Glossary of Process Attributes

Details of the following process attributes are among those stored in Granta's extensive range of materials reference data modules.

Classes of processes in the database

Process & class attributes

   

Casting
Composite Forming
Deformation Processing
Deposition
Fabrication
Machining
Moulding
Powder Methods
Rapid Prototyping

Process Class
Material Class
Shape Class

Adjacent Section Ratio
AspectRatio
Capital Cost
Economic Batch Size
Hole Diameter
Lead Time
Mass Range
Material Utilisation
Maximum Dimension
Minimum Corner Radius
Production Rate
Quality Factor
Roughness
Section
Tolerance
Tooling Cost
Tool Life

Casting

Casting involves pouring molten material into a mould, where it solidifies, taking the shape of the cavity. This can be achieved under the self-weight of the metal (gravity casting) or under pressure. The mould can be made from a collapsible material, such as sand, in which case a new mould is made for each casting; or the mould can be made of a metal and thus can be used repeatedly. Internal details in a casting are achieved using 'cores'. Castings of very complicated shapes can be produced and most materials which can be melted can be cast. Castings can vary in mass from fractions of a gram to many tonnes.

 

Composite Forming

Composite forming methods vary depending on the form of the fibres used. Chopped fibres are mixed with resin and shaped by polymer moulding techniques; resin-impregnated mats of fibres are laid in a mould or pressed together and then allowed to cure; and continuous fibres coated with resin are wound on a mandrel to make spherical, cylindrical and other shapes.

Deformation Processing

Deformation is forming the solid material into shape by applying forces to it. Because the material is in the solid state, the forces required are high; for this reason metals with very high yield stresses are deformed hot. However, many commonly used metals can be deformed at room temperature thus eliminating the need for expensive heating equipment. The most well known deformation processes are forging, rolling and extrusion which can produce components of a variety of shapes. Forming sheets into various shapes is also a type of deformation processes. Cold forming gives a better surface finish than hot forming; and cold-formed parts generally have a higher yield strength than those which are hot-formed because the work hardening is retained.

Deposition

Deposition includes processes in which the desired shape is built up atom by atom, as in electroforming and chemical vapour deposition (CVD). A mould or a mandrel, onto which the material is deposited, creates the internal details of the component. CVD is mainly used as an additional process to deposit surface layers (to improve wear and corrosion resistance) on manufactured components.

Fabrication

A number of techniques allow a part to be assembled from smaller components. Welding, adhesive bonding and fastening by the use of bolts and rivets are the most widely used examples. Microfabrication refers to processes which are used to make micro components used in microelectronics and microdynamical systems.

Machining

This is the group of processes in which a shape is generated by removing unwanted material. Machining can be used to make a component from stock material but more often it is used as a secondary process to impart a shape or a level of precision to a manufactured component which cannot be achieved otherwise. Shape restrictions exist for some machining processes. For example turning can only produce axisymmetric shapes.

Moulding

Moulding resembles casting in that the material solidifies in a die. However, the term mainly refers to processes involving polymers or glasses. The method of introducing the polymer into the die varies depending on whether the polymer is thermoplastic or thermoset. Both polymers and glasses can also be blown into shape ( blow-moulding) or made into a foam. Complicated components with intricate internal details can be made in masses ranging from very tiny parts to 25 kg.

Powder Methods

As the name implies, these methods involve the use of powders which are normally pressed together and then sintered to form the desired shape. Small, intricate parts of high precision can be made. The processes are also used for forming materials which are difficult to manufacture otherwise. Also, powders of two metals can be combined together to give products with combined properties. In die pressing, the powder is placed in an open die and compressed by forcing axial plungers into it. In hot isostatic pressing (HIPing) the powder is placed in a thin-walled preform and compressed using hydrostatic pressure.

Rapid Prototyping

This includes a number of rapidly evolving techniques for making prototypes and models quickly thus allowing designers to check their designs and make any necessary changes before investing in expensive tooling. A CAD model of the part is required and the model is usually built layer by layer.

Process class

Each process is assigned a group of process classes, the structure of which is shown in Figure1. Primary processes take unshaped material (liquid metal, a powder or a solid ingot) and give it shape. Thus casting processes are primary, though they can be discrete or continuous. Secondary processes modify, refine or add features to an already-shaped body. As an example: 'fine machining' is a secondary process, and it is one that can modify, refine and add features. Thus a single process can belong to more than one class. In the same spirit, tertiary processes add quality (hot isostatic pressing of castings and polishing of surfaces are examples), and fabrication (joining) processes have the special feature that, among other things, they add size.

Process Class Structure

Figure 1

The classification of process types. Each process is linked to one or more members of each.

Material class

Each process is linked to one or more broad material classes. Figure 2, shows the class-structure. The class of metals is subdivided into ferrous and four subclasses of non-ferrous metals; that of polymers into thermoplastics, thermosets, foams and elastomers. The other classes are similarly subdivided.

Material Class Structure

Figure 2

The classification of material types. Each process is linked to one or more members of each.

Shape class

Like other researchers in this field, we have explored alternative approaches to the characterisation of 'shape' and 'complexity', some based on ideas of symmetry, others on information theory, still others based on an amalgam of experience and intuition. None gives complete satisfaction. Figure 3 shows the one we find works best, because it relates most closely to the ways in which real manufacturing processes work. Prismatic shapes result from extrusion, rolling and drawing and turning. Products made from sheet are flat or dished, with or without cut-outs; they are made by processes such as pressing, stamping, rolling and spinning. Other solid shapes, not made from sheet, are 3-dimensional; those with features which lie parallel to a single direction (and thus can be made in simple 2-piece moulds or dies) are distinguished from those with transverse features and re-entrants.

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Figure 3 The classification of shapes. Each process is linked to one or more members of each shape class

Mass range (normal and extreme)

Units: kg (SI), lb (Imperial)

The 'normal' range of component mass which lies within the capacity of the process. Most processes can be pushed to an 'extreme' mass outside the normal range, but with an associated cost penalty which can be large. This 'extreme' range is also stored. Mass range is determined by the capacity of the caster, press, machine tool, etc. It can be extended by fabrication (joining).

Section (normal and extreme)

Units: mm (SI), inches (Imperial)

The 'normal' range of section thickness which lies within the capacity of the process. As with mass, an 'extreme' range is also stored. Minimum section is determined by considerations of fluid flow in castings, of plastic constraint in forgings and so on. It can usually be reduced by machining.

Roughness (normal and extreme)

Units: mm (SI), mils (Imperial)

The 'normal' range of RMS (root mean square) surface roughness which lies within the capacity of the process. As with mass, an 'extreme' range is also stored. Surface roughness is determined by the nature of the process: the smoothness of mould surfaces in casting and moulding or the depth of cut in machining. It can usually be refined by machining, grinding and polishing.

Tolerance (normal and extreme)


Units: mm (SI), inches (Imperial)

The ' normal' range of precision which lies within the capacity of the process. As with mass, an 'extreme' range is also stored. Precision, like surface roughness, is determined by the nature of the process: the accuracy of mould surfaces in casting, distortion in moulding or the machine precision in cutting. It can be refined by fine machining or precision grinding and polishing.

Aspect ratio


Dimensionless

The maximum length-to-thickness ratio of which a process is capable. Batch processes like casting have limits imposed by the physics of the process. Continuous processes like rolling, extrusion or wire-drawing have no real upper limit. For these, a cut-off of 1000 has been used.

Adjacent section ratio


Dimensionless

The maximum practical ratio of adjacent section thickness at a change of section. It is limited by shrinkage stresses in casting and moulding and by plastic flow in forging. It can be altered by machining.

Hole diameter


Units: mm (SI), inch (Imperial)

The minimum hole diameter which can be created by the process. Casting, stamping and moulding impose limits on minimum hole size which can be overcome by creating the holes with a secondary process such as drilling or laser cutting.

Minimum corner radius


Units: mm (SI), inch (Imperial)

The minimum radius of curvature at a corner which can be created by the process. Casting, stamping and moulding impose limits on minimum corner radius.

Maximum dimension


Units: mm (SI), inch (Imperial)

The largest dimension of a component which can be created by the process. In batch processes it is limited by the capacity of the machine, but it can be increased by joining. In cases where there is no defined limit, a cut-off of 10,000 mm has been used. Continuous processes like rolling, extrusion or wire-drawing have no real upper limit on length so, instead, maximum width is stored.

Quality factor


Dimensionless

Quality is difficult to quantify. Processes prone to porosity (certain sand-casting processes, for example) or other defects are assigned a low value. Processes which minimise the probability of defects (closed-die forging and Cosworth casting are examples) are given a high value. Quality can sometimes be enhanced by using tertiary processes such as HIPing or surface treatment. The quality factor is assessed on a semi-quantitative numerical scale of 1-10.

Economic batch size


Units: number; or kg (SI), lb (Imperial); or m (SI), ft (Imperial)

The economic batch-size is a measure of the output required before a process becomes competitive. For batch processes, it is quantified by the number of units, or by the total mass of product. For continuous processes, it is measured by the total mass or length. Processes with high tooling costs have high economic batch sizes, those with low tooling costs enable low batch sizes.

Capital cost


Units: Currency (UK£, US$ etc.)

The capital cost is the total cost of the equipment necessary to perform the process. Manual processes have lower capital costs than their automated counterparts. In cost estimation, the capital cost is converted to a time-cost by dividing it by the capital write-off time, except when the equipment is totally dedicated to a single product, in which case it is treated in the same way as tooling cost.

Tooling cost


Units: Currency (UK£, US$ etc.)

The tooling cost is the cost of the tooling (that is, moulds, dies, jigs and fixtures) which are totally dedicated to the production of a single product. This cost must be pro-rated over the number of products produced in the production run. Processes with high tooling costs have high economic batch-sizes.

Lead time


Units: weeks

The lead time is the advance-planning time necessary to prepare and set up tooling to allow a production run.

Material utilisation


Dimensionless

The material utilisation is the mass-fraction of primary material entering the process which remains in the final product. It is measured on a scale of 0-1. Machining from solid leads to low material utilisation; near net-shape processes allow a utilisation approaching 1.

Production rate


Units: hr--1; or kg/hr (SI), lb/hr (Imperial); or m/min (SI), ft/min (Imperial)

The production rate is the output-rate of the process. For batch processes it is measured in number of units per hour, or in total mass per hour of product; for continuous processes it is measured in total mass or length per hour. Automated processes have higher output rates than their manual counterparts.

Tool life


Units: number; or kg (SI), lb (Imperial); or m (SI), ft (Imperial)

The tool life is a measure of the output which is possible before the tooling must be replaced. For batch processes it is measured in number of units, or in total mass of product; for continuous processes it is measured in total mass or length. When the production run exceeds the tool life, tooling must be replaced and the cost adjusted.