One of the most important factors in the success of any twin-screw extruder application is the materials of construction used on the screws, barrels and other wear parts. The success or failure of the twin-screw extruder, especially when processing highly abrasive or corrosive materials, depends on the ability of the parts to perform well under difficult processing conditions. This article provides an overview of what we at ENTEK have learned over the years about manufacturing screws and barrels.
A Brief History of ENTEK Materials of Construction
ENTEK’s screw and barrel business has changed from 70% nitrided tool steels, 20% through-hardened tool steels, and 10% Hot Isostatic Pressed (HIP) alloys 12 years ago to a very different product mix today: 10% nitride, 30% through-hardened, and 60% HIP. 9V screws / 10V barrels were the most common HIP metallurgy choice 10 years ago. Now, HIP alloys with better wear resistance and higher corrosion resistance are more frequently requested by Customers.
ENTEK started by purchasing HIP materials from vendors – this made sense when volumes were relatively low. As volumes for HIP parts grew as a result of ENTEK’s growth and our Customer’s move from nitrided tool steels to the higher performance HIP materials, transportation time and cost spurred ENTEK to develop in-house processes for manufacturing HIP blanks ready for final machining.
Doing this allowed us to lower costs for our Customers, shorten the supply chain, reduce lead-times, and manage inventory levels to better match Customer demand.
Corrosive applications make total wear resistance more complicated. Corrosive wear is indicated by pitted surfaces and the rounding-off of sharp corners. High-chromium tool steels are typically the best and most cost-effective defense against corrosive wear. Note that traditional Stainless Steels, for example 304SS, are almost never used for extrusion. Although 304SS is difficult to machine, it offers very low abrasive wear resistance and does not offer good value for either an extruder barrel or screw.
More exotic materials may be required where extreme corrosion is present, for example when processing fluoropolymers and their monomers. Iconel and Hastelloy are often used for these applications. These alloys offer superior corrosion protection through high nickel and chromium content. However, these exotic alloys have Rockwell C hardness values of 35 or less, so they don’t offer good wear resistance. They are also expensive; the raw materials are very costly and are difficult to machine. For these reasons, the high nickel alloys are not widely used in extrusion except where corrosion is so severe as to justify the expense and reduced wear resistance.
Barrels and Screws
Ten years ago, ENTEK made barrels in one piece for a single-use and then discard. Today, barrels are made almost exclusively with replaceable liners. This is a bit more expensive up front because of the extra machined surface where the liner and the holder meet, but the long-term cost of ownership is significantly reduced. A lined barrel can be refurbished by removing the worn liner and inserting a new one. The body and flanges of the barrel – which contain the cooling passages, thermocouple wells, injection ports, and counter-bored holes for bolting barrels together – can be re-used indefinitely. There is considerable expense in machining all of these features, which is why the higher upfront cost of the lined barrel can be recovered quickly compared to installing a completely new barrel every time.
ENTEK makes most of its screw elements in a bi-metallic construction. The core where the screw spline resists torque overloads better than the high-wear but more brittle HIP material on the outside of the element. Ordinary solid HIP screws are very prone to breakage when torque exceeds normal limits, for example when an extruder is over-fed with material and the screws stall.
Typical practice in extrusion is to make the screw out of material that is equal or slightly less wear resistant than the barrel material. Screw geometries are more complicated than barrels and the tolerances are similar, but because of the large amount of material removal required to make a barrel it is less expensive to replace screws than barrels.
Years of experience have helped us learn what works best for materials of construction for screw and barrel wear resistance. We use this knowledge to manufacture the best quality wear parts we can for our customers. However it is important to note that every application is unique. Testing of any materials compound by processing it in a controlled lab environment is essential to compile the data necessary to determine the best materials of construction. The ENTEK in-house development lab is ideal for this type of activity.