Cavity Transfer Mixers


Single Screw Extruders are basically polymer melting and pumping machines with fundamentally limited mixing performance. In today's economic climate these machines are expected to provide adequate mixing to incorporate masterbatches for colour, flame retardancy, slip, antiblock, and many other additives in addition to recycled scrap sometimes consisting of several different co-extruded
polymers. The most certain way of raising an extruders mixing performance to meet these demands is to fit a Cavity Transfer Mixer. (CTM)


Twin Screw Extruders usually fall into one of two relevant main types:
Corotating for compounding and Contrarotating for rigid PVC. The former have special mixing features and the latter are very specific to PVC. Nevertheless, there are instances where filting a CTM can be highly beneficial:- 

eg. for the incorporation of high levels of liquid additives, very accurate colour shade control and advancing PVC gelation.





  • Short unit ( normally about 3D ) fitted between barrel and die ( or screen changer ) flanges.
  • Mixing carried out at screw tip with no loss in output rate.
  • Cavities are much deeper than screw channel and land areas minimal so that shear heating is easily contained .
  • Clearances are much greater than for the screw so alignment is simplified and wear risks minimised.
  • Over 100 CTM's made by Aspin Engineering Limited - ranging in size from 20mm to 600mm diameter.

The main reason for using the Cavity Transfer Mixer has been unchanged since its inception 20 years ago:

"to improve profitability of extrusion businesses through better mixing."

Its invention at Rapra Technology enabled pipe extruders to make materials savings of £50 per tonne (in 1980's money) by direct extrusion of natural polymer and carbon black masterbatch, whilst still meeting British Standards.

Furthermore, there were no penalties of mechanical unreliability, uncontainable shear heating, or hang-up.

It also proved suitable for mixing a variety of viscous fluids, including; elastomers, adhesives, soap and edible fats.

Its reliability was proven by 12 years of round the clock operation with a 114mm prototype.

Unusually, for plastics processing machinery, scale-up has not been a problem.

Units have been fitted to extruders ranging from 20mm to 305mm, plus independently powered CTM pipeline mixers up to 600mm have been supplied.

The CTM is fitted between extruder flange and die entry, and its length is typically 3D, although far longer units are sometimes used for very specialised compounding. It has a rotor attached to, and turning with, the extruder screw. The rotor is located within a stationary stator.

Mixing is performed within overlapping rows of hemispherical cavities in the rotor and stator surfaces. Its excellent distributive mixing performance can be explained by mixing theory which dates back to 1955.

Overall financial gains are normally achieved by enabling production extruders to carry out mixing tasks more effective ly. These include the following:

  1. Improved distribution of masterbatched additives to eliminate precompounding or allow less of these to be used. Better mixing results in consistent flame retardancy, controlled electrical properties and no pigment "given away".
  2. Polymer blending and direct recycling of co-ex edge trim and skeletal waste is a growing application to reduce material and landfill costs.
  3. Incorporation of liquid additives such as colours, foaming agents, lubricants, antioxidants, polybutene tackifiers etc by direct injection into the CTM. This method of incorporating silicone gives significant cost savings with no loss or variability of output rate.
  4. Reduction of "nerve" in flexible PVC and rubbers can allow higher running speeds.

We appreciate that potential users will want further technical information and advice and consequently will be pleased to answer questions from both the polymer processing or engineering aspects.

We can draw on 20 years of practical application and expertise, so please contact our offices and ask about information specific to your potential application.