Thursday, August 6, 2009

Need Help Extruding Rigid Tubing Using a Vacuum Tank

Buzz: You know Skip, we are working on extrusion materials from the last NPE show, but we often get questions that show the need for making sure just the basics are covered as well - like this one:

"We just got a vacuum tank and have had several jobs tool really well on it. On the past few jobs, we are getting a fold in the wall of the tubing that we cannot seem to work out. Materials are PP, PE and Nylon. I know that we are probably missing something simple. Any suggestions or resources that can help me trouble shoot?"

Skippy: Oh boy, I hope that our 30 years of extrusion experience can be of some benefit. We do this for a living, and would be glad to come in and help - Note these comments are not meant to criticize, but we will assume nothing -

Mentioned in your post:

"We just got a vacuum tank and have had several jobs tool really well on it. On the past few jobs, we are getting a fold in the wall of the tubing that we cannot seem to work out. Materials are PP, PE and Nylon. I know that we are probably missing something simple"

Buzz: Let's talk about a couple of these elements to see if you are in fact "missing something".

Skippy, ok, A) You got a new vacuum tank. Was this the only addition to the line, or did you get some new tooling with it to make particular parts as well? You mention the first few jobs ran "well" - were they in the same materials? Using the same tooling? Do you have a program in place to tear down the machine from job to job and a set up sheet that has been gone over to be sure that all processing conditions are "correct"? Are you running more or less lines now that in the past (see calibration - aka water)

Buzz: right, then B) Suddenly, you are having problems with the extrudate folding over in the wall of the extrudate - is it in one area only, or all around the tube - A fold usually indicates one of several things -

1) The wall balance versus the "draw down" is not correct.

When you shear off some material coming out of the die at rate, the parison (tube shape) pushed out should flow out roughly horizontally for a small way (until gravity kicks in).

Question? Do you clean the machine up totally from material to material (clean means all plastic out of the tooling to bare metal, pull the screens, pull the screw and clean all down to the metal?) If not, you might have material hung up in the barrel or at the tip of the screw, in the screens or breaker plate that isn't moving. Since the screw is imparting ribbon flow that must be converted into plug flow through the breaker plate, if you interrupt this plug, erratic and generally uneven flow through the balance of the tooling may occur. You might also have material caught up in one or more spider legs - causing whacked out flow that an operator is trying to correct with pin/shell relationships -

Once the material flows through the gate adapter in plug flow, through the spider and pin/shell in even distribution and assuming that your tooling is approximately 5 degrees above the correct melt temperature, then you should be delivering a "tube" out of the tooling that is ready for draw down and calibration -

Skippy: Good point, but it also might be
2) Die Gap - this is the distance between the Die Face and calibration. Assuming that the wall is even, then we would want the Die Gap to be even from every point on the die to every point on the calibration. Since the tooling is larger than the calibration, you need to back the tank AWAY from the die so that "draw down" can occur. The correct distance away will be somewhat variable based on

a) the melt temperature and "stiffness" of the melt"
b) the wall thickness versus the cross sectional area
c) the lot of material - yes I know that your materials should be fractional melt, but since they are "fractional" even a small change can affect the apparent "stiffness" of the melt at any given temperature.

In the materials that you mention, it is not uncommon for the OD and ID of the tubing tooling to be larger (in some cases up to 60% or more) than the final size of the tubing so that the material can be drawn down prior to entering the calibration. Are you using common tooling for all of the materials, or do you have different pins and shells for different sizes of tubing from different materials?

Also the die gap is established by making sure that the calibration and extrusion centerlines in both planes (side view and top down) are straight in line with one another. Vacuum tanks, takeoffs and cutting devices are notorious for vibration and will "creep" along the floor if able - is the line "in Line?" if not, you are perhaps not applying cooling evenly down line.

Buzz: yes sir, and doen't forget
3) Lubrication - two of the materials - PP and PE have an affinity to move from being cohesive to adhesive as they get hotter - generally, most custom extruders use a lubricant (water or other proprietary) systems to lubricate the material as it enters the calibration. Is this system working correctly? Nylon is VERY sensitive to moisture - is your drying system capable, is the (lack of) moisture level very well maintained? It goes without saying that PP, PE and Nylon all require different pins and shells and draw downs to make the "same" size tubing. Anyone that tries to run all three from the same tooling perhaps can by sacrificing rate or quality. Best to optimize for each material and go clean to clean between materials on machines.

Skippy: yup, and this is key -
4) Calibration cooling - is it dependable and repeatable? Is it actually working? Your vacuum tank calibration cooling might be hooked up to

a) well water; coming up out of the ground, it is likely to be around 55 degrees all year round - good system if dependable in terms of gallons per minute

b) city water; likely to change temperature with the seasons - 55 or so in the winter to possibly 75 degrees of more in the summer. Again, are you sure that your supply in gallons per minute is known and repeatable?

c) some sort of internal water recovery system (cooling tower, chiller); do you do preventative maintenance on the system, again do you know day to day what the temperature and actual gallons per minute availability is

Once the supply of water is maintained, then it has to be APPLIED to the calibration in a known manner. I like to put gallons per minute meters in line supplying my calibration - for a couple of reasons -

a) once we know for instance that 7 gals per minute at 55 degrees on a part running 150# per hour at "X" feet per second through the calibration at 300# per thousand feet is running WELL, I note that as being critical to being repeated in the future.

Buzz: ok, all together now, b) are we ready for "silly?"

We often encounter plants with a problem like this, where everything is running well as in a) above, and then the process "goes south". I always ask what is "new at the line" - "Nothing" is generally the reply. Then I ask "what else is new in the plant?" and we find out that three other lines were being started at the same time, and the SUPPLY of water which was capable of giving me 7 gal per minute with valve open 1/4 is now being diverted into filling three more lines. A quick check on my gal per minute meter shows that at my line, we've dropped to 3 gals per minute and the parts are "folding" (too hot). I open the supply valve at my line from 1/4 to say 1/2, bringing my supply back up to 7 gal per minute and line settles out. Don't forget that when the other three lines eventually settle into production and turn their valves from FULL to fill to 1/4 to run that we will suddenly have TOO much water (check the gage) and have to make an adjustment again. One of these gages on every line allows each operator to know what effect others are having on his line, and what to do to get back in control rather than just "trying zone three for 5 degrees" etc.

Skippy: wow, just goes to show you that laying down the foundations is really important. Other simple thoughts include making sure that the water application in the tank is even - water rings get plugged up with rust, shavings, dirt etc, and make sure water is cooling all sides evenly.

Buzz: We haven't really talked much about how to set up the calibration, the materials of construction to seal around the tube after the calibration and from compartment to compartment etc, but have been focused on the "fold" probably happening with either uneven flow out of tools or uneven cooling in the Calibration.

Once again, we can be called upon for additional help, training etc in your plant as required.

Just our "two cents" -