|Soil Block Presses (GTZ, 1988, 38 p.)|
In view of the vast choice of machines available, it seems difficult to decide which one should be bought. If there is not enough money to buy expensive equipment, the choice is smaller and the decision much easier. But generally, the following points need to be considered, especially when the available resources allow for the purchase of higher priced equipment.
Design of Press
· Compressing blocks is only part of the operation, hence apart from the press, additional equipment (eg crushing machine, sieve, mixer, measuring scoop) is needed. In the case of machines which incorporate all these functions in a single unit, consideration should not only be given to the costs, but also to the required storage and working space, ease of transport, production efficiency and quality, output rate and the like.
· However, sophisticated mechanical and electronic control devices often necessitate special training and experience for maintenance and repairs. Spare parts are usually expensive and difficult to procure (import).
· The filling of moulds both manually and automatically is commonly by volume (less frequency, though more accurately by mass). In rotating or drawer moulds, which only pass once under a fixed trooper, the quantity of fill on each side is likely to differ, thus producing non-homogeneous blocks.
· Moulds have to withstand high pressures over long periods, hence preference must be given to thick, reinforced mould walls for durability and resistance to deformation. Speed and ease of changing moulds also need consideration, which can be a drawback, the more moulds a rotating table has.
· Compression and ejection can be in vertical and/or horizontal direction. Ideally the blocks should be laid in masonry such that the structural forces follow the same direction as the compression force during production. Also the exposed side of the block should preferably be smooth for greater durability.
· Smaller presses are invariably manually operated (muscle power), while larger units are usually motorized (electric motor, diesel, or petrol engine). Manual presses depend on the weight, strength, stamina and motivation of the operator, while mechanized presses overcome the problems of human fatigue and non-uniform products.
· Energy transmission to the block can be via a lever, toggle, cam, pivot, ball and socket joint, piston, etc. But principally there are two systems of energy transmission: mechanical and hydraulic.
· Mechanical systems are usually simple but relatively heavy, unless special alloys are used, in which case repairs may be difficult.
· Hydraulic systems are susceptible to dust, sand and heat, under harsh conditions the hydraulic fluid must be changed once a month, so that maintenance can be difficult and costly. The systems are usually designed for operating temperatures around 70° C, but under tropical conditions temperatures can reach 120° C, requiring cooling mechanisms and/or special spare parts and oils to withstand the heat. Flexible tubing, joints, etc. that need frequent replacement should best be standardized.
Compression of Blocks
· There is a difference between the real pressure acting on the brick and the theoretical pressure, which neglects losses due to friction and inertia. The difference can be about 50 % of the theoretical value. The required standards generally specify the real pressure at the end of the cycle, hence manufacturers should be asked to explain their measurement procedures in order to check their validity.
· The compression ratio, which is the difference between the depth of the mould (with uncompacted earth) to the height of the compressed block, should not be less than 1.65, preferably around 2, which is rarely achieved by mechanical compaction. Hence, pre-compression (eg by forceful closing of fold-back lid) can be advantageous. Machines in which the compression ratio can be adjusted according to the soil type, can be especially useful.
· The speed of the compression process is of significance for the rate of production, but it must be noted that compression cycles below 2 seconds for 10 cm thick blocks run the risk of lamination. If the compression process is too rapid, the cycle should be interrupted after pre-compression to allow the compressed air to escape, after which the cycle is completed at a slower pace.
· The quality of the block improves proportionately with the increase of moulding pressure. However, this is true only up to a critical point, which lies between 4 and 10 N/mm² (depending on the soil type), after which lamination can occur, especially when applied too rapidly.
· Double-sided compaction from above and below produces more homogeneous and durable blocks, than one-sided compression.
· Even though the compressive strength of blocks, in most cases, need not be high - the quality of the CINVA-Ram type blocks is structurally quite adequate - it is important to note that insufficiently compacted blocks are porous and easily absorb moisture, the coarse surface is difficult to keep clean and can be abraded easily, while cracks and cavities are likely to harbour vermin. Such surfaces usually need some protective coating, which naturally incurs additional costs. Denser blocks, which have been compacted with pressures upwards of about 3 to 4 N/mm² can remain untreated, offer no refuge to insects, and can do with only small quantities of binder (in cement or lime).
· Alternatively, in case of low compaction pressures, a chemical additive (eg asphalt-based) can provide the necessary moisture resistance. However, such additives do not increase the compressive strength of the block, and it should also be remembered that these substances invariably have to be imported, thus making the production of blocks more expensive and dependent on supplies.
· Small sizes require a greater number of blocks per cubic metre than larger ones, so the overall effort needed to produce small blocks is greater than that of making larger ones. Furthermore, masonry constructions with small bricks require more mortar, since the proportion of joints is higher. Therefore, the best block format is determined by the maximum weight and size that can be easily handled by a single person using only one hand. The most common dimensions are 29.5 x 14 x 9 cm (I x b x h).
· When pressure is applied only on one side, the height of the block is limited to 10 cm, above which the opposite side remains weakly compacted.
· It is useful to be able to produce hollow blocks of all kinds, partially or totally hollowed out from one side to the other. The total volume hollowed is generally limited to about 30 % for the most efficient processes. Frogs (removeable mould inserts) up to 5 % are usual.
· Certain presses can produce a complete range of products (large and small blocks, paving and roofing tiles, etc.) which is a distinct advantage, but which has its price.
· Under theoretical conditions, given a 15 second cycle, for example, 2000 production cycles can be run in an eight hour working day. If one increases the complexity of the compression action (double compression, compression in two steps, dynamic compaction) the cycle is longer, in this event, the constructors provide for the production of several blocks per cycle.
· The machine's output is often indicated according to the theoretical production cycle. The real productivity in the field is different and depends upon a number of factors that are totally independent of the machine's theoretical capacity, eg breakdown time, manpower organization, maintenance, etc. Real productivity lies quite often around or under 50 % of the theoretical production cycle.
· The theoretical production cycles of the most efficient manual presses lie between 30 and 60 seconds.
· As concerns the motorized presses calling for human intervention, it is difficult to run a complete cycle in less than 15 seconds - filling, compression (1 to 2 seconds) and turning out included.
· Automatic production units can, in the best of cases, complete this same cycle in 4 or 5 seconds.
· The normal work force comprises about 5 people excavating, sieving and mixing the soil, as well as curing and stacking the previous day's production, and at least 3 people to fill the mould, operate the press and remove the block to the curing area.
· Considering that a manually operated press requires the person, who pulls or pushes down the lever, to exert a great force, up to about twice every minute, it becomes clear that gradual exhaustion causes diminishing performance and lower quality blocks. In view of this, every means of facilitating manual operations should be given priority, if the financial resources permit. If a motor-driven machine is chosen, it would be advantageous to also be able to operate it manually, in case of short supply of energy, or failure of the motor.
· Special attention should be given to safety measures, such as avoidance of projecting moving parts, designing manual operations such that hands cannot get jammed between moving parts, clearly marking and/or protecting dangerous points, incorporating thermal fuses, security pins, etc. Automatic machines must at all cost be equipped with an emergency stop switch, which is easily accessible.
· The advantages of large machine producing companies are
- a strong capital base,
- large, efficient working team with modem equipment,
- high sales figures and good (international) references,
- good administrative backing,
but the machines are likely to to be more expensive than those of smaller firms.
· In small machine producing firms, the manager is usually
part of the workshop team. The advantages are generally:
- high motivation,
- low overhead and production costs, thus lower price of machine,
- frequent modification and improvement of machine.
Small firms or their machines are often not so well-known, because of small advertising budgets, hence their list of references can be small in spite of a good product.
· Personal visits to the manufacturer and/or sites at which the machine is in use should be undertaken as far as possible. The value of reference lists is to be able to meet or correspond with users, to learn about their experiences. If reference lists do not contain addresses, these should be specifically asked for.
· Of special advantage are training courses, offered by some manufacturers. To be effective, they should not only include the production of blocks, and handling and maintenance of the machine, but also the testing and use of problem soils, as well as design guidelines for building construction. Trainees should also learn to dismantle and assemble the machines, to understand their function and conduct repairs by themselves.
Purchase of Machine
· The "FOB" price (free on board) includes packaging, transportation and insurance costs of the machine within the retailer's country. This price can be artificially inflated in order to compensate for the reduction offered on the factory price.
· As regards sales or rental conditions, one must be suspicious of contracts providing for price indexing based on the number of blocks produced or for payment of royalties for patent use, which is often not justified. A patent is not necessarily a proof of guaranteed quality and constructors frequently apply for patents for processes that are already of the public domain.
· It is advisable to include a penalty clause in the contract, to safeguard against late delivery.
· In the case of an after sales service contract, the waiting period for repairs and maintenance must be clearly indicated. A detailed handbook should be provided, including specifications of all spare parts and a maintenance plan, indicating operations necessary and expected maintenance frequency.