I just began my new job. We have a rigid PVC product ('W' shape). The engineering quote extrusion speed is 1500ft/hr, while our actual extrusion speed is 700ft/hr. This made we lost a lot of money. We found if we speed it up, the product would be burned and smoked. Someone said the problem is we don't have good cooling system for the die (when the speed is high, the temperature of the die will go up and can not drop down). While I think it may have some relationship with the die design. Would someone give me some idea? If it was the cooling system, what we should do (buy new oil cooling system is very expensive)?
The wall thickness of the product is 1mm. We are using the 3 1/2" single screw extruder. We can use this extruder extrude thick products without any problem with 1500ft/hr.
A number of things could be going on here. Since this is a single screw extruder, I'm assumming that you are using precompounded pellets. Are you extruding a rigid or flexible PVC compound? How hot is the die or melt temperature at 700 ft/hr and at 1500 ft/hr. I wouldn't push PVC much past
200 C. Also, your screw may be generating too much shear which is causing the PVC to over heat. You may need a less agressive screw design for your compound.
That said as L Alpert indicated you'll need to have water cooled barrels on the extruder. Have you tried a reversing barrel temperature profile? Something like 190 C at the feed throat moving down to about 160-150 C at the metering zone and then 190-200 for the adapters and die?
We use precompounded PVC. Air cooling system is used for the barrel. The scew temperature is 332, 336, 336F. The die temperature is 341,
338F. The PVC melt temperature range is 360-380F.
The PVC supplier don't want to put too much on the formulation (such as stablizer). I suggested using the water cooling system, but the Plastic Specilist ( he has a lot of experience without much education as my boss said) did not agree. He said that would not help and would cause maintenance problem.... He wants to try using air to cool the screw..... That is all. The screw we are using is designed and made outside for our rigid PVC material. We don't have any parameters about the screw (funny?). I am going to contact with the supplier to provide... That is it.
Air cooling the screw will usually cause a buildup in the channels of the metering end which will increase the compression ratio (not as bad as liquid cooling) and will lower output and increase your problem. You need to carry heat away from the process more effectively if you are even to have a chance. You will need a cooling jacket with a heat transfer fluid on the barrel. 1500 lbs an hour on a 3.5" single screw without effectively dissipating the shear energy will be a costly mistake. Your "specialist" is incorrect. there is no substitute for the energy you need to carry away from the process.
Check with the SPE website or Antec for white papers on shear energy in extrusion....
Right now the screw running speed is 32 rpm. The output is 20KG/hr (500feet/hr). There is no good pressure gauge on the head (broken). I will let them get in new guages. I found all the products which we are made from this rigid PVC have the same problem (cannot speed up). The other problem is when the speed was very high the product surface was rough ( As the 'specialist' told me, I have had a chance to see that, yet). It seemed the white colorant master did not melt completely ( I guess). Our screw is only one stage. Will change the screw desgin help? My boss really supports me and asks me do what I want, but I don't want to make some serious mistake. In the meanwhile, the 'specialist' is not happy with any change. I have to find sufficient evidence to convince him.
I'm not too surprised. My recollection from processing PVC on single screw extruders was that it was difficult to go much faster than 35 rpm without the compound burning up. My recollection is that with a 6" extruder at 35 RPM we were only doing about 2200 lb/hr. This is why most companies have switched from single screw extruders to conical or parallel twin screws. You can get more output with less shear. The rough surface at higher speed sounds like you have over fused the PVC and you are at the onset of degradation.
In order for you to go faster, you'll need to be cooler. So, you must drop your barrel zone temperatures and allow the compound to heat up via shear heating. You might first try leave your feed zone where it is at 332 F and then setting the rest of the barrel zones at that temperature as well (leave the adapter and die at 340 F) and bump your speed up by 5-10 rpm. See what you get (be patient you may have to wait 1-2 hrs to see results). Keep an eye on your melt temperature to avoid burning up. Next try dropping the middle and metering zone by 10 and 20 F respectively. Again see what you happens as you bump up the screw speed. For more improvements drop all the zones by 5-10 F. Go in small increments. That way you can hopefully recover before falling over into dehydrochlorination.
I've done this with lines making PVC pipe and in one case went from running with zones at about 350-370 F to 300-320 F by the time we were done with output up about 20%.
Your other alternatives are to work with your compound supplier to see if they have a compound that is more lubricated and as such not as shear sensitive. Or to switch to a screw design that imparts less shear.
It sounds as if you have a high shear rate as it is. What is your screw clearance? If it is over .010", either your barrel is worn or you screw is undersized. 32 RPM? Awful low. What is the motor load? No barrel or die pressure? I hope you at least have blow plugs.
Of course, there may be other issues (compression ratio of screw, metering depth, feed section wear, and lack of proper cooling to name a few (cooling is moot if the screw and barrel clearance is too large).
Thank you. I will take both of your suggestion. I will contact with the material supplier about the compounding (lubriciant & stabilizer) and think about the water cooling system. Our equipments are just toooooo old. Except the temperature and screw speed, I can not find anything from the control panel. I asked the mechanical department get in the pressure gauges. I will keep updating my experiment result. Thanks again.
I can't see how they can give you a clearance without knowing the barrel ID. You need to use a bore scope, preferable one that can get measurements at various depths into the barrel (all the way up to the feed throat if possible). Look in MSC or McMaster Car catalogs for one. Make the measurements at room temp, and also get a flight micrometer for the feed screws and get some real numbers. (Barrel ID/Screw OD)/2 is your clearance.
A 3.5" barrel and screw is a pretty good capital expense. There should be a replacement invoice somewhere. If they have been in heavy use for 15 years and have not been replaced, I can't see how they are not worn. It's hard to fight the "it's been working all this time" management mentality....