The barrier screw is basically a double flighted screw. One flight has a smaller diameter with the aim to separate the melt from the solid polymer. This separation makes the plasticizing faster but is not suitable for all polymers. Variants have been developed from the barrier screw with multiple (1-4) inlet channels. This screw type is used increasingly in the ‘packaging sector.
The barrier screw, also called “transfer”, has been developed by the Maillefer company in 1962 to optimize and speed the plasticizing. It is widely used for extrusion applications and since the early 80 also used in injection molding.
The barrier screw geometry is defined as a function of the characteristics of the polymer. The position should be defined where to start the second flight (barrier), the depths of the two channels, the end position of the second flight and the diameter difference between both flights. This geometry is specially made for the transformation of a single type of polymer or a limited group of polymers and can not be used as a universal screw.
The advantages of the barrier screw with respect to a single flighted screw are:
- Increased plasticizing capacity
- Better temperature control
- Better melt homogeneity.
The increased use of barrier screws is mainly in the packaging marked who requires high production speeds. As noted above the screw is not universal and therefore not suitable for plasticizing polymers sensitive to overheating, such as POM, PC, PMMA, PVC.