Plastics cover a broad field of organic synthetic resin and may be divided into two main classifications - aerospace thermoplastics _ and aerospace thermosetting plastics. Thermoplastics may be softened by heat and can be dissolved in various organic solvents.
Thermoplastics may be softened by heat and can be dissolved in various organic solvents. Two kinds of transparent thermoplastic materials are commonly employed in windows, canopies, etc. These are known as acrylic plastics and cellulose acetate plastics. Cellulose acetate was used in the past but since it is dimensionally unstable and turns yellow after it has been installed for a time, it has just about passed from the scene and is not considered an acceptable substitute for acrylic. Acrylic plastics are known by the trade names of Lucite or Plexiglas and by the British as Perspex and meet the military specifications of MIL-P-5425 for regular acrylic, MIL-P-8184 -~ 184 for craze-resistant acrylic.
Aerospace Thermosetting Plastics.
Thermosetting plastics do not soften appreciably under heat but may char and blister at temperatures of 240 to 260 'C (400 to 500 °F). Most of the moulded products of synthetic resin composition, such as phenolic, urea-formaldehyde, and melamine formaldehyde resins, belong to the thermosetting group. Once the plastic becomes hard, additional heat will not change it back into a liquid as it would with a thermoplastic.
Storage and handling.
Because transparent thermoplastic sheets soften and deform when they are heated, they must be where the temperature will never be excessive.
Transparent acrylic plastics get soft and pliable when they are heated to their forming temperatures and can be formed to almost any shape. When they cool, they retain the shape to which they were formed. Acrylic plastic may be cold-bent into a single curvature if the material is thin and the bending radius is at least 180 times the thickness of the sheet. Cold bending beyond these limits will impose so much stress on the surface of the plastic that tiny fissures or cracks, called crazing, will form.
Simple Curve Forming. Heat the plastic material to the recommended temperature, remove it from the heat source, and carefully drape it over the prepared form. Carefully press the hot plastic to the form and either hold or clamp the sheet in place until it cools. This process may take from ten minutes to one-half hour. Do not force-cool it.
This type of forming is normally used for such parts as canopies or complex wingtip light covers, and it requires a great deal of specialized equipment. There are four commonly used methods, each having its advantages and disadvantages.
Stretch forming. Preheated acrylic sheets are stretched mechanically over the form in much the same way as is done with the simple curved piece. Special care must be taken to preserve uniform thickness of the material, since some parts will have to stretch more than others.
Male And Female Die Forming. This requires expensive matching male and female dies. The heated plastic sheet is placed between the dies which are then mated. When the plastic cools, the dies are opened.
Aerospace Vacuum Forming Without Forms. Many aircraft canopies are formed by this method. In this process a clamp with an opening of the desired shape is placed over a vacuum box and the heated sheet of plastic is clamped in place. When the air in the box is evacuated, the outside air pressure will force the hot plastic through the opening and form the concave canopy. It is the surface tension of the plastic that shapes the canopy.
Aerospace Vacuum Forming With A Female Form. If the shape needed is other than that which would be formed by surface tension, a female mould, or form must be used. It is placed below the plastic sheet and the vacuum pump is connected. When air from the form is evacuated, the outside air pressure will force the hot plastic sheet into the mould and fill it.
Getting goods from A to B is hard enough. Finding the right way to carry those goods is another headache. Once, we loaded and unloaded goods item by item - those were the days when labour was cheap. Then the Second World War came. This mother of many inventions brought us the wooden pallet. This, combined with a fork lift truck, enabled goods to be moved quickly and with less labour.
The wooden pallet was a great idea. So good was the idea that it is still going strong to this day. However, things have changed. Pallet pooling can be an expensive business. Rental and purchase costs have risen to reflect the increase in timber costs and the extra burden of fumigation and quality control.
Escalating fuel costs have taken their toll on the inevitable dedicated trips that are made to repatriate empty pallets when imbalances occur in the supply chain. These fuel costs - and driver shortages - have also forced an examination of the space and weight taken up by the ubiquitous pallet.
The pallet is a bit like the air that we breathe. It is all around us yet we don't see it. It is easy to forget the extra cost that the pallet adds to the value chain. Perhaps now the time has come when, in some eyes, the pallet has outstayed its welcome. Rather than being the carrier, is the pallet itself being carried by the value chain? Rather than wait for another world war, a ready solution has been found and is tried and tested. That is the Slip Sheet.
Enter the Slip Sheet
Slip sheets have been around for some time, especially in the
U.S. where the free market has always had the knack of taking the path of least resistance. European countries tend to prefer centrally planned systems and standardization. Slip sheets lack firmly agreed international standards and this may be one of the reasons why they have not been popular in Europe. The greatest reason is ignorance. Many simply have no understanding of slip sheets and how they work - so they don't bother.
So perhaps it's time to right this wrong here and now.
How Slip Sheets Work
1. Slip sheets are durable sheets of material that are designed to carry a uniform load of goods with plan dimensions (or "footprint") of roughly 1 metre square/40 inches square (typically they may be designed to carry a footprint of 40 inches x 48 inches - 1m x 1.2m). The slip sheet is made to these dimensions plus a pull tab - an extension to the sheet - of up to 6 inches/15cm. There may be just one pull tab sticking out from under the load on one side. There could, however be pull tabs on 2, 3 or all 4 sides. Slip sheets can be made of
fibreboard or plastic.
2. Goods are loaded directly onto the slip sheet within the creases that border any pull tabs. A push-pull attachment on a forklift truck is used to drag the load onto the truck by gripping (one of) the pull tab(s). The push-pull attachment has very flat ground-level forks that are designed to go under the
slip sheet whilst it is under load
3. To unload, the forklift truck pushes the load, complete with slip sheet off of itself and withdraws
Pros and Cons
Slip sheets are not the panacea to all distribution needs. For one thing, all parties in any supply chain will need to invest in the push-pull attachments, which are not cheap. The staff training that will be required, especially for forklift truck operators, can also be expensive. Perhaps the greatest factor, though, is the uniformity of the load itself. This exposes the slip sheet's greatest advantage - and greatest disadvantage. Slip sheets may not be able to cope when boxes are awkwardly shaped or where there is a mixed load of different shapes. Conversely, if a company is shipping out large quantities of the same sized box, then slip sheets are ideal. As such, slip sheets lend themselves to operations further upstream of the supply chain. As loads are broken down further downstream, then pallets may be more appropriate. The good thing is that loads of slip sheets can be easily transferred to pallets. A good example of the advantages of using slip sheets is where parts for goods are manufactured and shipped through a long distance sea route. The space and weight saving can result in a significant financial boost for the value chain.
Co-operative Value Chains
Another great factor in all this is uniformity of slip sheet systems. All parties in the supply chain need to be using the same specification of slip sheet and ensure that all staff are fully trained to ensure delivered loads are configured correctly for the receiving party. This requires value chains that are highly co-operative, or, to be realistic, it requires a value chain that has one dominant party who will simply dictate their requirements. The U.S. has pioneered slip sheets and has used them for many years. However, the dominance of the supermarket sector in some European countries may be forcing the change there also. The ability of large supermarkets to influence their supply chains means that they are in a position to take a strategic view of the benefits that slip sheets can bring. They are in a position to arrange for both senders and receivers to be ready to handle products on slip sheets. Things are changing. Perhaps the pallet industry is about to get a jolt.
Watch this space. There are tremendous savings to be made and significant environmental benefits to be gained. Full conversion to slip sheets for any supply chain is not going to happen, but a partial conversion for particular product lines is not only desirable, but inevitable - even in Europe.