METAL CASTING AND FABRICATION

 

Metal fabrication is a value added process that involves the construction of machines and structures from various raw materials. A fab shop will bid on a job, usually based on the engineering drawings, and if awarded the contract will build the product

Steel fabrication shops and machine shops have overlapping capabilities, but fabrication shops generally concentrate on the metal preparation, welding and assembly aspect while the machine shop is more concerned with the machining of parts.

Metal Technologies began as a metal fabrications company and over the years has continued to develop their fabrication and powder coating competencies. Projects have covered an array of applications from the Power industry to the Gaming industry and have been both contract and non-contract jobs. 

 

Fabrication Technologies

 

Fabrication Technologies is a leader in contract manufacturing, engineering and assembly of fabricated sheet metal products. With an outstanding combination of “state of the art” technology complimented by the best people in the business, we’ve established the benchmark for the industry.

 

 

Forming

 

 

Hydraulic brakes with v-dies are the most common method of forming metal. The cut plate is placed in the press and a v-shaped die is pressed a predetermined distance to bend the plate to the desired angle. Wing brakes and hand powered brakes are sometimes used.

Tube bending machines have specially shaped dies and mandrels to bend tubular sections without kinking them.

 

 

Rolling machines are used to form plate steel into a round section.

Sheet metal forming refers to various processes used to convert sheet metal into different shapes for a large variety of finished parts such as aluminium cans and automobile body panels. Key to the formability of sheet metal is its ductility

 

Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.

Casting may be used to form hot, liquid metals or meltable plastics (called thermoplastics), or various materials that cold set after mixing of components such as certain plastic resins such as epoxy, water setting materials such as concrete or plaster, and materials that become liquid or paste when moist such as clay, which when dry enough to be rigid is removed from the mold, further dried, and fired in a kiln or furnace

 

Substitution is always a factor in deciding whether other techniques should be used instead of casting. Alternatives include parts that can be stamped out on a punch press or deep-drawn, forged, items that can be manufactured by extrusion or by cold-bending, and parts that can be made from highly active metals.

Investment Casting

Main article: Investment Casting

Valve for Nuclear Power Station produced using investment casting

Investment casting (lost-wax casting) is a process that has been practised for thousands of years, with lost wax process being one of the oldest known metal forming techniques. From 5000 years ago, when bees wax formed the pattern, to today’s high technology waxes, refractory materials and specialist alloys, the castings ensure high quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.

The process is suitable for repeatable production of net shape components, from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.

Casting is the process of production of objects by pouring molten material into a cavity called a mold which is the negative of the object, and allowing it to cool and solidify.

 

Sand casting is a means of producing rough metal castings using a mold usually made from sand formed around a replica of the object to be cast that is removed once the sand has been compacted. Castings made by this process can be further refined by any or all of hammer peening, shot peening, polishing, forging, plating, rough grinding, machine grinding or machining. Sand castings not further worked by polishing or peening are readily recognized by the sand-like texture imparted by the mold. As the accuracy of the casting is limited by imperfections in the mold making process there will be extra material to be removed by grinding or machining, more than is required by other more accurate casting processes. Furthermore, because the mold is destroyed in order to retrieve the object, a new mold must be made for each casting

Patterns

From the design, provided by an engineer or designer, a skilled patternmaker builds a pattern of the object to be produced, using wood, metal, or plastic; other materials to be used can be polystyrene or even sand strickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as contraction allowance. Patternmakers are able to produce suitable patterns using 'Contraction rules'. Different scaled rules are used for different metals because different metals / alloys contract at different rates. Patterns also have coreprints; these create registers within the molds, into which are placed Sand cores. Sand cores are used to create under cut profiles and holes which cannot be molded.

Paths for the entrance of metal, during the pouring (casting) process into the mold cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed' and 'runners', and ingates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via the riser, are added either in the pattern itself, or as separate pieces.

 

Molding box and materials

 

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag) is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand—a slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes possible casting without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.

 

Cores

 

To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.

 

Two sets of castings (bronze and aluminium) from the above sand mold

With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze, brass, aluminum, magnesium alloys, or various pot metal alloys, which often include lead, tin, and zinc. After filling with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed revealing a rough casting that, in the case of iron or steel, may still be glowing red. When casting with metals like iron or lead, which are significantly heavier than the casting sand, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.

 

 

Cold Uniaxial Pressing

 

Elemental metal, or an

atomised prealloyed, powder is mixed with a lubricant, typically lithium stearate (0.75 wt.%), and pressed at pressures of say, 600 MPa (87,000 lb/in2) in metal dies. Cold compaction ensures that the as-compacted, or ‘green’, component is dimensionally very accurate, as it is moulded precisely to the size and shape of the die.

Irregularly shaped particles are required to ensure that the as-pressed component has a high green strength from the interlocking and plastic deformation of individual particles with their neighbours.

One disadvantage of this technique is the differences in pressed density that can occur in different parts of the component due to particle/particle and die wall/particle frictional effects. Typical as-pressed densities for soft iron components would be 7.0 g/cc, i.e. about 90% of theoretical density. Compaction pressure rises significantly if higher as-pressed densities are required, and this practice becomes uneconomic due to higher costs for the larger presses and stronger tools to withstand the higher pressures.

 

Isostatic Pressing

 

 

Metal powders are contained in an enclosure e.g. a rubber membrane or a metallic can that is subjected to isostatic, that is uniform in all directions, external pressure. As the pressure is isostatic the as-pressed component is of uniform density. Irregularly shaped powder particles must be used to provide adequate green strength in the as-pressed component. This will then be sintered in a suitable atmosphere to yield the required product.

Normally this technique is only used for semi-fabricated products such as bars, billets, sheet, and roughly shaped components, all of which require considerable secondary operations to produce the final, accurately dimensioned component. Again, at economical working pressures, products are not fully dense and usually need additional working such as hot extrusion, hot rolling or forging to fully density the material.

 

Sintering

 

Sintering is the process whereby powder compacts are heated so that adjacent particles fuse together, thus resulting in a solid article with improved mechanical strength compared to the powder compact. This “fusing” of particles results in an increase in the density of the part and hence the process is sometimes called densification. There are some processes such as hot isostatic pressing which combine the compaction and sintering processes into a single step.

After compaction the components pass through a sintering furnace. This typically has two heating zones, the first removes the lubricant, and the second higher temperature zone allows diffusion and bonding between powder particles. A range of atmospheres, including vacuum, are used to sinter different materials depending on their chemical compositions. As an example, precise atmosphere control allows iron/carbon materials to be produced with specific carbon compositions and mechanical properties.

The density of the component can also change during sintering, depending on the materials and the sintering temperature. These dimensional changes can be controlled by an understanding and control of the pressing and sintering parameters, and components can be produced with dimensions that need little or no rectification to meet the dimensional tolerances. Note that in many cases all of the powder used is present in the finished product, scrap losses will only occur when secondary machining operations are necessary.

 

Hot Isostatic Pressing

 

Powders are usually encapsulated in a metallic container but sometimes in glass. The container is evacuated, the powder out-gassed to avoid contamination of the materials by any residual gas during the consolidation stage and sealed-off. It is then heated and subjected to isostatic pressure sufficient to plastically deform both the container and the powder.

The rate of densification of the powder depends upon the yield strength of the powder at the temperatures and pressures chosen. At moderate temperature the yield strength of the powder can still be high and require high pressure to produce densification in an economic time. Typical values might be 1120°C and 100 MPa for ferrous alloys. By pressing at very much higher temperatures lower pressures are required as the yield strength of the material is lower. Using a glass enclosure atmospheric pressure (15 psi) is used to consolidate bars and larger billets.

The technique requires considerable financial investment as the pressure vessel has to withstand the internal gas pressure and allow the powder to be heated to high temperatures.

As with cold isostatic pressing only semifinished products are produced, either for subsequent working to smaller sizes, or for machining to finished dimensions.

 

Hot Forging (Powder Forging)

 

Cold pressed and sintered components have the great advantage of being close to final shape (near-nett shape), but are not fully dense. Where densification is essential to provide adequate mechanical properties, the technique of hot forging, or powder forging, can be used.

In powder forging an as-pressed component is usually heated to a forging temperature significantly below the usual sintering temperature of the material and then forged in a closed die. This produces a fully dense component with the shape of the forging die and appropriate mechanical properties.

Powder forged parts generally are not as close to final size or shape as cold pressed and sintered parts. This results from the allowances made for thermal expansion effects and the need for draft angles on the forging tools. Further, minimal, machining is required but when all things are considered this route is often very cost effective.

 

 

Rolling

 

From Wikipedia, the free encyclopedia

Rolling is a combination of rotation (of a more or less cylindrically or spherically symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that the two are in contact with each other without sliding. This is achieved by a rotational speed at the cylinder or circle of contact which is equal to the translational speed. Rolling of a round object typically requires less energy than sliding, therefore such an object will more easily move, if it experiences a force with a component along the surface, for instance gravity on a tilted surface; wind; pushing; pulling; an engine. Objects with corners, such as dice, roll by successive rotations about the edge or corner which is in contact with the surface.

Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling. In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called "strip") or plate (typically thickness more than 3 mm). In profile rolling, the final product may be a round rod or other shaped bar such as a structural section (beam, channel, joist etc). Rolling is also classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature then the process is termed as hot rolling, If the temperature of metal is below its recrystallization temperature the process is termed as cold rolling.

Other processes also termed as 'hot bending' are induction bending, whereby the section is heated in small sections, and dragged into a required radius.

Heavy plate tends to be formed using a press process, and is termed forming, rather than rolling.

 

 

Roll forming or Rollforming is a continuous bending operation in which a long strip of metal (typically coiled steel)is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities.

A variety of cross-section profiles can be produced, but each profile requires a carefully crafted set of roll tools. Design of the rolls starts with a flower pattern, which is the sequence of profile cross-sections, one for each stand of rolls. The roll contours are then derived from the profile contours. Because of the high cost of the roll sets, simulation is often used to validate the designed rolls and optimize the forming process to minimize the number of stands and material stresses in the final product.

Roll forming lines can be set up with multiple configurations to punch and cutting off parts in a continuous operation. For cutting off a part to length, the lines can be set up using a precut die (single blank runs through the roll mill) and a post cut die which the profile is cutoff after the roll forming process. You can add features in a part (holes, notches, embosses, shear forms, etc) by punching in a rollforming line. These part features can be done in a pre punch application (before roll forming starts), in a mid line punching application (in the middle of a roll forming line/process) and a post punching application (after roll forming is done). Some roll forming lines may incorporate only one of the above punch or cutoff applications or all of the applications in one line.

 

 

A rolling mill is a machine or factory for shaping metal by passing it between a pair of rolls.

Rolling mills are often incorporated into integrated steelworks, but also exist as separate plants and can be used for other metals, and other materials.

Rolling mills historically have been of several kinds:

Depending on the temperature of the metal being rolled, rolling mills are typically hot or cold rolling mills.

A slitting mill was used to cut flat bar iron into rods for nail-making.

A tinplate works normally contained at least two rolling mills, one for hot rolling and the other for cold rolling single plates, prior to tinning.

From the Industrial Revolution, puddled iron, after consolidation with a powered hammer (shingling), was rolled into bar iron using a rolling mill with grooved rolls. The grooves provided were progressively smaller, so that on successive passes through the rolls, the cross-section of the bar became smaller and the bar itself longer. By designing the rolls appropriately, it is possible to obtain iron or steel with various cross-sections, including I-shaped girders and railway rails.

 

 

Deep drawing is a compression-tension metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch[1]. It is thus a shape transformation process with material retention. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.

The total drawing load consists of the ideal forming load and an additional component to compensate for friction in the contacting areas of the flange region and bending forces at the die radius. The forming load is transferred from the punch radius through the drawn part wall into the deformation region (sheet metal flange). Due to tensile forces acting in the part wall, wall thinning is prominent and results in an uneven part wall thickness. It can be observed that the part wall thickness is lowest at the point where the part wall loses contact with the punch, i.e. at the punch radius. The thinnest part thickness determines the maximum stress that can be transferred to the deformation zone. Due to material volume constancy, the flange thickens and results in blank holder contact at the outer boundary rather than on the entire surface. The maximum stress that can be safely transferred from the punch to the blank sets a limit on the maximum blank size (initial blank diameter in the case of rotationally symmetrical blanks). An indicator of material formability is the limiting drawing ratio (LDR), defined as the ratio of the maximum blank diameter that can be safely drawn into a cup without flange to the punch diameter. Determination of the LDR for complex components is difficult and hence the part is inspected for critical areas for which an approximation is possible.
Commercial applications of this metal shaping process often involve complex geometries with straight sides and radii. In such a case, the term stamping is used in order to distinguish between the deep drawing (radial tension-tangential compression) and stretch-and-bend (along the straight sides) components.

Deep drawing has been classified into conventional and unconventional deep drawing. The main aim of any unconventional deep drawing process is to extend the formability limits of the process. Some of the unconventional processes include hydromechanical deep drawing, Hydroform process, Aquadraw process, Guerin process, Marform process and the hydraulic deep drawing process to name a few.

Industrial uses of this process include automotive body and structural parts, aircraft components, utensils and white goods. Complex parts are normally formed using progressive dies in a single forming press or by using a press line.

Wire drawing is a manufacturing process used to reduce or change the cross section of a wire by using a series of draw plates or dies.

 

Diagram of drawing wire

Rod or wire stock is drawn through one or more tapered wire drawing dies. For steel wire drawing, a tungsten carbide inner "nib" is inserted inside a steel casting. The hard carbide provides a wear

 

Extrusion

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For the process that creates volcanic rock, see extrusive (geology).

 

Extruded aluminium; slots allow bars to be joined with special connectors.

Extrusion is a manufacturing process used to create long objects of a fixed cross-sectional profile. A material, often in the form of a billet, is pushed and/or drawn through a die of the desired profile shape. Hollow sections are usually extruded by placing a pin or piercing mandrel inside of the die, and in some cases positive pressure is applied to the internal cavities through the pin. Extrusion may be continuous (producing indefinitely long material) or semi-continuous (producing many short pieces). Some materials are hot drawn while others may be cold drawn.

 

Extrusion of a round blank through a die

The feedstock may be forced through the die by various methods. A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (for steel alloys and titanium alloys for example), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.

Extrusion simulation tools help to understand the extrusion process and to optimize development of tools and products.

Extrusion is also the first step in the process of extrusion and spheronization, a commonly used process in the pharmaceutical industry.

Commonly extruded materials include metals, polymers, ceramics, and foodstuffs

 

Plastics extrusion is a high volume manufacturing process in which raw plastic material is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, window frames, plastic sheeting, adhesive tape and wire insulation.

Tubing extrusion

Plastic tubing, such as drinking straws and medical tubing, is manufactured by extruding molten polymer through a die of the desired profile shape (square, round, triangular). Hollow sections are usually extruded by placing a pin or mandrel inside of the die, and in most cases positive pressure is applied to the internal cavities through the pin.

Sometimes tubing with multiple lumens (holes) must be made for specialty applications. For these applications, the tooling is made by placing more than one pin in the center of the die, to produce the number of lumens necessary. In most cases, these pins are supplied with air pressure from different sources. In this way, the individual lumen sizes can be adjusted by adjusting the pressure to the individual pins.

 

 

Sheet/Film extrusion

 

For products such as plastic sheet or film, the cooling is achieved by pulling through a set of cooling rolls (calender rolls), mostly 3-4 in number. In sheet extrusion, these rolls not only deliver the necessary cooling but also determine sheet thickness and surface texture (in case of structured rolls).

Often coextrusion is used to apply one or more layers to obtain specific properties such as UV-absorbtion, soft touch, matt surface, energy reflection, ...

A common post-extrusion process for plastic sheet stock is thermoforming, where the sheet is heated till soft (plastic), and formed on a mold into a new shape. When vacuum is used, this is often described as vacuum forming. Thermoforming can go from line bended pieces (e.g. displays) to complex shapes (computer housings), which often look like being injection moulded, thanks to the various possibilities in thermoforming, such as inserts, undercuts, divided moulds.