Development of a Small-scale Machine for Extraction and Brushing of Sisal Fiber

Abstract

Sisal fibre is a raw material used in the production ropes, mats, gunny bags, carpets and polymer composites. UNIDO, in 2001, revealed that there is large amount of small-scale sisal in East Africa that is inadequately exploited due to lack of appropriate decortication technology. Small-scale have for a long time relied on tedious and low yielding manual methods that only contributed less than 25% to the total Kenyan sisal fibres exports in 2003. To increase processing of hedge sisal, UNIDO recommended that appropriate small-scale decortication technology should be developed. However, the raspadors developed so far have only focus on extraction ignoring brushing that greatly improves fibre quality. Low grade fibre obtained by these machines is sold at very low prices (Kshs 17 per kg) compared to a better under grade that would be sold at Kshs 175 per kg for brushed fibres. Moreover, research has shown that environmental conditions, fibre surface modification, age and processing methods cause the variability of sisal fibre properties. Howbeit, inadequate research quantifying the variability of sisal fibre properties with processing method was found. Therefore, this project aimed at developing a raspador for extraction and brushing of sisal fibres and quantifying the effects of raspador processing variables on sisal fibre properties. A raspador for extraction and brushing of sisal fibres was designed based on the raspador principle using Autodesk Inventor. It was fabricated and extensively tested at Mechanical Engineering Workshop. Tensile tests were conducted using Hounsfield Tensometer (type W) in accordance with the ASTM C1557 standard. The raspador achieved extraction and brushing capacities of 10.85 kg/hr and 24.64 kg/hr respectively with a design efficiency of 39%. The power consumption per sisal leaf was 1.368 kW and 1.018 kW for extraction and brushing respectively and increased with drum speed. Moreover, the increase in power due to incorporation of a brushing unit is 16%. The fibre properties highly correlate with the gap size, number of blades/brushing elements and drum speed. For instance, the properties reduced with increase in speed and number of blades/elements but increased with gap size. Furthermore, brushing improved the grade of fibres from UHDS to UG. The average σu, E and ε for fibres obtained by the raspador were 124.04 MPa, 1.48 GPa and 0.085 for extraction and 142.68 MPa, 1.78 GPa and 0.081 for brushing as compared to 236.65 MPa, 1.58 GPa and 0.09 for manually extracted fibres and 87.23 MPa, 1.48 GPa and 0.07 for estate fibre. The values obtained are vii comparable to 161 MPa σu, 3.6 GPa E and 4.5% ε reported by a research on Rea Vipingo Kenyan estate sisal. The machine’s tensile properties were lower than those of manual method but comparable to UG estate sisal. Optimum results were obtained at a gap size of 1.5 - 2 mm, speed of 1000 - 1200 rpm and 6 blades and a gap size of 2 - 3 mm, speed of 1000 - 1200 rpm and 3 brushing elements for extraction and brushing respectively. The total cost of the raspador is Kshs. 115,908.10. In conclusion, a farmer who manually extracts 3 kg/day of fibre earning Kshs. 150/day, will process 80 kg of brushed fibre earning Kshs. 4,588.83/day. A positive NPV was obtained implying that the raspador makes economic sense to the small-scale farmers.