ABSTRACT
A good starting level is an excess of 0.3-0.5% of the specified fibre content, and this requires setting the fibre content in the given example to between 5.3% and 5.5% or, from Figure 11.4, at 750g/min. of fibre output.In conclusion, when the output of fibre has been decided, it must be determined that the chopper gun can in fact operate at that particular level and give a consistent output (refer to Chapter 3, Section 3.2.1 and Figure 3.15). This only becomes a problem when operators set the mortar output up to a high level in order to quicken the spraying time. It is then found that fibre contents obtained from wash-out tests reveal a variation and lower fibre content than is specified. Spray ~gun I ay down Before components can be sprayed-up with the "confidence that a uniform ‘wet-out’ and ‘laydown’ pattern is being achieved, a test should be made on a piece of card or polythene. Reference to Chapter 3: Figure 3.12 shows the basic details of a hand spray head. With the mortar spray head vertical and maintained at right angles to the surface at a distance of 300-400 mm, two passes across are made, one with the chopper gun leading and the second to one side of the first, with the chopper gun trailing. The ideal laydown is achieved when uniform mixing of the fibre and mortar occurs before depositing on the surface, thus ensuring no ‘dry fibre’ is visible. If dry fibre is being laid down then the inclusion angle may require adjustment and the spray pattern again examined as before. The mortar spray cone may also need alteration by either relocating the air nozzle (as is possible on the earlier mortar spray heads) or by adjusting the air pressure to the mortar spray head.It may be noticed that the fibre is being bounced out of the mortar spray cone and this can be remedied by either lowering the air pressure to the mortar spray head, or increasing the air pressure to the chopper air mover. The inclusion angle a should not require changing once an optimum setting has been found, and trimming air pressure settings should enable the spray pattern to be finalized. Misalignment of the mortar spray cone and the chopped fibre spray can sometimes occur in the plane of the illustration Figure 3.12. This should be checked by viewing the laydown process in action. The modern spray heads are not likely to cause this unless nozzles have worn irregularly and require replacement. The older type of spray heads, as shown in Figure 3.19, which use a connecting plate to locate the mortar spray head and the chopper gun unit, have been known to suffer from this problem. Similar problems have occurred with the traversing head unit shown in Figure 3.14. 7.2.4 Component manufacture Component manufacture involves the following stages which are now examined — pre-spraying checks, test
board manufacture, spraying, finishing, demoulding and making good. Pre-spraying checksNaturally it is hoped that the majority of making good and mould checking would take place prior to moving the moulds into the area adjacent to spraying. These checks can be carried out either at the demoulding and making good stage or during mould cleaning and oiling (as shown in Figure 2.1), and would be the responsibility of the production Q/C laboratory staff. However, a basic assessment of the mould should be made by the spray team in advance of spraying. Recycled moulds in particular require special attention. These tests should ensure: 1. The mould has been assembled correctly (good mould design can make certain that this becomes foolproof). 2. The mould oil or release agent has been applied satisfactorily and excessive ponding has not occurred in recesses for example. 3. Overspray, dust and the like have not contaminated the mould surface. Test board manufactureEarly on in the preparation of the first product at the start of a shift a test board should be sprayed-up and roller compacted by the same labour force using exactly the same methods. Samples are taken from the test board to determine the glass content and water/solids ratio and hence the production output. The remainder of the test board is then cured and tested in accordance with the details in Chapter 11, for strength and density properties. Spraying procedureIt is customary first to apply a ‘mist coat’ of pure sand and cement slurry to the mould. This is intended to pick up the surface details of the mould and prevent the subsequent fibre addition from appearing at the surface and reducing the moulding of any surface detail. The mist coat is usually a single pass of the mortar spray head. Often, in order to reduce crazing of the surface of a grc component, a polymer addition of an acrylic or S.B.R. latex is added to the mist coat formulation. In this case it may be desirable to spray the mist coat with a hopper-fed texture surface gun.Once the surface of the mould has been coated with the mist spray, the main spray-up of the grc must follow directly in order to achieve an indistinguishable bond between the mist spray and the grc backing.The grc is sprayed by making straight lines across the mould face, with the mortar nozzle perpendicular to the mould face, and at about 200 mm from the surface of the mould. Often it is found beneficial to spray into the corners and return details of the mould first, and compact these, then following on with the bulk of the spray work.Layers of 3 mm thickness are laid down and then the material is compacted with rollers, to remove any air voids. Further layers of 3 mm are laid down and compacted until the design thickness has been achieved. Thickness checks are made, as described in Chapter 11, and low thickness areas are sprayed over to produce a minimum equal to the design thickness. Corners tend to become bridged with dry fibres
during the spraying process and should be well compacted with a special tool. Chapter 4 gives details of these tools and also the techniques used for component spray-up. These techniques should beunderstood and implemented. The following points are worthy of particular note: • Never use overspray to assist in the filling of moulds • Remove all dry glass build-ups from corners etc. • Do not transfer sprayed material from one location to another in the mould as bonding will not be formed between the layers. FinishingWherever possible, overspray can be turned into the mould to achieve a good edge detail. Surplus material can then be cut off when ‘green’ or allowed to set before removal with a sharp knife. Following this the back face can be trowelled flat and the overspray around the mould perimeter removed before setting. Sometimes it is required that the back face receives a rollered finish. This would be performed while the grc is still green.The moulding tools are best stored and returned to a bucket of water adjacent to the mould during spray-up to keep them clean and serviceable. Dirty tools tend to pick up the composite during compaction and trowelling.When the product is finished, the back face is covered in polythene and transported to the overnight curing area, which is maintained at 20 °C or thereabouts. DemouldingProducts are usually demoulded between 16 and 24h after spraying. Naturally care must be taken during this operation since the stress induced while demoulding can be critical and lead to product breakages and failure. Practical mould design combined with the correct release agent reduce the risk of damage. Grc products are, however, in the main thin planar products. This contrasts with concrete which is a bulk product and consequently the demoulding team should exercise the necessary care. Making goodThe most important aspect about making good is that it should be done as soon after demoulding as possible, and certainly before the products are put in for final cure. It must be emphasized that some defects cannot satisfactorily be made good and these are usually caused by excessive voidage in the matrix. Places of particular concern are around fixing points and in areas that will be highly stressed. The usual amount of voidage that is found on demoulding, however, is easily made good with the normal matrix proportions of sand, cement and water and is preferably boosted with a polymeric admixture to improve bonding. 7.2.5 Plant cleaning and maintenanceThe quality of the output obtained from a grc production facility is directly dependent on the manufacturer’s attitude towards plant cleaning and maintenance. Sufficient time for example must be set aside in the schedule to allow for both to be carried out.At the end of each shift the production equipment must be washed down. All the components that come
into contact with the grout, such as the pump hopper and filter, pipes, stator assembly, spray gun and nozzle, should be included. At the same time checks can be carried out for wear. After this the equipment needs reassembling correctly ready for the next shift.The two most common problems that are incurred during spraying are: 1. Blockage of the mortar spray system due to flakes or lumps of grout entering the delivery pipe and nozzle. 2. Inconsistency of output from the chopper gun. This is usually the result of maladjustment or worn components. The cutter blades are best replaced at the end of each shift (or sometimes sooner depending on output rates). The backing roller should also be checked for wear and the pressure that it exerts on the cutting blades requires setting whenever the cutting blades are replaced. 7.2.6 Plant lay-outs and output ratesLay-out of any plant depends on the space available and the production process in mind. Hand spray grc can be carried out in four basic ways: • Static mould • Rotating mould • Production line moulding • Station moulding. Static mouldIn this process the mould is brought to the spray area and deposited in front of the operator, who then moves around the mould spraying-up the component. Rolling is carried out during the spraying but final finishing is performed after the product has been removed from the spray area. Rotating mouldWith this system, the sprayer benefits from the placing of the mould on to a revolving turntable that is either mechanically indexed round or can be turned by hand. In this way the sprayer need not move and therefore is not struggling with heavy delivery hoses and such like.Consideration should be given to the setting up of two turntables, one either side of the sprayer. This would enable spraying to be carried out on one while roller compaction or finishing takes place on the other. Production line mouldingThis technique was developed for cladding panel manufacture. A linear carriage or roller table carries the moulds in a conveying system to each stage of the process. Typical stages are: 1. Spray main outside shape into mould 2. Move along to filling location 3. Fill with core material 4. Move to second spraying location (or cycle back to
1-)5. Spray final back coat and trowel 6. Move away for curing. Station mouldingThis method is similar to production line moulding in that each particular process is carried out at a set location in the factory. However, in production line moulding, the sequence is laid out in a line along a
conveyor system, whereas in station moulding the sequences are located at various points around the production area, and the mould is transported from one to the next by either carrying, fork lift, overhead crane or hand barrow. This may mean that once set-up the production facility is cheaper and more flexible. 7.3 AUTOMATED SPRAY-UP GRCWhen a particular product has been decided upon after market research it becomes obvious that production costs can be reduced by automated production facilities.Automated production of grc is a vague term and can vary between merely replacing the function of the sprayer by using an automatic traverse unit to settingup a complete facility. Various techniques have been investigated and proven for a range of products from flat sheet to cladding panels and concrete pipes. The equipment used in such processes is discussed in Chapter 3.It is worth mentioning here that when a rapid and proven system is developed and successfully employed the benefits of using a rapid setting cement should be considered. This rapid set can make early handling and demoulding a relatively simple process and cheaper than endeavouring to struggle on with oPc formulations. One such cement is known as Regulated Set Cement, which is available in Germany and the USA. This cement can be formulated so that its own internal chemistry can produce a set at any time from5 minutes to 45 minutes. Another way of inducing the set is to initiate the reactions by infra-red heaters. This causes the set to occur at a particular stage in the process which then becomes predictable and consistent. The cement is expensive but the rapid turn round of moulds and product output rates have led a few manufacturers to tool-up production to utilize this material. 7.4 EXTRUSIONThis process has been developed in order to produce products from premix material. The equipment has been discussed in Chapter 3. A range of products have been developed using the extrusion technique, but production has not as yet developed sufficiently to make the process cost effective. The end result has, however, shown little benefit, if any, over that of spray-up production. 7.5 PRESSINGPressing grc is best considered as a stage in the production process of either spray-up or premix. It should be considered as a method of compaction rather than a total technique. It is possibly better suited to the processing of premix material and more likely to be of benefit in the production of cast products rather than sheet, 2D components. 7.6 VACUUM FORMINGAs with pressing, vacuum forming is best considered as a stage in the production process, more likely to be suited to spray-up work.Vacuum forming can be utilized to produce products from male or female moulds. Normally female moulds would be used in downwards vacuuming (as carried out for example on vacuum tables). Upwards vacuuming would usually employ male moulds, that would
be drawn up into a backing structure.Besides compacting the grc it also removes excess water and produces a good quality dense product with superior properties. 7.7 WINDINGThis process consists of winding the reinforcement round a rotating mould, which determines the internal dimensions of the product and the internal surface profile. The reinforcement may be in a variety of forms; rovings, matts or fabric (Figure 7.1A). It is drawn in a taut condition through a bath of cement paste, which is continuously stirred to prevent segregation by pumping it through a mortar pump, and returning it under pressure to the bath.This system can be adapted further by winding the reinforcement round a cylindrical mould or roll. At the same time, chopped strands are sprayed onto the revolving mould as well as cement paste. The composite is compacted by means of compressing rollers (Figure 7.1B).The advantages of this system are: 1. It enables different types of product to be made from sheets taken from the drum in the green state (flat sheets, troughs, pipes and shells). 2. Production can be easily mechanized to produce a consistent quality of grc. 3. It can employ continuous rovings that can be uniformly wound onto the mould, thus ensuring firstly a uniform dispersion of the reinforcement throughout the composite, and secondly the most efficient use of the fibre. 4. The fibre content that is obtained by this method, due to its uniformity throughout the product, yields a
composite that has consistent strength values throughout. 7.8 PERIODIC DELAMINATIONThis method is suitable for making thin-walled structures, for example shells, irrigation channels and embossed units, as well as sheeting. It operates on the following principles: 1. A glassfibre mat is placed between two planes and immersed in a bath of cement paste. A certain amount of paste therefore penetrates into the mat. 2. The planes enclosing the mat are then moved further apart. The cement paste wets both surfaces of the mat and partially penetrates into it. 3. The planes enclosing the mat are now compressed onto the mat. Its external layers are impregnated with cement paste which will adhere to these planes so that, when the latter again moves apart, the mat will segregate along those layers of fibre which the paste has notyet reached.
