In engineering, a shim is a thin and often tapered or wedged, piece of material, used to fill small gaps or spaces between objects. Shims are typically used in order to support, adjust for better fit, or provide a level surface. Shims may also be used as spacers to fill gaps between parts subject to wear.
Many materials make suitable shim stock, or base material, depending on the context: wood, stone, plastic, metal, or even paper (e.g., when used under a table leg to level the table surface). High quality shim stock can be bought commercially, for example as laminated shims, but shims are often created ad hoc from whatever material is immediately available.
Examples of the usage of shim from different engineering disciplines are outlined below:
In automotive engineering shims are commonly used to adjust the clearance or space between two parts. For example, shims inserted into or under bucket tappets control valve clearances. Clearance is adjusted by changing the thickness of the shim.
In carpentry or joinery small pieces of wood may be used to align gaps between larger timbers.
Thin pieces of metal called shims are used to test that epee tips conform to specifications. If the shim can be inserted into the narrow gap of the weapon tip, the gap is the correct size. Foil tips were initialy tested with shims as well, but this was abandoned as being useless.
A small metal device used to quickly open a lock is called a shim.
In luthiery shims made of various materials are often used to adjust neck alignment.
In masonry small stones may be used to align or fill gaps between larger bricks or slabs.
In the Robert M. Pirsig novel Zen and the Art of Motorcycle Maintenance one narrator discusses the use of a piece of beer can, as a shim, to tighten the handlebars.
In NMR or MRI, "shimming" is used prior to the operation of the magnet to eliminate inhomogeneitites in its field.
In plumbing shims of metal are used to align pipes.
In web design a well known hack or kludge is to use a single-pixel transparent GIF as a shim to align spaces between tables in HTML to ensure a desired page layout. See spacer GIF.
In software engineering or hypermedia a piece of code (such as an API shim), placed between layers after the manner of a physical shim, in order to create a better fit or a smoother flow of code execution across layers.
In computer networking, a piece of extra code inserted between existing layers of the protocol stack to translate a communications protocol as the data is passed between the layers.
See also CPU shim for shims used to protect the core of CPU's from damage.
Aluminium foil (Aluminum foil in North American English) is aluminium prepared in thin sheets (on the order of 0.02 mm in thickness). As a result of this, the foil is extremely pliable, and can be bent or wrapped around objects with ease. Aluminium foil is sometimes known as al-foil or alu-foil. It is also often called tinfoil, although it is not made from tin, or as silver paper although it is not made from silver; or in North America, as Reynolds wrap after Reynolds Metals Company, the leading manufacturer when it was introduced on the American market (much to the chagrin of Alcoa, Reynolds' main competitor, which had its brand "Alcoa Wrap" referred to as "Alcoa Reynold's Wrap").
Aluminium foil typically has a highly reflective side and a more matte side. This is a result of common manufacturing processes. As aluminium foil is easy to tear, the foil is sent through machines in pairs. The side where the aluminium foil was in contact with the other sheet is more matte than the exterior side. This unconformity of finish has led to the perception that favoring a side has an effect when cooking. While many believe that the shiny side's reflective properties keep heat in when wrapped on the interior and keep heat out when facing exterior, the actual difference is imperceptible without instrumentation.
Millions of tons of aluminium foil are used throughout the world in the protection and packaging of foods, cosmetics and chemical products. Usually, an extremely thin layer (0.0065 mm or 6.5 Ám) is laminated to other materials, plastics and paper, to make long life packs for drinks, dairy products, and many other sensitive foods. The foil acts as a complete barrier to light (which spoils fats), odours, loss or gain of moisture, and bacteria. Aluminium foil containers and trays are used to bake pies and to pack takeaway meals, ready snacks and long life pet foods.
Aluminium foil is also widely used for insulation (barrier and reflectivity), heat exchangers (heat conduction) and cable liners (barrier and electrical conductivity). Foils in special alloys are even used for structural honeycomb components for aircraft. Aluminium foil's heat conductive qualities make it a common accessory in hookah smoking: a sheet of perforated aluminum foil is frequently placed between the coal and the tobacco, allowing the tobacco to be heated without coming into direct contact with the burning coal. It may be used to smoke heroin or freebase cocaine too.
Aluminium foil is widely sold into the consumer market, usually in rolls of around 50 centimetres width and several metres in length. It is used for wrapping food in order to preserve it, for example when storing leftover food in a refrigerator (where it serves the additional purpose of preventing odour exchange), when taking sandwiches on a journey, or when selling some kinds of take-away or fast food. Mexican restaurants in the United States, for example, typically provide take-away burritos wrapped in aluminium foil. It is also used for barbecuing more delicate foods such as mushrooms and vegetables; food is wrapped in foil then placed on the grill, preventing loss of moisture that may result in a less appealing texture.
Heavier foils made of aluminium are used for art, decoration, and crafts, especially in bright metallic colours. Metallic aluminium, normally silvery in colour, can be made to take on other colours through anodization. Anodizing creates an oxide layer on the aluminium surface that can accept coloured dyes or metallic salts, depending on the process used. In this way, aluminium is used to create an inexpensive gold foil that actually contains no gold, and many other bright metallic colours. These foils are sometimes used in distinctive packaging.
In the late 19th century and early 20th century, tin foil was in common use; aluminium foil largely supplanted it, but some people long continued to call aluminium foil by the name of its tin counterpart (perhaps due to its being shorter to say).
Aluminium foil is also sometimes used in the training of cats; as cats have an inborn dislike of either the texture or noise caused by sheets of aluminium foil, it is possible to prevent cats from jumping on or otherwise damaging furniture by covering its surfaces. Aluminium foil is also used in tinfoil hats which purportedly reduce the effects of "mind control rays". Aluminium foil, like all metals, reacts when microwaved. This is due to the effect of electric fields of the microwave causing a build up of charge to form between sharp points in the aluminum, if enough charge accumulates it will discharge to a different place on the foil, creating a spark (i.e. arcing). Due to frequent use in food services, this commonly leads to kitchen fires.
The extensive use of aluminium foil has been criticised by some environmentalists because of the high resource cost of extracting aluminium, primarily as a result of the large amount of electricity used to decompose bauxite. However, this cost is greatly reduced via recycling and the fact that many foods that would otherwise perish can be protected over long periods without refrigeration thanks to the total barrier properties of aluminium foil. Many aluminium foil products can be recycled at around 5% of the original energy cost.