7.1.1 Scutching

Scutching consists of a number of steps. The schematic structure of a scutching line is presented in scheme below. The bale with parallel hemp stalks is unwound (Figure below) and the stalks are broken. In this process, the woody core is broken into so-called shives so that they can be more easily separated from the bast fibres. This process already removes most of the shives from the bast fibres.

The bast fibres are then passed through turbines equipped with counter-rotating blades (Figure below). The blades continuously strike the fibres, separating the shives and also short fibres (also called 'tow’ or ‘scutching tow’) from the long fibres (Figure below). The scutching opens the fibre bundles, making them finer and softer. The speed of the rotating blades for scutching hemp is lower than for flax.

The long fibres will be hackled (§7.1.3) for producing fine yarns. The short fibres (tow), after further cleaning, can be used to produce coarser yarns (§7.2) or they can be cottonised (§7.3) into fibres approximately the size of cotton. Other applications of the short fibres include insulation material (§6.2) or composites (§6.3). The shives can be used in building blocks (§6.5) or animal bedding (§6.6).

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Schematic representation of a scutching line. Depoortere.^[1]

The main steps include:

• Tilt table (2): Bales of hemp straw are tilted from the flat position to the side position, in the right direction to be unrolled for scutching.

• Reserve tables (1): Storage of bales that will be scutched (see also Figure 26).

• Preparation table (3): Removal of twine so that the bale can be unwound.

• Unrolling machine (4): Unrolling the bale into a layer of hemp straw.

• Feeding table (5): Control the feed speed.

• Deseeding machine (7): Parallelising the stalks and extracting the seed heads.

• Divider (9): Refining the layer of stalks ('swath') by progressive acceleration, by means of star wheels where speed of every following star wheel is higher in comparison to the previous.

• Breaker (10): Breaking the woody part of the stalks.

• Scutching drums (11): Blades continuously beat against the fibres separating the shives and short fibres from the long fibres (see also Figure 28).

• Mills with leather blades (12): the mills clarify the fibres.

• Uproller press (14): Fibres are packed into round bales, with a web weight of about 1 kg/m.

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Feeding bales into the scutching line: breaking + scutching. Photo HOGENT.

The quality of scutched hemp fibre (and flax) is divided into classes according to the expected yarn fineness which can be achieved by wet ring spinning (Table below).

[1] https://www.depoortere.be/Zwingelmachines

[2] E. Dijkmeijer, Textiles Part 2 Spinning and weaving (Dutch).