PLE – 305: Extrusion Theory and Application

Screw Design for a Given Material and Application

Focus: VICTREX PEEK Extrusion

Dr. Wickman

October 29, 2010

Jason McNulty, Jesse Pischlar

What polymeric material is being processed (include a copy of the IDES data sheet)? VICTREX PEEK 381G (IDES datasheet attached behind works cited) What is the Melt Flow Index (MFI) of you chosen material?

The MFI data for VICTREX PEEK could not be identified. However, the MFI for similar PEEK materials, AvaSpire AV-722 CF30 and AvaSpire AV-848 CF30 are as follows:

Is the material of choice amorphous or semi-crystalline?

Semi-crystalline (IDES)

What is the melting point temperature of the material?

649˚F/343˚C (IDES)

What is the thermal conductivity of the material?

2.0 btu-in/hr/ft^2/F or .29 W/m/K (IDES)

What is the thermal diffusivity of the material?

No property search option exists for thermal diffusivity on IDES or MATWEB. Also, the thermal diffusivity value for VICTREX PEEK is not indicated in the datasheet from the manufacturer. What is the viscosity of the material?

@ 752˚F/400˚C, 300 Pa Seconds (IDES)

What additives are typically found in the material of choice?

None – virgin no fillers added (IDES)

What is the hardness of the material?

Shore hardness (shore D, 73F 23C), 87 (IDES)

At what point can one assume the material will begin to thermally degrade?

In order to determine the point at which the material will begin to thermally degrade DSC analysis is required. No information regarding the degradation temperature of VICTREX PEEK

was uncovered. Considering the melting temperature of the material is 343˚C, the thermal degradation temperature must be above that. What is the melting characteristics curve for your specific material?

Westland indicates that when extruding polymers, a hump or reverse type heat profile aids in the efficient melting of the resin and in the achievement of a high quality extrudate. Ideally, the pellets in the feed zone should stick to the barrel wall and move freely on the screw. Heater band settings that are too low in the feed and transition zones can cause excessive shear in melting the resin. The overabundance of shear causes abrasive wear on the root and flight radii of the screw and on the barrel lining. The same condition can result from heater band failure where inadequate conductive heat is used to melt the resin. Many molders of reinforced materials also utilize a hump or reverse type heat profile. Due to these reasons the extrusion company will utilize one of the profiles mentioned above.

How shear sensitive is the material?

According to Westland Corporation, amorphous materials are considered to be more shear sensitive than crystalline materials. Higher shear rates result in rapidly increased resin temperatures which amorphous resins do not tolerate. In contrast, the higher crystalline materials such as Victrex PEEK, can be processed more effectively by screws with shorter transition zones, more shallow channel depths and higher compression ratios. This is due to the fact that PEEK and other crystalline materials do not react as unfavorably to higher shear rates as the amorphous polymers (Westland Corporation, 2006).

Does the polymeric material present any “special considerations” when processing?

The processing temperatures of VICTREX PEEK are high (see below). While no “special considerations” were identified, precautions should be taken to ensure staff safety during the processing of this material.

(Victrex)

IDES recommended, but did not specify why, to refrain from using continuous compression or PVC type screws. Is it necessary to have a vented screw/barrel to process your material?

The Westland screw handbook indicates that a vented screw and barrel setup is designed to remove the need for drying material before processing it through an extruder. In order to do this, the screw is essentially comprised of two parts: one for drying the material, and the other for metering the molten material. In order to accomplish this, the screw must be lengthened on the order of 26:1 to 32:1. VICTREX PEEK (as mentioned later in this report) is processed most efficiently utilizing a screw between 18:1 and 24:1. Therefore, lengthening the screw and barrel may compromise extrudate quality. Additionally, since PEEK has a tendency to wear out screws and barrels relatively quickly compared to other materials; utilizing an unnecessarily long screw and barrel would just yield higher maintenance and replacement part costs (Westland Corporation, 2006). Are there any mixing sections that need to be added to the screw?

Westland screw and barrel handbook states that the main objectives of a mixing screw are to:

1. Achieve distributive mixing of melt 2. Provide dispersive mixing of melt 3. Achieve isothermal melt quality and uniform viscosity at lowest possible melt

temperature 4. Accomplish pumping action with minimum pressure drop to achieve maximum

throughput 5. Avoid dead spots or hang-up areas causing thermal degradation or impeding color

changes (Westland Corporation, 2006)

The extrusion company should consider incorporating a mixing screw when processing VICTREX PEEK. Not only does a mixing screw provide a uniform melt and eliminate gels before the screen pack, it also contributes to creating a homogenous melt inside the barrel. This is essential when processing resins similar to PEEK, because the extreme temperatures are difficult to produce throughout the entire melt without aggressively stirring the liquid. In order to obtain the proper mixing screw, the extrusion company will research companies such as Westland, Spirex, Xaloy, Cincinnati Milicron, Glycon, and other screw manufactures. Is it a barrier type screw? A barrier screw is one that separates the main flutes with undercut sections to prevent non melted polymer to pass through. This is important when working high flow, low viscosity polymers which are difficult to ensure proper melt. PEEK is not a material that would be processed with a barrier type screw, because by restricting the flow area for the polymer, the hardness of PEEK would undoubtedly damage the screw through abrasion.

Identify the major extrusion screw manufacturers in the U.S. by company name and address.

Westland Corporation 1735 S. Maize Rd. Witchita, KS 67309

Spirex Corporation 8469 Southern Blvd. Youngstown, Ohio 44512 USA

Xaloy Corporation Xaloy Incorporated 1291 19th Street Lane NW

Cincinnati Milicron 4165 Half Acre Road Batavia, OH 45103

Glycon Corporation 912 INDUSTRIAL DRIVE TECUMSEH, MICHIGAN 49286

United Feed Screws 487 Wellington Avenue Akron, OH 44305 USA

Davis Standard, LLC #1 Extrusion Drive Pawcatuck, CT 06379-2313 UNITED STATES

Describe the metallurgical requirements of the screw.

Screw core material Most screws are manufactured from a 4000 series alloy steel (usually 4140) or nitriding steel (such as Nitralloy 135M). In most cases these materials are either chrome-plated or nitrided. Some screws are made from tool steels which, in a heat-treated and hardened condition, they are very resistant to abrasive and adhesive wear (Westland Corporation, 2006). The company United Feed Screws recommends when processing PEEK that the screw be made out of one of their 4340 heat treated steels with a high hardness alloy coating such as Colmonoy83 or Colmonoy56 to prevent wear in high wear applications (United Feed Screws). Coatings/Method of coating application/Coating thickness over all geometry of screw According to Westland, common methods for coating screws include gas nitriding, ion nitriding and chrome-plating. Out of these methods, the extrusion company‟s screw design will utilize the gas nitriding technique. This will create a uniform coating across the entirety of the screw. In addition, this method of coating can allow for an alloy to display an excess of 60 Rockwell hardness and sometimes upwards of 70 (Westland Corporation, 2006). The reason for this is the entire screw will have a uniform hardness over its entire surface. In order to reduce abrasive wear in certain areas when processing a harder material, the coating thickness may need to be altered. Examples of these areas include the compression zone, the flights and wipers of the screw and the face of the flights. These areas may require a thicker coating to prevent wear due to the fact that they are where the majority of the abrasion would occur. The flights and wipers are constantly smearing pellets against the barrel causing drag and adhesion wear. Additionally, the compression zone on our companies modified nylon screw is only ¼ of the length of the screw; because of this, the pellets are being sheared and melted to a liquid in a short amount of time.

Secondary treatment of coating The company United Feed Screws will coat screws using a full cap flight encapsulation method known as plasma transfer automated welding. In this process, the least amount of base material dilution into the hardened alloy can be achieved. The alloys United Feed Screws uses in this system are Colmonoy83, Colmonoy56, Stellite1 and Stellite2. For processing PEEK, the extrusion company will utilize the full Colmonoy83 encapsulation technique. Adopting this technique will yield the longest screw life. Additionally, United Feed Screws will rebuild and remanufacture old, worn out screws, which would generate a 40-50% cost savings opposed purchasing new ones (United Feed Screws). Another method for encapsulating a feed screw is utilizing a carbide coating. This will yield the highest hardness number, but will also be the most expensive. The Westland Handbook states that as long as the screw and its coating is intact, the encapsulation process produces a superior solution to preventing adhesive, abrasive and/or corrosive wear. If the encapsulation is cracked or nicked, a rather rapid deterioration of the base metal can occur which encourages a further “shelling off” of the encapsulation and, ultimately, severe wear of the screw. To combat this issue, the extrusion company will implement procedures to ensure proper process setup and screw/barrel heat soak to prevent excessive wear (Westland Corporation, 2006). Surface coating finish Surface finishes for sealing surfaces of extrusion dies should be 32 microinch or better. Melt flow surfaces should be 16 microinch or better, with 4 to 8 microinch preferred (Hendess, 2002). It can be inferred then that the surface finish for an extrusion screw should be in the approximate region of 4 to 32 microinches. Type of screw wear expected and why/Location on screw where wear will take place The Westland Screw and Barrel Handbook discusses adhesive, abrasive, and corrosive wear. After analyzing the types of wear and how they are caused, the leading cause for the extrusion company will be abrasive wear. The processing of heavily reinforced resins will result in abrasive wear to the barrel and screw and even though the extent may be reduced by proper processing, it is inevitable. Due to the short compression zone on the modified nylon screw to be used (more on screw selection below), this portion of the screw and corresponding section of the barrel are likely to show the first signs of wear. This is due to the fact that the flights, lands and wipers of the screw are compressing, shearing and smearing solid pellets against the barrel causing scoring (Westland Corporation, 2006).

Screw Design for VICTREX PEEK

Victrex, the material manufacturer, indicates that “most general purpose and „nylon‟ type screws are suitable for processing VICTREX PEEK grades” (Victrex). However, it has been stated in the literature and PLE-305 lectures over and again that a “general purpose” screw does not exist. Each material should be processed with a screw engineered, tested, and optimized for the processing of that material. As a result, the extrusion company should investigate the usage of “nylon” (also known as rapid compression) screws for the processing of VICTREX PEEK. The “nylon” screw (Figure 1), was for many years the standard screw for nylon extrusion. This screw has a very rapid transition which takes place within ¼ flight of the screw. It has a deep feed section which conveys the pellets and partial melt to the metering zone. Standard design

calls for feed section to be ¾ of the total screw length, with the remaining ¼ serving as the metering section. The metering section is the most critical area of the screw since it acts as a pump and is also the region where shear is applied to the material to prepare a homogenous melt. It is recommended that the screw have a long, shallow metering section to improve the homogenizing action (Firestone Textiles Company).

Figure 1: Standard Metering or “Nylon” Screw

A newer screw design (Figure 2) has been used for a number of years, particularly at higher screw speeds. The screw is quite similar to the “nylon” screw and is commonly referred to as the “modified nylon screw”. The metering section is still ¼ of the screw length but has a long, gradual transition zone approximately ¼ of the total screw length. The feed section now is about ½ of the screw length. This screw design provides greater shear and homogenization, delivering a more uniform melt at higher rates than the older design (Firestone Textiles Company).

Figure 2: “Modified Nylon Screw”

According to Victrex, the minimum recommended L/D ratio for the screw is 16:1; with L/D ratios between 18:1 and 24:1 preferred. Long feed sections are required to prevent compaction of unmelted pellets in the compression section of the screw. Also, the compression ratio should be between 2:1 and 3:1 (Victrex). It has been discovered that the standard “nylon” screw design is a contributor to surging in the extrusion process (Rauwendaal, 2002). Overfeeding of pellets from the feed section of single screw extruders into the compression section appears to be a common cause of large observed random fluctuations of melt pressure and rate (surging) in single stage screw designs. Better balancing of the flow capacities of feed and metering sections should reduce surging (Carley, 1971). The “modified nylon” screw is an attempt to address the issues invested by Carley. Due to the surging complications of the standard “nylon” screw, the “modified nylon” screw will be adopted by the extrusion company.

Helix Angle While numerous screws simply use a square pitch with a 17.66˚ helix angle, an optimum helix angle in the metering section exists for each polymer based on its power law index (Giles, Wagner, & Mount, 2005).

(Giles, Wagner, & Mount, 2005)

In consulting the Moldflow Insight software in the University of Wisconsin-Stout CAD lab, it was determined that the power law index for VICTREX PEEK 381 G is 0.525. Therefore, according to the table above the optimal helix angle in the metering section for prcessing VICTREX PEEK is approximately 18-19˚. Since “modified nylon” screws are considered constant pitch metering screws, the helix angle is the same throughout the length of the screw.

Power Law Index

Number of Flights The lead on a screw is defined as the distance between the flights. As an approximation it is equal to the ID of the barrel for a single-flighted screw (Cheremisinoff, 1987). The barrel ID for the company‟s extruders will be 1.5 inches (as stated in the assignment outline). Thus, the screw will be comprised of the following number of flights: All dimensions in inches:

All calculations performed by the authors using PTC Mathcad 14 software. Screw Speed A screw speed between 50 and 100 rpm is optimum for the transport and melting of VICTREX PEEK. However, low screw speeds (50-60 rpm) are recommended for the reinforced grades to prevent excessive fiber breakdown. Screw speeds lower than 50 rpm should be avoided since it results in longer cycle times. Screw speeds higher than 100 rpm are not recommended because they can cause excessive localized shear heating (Victrex). Shear Heat Generated A longer transition zone results in less shear heat and more time to compress and melt the resin (Westland Corporation, 2007). Considering the length of the transition zone for a “modified nylon” screw is similar to that of a so-called “standard” screw (see Figures 2 and 3) and it can therefore be said that the shear heat generated by the two designs is comparable. The shear in extrusion typically generates 75 to 80% of the energy required to plasticize a material (Westland Corporation, 2006). Melt Pool Theory Supporting Design Contiguous Solid Melting (CSM) rather than Dispersed Solid Melting (DSM) is more likely to occur in single screw extruders where the material is conveyed along a continuous screw channel (Rauwendaal, Polymer Extrusion, 2001). Considering the company‟s extruders will use single “modified nylon” screws, it can be said the melt pool theory associated with them is the CSM model. The following is an excerpt from Rauwendaal‟s work, Polymer Extrusion, explaining the

theoretical model of CSM:

(Rauwendaal, Polymer Extrusion, 2001)

Channel Depth As a rule of thumb, the feed section of a screw should not be deeper than:

(Womer)

Furthermore, “modified nylon” screws tend to have a channel depth of 3.5 to 5% of the screw diameter in the metering zone (Westland Corporation, 2007). Thus, the channel depth in the transition zone would gradually decrease from 20% of the screw diameter to a minimum of 3.5% of the screw diameter.

Compare and Contrast Your Chosen Screw design to a General Purpose (GP) Screw Design The following table compares and contrasts the fundamental characteristics of screw design for a “General Purpose” and “Modified Nylon” screw.

Characteristic General Purpose Screw “Modified Nylon” Screw

Total Length 20 – 30 D 20 – 30 D

Length of Feed Section 4 – 8 D ½ of total Length

Length of Metering Section 6 – 10 D ¼ of total length

Number of Parallel Flights 1 1

Flight Pitch 1 D (helix angle 17.66˚) Approximately 22 - 24˚ (helix angle)

Flight Width 0.1 D Approximately 0.1 D

Channel Depth in Feed Section 0.15 – 0.20 D 0.2 D

Channel Depth Ratio 2 – 4

Note: D = screw diameter For General Purpose Screw: (Rauwendaal, Polymer Extrusion, 2001). For “Nylon” Screw: (Firestone Textiles Company), (Giles, Wagner, & Mount, 2005), (Womer). The following chart displays the difference in heat profile of nylon along the length of the extruder as it is processed using a “nylon” screw and a “general purpose” screw. A stable heat profile is maintained when the nylon material is extruded with a “nylon” screw.

(Universiti Sains Malaysia School of Materials and Mineral Rescources Engineering)

Heat profile for nylon using a nylon-type screw

Heat profile for nylon using

a general purpose screw

Heat profile for polyethylene using a general purpose screw

Heat profile for polyethylene using

a polyethylene-type screw

The following are diagrams outlining the differences in geometry relative to “general purpose”/”standard” screws and “modified nylon” screws:

Figure 3: “Standard” Screw

(Rauwendaal, Polymer Extrusion, 2001)

Figure 2: “Modified Nylon” Screw

(Firestone Textiles Company)

Works Cited

Carley, J. (1971). Extruder surging. Causes and cures. Modern Plastics , 48 (7), pp. 56-58. Cheremisinoff, N. P. (1987). Polymer mixing and extrusion technology. CRC Press.

Firestone Textiles Company. (n.d.). Wire Jacketing With Firestone Nylon 6. Retrieved October

27, 2010, from Firestone Textiles Company: http://www.firestone-textiles.com/Resin/pdfs/Wire%20Jacketing.pdf Giles, H. F., Wagner, J. R., & Mount, E. M. (2005). Extrusion: The Definitive Processing Guide and Handbook, Volume 1.

Hendess, P. M. (2002). Plastic Extrusion Forming Heads For Pipe and Tube: Designs and Materials. IDES. (n.d.). VICTREX PEEK 381G Polyetheretherkeytone Victrex Plc. Rauwendaal, C. (2002). Innovation in Extrusion. ANTEC.

Rauwendaal, C. (2001). Polymer Extrusion. Hanser.

United Feed Screws. (n.d.). United Feed Screws. Retrieved October 28, 2010, from Feed Screws: http://www.unitedfeedscrews.com/feed-screws.html Universiti Sains Malaysia School of Materials and Mineral Rescources Engineering. (n.d.). Universiti Sains Malaysia School of Materials and Mineral Rescources Engineering. Retrieved October 28, 2010, from http://webcache.googleusercontent.com/search?q=cache:UZmh2esHpr8J:material.eng.usm.my/stafhome/mariatti/EBB427/Week%25204%2520Processing.ppt+nylon+screw+flights&cd=20&hl=en&ct=clnk&gl=us Victrex. (n.d.). High Performance PEEK Polymers Processing Guide. Westland Corporation. (2006). Cylinder and Screw Handbook.

Westland Corporation. (2007). Screw Geometry Resins. Retrieved October 27, 2010, from Westland Corporation: http://www.westlandusa.com/Vol%204%20Iss%201%20Screw%20Geometry%20Resins.pdf White, J., De, S. K., & Naskar, K. (2009). Rubber Technologist's Handbook, Volume 2. Smithers

Rapra Technology. Womer, T. W. (n.d.). Retrieved October 27, 2010, from Xaloy: http://www.xaloy.com/pdf/ThingsYourScrewDesigner2.pdf