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Thermwood

Bespoke System to Aid Wood Composite Research

27 August 2004

Celsum Technologies has delivered and commissioned a purpose-built instrument system to the Wood Materials and Engineering Laboratory at Washington State University.

The system will be used for research and development of highly filled wood composites with improved resistance to moisture and biological decay.

The CEST (combined extrudate swell and temperature) system comprises an optical micrometer capable of measuring to 0.2um, an infra-red pyrometer which monitors a target of diameter only 0.8mm, an instrumentation pack and a computer system with purpose-written software.

Richard Carter, Celsum's Marketing Manager, said: ""CEST is a synthesis of two of Celsum's measuring instruments, the CADS optical micrometer for die swell and the CIRP close-focus, small-target IR pyrometer"".

The optical micrometer uses a low-power blue-green LED light fan (which is more stable and safer than a laser light source) to illuminate the target (ie the extruding material), projecting it onto a precision CCD array via a sealed lens system.

The instrumentation pack includes ""glass logic"" in order to compensate for spurious signals caused by light transmission through transparent or translucent specimens.

The light transmitter and receiver are mounted 300mm apart on an especially-designed flat bracket made from hard-anodised aluminium tool plate, with a mount for a photographic tripod included.

The distance between transmitter and receiver was chosen so that the electronics and optics would fit snugly on either side of the barrel of an Acer capillary extrusion rheometer (which was codesigned by one of Celsum Technologies' directors), to enable the diameter of the extruding material to be measured as soon as possible after exit from the extrusion die.

The use of a photographic tripod allows the use maximum flexibility in order allow the equipment to be used on a pilot extrusion line as well as in the laboratory.

The measuring head of the CIRP infra-red pyrometer is fitted with an aluminium cone to exclude extraneous radiation, and measures surface temperature of suitable materials from sub-ambient up to 500C.

The unit takes readings at approximately 4Hz, and is equipped with built-in functions to allow for surface emissivity and to operate in ""peak"" or ""trough"" mode.

The instruments are used to measure some aspects of the viscoelastic properties of the materials during processing.

Long-chain molecules such as polymers need time when their shape is changed for the molecules to align themselves to their new geometry.

If the force which is creating the change of shape relaxes before the molecules are newly aligned and before, for example, a molten material cools, the molecules will tend to snap back to their original shape.

In the case of extrusion, this produces and effect known as ""die swell"", ""extrudate swell"", or the ""Barus effect"", where the material expands after exiting the extrusion die.

Of course, this can have dire repercussions on product quality. The CADS optical micrometer in the CEST system will measure the die swell for different compositions over a range of extrusion conditions, and will provide information to allow the die geometry for the manufacturing plant to be designed in order to produce the required product dimensions.

Similarly, when materials, especially high filled ones, are extruded under pressure through an extrusion die, friction both with the die wall and internal between particles can cause significant increases in temperature, particularly near the surface of the profile.

This temperature increase can cause premature curing or ""scorch"" of thermosetting materials such as rubbers and phenolic resins, can cause surface degradation and discolouration, and can lead to in-built stresses in the product which can have deleterious long-term effects, and, in some processes with certain materials, can lead to health and safety problems such as starting fires and generating toxic fumes.

The CIRP IR pyrometer within the CEST package will measure the temperature of a small target spot on the moving extrudate for different formulation combinations under a range of test and processing conditions, and will provide information to allow the processing geometry and conditions to be optimised.






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