperature or mold temperature is too high
Make adjustments to the respective temperatures of the mold and the material
How Do Bubbles Form in plastic?
When the mold cavity fills up, it is possible that the flow of the plastic is too quick and air can’t escape quickly enough from the mold. When this happens, bubbles can form in the materials.
The presence of air sets this issue apart from vacuum voids, which are caused by a similar yet not identical set of causes.
Some examples are pictured below:
The bubbles in the first picture are ones from the vacuum vesicle and the other is small bubbles in the plastic. The reasons and improving method of the plastic with bubbles in the below table:
Back pressure is low or the melt temperature is too high
Increase back pressure or lower the melt temperature
The screw rotation speed or the injection speed is too fast
Decrease the speed of the screw or the injection pressure
The gas can’t be expelled quickly enough
Make the mold vent larger to allow air to escape
Incorrect gate size
Decrease the size of gate or use a different shape
Raw materials are not fully dried
Increase the intensity of the drying process
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