Feeding Function of Single-Screw Extrusion
A polymer feed, usually in the form of pellets, drops from the hopper through the feed throat into the rotating screw. This feeding function occurs by gravity in most cases for single-screw extruders. Some feeds, such as sticky powders or recycled film flakes with a large surface to volume ratio, tend to bridge inside the hopper and do not drop freely from the hopper into the screw by gravity. Such non-free flowing feeds require a forced feeding device. A short conical screw installed inside the hopper, called a “crammer feeder”, is widely used for non-free flowing feeds. Single-screw extruders do not require starved-feeding, and they usually operate with a full hopper in a flood-feeding mode. A metered feeding device, such as a volumetric feeder or a loss-in- weight feeder, is used to control the feeding rate and to run the screw in a starved feeding mode in special situations. Many polymers react with oxygen at high temperatures during extrusion, causing undesirable oxidation, degradation, or crosslinking of the molecules. Purging of the feed at the feed throat by an inert gas like nitrogen may be necessary, especially when the screw is run in a starved feeding mode.
The feed throat is directly attached to the heated barrel, and it becomes hot. Feed materials with a low melting point stick to the wall of the feed throat, reducing the feeding rate or completely blocking the feed stream in the worst case. The feed throat must be cooled by water to avoid such feed sticking problem.
A feed stream is often made of several component materials. Even if the component materials are well blended/mixed coming into the hopper, they could segregate inside the hopper. Two different materials with the same shape but different densities, or with the same density but different shapes, readily separate upon flow. “Flow segregation” of the feed materials inside the hopper is a common problem in extrusion.
Because an extruder is a continuous pump without back mixing capability, the first condition for a successful extrusion process is to provide a consistent feeding rate into the screw from the hopper, in terms of both a constant composition and a constant weight. Extrusion problems often arise from an inconsistent feeding rate.
The feeding rate of a polymer feed is determined essentially by the physical characteristics of the feed, such as size and shape, and their distribution, controlling the bulk density (the weight divided by the apparent volume), and the internal friction between the feed constituents. The feeding rate also depends on the inherent properties of the polymer (the solid density, the external friction on the metal surface), the hopper design, and the feed throat design. The external friction of the feed on the hopper wall mainly depends
on the inherent properties of the polymer and the roughness of the hopper wall. A tiny amount of lubricant or additive, especially if it is coated on the surface of the feed, can drastically alter both the internal friction and the external friction.
Because a polymer feed, in pellet, powder, or flake form, becomes interlocked in the hopper, almost supporting its own weight, the pressure at the bottom of the hopper is very low and the feeding rate is usually independent of the amount of feed in the hopper.
The driving force in flood-feeding is gravity. The opposing forces are the centrifugal force exerted by screw rotation and the back-flow of air flowing out of the screw into the hopper through the feed throat. Feed materials contain 30–70% air by volume, and the air is squeezed out of the feed as the feed is compacted into a solid bed along the screw. Continuous flow paths from the solid bed back to the hopper are necessary for the back-flow of the air. If the flow paths are blocked, the air is entrapped in the melt. Feed forms with a large surface area per unit volume, such as powders and film regrinds, are prone to the air entrapment problem.
Unfortunately, no mathematical model is available to simulate the feeding function at present. Development of a feeding model will greatly improve the computer simulation of extruder performance.
Preferred conditions for the feed material to exhibit good feeding characteristics are:
- Small pellet size in comparison to the screw channel area
- High bulk density
- Small surface area to volume ratio
- Low internal friction between the pellets
- Low external friction on the hopper surface
- High melting point
