Under the top casing?
The purpose of using a plastic casing depends on what product the casing is for. Generally plastic is used because it is a very versatile material that can both protect and look good. Plastic is also easy to color and a relatively cheap material.
It will be made of many components the casing material will not matter much.
Metal casing on kettles is common. Metal casing is desired on anything to put on the stove. This is because the material conducts heat so well and efficiently stays warm.
Usually polyethylene (casing) and polyurethane foam (insulation).
In order to reduce material stresses. Under pressure, a casing has to seal against the pressure inside the casing and atmosphere. As plant pressures increase, besides geometrical designs differences, the other basic items left for the designer to minimize stresses are material selection and casing thickness. A double casing provides another option since it essentially places an intermediate casing in between the single casing. In that way, one casing seals against steam pressure and a lower partial pressure, a few stages downstream of the inlet stage, and then the intermediate casing seals against this lower pressure an ambient pressure.
You will have to remove the air filter casing to get at them they are At the end of the HT leads from the coil
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It all depends on where the speakers will be, and If you mean sound quality or protectivness.
mud and fibers
Batteries, specifically wet cell batteries, mostly corrode due to leaks in their casings. These leaks allow the material inside the battery to seep out and contaminate the battery's metallic casing. This leaked material acts as a catalyst for the oxidation of the metallic casing of the battery, otherwise known as corrosion.
the reason is because the molecules casing the pressure inhibit the molecules of the material.