A Producer of BOPP asks; We are using EVA, extruded on BOPP or BOPET and we are always struggling to minimize gel. What is the cause of gel and how to eliminate?
Reply: In general EVA is sensitive to gel formation due to thermal degradation and crosslinking. So first you should look at the melt temperatures you are extruding at and lower them to what your supplier would recommend. The higher the Vinyl acetate content the more sensitive the EVA resins are.
Secondly if the screw design or the melt system is not streamlined and has restrictions to flow where eddies can form this is a place where resin will slowly oxidize and form gels which will form and then break loose into the flow stream. You will need to evaluate your extrusion system to make sure that it is streamlined and that the screw design gives good melting and mixing and that the melt temperature is not excessive for the polymer.
To determine what the gels are, first you have to isolate and analyze the “gel” to make 100% certain what it is. That is the only way you can be sure you are solving the correct problem.
You did not say if the gels were colored or clear. If the gels are colored it is likely due to oxidation of the polymer in the extrusion system. Some clear gels will come from the supplier as crosslinked polymer in the resin but they generally have a good handle on that, but that should be confirmed with the resin supplier.
If the gels are clear or a little opaque, it might indicate poor melt quality from the screw (unmelted polymer) and this has to be evaluated from a process study and performance characterization of the extruder and screw (output, melt temperature pressure and melt temperature stability as a function of screw speed and head pressure if you have a back pressure valve)
If it is an unmelt then it is the fault of the screw design or screw wear. The screw should be pulled and measured to make sure it conforms to the design and that it is not worn out. Once you have determined that it is not worn out you need to determine at what speed and back pressure you can run the screw with no unmelts (i.e. is the screw design correct for the resin).
You need to determine if the back pressure can be increased to eliminate the unmelts. You may need to experiment with barrel temperatures as well, perhaps lowering them to get better solid bed formation and higher torque. If the bed breaks up too soon and solids enter the melt it can be very difficult to eliminate them by just the metering flow.
If you cannot find conditions where you can melt the polymer 100% you will need to change the screw design.
But first you really have to determine what the “gels” are and where they are coming from. Are you extruding 100% of the EVA or is it blended with LDPE?