Timothy Womer

Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Calibrate Those Instruments

By: Timothy Womer 28. June 2013 09:37

I was recently asked to visit a sheet processor to determine the cause of a major screw design problem. So, as always, I started at the beginning to gather all of the technical information to determine the root cause. This facility had 5 large extrusion sheet lines, and they were issues with all 5 extruders.

With the extruder at room temperature, I set up three dial indicators on the discharge flange of the barrel in the X,Y and Z axis.  Then I turned on the barrel heaters to the standard zone setting to make sure that the barrel thermally expanded in the Z-axis direct as much as it should theoretically, and that the X and Y indicators move minimally.

 

The simple equation to determine the amount of expansion that a barrel should grow is:

 

ΔL=0.00000633 X ΔT X L

where:

                                             ΔL = The change in length

                                             ΔT = The change in temperature, in this case from room

                                                       temperature to the barrel zone setting

                                                     

                                               L =  The heated length of the barrel

 

Amazingly the barrel grew within about  0.030-in. of the theoretical change in length, which in this case was approximately 0.750 in.

 

Then I measured the flight OD on several of the screws for various designs to determine if there was a consistent wear pattern. There was, so that was noted.

 

Then I gathered all of the process data.  This is a very important part of doing a “CSI” on screws.  This is where you collect the given throughput rate at a given screw speed against the headpressure during that timed rate check, motor load and melt temperature.

 

The motor load reading is taken from ammeter on the control panel; the screw speed is taken from the tachometer.  If at all possible, it is best to have the customer’s plant manager to check the motor load with a hand held meter to verify that the ammeter on the control panel is reading correctly.  As for checking the screw speed, this typically can be done by using a stop watch and counting the rotation of the drive quill at the back of the gearbox.

 

In this case the control panel ammeter was reading correctly, but the screw speed was not.  The customer’s setup sheet showed that their standard setup was to have the extruder operating at 70 rpm, but when I counted the revolutions of the drive quill, I was getting 92 rpm.  This is an error of 24%! 

 

I then checked the tachometer on the line next to the one that I was gathering the process data from and the tachometer on it read 86 rpm but when I did the count, it was only rotating at 70 rpm. This meter was mis-calibrated by 23%!!!

 

So, the moral of the story is, the only thing worse than no data is BAD data.  In this case, the customer immediately had their maintenance people re-calibrate all of their control instruments.

 

NOTE: Sometimes the screw rotation is faster than what a person is able to visually observe. In these cases, I take the advice given to me when I was a kid by an old mechanic mentor of mine (who only had a 4th grade education)...I  “count the clicks.” I had no idea what he meant until he showed me.

Howard took this machinist scale (a pencil or pen will work) and turned on the chuck of the engine lathe in his shop, then took the scale and let it rub against the chuck. On an extruder it can be a small bolt in the back of the rotating drive quill or the drive key on the shank of the screw.  Then with your stopwatch in one hand the “clicker” in the other, you can count the number of times that bolt or key hits the end of the scale, pencil or pen...or the number of clicks.  “Count the clicks.”  Very simple but very effective.

 

Just make sure that your instruments are calibrated on a regular bases and also do a check and balance when gathering data.  Never trust what you think  you see the first time.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com
 

Overhung Loads

By: Timothy Womer 10. June 2013 11:06

Recently a processor installed a new screw into its 6-in., 32:1 L/D extruder. Within a few weeks the hard facing that had been welded on the flight OD started to pop off. The flight failure was in an isolated area, so it was then assumed it was due to a poor weld bond. The screw was ultrasonically inspected and the remaining flights showed good bore, but the entire screw had to be rebuilt.
 
Within about 4 weeks after it was returned, the customer called and said that the flights had failed again in the same area, which was located about in the middle of the screw and within about a 12-in. length. The screw was returned to the manufacturer and repaired a second time, sent back to the customer and reinstalled.
 
Once again, in about another 4-5 weeks the customer called again to say the screw had been pulled and yet again the flights had failed in the same exact location. This was unbelievable! 
 
After much research and review of the screw design, it was considered that there was something wrong with the extruder—and not the screw—and most likely the problem  was due to thermal expansion.
 
It was time to make a plant visit.  The first thing I noticed was that the 900-lb, 6-in. screen changer—located approximately 36 in. in front of the front barrel support— had no support under it, causing a cantilevered overhung load.
 
So to determine if the barrel was expanding properly, the die and adapter were disconnected from the extruder. Then, three dial indicators were mounted so that they contacted with the screen changer. The indicators were mounted independent to the extruder at 12 o’clock, 3 o’clock, and also on the face of the extruder, so that the movement could be measured in the X, Y and Z axis. The die and cart had been pushed up close to the screen changer so that it could be used as the independent support for the indicators.
 
Also, a dial indicator was mounted independently near the middle of the barrel between the front barrel support and the face of the feed throat housing, at the 12 o’clock position. This was done to observe if the barrel would bow upwards, which be an indication that the barrel was not thermally expanding forward properly.
 
Calculations were made to determine the theoretical amount of thermal expansion that should be expected when the barrel zones were set at the processing temperatures of the extruder. The expected expansion was to be approximately 0.434 in. at an average barrel temperature of 410°F. Once all of the indicators had been properly “zeroed” on the “cold” extruder, the barrel zones were turned on and allowed to heat up.
 
Within an hour, the barrel only expanded forward approximately 0.400-in., but the screen changer had dropped 0.045-in. and the middle of the barrel had lifted 0.032-in. for a total deflection of 0.077 in. Also, it was evident and measured that the barrel had only moved forward at the front barrel support a distance of 0.253 in.
 
From all this it was concluded that the overhung load from the screen changer was causing a bind in the area of the front barrel support and not allowing for smooth and uniform expansion of the barrel in the axial direction. 
 
A support for the screen changer was fabricated and installed to eliminate the overhung load. Also, it confirmed by a major screen changer manufacturer that they recommend a screen changer cart for all screen changers 6-in. diameter and larger. and even for smaller extruders also.
 
Lesson learned: Excessive overhung loads and non-uniform thermal expansion will cause premature screw and barrel wear.
 
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Screw Speed vs. Recovery Time

By: Timothy Womer 29. April 2013 14:24

Many times I have visited molders to help them with processing issues. In reviewing their setup parameters, I find many times that the screw speed is not setup to operate at its most effective rate.
 
There have been times, for example, when a customer was processing Xenoy— Sabic’s blend of semi-crystalline polyester (typically polybutylene terephthalate, or PET, and polycarbonate)—and the molder was complaining that the screw was degrading the resin. The screw was turning at a fairly moderate speed and was recovering in 5 seconds, then sitting in the back position before injecting for an additional 8 seconds. Then during the injection stroke, degassing and smoke was evident at the nozzle.
 
After witnessing several cycles of operation in the above manner, I suggested that the screw rotation be reduced to the point where the screw got back just in time so that a count of “3” could be obtained. Then the screw was stroked forward to fill the mold. The screw speed was practically reduced by half, and within about three shots the degraded material was flushed out of the barrel, and the smoking stopped.
 
This simple change in the processing parameter reduced the shear on the resin and allowed the process to be improved. Not only was the degrading eliminated, but most of all the overall melt temperature was reduced which in turn reduced the cooling time and, therefore, reduced the overall cycle time.
 
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Ensuring A Consistent Polymer Feed

By: Timothy Womer 15. April 2013 14:34

The material handling of the feedstock is a very critical part of the extrusion process. If the feedstock is not introduced to the feedsection of screw in a smooth and uniform matter, then the likelihood of a stable and consistent output is low. This is the reason why it is very important that if regrind is added to the virgin feedstock, it must be done very consistently and uniformly. 

 

It should also be mentioned that a very important part of the extrusion equipment is the hopper and feedthroat section. If the hopper and feedthroat sections are not design properly, inconsistent material flow to the screw can take place. For example, if the conical section of the hopper does not have the proper transition, the resin will not flow smoothly into the extruder.

 

Many times processors will modify the OEMs hoppers in order to increase the residence time in the hopper. But by doing this re-engineering the processor is actually doing itself a disservice and a lot of times causing problems down the road. 

 

The feed section of the screw is where the polymer is introduced to the feedscrew. Before discussing the mechanical portions of the extruder it is necessary to mention the importance of the consistency of the feedstock. Extruders are just like computers in one main feature, GIGO or “Garbage In, Garbage Out”. 

 

The feedstock entering the extruder must be delivered to the screw consistently and uniformly. The resin cannot play leapfrog over the channels of the screw and balance itself out. Therefore, the material handling and feedstock my present the resin to the feedthroat section of the extruder precisely.  One of the biggest problems in extrusion of polyethylene blown or cast film is that edge trim, off-spec and start material typically has to be feed back into the extruder as some form of regrind. Due to the fact that most of this material has been produced as very thin film, causes this regrind to have a very light bulk density. The bulk density of this regrind can be as low as 2–3 lb/ ft³  versus its original pellet bulk density of about 30 lb/ft³. This lightness causes troubles not only in material storage and handling, but also in the area of reintroducing it back into the extruder. Several different methods have been used over the years, such as densification, repelletizing, fluff/pellet feeders and side arm extruders.    

 

In the case of the smooth bore feedscrew, this is typically the deepest portion of the screw. Also, it might be noted here that since it is the deepest section and closest to the drive end of the feedscrew, it is most liable to torsional breakage. Therefore, when designing the feedscrew it very important to take this into consideration. Sometimes, in the case of very small screws (2-in. diameter and smaller) it might be advisable to have the screw manufactured from a steel alloy which has a higher yield strength. tainless steel is a good choice sometimes to help combat this design problem.  

 

In the feed section of the screw, the primary function is to forward the polymer. This is where solids conveying takes place. The basic theory of solids conveying is that the polymer must stick to the inside diameter of the barrel (or barrel wall) and slip on the screw root. If this simple phenomenon does not occur then the resin cannot be transported down the screw channel.  In some cases, if the root of the screw is too hot and the resin melts prematurely onto the root of the screw. This forms a melt plug and as a result no material at all is conveyed forward.   This phenomenon was first investigated by Darnell and Mol and present to the industry in a technical paper that was presented at an Antec many years go. Their theory and approach has been the basis of many studies over the years.      

 

Since then many others has also study the feeding mechanism of single stage screws, such as Chung, Kun, Spalding, Campbell and others.   But after many dollars of research with very similar conclusions, one item hasn’t changed and that is, the resin must stick to the barrel wall and slip on the screw in order for the resin to be forward. 

 

Sometimes it is possible to enhance the slippage of the resin on the screw root by reducing the coefficient of friction (COF) between the resin and the screw root.   This COF reduction can be done by means of either screw root coatings or screw cooling. As for screw coatings, the most common is chrome plating with other choices being Polyond or Armolloy, all of these are surface treatments which can be applied to the basic metal of the screw and give a better release between the resin and the screw interface.  Another means of improving the coefficient of friction between the resin and the screw interface is screw core cooling, which we covered in a previous blog.  ___________________________________________________________________________________

Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. He was inducted in the Plastics Hall of Fame in 2012. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com.

 

 

 

The Screw Isn’t Always the Problem

By: Timothy Womer 24. March 2013 20:44

For months now I have written articles offering bits of advice that I hope can be used to improve a process issue, but this month I want to pass on an experience that happened to me several years ago.

 

Most of the time when there is a process issue, the screw is accused of being the culprit. It does not matter if it is an extrusion, injection molding, or blow molding application.  But like the old German saying goes, "Der Teufelsteckt im die details” or “The devil is in the details.”

 

Over 30 years ago, the company that I was working for designed and built a one-of-a kind polyethylene bag reclaim system for a customer located in Wisconsin. This was a very elaborate piece of equipment and included a crammer feeder, a dual-diameter vented extruder, and a pelletizer. As a first time project “bugs” were expected. The customer was advised of this, and they accepted the fact that what they were asking us to build was on the cutting edge of technology at that time.

 

The system was designed, built, and quickly tested...but nearly not long enough to ring out completely. The system was disassembled, shipped, and installed at the customer’s facility. Once the installation was completed, production began. As expected, “things” happened. First, the bearing area of the screw galled in the “one-of-a-kind” feed throat, the key sheared in the quill of the crammer feeder, and the vent port bled (again, the first time a dual diameter extruder had ever been vented). The last blow was when the weld on the large-diameter barrel flange cracked. The entire extruder had to be disassembled to make the repair. All of these “devils” were corrected and improved, which lead to a learning experience that was never forgotten. 

 

After the final defect was corrected, there was nothing heard from the customer for about three months; then came “the” call! I received a call from the company’s general manager, who was not a happy man to say the least. I could “see” his frustration through the phone. The reclaim system was not meeting the daily production rate which had been quoted, he said, and he wanted me at his plant immediately. 

 

I flew to the customer’s plant, and instead of going to the main building I went directly to the Repro Building, which was across town where the “Reclaim System” had been installed. As I entered the building, I could hear the reclaim line in operation. “Joe” the day-turn reclaim operator, with whom I had befriended months before, looked at me and asked why I was back. I told him that his boss had called and was very upset because he had been reviewing the daily production reports and was not getting the production capacities he had been guaranteed for the past two months.

 

Well, “Joe” had to excuse himself because he had to take the gaylord of pellets, which just had been filled by the Repro System, to the warehouse storage area using the pallet jack. While he was gone, I started to investigate the system. The very first observation that I made was that the extruder was only operating at about 75% of full speed. Therefore, I gradually started to increase the screw speed and everything increased and performed perfectly until I was able to reach the guaranteed production throughput.  

 

After several minutes of absence, “Joe” returned with his empty pallet jack and wanted to know why I had increased the screw speed. Again, I told “Joe” that his boss had called me and was very upset because the repro production on this daily production report had not been at the capacities which had been guaranteed to him, and I had to figure out why.

 

“Joe” looked at me, smiled, and said, “There is nothing wrong with the repro line.  My fork truck broke down a couple of months ago, and maintenance has been too busy to fix it. So, now I have to pull the gaylords of pellets to the storage area with the pallet jack!”

 

I could not believe my ears.  I went to the main facility to meet with the “boss.” As soon as he saw me walk in, I knew that I was going to get it with both barrels.  After I let him vent, I asked him when the last time he had been in the Repro Building. He said that he didn’t have time and hadn’t been over there for a few months. When I told him what “Joe” had just told me and what I had witnessed, he would not believe me; so we took a ride across town to the Repro Building.

When we got there, he talked to “Joe” as I stood off to the side and just watched the body language. After a few minutes, looking at the floor, the “boss” walked over to me and apologized for jumping to conclusions. He asked me to send him the bill for the travel expenses and time. As far as I know, that line is still in operation today with only replacing the screw and barrels a couple of times.

 

The point of this story is to make sure that you ask “The 5 Why’s” and don’t jump to the first conclusion. Usually, if you ask “Why” five times, you will get to the root cause and many times “The Screw isn’t Always the Problem!”

 

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. He was inducted in the Plastics Hall of Fame in 2012. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com.




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