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Foam, tr E pywoRK)

15th January 1971
Page 66
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Page 66, 15th January 1971 — Foam, tr E pywoRK)
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Which of the following most accurately describes the problem?

slab or spray

A leading bodybuilder's assessment of insulation methods

by Paul Brockington, MIMechE IN the field of insulated and refrigerated bodywork, different materials and methods of construction have their own champions. It is therefore valuable to have the views of a company which has spent many years in assessing the pros and cons and is still making detailed improvements as its considerable production experience and operational feedback continues.

After 13 years of exploration and testing, in 1963 J. H. Sparshatt and Sons Ltd, of Portsmouth, settled on polyurethane foamedin-situ to provide insulation of the company's latest type of container-in-body structure, and this type of structure and insulant is still in use today. Polyurethane has an insulating efficiency around double of that of polystyrene; using the K factor (a measure of the heat loss across an insulant in British thermal units (Btu) under test conditions) polyurethane has a factor of 0.13 and polystyrene 0.24/0.25.

For the past 15 or more years Sparshatt has built bodies with an insulant in slab form and has more recently carried out a thorough investigation of the spray method of applying polyurethane. Technicians have very decided views on whether to foam, slab or spray, the method advocated depending on the type of bodywork and workshop organization. While the spray process is not employed, its advantages for some types of application, and for bodybuilders who are not in a position to train, personnel to the requisite standard, are fully acknowledged.

The decision to employ the foamed-in-situ method of applying polyurethane insulation to container-in-body structures was taken after successful field tests that were made in conjunction with SPD Ltd. In the early 1950s SPD was using conventional steel-framed bodies but later a twin-framed timber structure was produced, insulated with onazote (expanded rubber) and equipped with a holdover-plate refrigerating system designed for plugging into the mains overnight at the cold store.

This system was later superseded by a mechanical built-in refrigerating unit and a number of vehicles of this type were operated by SPD for five years and subsequently for a further five years by another company. While the original hold-over-plate system was "not very satisfactory", mainly because of the shortcomings of the breakaway couplings, it was considered that refrigeration by holdover plates in modern form has much to commend it for appropriate types of delivery work.

Under pressure from leading operators for a higher standard of temperature control, polystyrene in slab form was imported from Germany and for the first time thermal efficiency was taken into account as a basic design factor. In the mid-1950s timber was replaced by aluminium as a structural material and towards the end of the decade further pressure by operators to produce a body with lower heat-transfer characteristics inspired Sparshatt to attempt a breakthrough with a new design concept. The main problem was to minimize the reduction in the effective thickness of the polystyrene slab by metal penetration without increasing weight or complexity of production. This was achieved by using 6ft by 3ft Aluminium panels and by folding the outer sides to form flanges that also acted as joining members and pillars. The slabs were located between two inwardfacing panels with staggered flanges that penetrated the slabs but had a minimal section area equal to two thicknesses of 18g material. Penetration of the flanges had, therefore, little effect on overall thermal efficiency.

Heat loss Tests were carried out by SPD of a body Insulated with 5in, of polystyrene slab that was rebated to form half joints, a vapour barrier being provided by Kraft paper and bitumastic cement. The hot method of deter mining heat leakage was used and the efficiency obtained was 91 per cent of that which would have been provided by the insulant with no heat paths, the main sources of heat loss being through the door frames and bearers.

Because polyurethane is a better insulator than polystyrene, a relatively poor structure using this material in slab form can be more effective as a temperature barrier than a better structure with an inferior insulant; there has been confirmation of the generally accepted assumption that the quality of slabs is normally more consistent, because they can be produced without voids, than the quality of the insulant when it is foamed-in-situ. A body structure can be built by semi-skilled labour, but slabs do not add to the strength of the structure. If, however, fully trained personnel are employed the quality of foamed-in-situ insulant is of comparable consistency. There is, however, a greater margin for error in its application. The operative must, therefore, be "reliable" as well as skilled. Spraying may be compared with slabbing with regard to the skill required. Slabbing is restricted by Sparshatt to particular types of body, current examples of which include a standard Austin EA van having interior surfaces which can readily be fitted with slabs.

Foamed-in-situ insulant has the valuable advantage that it adds to the strength of a structure, the overall weight of which can therefore be reduced. Moreover, the equipment is fully portable (the same type of equipment can be used for spraying) and this facilitates production. Foaming-in-situ is virtually non-toxic, the material being discharged from the gun being in small lumps. The fine spray from a spray gun can contaminate the air breathed by an operative.

Vapour sealing In the foaming-in-situ process the material is fed into the cavities between the inner and outer panels of the body after assembly of the structural members and adheres to both surfaces. The spraying process is initially applied to the inner or outer skin in a spray chamber (which adds to the cost) and the other skin (generally the outer) is subsequently placed in position after the surface of the material has been "levelled off". Although sprayed material adds to the strength of the structure, vapour sealing is considered less efficient than is that provided by the foamed-in-situ process because of the difficulty of obtaining a smooth outer surface. Moreover, foaming can readily be used in the production of heavy-duty bodies, designed for tropical climates, which incorporate an air gap between the insulant and the outer panels, and this gives added protection against solar heat. Air is circulated round the inner struc ture by induced draught when the vehicle is travelling, and superficial damage to the outer skin is easily repaired.

Unusual applications of the foamed-in-situ process include Commer Walk-Thru vans of the standard type. The roof of the body is cut off, a container is lowered into the interior and foaming is applied in the normal way. An extended roof of glassfibre is then fitted to the body.

The main advantage of spraying, cited by Sparshatt, is that the process facilitates application to "difficult shapes" because the material is added in layers and the process can be accurately controlled. Sparshatt's decision to employ the foamed-in-situ process instead of spraying was in part influenced by the necessity to provide a spray both on each production line for the spraying process.

Insulation A technical spokesman of the company points out that the improved overall insulating characteristics that have been obtained highlight the fact that typically the main heat gain is frequent opening of the doors in multiple drops, notably in the summer, and the long periods that doors are left open, which nullify the advantage of better insulation. Various forms of system of interlocking the inner and outer doors have been devised by the company to obviate opening both doors at the same time. But such systems are commonly regarded as a safety hazard and a sliding door has been available for some time that self-seals when it is pulled into the closed position.

To sum up, foaming-in-situ is regarded by :he company as the best means of applying an insulant to a container-in-body type of structure given that trained personnel and up:o-date techniques are employed because the process does not involve the use of a spray booth, is non-toxic and adds to structural strength. But using insulant in slabs has the merit that the quality of the insulant is invariably consistent and the bodies can be built by shape and adds to structural strength, vapour sealing is less efficient, the spray is toxic and the process involves the use of a spray booth.

Left and below left: These photographs show the foamed-in-situ process being applied to a demountable container body with a Dobson base assembly. (Below left) the operative is injecting polyurethane into a side cavity and (left) the material is being fed into a floor cavity. Below right: The foamed polyurethane insulation of the roof of the demountable container is seen being vapour-sealed with biturnastic paper. Bottom left: In the case of Commer Walk -Thru vans the roof of a standard body is removed, a container is lowered into the body and polyurethane insulant is applied in-situ in the normal way. The body is finally fitted with an extended roof of glassfibre. Bottom right: This is the interior of a standard Austin EA van that is being insulated with polyurethane in slab form. Insulating the body with slabs can be preferable to foaming-in-situ on the score of the simplicity and cheapness of application.

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Organisations: US Federal Reserve
Locations: Portsmouth

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