Staple Length & Strength Measurement

After Yield and Diameter, one of the most important characteristics that determines the value of combing wool is the average fibre length which will be achieved after processing. This length is known as Hauteur. Research has shown that Hauteur is closely correlated to the staple length and staple strength of the greasy wool.
Prior to the measurement of Staple Length & Strength, three levels of sampling occur:

• grab sampling; 
• tuft sampling; and
• staple preparation.

1. GRAB SAMPLING

Grab sampling is conducted at the time of core sampling, under the supervision of AWTA Ltd Sampling Officers. Every bale is sampled with a minimum of 20 grabs taken from each sale lot, ensuring that the grab sample taken is representative of the wool that constitutes the sale lot. A reduced grabbing schedule can be used for 1, 2 and 3 bale grower lots.

2. MECHANICAL TUFT SAMPLING (MTS)

AWTA Ltd has MTS machines located throughout Australia in brokers’ stores, as well as in the testing laboratories.

The purpose of the machine is to randomly draw approximately 60 tufts of wool from the grab sample and then package the tufts in a form suitable for transportation. This is achieved by spreading the grab sample in an even layer over the conveyor belt so that the full sample is conveyed under the sampling head in 20 equal steps. At each step, three tufts are mechanically drawn from the sample by jaws and blown onto a flyscreen belt which is wound onto a reel for shipment to the laboratory.

The jaws withdraw slowly for the first 200mm to ensure that tender staples are not broken. In addition, the jaws are heated to 60°C which is beyond the melting point of wool grease so as to reduce the risk of greasy tufts sticking to the jaws.

After tufting, the grab is placed in a sample box on the show floor where it becomes the display sample for buyers to appraise.

3. STAPLE PREPARATION

The purpose of this sampling stage is to randomly take one staple from each tuft and prepare it to a standard suitable for the measurement of its Length & Strength. A variety of staple shapes are acceptable, as are second cuts.

Of all the functions in the laboratory, staple preparation is the most manual and therefore requires the most stringent quality control. Testing Officers are trained to grasp the first staple end they identify in the tuft, and, having done so, remove any wispy fibres from either end and place it in the staple tray. Strict adherence to this procedure ensures that staples are chosen at random. Four Testing Officers prepare a single lot, further ensuring randomness of staple selection.

To confirm that operators are not imparting any bias on the results, they participate in regular quality control trials. In these trials the same lot is prepared by each operator, and any who differ significantly from the overall average are retrained and re-trialled before returning to commercial work.

Staple trays, containing the staples, are then moved to a controlled atmosphere room (20°C, 65%RH) and conditioned for 8 hours prior to measurement. The design of the trays allows the staples to relax while keeping them straight. This procedure ensures staples from all types of wool are measured under the same conditions.

4. MEASUREMENT (ATLAS)

Length and Strength is measured using an instrument called the Automatic Tester of Length and Strength (ATLAS). These machines were originally designed and built by CSIRO. Manufacture is now undertaken by AWTA Ltd’s Research and Development division.

Length is measured by conveying the staple, tip first, through a vertical array of eight light beams and then electronically measuring the distance the conveyor moved while the light beams were interrupted by the staple. All staples are measured for Length regardless of the staple dimensions. Length is measured in millimetres. The IWTO standard requires that a minimum of 55 staples are measured for Length to produce a certified result.

After Length measurement, the staple is picked up by two rubber belts which feed it through to a jaw which grips the tip of the staple. A jaw then moves away until the base of the staple is clear of the rubber belts. The base jaw then grips the base of the staple. The tip jaw moves away and the staple is broken in two. The peak force required to break the staple is measured in Newtons (N) by a force transducer attached to the stationary (base) jaw.

On its own, this information is of little value since the staples that are measured vary in thickness. All things being equal, thick staples require a greater force to break than thin staples. Therefore, the force required to break a staple must be related to staple thickness before it can be used more meaningfully.

Staple thickness is determined from the weight of the staple and the Length of the staple. That is, the more grams of weight per millimetre of Length, the thicker the staple. Staple thickness is measured in kilotex (ktex). The total Newtons of force is divided by the number of kilotex to give a Strength value per unit of thickness (N/ktex). This figure is known as the greasy Staple Strength because it was derived from the greasy staple weight. This is then converted to a measure of clean Staple Strength using the Wool Base and VM Base information.

Only staples longer than 50mm are measured for Strength. This is due to the fact that 25mm of the staple is held in the tip and base jaws and cannot be broken, so only the middle 25mm is measured on a 50mm staple. Strength measurements on staples shorter than 50mm are of very little value. A minimum of 40 staples must be measured for Strength in order to produce a certified result.

The weights of the tip and base portions are measured and are used to determine the weight of the staple and the Position of Break (PoB). For example, if the tip is very light and the base is very heavy, then the PoB is close to the tip.

The PoB is reported as the percentage of staples which break in the tip, middle and base thirds of the staple. From the processors point of view, the worst case is to have the majority of staples breaking in the middle, as this reduces the fibre Length in the processed top (Hauteur). However, this is only of major importance to the processor if the Staple Strength of the sale lot is low.

APPLYING STAPLE MEASUREMENT IN PROCESSING

Two major trials were conducted in the 1980’s by AWTA Ltd, CSIRO and AWC together with 20 local and overseas processors. Known as the Trials Evaluating Additional Measurement (TEAM), these trials demonstrated that processing performance can generally be predicted from Staple Measurement together with the Core Test results.
The TEAM general formula for estimated Fibre Length in the top (Hauteur) derived from these trials is