Die Swell, more correctly called Extrudate Swell or the Barus Effect, is a result of the viscoelastic properties of a material. As the material is, for example, extruded out of a rheometer barrel through an extrusion die, there is often insufficient time for the long-chain molecules to relax into their new geometry and so, when the confinement of the die is escaped, the molecules recover in part some of their original positions, which results in the diameter of the extruded material being greater than the die diameter. The ratio of extrudate diameter to the die diameter is referred to as the Die Swell Ratio, often quoted as a percentage. Another effect of interest, for example in blow moulding, is the Draw Down Ratio, a measure of the change in the extrudate diameter as it is pulled downwards by the weight of extrudate previously extruded from the rheometer die.

NODS, above right, Aptifirst's improved state-of-the art, stand-alone Advanced Optical Die Swell monitor, is based around a laser micrometer, easy to set up, maintenance free, will measure consistently to an accuracy of 2 microns (0.00008"), has a high repeatability of ± 0.15 microns ( 0.000006") and can read at speeds in excess of 2 kHz. This particular NODS system was designed with a gap of 300mm between the transmitter and receiver modules specifically so that it would fit around the barrel fascia of an ACER 2000 capillary extrusion rheometer in order to allow measurements closer to the die exit. We are always happy to look at customizing our designs to suit your specific needs.

Aptifirst has NEST, a Combined Extrudate Swell and Temperature system which combines the NODS optical micrometer with the AptiIR miniature Infra-Red Pyrometer, together with custom-designed mounts and a computer system with purpose-written software.

Additionally, the system is self-calibrating and automatically compensates for any changes in ambient temperature. The associated instrumentation package, which communicates with our rheometer software, has many features including edge-detecting logic for measuring transparent samples. We have selected this system as the transmitter and receiver may be placed quite a distance apart compared to conventional laser micrometers, and this allows plenty of room for changing the die and cleaning your rheometer without having to move your optics, and without the danger of your hot extrudate sticking to the optical windows!

The NODS system can be adapted for other applications, of course. For example, we incorporated it into the design for a unit to measure the height and diameter of cakes and puddings after they were cooked on a production line, as well as using our AptIR miniature infra-red pyrometer for measuring the temperature in a non-intrusive manner, and with a digital electronic balance for measuring the unit weight.