Thursday, November 12, 2009

Worn extrusion equipment and surging

Skippy: Hey Buzz - happy almost Thanksgiving - we saw an interesting question in another plastics forum -

"Can a worn extruder screw cause surging? I've heard this for years but can't see how a linear variation with a period of a minute or two can be caused by a slow radial change over many weeks or months or even years. If anyone can come up with a reasonable and technically sound explanation, I would like to see it. ALG"

Buzz: - Well Skippy, I tend to agree with most that the slow evolution of “wear” is not likely to create a situation where we have good melt pumping on Tuesday, only to have it fall apart on Wednesday. Screw melt pumping conditions DO change over time with wear, and particularly when aggressive screw and screw tip cooling are employed to “throw material around” in profile tools.

Skippy: To be fair, don't we have to acknowledge here that there are a number of critical process areas that contribute perhaps as much to "surging" based on material delivery (aspect ratio of material components, mix of materials, conveying method/stability, existence of fines, drying temperature, air flow, residence time in the dryer, feed throat temperature control, zone 1 and or 2 temperature control, etc etc etc) to the feed section of a screw that are ignored or not very well understood in too many shops?

Buzz: Of course, although that conversation is way beyond the scope of this particular discussion and will be investigated more fully in the future. On the other hand, in custom plant environments where screw and barrel wear were faithfully measured by quarter on extrusion lines that ran a variety of materials from week to week (aka not just one material from beginning to end of each screw/barrel rebuild life) we observed the following –

The most amount of wear from quarter to quarter occurred at the END of the screw and or barrel life measurement period, not at the beginning. (by production hours)

R&D staff tend to prefer to “run in new items on new screws and barrels” and in short order, the “process” is/was not as "stable" to established run conditions in future runs and required “tweaking”

Many US firms have gotten caught in the trap of increasing rates to “increase contributions per hour” (false economy of course if no new sales fill the new empty machine time) at the expense of significantly closing the ‘window of process ability’ with any particular brand of elements

Skippy: I'm wondering whether the economic climate we're experiencing contributes in any way as well -

Buzz: well, we have observed that batch to batch variation in raw materials from manufacturers quite frankly (despite their “certifications” on lab materials) DO exist, sometimes in great measure and

Business conditions (and perhaps lack of cash flow?) in custom extrusion in poorly run shops affects the run to run use of varying raw material virgins and regrinds –

Skippy: How does that affect running an extrusion line?

Buzz: Well sometimes, rather than running a standard mix of regrinds back into product from the beginning of the run until the end, some houses accumulate and “rev the meter” from nearly 0 to nearly 100% which further requires "tweaking" in general to re-achieve proper melt thixotropy with varying amounts of regrind.

This can result in some rather aggressive approaches to achieving a stable melt pumping mechanism at the correct melt temperature and pressure to have the material “be one” with the intended tooling flow paths –

Skippy: So all that translates to what?

Buzz: In the case(s) above, we find that the requirements for process adjustment in a process where we KNOW that all else (listed in the opening above) is in good working order and under “control” end up being “justified” due to “changing outputs”. The bottom line is that although a good deal of data can be accumulated in “screw and barrel wear” verus "surging" it is meaningless, unless the contribution to wear can some how be tied back to the requirements for any number of the destabilizing run approach problems above in the plant as well.

Your question is an excellent one, because many shops quite frankly do not do a very good job of monitoring, mixing, drying etc the input of raw materials and then “change the process and blame the extruder” aka worn barrel and screw.

Skippy: is this likely to be a standard answer for extrusion in general?

Buzz: Quite frankly, we would guess that the pipe folks who will routinely rebuild screws and change out barrels on an “X” pounds schedule on lines running the same product 24/7 for months on end would have a totally different answer than the “custom gang” but that's just our two cents