Casting of Tap and Faucet Page 1.) Introduction 2 2.) Product Brief 2 3.) Executive Summary 2 3.1) Final Selection 2 3.2) Advantages and Disadvantages 2 4.) Design Process 2 4.1) Selecting a material 3 Option 1: Stainless steel 3 Option 2: Copper 3 Option 3: Brass 3 Option 4: Aluminium alloy 4 4.2) Selecting a Casting Process 4 Option 1: Sand casting 5 Option 2: Shell mould 5 Option 3: Die casting 5 Option 4: Permanent mould casting/ gravity die casting 5 Option 5: Investment casting 5 5.) Bibliography 5

1.) Introduction

This report aims to explore and select an economical manufacturing method for a hypothetical tap and faucet design. Many considerations come into play when designing a tap and faucet for manufacture. Well before any final drawings are produced for the manufacture of the tapware, the materials and their treatments, the methods of manufacture, and associated costs, availabilities, time and business issues must be addressed. In order to commence the assessment of casting method, assumptions have been made in the following Product Brief.

2.) Product Brief

• The tap and faucet are destined for general use in mains- supplied homes, schools, offices etc. • The brand is mid-market in placement. Affordability, along with reasonable product quality and lifespan, are as critical as appearance.• An initial production run of 35,000 units is planned; per unit cost is an essential consideration as investments in dies must be covered by returns from this production run.• The selected casting method should be capable of withstanding a further 6 production runs, at minimum.• An 18 week period is the maximum time frame, from final design to completed production run. Tooling, availability of materials, and finishing requirements must be achievable within this period. • A common sense approach must be applied to issues of product safety such as toxicity. • Where possible sustainability issues such as manufacturing wastes and recyclability must be considered. Manufacturing wastes are becoming increasingly expensive to dispose of, and a considera-tion of ‘green’ issues demonstrates responsibility worthy of marketing.

3.) Executive Summary

3.1) Final selection

The material selected for casting the taps and faucets is DCB3 Die Casting Brass (CuZn39Pb1Al-C CC754S). Chrome plating will impart an appealing and resilient finish.

The casting process will be Permanent Mould Casting, also known as Gravity Die Casting.

3.2) Advantages and disadvantages

DCB3 Die Casting Brass has excellent corrosion resistance- the most important functional aspect when considering the constant contact with water. It is readily plated for an attractive, long wearing finish. Although chrome plated brass lacks the visual finesse of stainless steel, it is a very cost effective choice for our chosen market.

The initial cost of the Permanent Mould Casting dies is high, but the proposed production volumes mean the process is inexpensive. Thousands of castings can be made without replacing the die. Although the finish is not quite as precise as die casting, it is superior to sand casting, and economical for taps and faucets.

4.) Design Process

Ideally the design process is not linear. In selecting materials and casting methods, the functions, costs, appearance, availability and time frames, environmental factors and product lifespan were considered simultaneously.

4.1) Selecting a material

Taps need to perform certain functions in a particular environment. • Continual contact with water dictates that the material must be resistant to corrosion. • Moving parts imply wear will be an issue. • The material must be suited to casting. • Finishing will need to be attractive and easily cleaned/ hygienic.

Option 1: Stainless steel

Stainless steel does not stain, corrode, or rust as easily as ordinary steel. Cast high alloy steels are widely used for their corrosion resistance in aqueous media at or near room temperature. (www.steel.keytometals.com) Stainless steel is defined as a ferrous alloy with a minimum of 11% chromium content. Most cast stainless steels are more complex compositionally than this, typically containing one or more alloying elements in ad-dition to chromium (for example, nickel, molybdenum, copper, niobium, and nitrogen) to produce a specific microstructure, corrosion resistance, or mechanical properties for particular service requirements. (www.steel.keytometals.com)

Service temperature provides the basis for selection of cast grades. Stainless material suited to tap casting does not need additional alloys specifically for excessively high temperature, strength, hardness, or chemically cor-rosive environments. The most commonly used identification system for cast stainless steels is the system of the Alloy Casting Insti-tute (ACI). (Engineering Materials, p 552- 555). The Cast-Alloy Designation ASTM A 743 would be specified on any final drawings, along with the Grade. The C series of grades designates the corrosion-resistant steels (www.steelforge.com). CA15 is a suitable selection, as it contains the minimum amount of chromium to be considered rust proof. This Cast Alloy is equivalent to the Wrought Alloy Type 410; a more familiar identifier.Widely used grades of stainless steel (such as Wrought Alloy Type 304) are only marginally more expensive than copper alloys (brasses). However the 410 grade selected for tap casting is approximately double this cost (Engineering Materials, p768-769). Machining costs are significantly higher (x4) for stainless steel than for brass (Engineering Materials, p771).

A modern and beautiful finish can be achieved with stainless steel without any further coating or plating. Electropolishing uses special chemicals and an electric current to uniformly corrode the surface, leaving a smooth, polished finish. This would also remove any carbon pick-up from casting moulds, which potentially reduce the corrosion resistance of the surface. Stainless steel is 100% recyclable.

Option 2: Copper

Pure copper is extremely difficult to cast as well as being prone to surface cracking, porosity problems, and to the formation of internal cavities. The casting characteristics of copper can be improved by the addition of small amounts of elements including zinc, chromium, beryllium, silicon, nickel, tin and silver. (www.nonferrous.keytometals.com). Important and useful copper alloys include the brasses.

Option 3: Brass

Brasses are ideal for a very wide range of applications, and are frequently the cheapest material to select (www.brass.org). The generic term ‘brass’ covers a wide range of copper-zinc alloys with differing combinations of properties, including:

• Corrosion resistance • Machinability • Wear resistance • Cost effective material • Colour • Readily finished/plated

Brasses can easily be cast to shape, and machine finishing is straight forward and quick. In fact, the machinability of brass sets the standard by which other materials are judged. Brass is easily finished to the attractive self-colour. In either a polished or lacquered state it is clean, hygienic and durable with a natural beauty. This would suit elegant, old-fashioned tap styles suited to period renovation retro fitting. Our main market, however, is fittings in new constructions. A more modern look is sought, which could be achieved by plating. Brass readily accepts chrome or gold plat-ing and its inherent corrosion resistance means no rust blisters or cracks commonly seen on inferior, steel-substrated products. Chrome plating in particular is relatively inexpensive, and best suited to our mid-range market.

There are over sixty Standard compositions for brass, with copper contents ranging from 58% to 95%. Apart from the major alloying element, zinc, small additions (less than 5%) of other alloying elements are made to modify the properties. For diecasting the 60/40 type alloys are normally used (http://www.brass.org). This has good fluidity while pouring, and hot strength to avoid hot tearing while solidifying. The higher zinc content lowers the casting temperature and gives essential hot ductility. Small additions of silicon or tin improve fluidity; tin also improves corrosion resistance. An addition of lead improves machinability. Aluminium is added to form a protective oxide film to keep the molten metal clean and reduce the attack on the die materials.

CuZn39Pb1Al-C CC754S (DCB3 Die casting brass) is used extensively for plumbing fittings (www.brass.org). This extensive use suggests the material would be readily available.

The initial die cost is high compared to aluminium alloy. (www.diecasting.org). This loses some signifi-cance when the large quantity of units is considered.The high value of any process scrap can be used to reduce production costs significantly. Further, the material can be recycled without loss of properties, at the site of production.

Option 4: Aluminium alloy

On its own, pure aluminium is prone to high shrinkage, and susceptibility to hot cracking. It is alloyed with silicon (9%), copper (3.5%) and several other alloy materials in relatively small proportions to im-prove the properties. Aluminium as a base material is inexpensive. Alloy ‘A380’ is the most common and cost effective of all die casting alloys (www.kenwalt.com) Machining costs are low- less even than for brass (Engineering Materials, p771), and the initial cost for dies is also less than for brass. Aluminum die casting alloys are lightweight, easy to cast, with good mechanical properties. Both the resistance to corrosion and dimensional stability is good, but less than that of brass.

4.2) Selecting a Casting Process

The casting process is ideal for the production of complex shapes. In the case of taps, detailed internal shapes cannot be machined; casting is the only method to repeat the form economically. The choice of process is determined by the materials to be shaped, the shape itself and the economics of the process ( Materials and Design, p 89).

Option 1: Sand casting

Many castings are made by pouring metal into sand moulds. Depending on the casting required, the sand may be bonded with clay or silicates or various organic mixes. The cavity in the sand is formed by using a pattern, typically made out of wood, sometimes metal. The cavity is contained in a box, called the flask. For hollow castings, cores are used. The core is a sand shape inserted into the mold to produce the internal features of the part. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting. (www.efunda.com) Sand casting does not impart a smooth finish, and a lot of finishing is required. It is economical for low pro-duction quantities.

Option 2: Shell mould

Shell moulding involves the use of a thermosetting resin bond in the sand. This process offers better surface finish and better dimensional tolerances than sand casting, and higher throughput due to reduced cycle times. A heated (200 ºC) metal pattern is covered with a mixture of the sand and thermoset plastic. This causes a skin of about 3.5 mm of the mixture to adhere to the pattern. This skin is removed from the pattern to form the shell mold. The two halves of the shell mold are secured together and the metal is poured in the shell to form the part. Once the metal solidifies, the shell is broken. A fairly high capital investment is required, but high production rates can be achieved. The process overall is quite cost effective due to reduced machining and cleanup costs. The materials that can be used with this process are cast irons, aluminum, brass, bronze and copper alloys.

Option 3: Die casting

Die casting is accomplished by forcing molten metal under high pressure into reusable metal dies. Die casting is very fast, This is in contrast to sand casting, which requires a new sand mold for each casting. Compared with sand casting, the process of die casting produces parts with thinner walls, smoother surfaces and even closer dimensional limits- as good as 0.2 % of casting dimension. Also there are no risers to machine off. Finishing costs are less and labour costs per casting are lower. (www.diecasting.org).These factors are also true when die casting is compared with permanent mold castings. However operational costs, utilising equipment more complex and expensive than for permanent mould castings, will be higher per unit. (www.efuna.com).

Option 4: Permanent mould casting/ gravity die casting

Permanent mould castings are also made in metal moulds, this time under a gravity head rather than high pressure injection. Tool steel moulds or dies are necessary for between 100 and 100, 000 pieces. Beyond this quantity, Stellite (an alloy based on cobalt, tungsten and chromium) is necessary. Although the die is expensive initially, thousands of castings can be made without die replacement. Tooling may take between 4 and 12 weeks, with samples typically available the week the die is available (www.efuna.com).

The metal flow under gravity is slower than for pressure injected die casting, but less complex machines are required, keeping prices lower. Because of solidification shrinkage and other considerations, not all brasses can be cast this way. Suitable alloys do, however, include our selected 60/40 brass.Cooling too quickly is the biggest problem faced by this casting method. A spiral mould test would allow flow rates to be tested prior to a large production run. There are advantages to quicker cooling; better grain size is achieved, therefore the part is stronger.

Correct taper, risers for shrinkage and webbing to moderate high stress points are other factors to con-sider in the final designs. The finish of permanent mould castings is not as quite precise as die casting, but is far superior to sand casting. There is a trade off between quality of finish, and cost.

Option 5: Investment casting

Investment casting by the ‘lost wax’ method has been used for centuries to make useful and decorative, high precision components in all sizes and weights. It can produce complicated shapes that would be difficult or impossible with die casting. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. This method is best suited to extremely high precision, low volume applications (www.wikipedia.org)

5.) Bibliography

Books:

Budinski, Kenneth G. Budinski, Michael K. ‘Engineering Materials.’ Pearson Education Ltd. New Jersey, U.S.A. 1979

Beylerian, George M. Dent, Andrew. Moryadas, Anita (Ed). ‘Material Connexion.’ John Wiley & Sons Inc. U.S.A. 2005

Manzini, Ezio. ‘The Material of Invention.’ The Design Council, London. 1986

Ashby, Mike. Johnson, Kara. ‘Materials and Design.’ Butterworth- Heineman. Oxford, England. 2003

Aspin, B. Terry. ‘Foundrywork for the amateur.’ Argus Books Limited, London. 1985

Clegg, A.J. ‘Precision Casting Processes.’ Pergamon Press, Oxford, England.1991

Benham, Paul. ‘Foundrywork design and practice.’ Jarrold & Sons Ltd, Norwich, Great Britain. 1966

Beadle, John D.(Ed). ‘Castings.’ Macmillan Engineering Evaluations. The Macmillan Press Limited, Hampshire, U.K. 1971

Web sites:

www.brass.orgwww.wikipedia.orgwww.steelforge.comwww.steel.keytometals.comwww.kineticdiecasting.comwww.kenwalt.comwww.diecasting.org