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

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.

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.

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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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.