When Selecting Screw and Barrel Extruders

A screw and barrel are two pieces of equipment that can play an important role in additive manufacturing processes. Both components are necessary for creating a successful product. There are several factors that influence the performance of a screw and barrel. These factors include high pressure, barrel alignment, and lubrication. High pressure can cause barrel wear and deflection.

Extruder

During the extrusion process, the screw and barrel are rotated in order to force the polymer into the barrel. This process requires energy that is converted into thermal energy as the screw turns. The drive power for a single-screw extruder is typically eighty to nine percent for melting, while the rest is allocated for pressurization and mixing. The barrel heaters, however, contribute almost no energy to melting once the screw is in rotation, as they are largely in cooling mode.

Types

Screws and barrels are two of the most common types of mechanical components found in plastic production units. These products are known to have high stability and require minimal maintenance. This makes them very cost-effective and easy to procure. To determine whether a barrel or screw is appropriate for a particular application, consider the purpose of the parts.

Screws and barrels can be made of a variety of materials. Some of these materials are surfaced with tungsten alloys for maximum protection against abrasive wear. Others are case-hardened and/or through-hardened. Bimetallic barrel liners are also available and offer cost/performance advantages. For improved corrosion resistance, HIP barrels contain nickel-rich boron alloy, molybdenum, or a mixture of borides.

Sizes

Screw and barrel sizes are a crucial consideration for ensuring proper operation. The wrong size can have severe consequences for production and quality control. The savings from using a smaller size could be negated by loss of profit due to poor quality. There are several factors to consider when selecting the right screw and barrel sizes for your machine.

The L/D Ratio (Length to Diameter Ratio) is the ratio of the screw's working flighted length to its outer diameter. The working flighted length is measured from the front edge of the feed opening to the forward end of the screw flight when the screw is in the forward position. The L/D ratio must be at least 0.6.

Measurement

Measuring the screw and barrel wear can be very valuable, but most processors are hesitant to dismantle their machines and remove screws and barrels. A hand-held measuring device can be used to determine the condition of both the screw and barrel. This tool allows for easy comparison of the screw and barrel, which can help in optimizing overall system performance.

In a screw and barrel assembly, the feed section is the cylindrical area that receives and conveys the material to be processed. The feed section will normally have a constant root diameter and channel depth. Most screws have a feed pocket at the intersection of the flight and the bearing. This feed pocket is an integral part of the screw and barrel assembly. The screw and barrel assembly may contain a polymer, which is a high-molecular-weight organic compound with repeating units. Polymers consist of monomers and copolymers, which are made up of two or more monomers.

Alternatives

A screw and barrel are two parts of the rotary screw. They are made from steel and are lined with a wear-resistant alloy. Typically, there is 0.005-0.010 inch clearance between the screw flights and the barrel. This clearance will be less for smaller screws and larger for large screws. A tighter fit would be too difficult and expensive to make, and it would also develop too much heat. While some wear is harmless, overheating is not.

In a production environment, it makes sense to regularly inspect screws and barrels to determine if they need to be replaced. A visual inspection of these parts can identify issues early and help to prevent unnecessary downtime. Additionally, regular visual inspections can reduce stress on other parts of the machine, while also reducing material waste.