Ensuring A Consistent Polymer Feed
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; firstname.lastname@example.org; twwomer.com.