UNPLUGGED: Defeating die buildup
Die buildup is, to put it bluntly, a pain in the a**.
Die buildup is, to put it bluntly, a pain in the a**.
Also known as drool, die bleed, or plate-out, buildup is the bane of every extrusion process, causing a wide range of problems from visible surface imperfections on extrudates to process disruption to outright product failure.
It’s not a situation that goes over well in today’s ultra-competitive plastics processing industry, where production delays are about as welcome as another Canadian election. “Frankly, I don’t meet a lot of customers who will tolerate shutting down their extrusion line because of die buildup,” said Jason Baird, process development engineer with Davis-Standard’s Extrusion Systems Group. Too often, though, many do just that, expending time and labor removing die buildup – including scraping out the accumulated gunk by hand – because they don’t know how to eliminate it any other way.
But it doesn’t have to be like this, and it definitely doesn’t have to happen to you. By honing in on three areas – your die, your process, and your material – you can improve your chances of stopping die buildup before it stops you.
Like all good villains, die buildup has a complicated origin. “Die buildup is related to stress at the die exit,” said John Perdikoulias, president of the North American office of polymer processing specialist Compuplast International Inc. “Resin flowing along the die inner surface moves relatively slowly and then is suddenly accelerated as it exits the die. This sudden acceleration causes stress within the melt, with low-molecular-weight polymer fractions and other components separating and being deposited at the die exit.”
Processors have a choice, Perdikoulias continued, as to how seriously they want to take the problem. “There are a number of ‘tricks’ that can negate some of the effects of die buildup,” he said. “Applying mold release or silicone to the die exit area after cleaning can reduce the rate of buildup and extend the time between cleanings; slowing down the process also reduces die buildup, but this hurts output; and the buildup can be intentionally pushed onto the product and removed further down the line.”
But these approaches treat only the symptom, not the problem. “Almost always, the better solution is to address the direct causes of the buildup,” Perdikoulias said.
THE DIE IS CAST
The first of these involves your die. And just like the silk purse and the sow’s ear, it’s almost impossible to prevent buildup in a badly-made die. To a certain extent, then, you’re at the mercy of your tooler from the outset. “A good tooling supplier goes to great efforts to machine tips and dies to a determined specification, ensuring perfect concentricity and alignment,” said Matt Fagan, a design engineer with Guill Tool & Engineering Co. Inc. “The extrusion material is then distributed into the proper location as part of the finished product, rather than building up on the die.”
So If you haven’t guessed by now, die geometry plays a huge role in helping or hindering buildup. “Increasing die land length can also lessen the amount of die buildup by reducing swell,” said John Perdikoulias. “Or you can use a larger die opening to reduce die-exit stress, and then draw the extrudate down to the desired size.”
On this note, enter die-flow modeling, which has been used to good effect to study the stresses that occur at the die exit, and to evaluate changes in die-exit geometry. “While it’s still a developing field, modified die exits said to decrease die buildup include a sharp square exit, sharp pointed exit, radiused exit, outward-stepped exit, inward-stepped exit, and outward-flared exit,” Perdikoulias said.
But even the best-made die still needs a little help – and sometimes it’s just a matter of applying some elbow grease. “Polishing the parts of the die that come into contact with the material, specifically the pin and the bushing, has a big impact on reducing die buildup,” said Jason Baird. “If I had to rank all of the various ways to tackle the problem, polishing would top the list.”
Adding a processing aid, which basically coats the surfaces with fluoropolymers like PTFE is beneficial in preventing die buildup, but it’s also an additional expense. “Another approach is to apply metallic coatings that incorporate fluoropolymers to the inside of the die to reduce stress at the die exit,” said John Perdikoulias.
PERFECTING THE PROCESS
Compared to the extruder die, your process is something you can control. Take heat. “Sometimes lower die temperatures can create a cool layer of resin on the inner surface of the die that creeps slowly to the die exit and then separates from the bulk flow, causing buildup,” Jason Baird said. “Higher die and melt temperatures are one way to reduce die-exit stress, although gains may be offset by additional degradation or burning due to residence time.”
When checking your process for ways to reduce die buildup, changes in melt and die temperatures should be explored independently. “Start by determining the true melt temperature and set the die body to that temperature – standard melt thermocouples are often wrong, so check the melt temperatures by hand,” Perdikoulias said. “Also, the die exit surface can be significantly cooler than the die itself, so check the die exit temperature with a surface thermocouple.”
And consider an air sweep – a wand contoured to the extrusion profile shape, with holes drilled in it to distribute compressed air. Pointed at the die exit, it can help reduce and control buildup. “The air sweep can direct any smoke and condensables away from the die,” Perdikoulias said. “Air sweeps also have the benefit of cooling die buildup so it doesn’t oxidize and turn dark. Nitrogen is also used to prevent oxidation, but take care not to blow too hard or cool the die so much that it distorts the extrudate.”
No surprise here: Since resin is what’s building up at the end of your die, odds are the resin’s chemical characteristics are playing at least a partial role in that accumulation.
To begin with, different materials produce different sorts of die buildup. “PVCs used in window lineages or decking are often formulated in-house, which can make them more susceptible to buildup than commercialized polyolefins or other engineered resins,” said Mike Pulhalla, general manager, technical director of global extrusions, with Milacron.
Making it more complicated, resins from some suppliers will generate more die buildup than others, even when the materials have the same characteristics. It’s a good rule of thumb in life, and true here too: try the simplest potential solution first. “If die buildup occurs, try a similar grade from another supplier,” said John Perdikoulias. “Resins from different suppliers can have similar shear viscosity but very different elongational viscosities. If no other differences can be seen between resins, try testing the elongational viscosity. The resin with the higher elongational viscosity will likely have more problems with buildup due to higher stresses at the die exit.” If you’re still stumped by inexplicable performance differences, try testing the die buildup chemically to see what’s in it.
And just to make it more complicated still, chemical compatibility of different components in a formulation also plays a role. “Extreme die buildup often occurs when highly incompatible polymers are melt-blended,” said John Perdikoulias. “In such cases, use of compatibilizers can help. Fluoropolymer processing aids can also be added in small amounts to reduce stress at the die exit.”
Lubricants can help many resins slide through the die without building up – but only to a point, after which they can actually add to the problem. “At some point, lubricants will build up and have to be removed from the die, which means more time and labor for die reinstallatio
n,” said Matt Fagan.
So has die buildup really met its match? “I wouldn’t say that die buildup will never be a problem in an esoteric application where, for example, you’re stuck with a certain material because only one supplier makes it,” said Jason Baird. “Those situations aside, if processors have the right tooling and process parameters, and follow a few simple steps, they can eliminate buildup almost entirely from most applications.”