Feed Screws, Designs

Extruding Polystrene Resins-Dow

Extruder Screw Design

Extruder screws work on the principle of rotary motion. A forward motion takes place with the assistance of raised flights having a helical profile. During rotation, plastic material from the hopper section flows vertically into the recess between flights. This material is then forced horizontally forward by friction on the barrel.

The material adheres to the heated barrel walls, and slips on the cool, smooth-surfaced, rotating screw. This way, a continuous flow is assured and the necessary shear for frictional heating and subsequent melting. There is a change in polymer bulk density from approximately 0.7 at the granule stage, to greater than 1.0 at the melt stage.

As screw rotation melts the plastic granules, air entrapped in the process is normally purged back through the hopper. Entrapped air is either removed at the vent on a vented extruder or entrapped in the extrudate.

Because screws take such hard abuse, they are made of a rugged alloy steel such as SAE 4140, with Rockwell C hardness of 35 to 40. Screw flights are further toughened by flame treatment to a Rockwell C hardness of 50 or higher, or are protected by application of special hard-facing alloys.

Commonly, the entire screw is polished to a finish of 5 to 8 microinches. Plating with chrome or nickel provides a smooth, hard surface that is easy to clean and is resistant to corrosion. Final dimensions of the screw are controlled so clearance between the barrel and outside flight diameter is 0.002 to 0.004 inch per side, about 1 to 1 1/2 mils per inch of screw diameter. This minimizes screw removal difficulties and keeps polymer leakage to a low level, yet is not so tight to cause overheating.

Different patterns of screw design have evolved over the years. Some of the early work was centered around double flights, which were not satisfactory for extrusion of polystyrene. Polystyrene flow in this type of screw is unpredictable, usually unsteady, and is more difficult to balance. Later developments involved constant root diameter and decreasing pitch between flights to achieve the necessary compression of granules.

Currently, the most popular screw design for extrusion of STYRON* and STYRON A-TECH* polystyrene resins centers around constant pitch and fully uniform flights. Compression of granules is achieved by an increasing root diameter in the transition section.

Single-Stage Screw
The following comments on screw dimension are general and purchasers should discuss specific screw requirements for their machines with their machinery suppliers.

The single-stage screw has three separate sections, as shown below:

* Feed
* Transmission (compression)
* Metering

Typical Single-Stage Extruder

The Feed Section
The feed section is located in the rear cylinder (hopper) zone. Flight depth is at a maximum, allowing material granules to fall directly on the screw. In most designs, the feed section has a constant root diameter throughout its entire length.

Experience supports the need for a minimum of four flights in the feed section to avoid non-uniform feed and/or unwanted temperature rise in the rear zone. Flight depths from 0.500 to 0.750 inch for 4 1/2-inch extruders are not uncommon.

The Transition (Compression) Section
The transition (compression) section of full-flighted screws is designed to promote both the compression and heating of the plastic granules. This is achieved by a uniformly tapered, increasing root diameter that reduces the available volume between flights, compressing the granules.

As the granules are compressed, air is purged back through the hopper. While the material is being compressed and moved forward, it is also being heated, partly by conduction, but mainly by friction from rotary shear. As it melts, it is also mixed into a homogenous melt. Some screw designs achieve transition within a single flight. A transition section that is too short can promote overheating. In other designs, the transition takes place in as much as eight flights. Normally, the transition section is from one-fourth to one-third the entire screw length.

The Metering Section
The metering section provides polymer melt stability and helps ensure a uniform delivery rate. Flight depth is at a minimum and is normally constant along the section length. The range for metering section 4 1/2-inch extruders is between 0.125 and 0.200 inch in depth, and five to twelve flights in length. On smaller 2 1/2-inch extruders, flight depth varies between 0.080 and 0.0120 inch, and length runs from five to ten flights. Shallow screws have a short metering length to avoid undue compression and consequent high polymer temperatures. Deep screws have a longer metering length to provide added flow stability.

Single-Stage Screw, Compression Ratio
Screws are frequently referred to by their compression ratio. This factor is calculated from the relative volumes per flight of one revolution in the feed and metering. In practice, compression ratio is often considered to be the ratio between the relative flight depth in the feed and metering sections.

With STYRON and STYRON A-TECH polystyrene resins' excellent stability, compression ratios between 3:1 and 5:1 achieve good mixing and uniform delivery. Higher compression ratios can provide feed uniformity if used to extrude polystyrene at high rates. Less stable and more viscous polymers require screws with lower compression ratios to avoid overheating.

Two-Stage Screw
The two-stage screws used in vented extruders can be considered as two separate screws in tandem as shown below. The first stage ordinarily comprises about 60 percent of the overall length, or about 17 flights in a 32:1 L/D ratio. The second stage is somewhat shorter, about 15 flights.
Typical Two-Stage Screw Design

Both stages are further subdivided into separate feed, transition (compression) and metering sections of varying length. For 4 1/2-inch diameter screws, flight depths in the feed sections range between 0.550 and 0.750 inch, with a greater depth in the second stage to provide a decompression zone at the vent. Metering section depths range between 0.150 and 0.250 inch, with a greater depth in the second stage to ensure forward polymer flow and eliminate vent flooding.

Pump Ratio
Pump ratio is the ratio of the flight depth in the second-stage metering section compared with the flight depth of the first-stage metering section. A factor greater than 1 is necessary to avoid having polymer flow out the vent. For polystyrene, pump ratios of 1.3 to 2.0 have been used. Pump ratios of 1.6 to 1.7 commonly provide good performance.

Screw Cooling
Some improvement in extrusion of plastic polymers is possible by circulating cooling water through the cored section(s) of the screw – typically the feed section. The amount of cooling required is dependent on screw design and operating parameters.

Cooling is more critical for larger diameter screws, because the larger volume of polymer flow requires more cooling. Superior extrusion may be achieved by optimized cooling, but reduced rates or surging may result unless proper processing temperatures are maintained. If screw cooling is used, it should be limited to the feed (hopper) section.

*Trademark of The Dow Chemical Company

Dow Chemical