A Screw Barrel Manufacturer Can Help You Select the Right Screw and Barrel Design For Your Extruder

A screw barrel manufacturer will analyze your application requirements and help you select the best design for your extruder. The right screw and barrel can reduce defect rates and equipment malfunctions, and extend production life.

A screw barrel is made of a high-wear steel or special performance alloy. The clearance between the screw flights and the barrel is between 0.005 and 0.010 in.

Injection Molding Screw

Injection molding screws are designed to transport, compact, melt, stir and apply pressure to plastics in a rotating barrel. Screw design and material selection influence all of these functions. They also determine how much friction is generated between the screw’s flight land edges, barrel surface, and the plastic. This friction produces heat that raises the temperature of the plastic and causes it to move forward in the barrel.

To make sure the screw does its job well it must have the right shape, size and material. It must have good grip on the plastic and be able to handle the injection pressure. Moreover, it must be able to keep the temperature of the plastic low enough for the process to be economical and consistent.

A screw’s length-to-diameter (L/D) ratio is another important factor. The feed section should have constant deep flights, while the transition and metering sections have tapered flight depths. It is recommended that a minimum L/D of 20:1 be used. Screws with shorter L/D ratios tend to have melt uniformity problems.

The screw’s metal must be harder than the barrel’s to prevent galling, but it should not be so hard that it cannot absorb energy and maintain a low temperature. Using bi-metal reciprocating screws can help with this problem. They have better wear resistance than steel and can resist corrosive plastics that produce chlorine gas as they’re heated.

Extrusion Screw

A basic extruder will have a single screw that will continuously melt plastic and shear it against the barrel walls to create a solid channel and liquid channel. During the process, turbulent back pressure will be build-up that pushes the molten plastic forward. It will eventually flow into a die that produces a finished product. This type of extruder can be used for producing sheet, tubing, pipe and profiles or blown film. A barrier screw can be added to the basic extruder to separate both molten and solid resins into certain channels. This is typically done by adding flights to the transition section of the screw.

Twin screws are at the heart of a compounding extruder and are modular in design. They are used to convey and pressurize polymer resins that are fed as pellets, powders or flakes from a hopper to a gap between rotating screw flights and a heated barrel. They are capable of multiple combinations of functions such as conveying, heating and melting, mixing-both distributive and dispersive, shearing and degassing (in vented extruders).

A metering zone is the third section of a twin screw and is the area where additives like fillers, fibers, flame retardants, stabilizers, lubricants, color pigments, reinforcements and other polymers can be mixed into the melted feed material. Depending on the screw geometry, the melt pressure may vary from high to low over the length of this zone. This can be a problem for the overall quality of the extruded product since it may affect the consistency and properties of the final product.