Comparison With Alternative Materials

Zinc alloys compete in the marketplace with other materials and diecasting competes with other processes to be the manufacturing route for a multitude of parts. There are many examples of zinc alloy diecastings being specified in replacement for aluminium diecastings (pressure and gravity), plastic injection mouldings, machined brass and steel, pressed steel assemblies, and cast iron. The advantages that zinc alloy diecastings show over these materials and processes that have lead to the substitution are listed below.

Zinc Alloy Diecastings vs Aluminium Diecasting

• Better precision
• Smaller draft angles
• Smaller and longer cored holes
• Much longer tool life
• Thinner wall sections possible
• Superior tensile strength and elongation
• Far superior impact strength
• Better machinability
• Better formability
• Lower casting costs
• Superior pressure tightness, no need to impregnate
• More finishing options
• Non sparking

Most aluminium diecasting alloys are made from scrap. Their cost per unit volume is generally lower than zinc alloys, which are high purity materials. They are lighter than zinc alloys and more capable of withstanding continuous stresses at elevated temperatures. However they can normally only be pressure diecast using the cold chamber process. Cold chamber diecasting is less productive than the hot chamber process used with zinc alloys, especially at the smaller end of diecasting machine sizes. In consequence the cost of a zinc alloy diecasting is frequently lower than that of an aluminium alloy diecasting especially for smaller components. When the other advantages of zinc alloy are factored in, such as the ability to accurately cast components to finished dimensions and hence avoid machining, and the much longer die life (around 5 times longer than for aluminium) the cost competitiveness of zinc alloy moves much further up the casting size range. In addition the superior formability and machinability of zinc confers cost advantages in finishing and assembly operations.

Zinc alloy diecastings are much more often pressure tight than those diecast from aluminium alloy. This means that fluid-handling devices made in zinc alloy are much less likely to leak than those made in aluminium. Also finish machining is more often required for aluminium diecastings, which tends to expose the porosity that causes the leaks. Hence it is normal for aluminium diecastings to be impregnated when they are used in such applications, adding further cost.

The tendency for aluminium to produce sparks when impacted with rusty iron or steel has lead to its prohibition from environments where there is a risk of explosive atmospheres developing. Eg coal mines, petrochemical plants etc. Zinc alloys containing less than 15% aluminium are free from such risk and hence are suitable for use in these environments.

Apart from vitreous enamel almost any finish can be applied to zinc diecastings. In contrast electroplating of aluminium alloys is impractical if not impossible, severely limiting the metallic finishes that can be applied to aluminium diecastings.

Zinc Alloy Diecastings vs Cast Iron

• Lower casting costs
• Vastly superior precision eliminates most machining
• Superior thermal and electrical conductivity

Many parts that are initially produced by machining iron sand castings are eventually produced by zinc alloy diecasting. As production requirements rise, the large per unit cost savings achieved by the switch to the diecasting production route justifies the cost of a diecasting die and the switch is made. It is difficult to compare the mechanical properties of “cast iron” with zinc alloys because the former title covers such a large range of materials with a wide range of properties, varying from coarse flake graphite iron with moderate tensile strength and poor impact strength, to nodular graphite iron with very high tensile strength and good toughness. However most if not all parts that are likely to be converted to zinc diecastings will initially have been made from flake graphite iron at the lower property end of the spectrum. Zinc alloy diecastings are well able to compete in most respects with such materials. However it is as well to bear in mind that cast iron has particularly good wear resistance because of its graphite content and the material is very rigid and has low thermal expansion.

When designing the replacement zinc alloy diecasting it is important not to simply reproduce the existing iron casting with tighter tolerances but to optimise its form for its purpose and to incorporate such extra features that would increase the value of the product. Often it is possible to incorporate mating components into the diecasting hence reducing the part count and assembly costs.

Zinc Alloy Diecastings vs Machined Brass

• Lower process costs (even for fairly simple screw machine shapes)
• Lower material cost
• Less process scrap
• Equivalent or superior tolerances achievable

The term “brass” covers a wide range of wrought and cast materials. They have a wide range of mechanical properties ranging from moderate to high. However the free machining bar stock varieties are notable for their superb machinability, which enables rapid metal removal and fine finishes with a single pass. This leads to low process costs. However the process costs involved with diecasting an equivalent shape from zinc alloy is usually if not always lower and the material cost is invariably considerably lower. Hence the diecasting tooling costs can be recovered over relatively few components.

Zinc Alloy Diecastings vs Machined Steel

• Lower process costs
• Superior corrosion resistance
• Equivalent or superior tolerances achievable

Steel is much slower to machine than brass, but it is this is still quite a frequently used manufacturing route because steel is cheaper than brass or zinc alloy. However, for all but the simplest of shapes, the greatly reduced process costs involved with zinc alloy diecasting means that the overall cost advantage lies with this manufacturing route once a fairly minimal number of components are required. The word steel covers an enormous range of ferrous materials with an enormous range of mechanical properties. For situations where zinc alloy is being considered as a replacement, the steel in question will undoubtedly be a low cost, low carbon, unalloyed type, or a free machining variety. Such steels are not especially strong but they are very tough. Hence when designing diecastings to replace stressed steel parts it is important to ensure that the geometry leads to a good stress distribution, avoiding stress-concentrating notches.

• Lower process costs
• Lower tool costs
• Much greater design freedom
• Better precision

Steel pressings are widely used because their strength and stiffness to cost ratios are very high and it is possible to achieve excellent surface finishes. However their design freedom is severely limited and more complex shapes are often produced by assembling several pressings, usually by spot welding. The basic geometry of a well-designed zinc alloy diecasting often resembles an assembly of pressings, but the ability to locally increase wall sections and to add detail features offers tremendous advantages. Although diecast zinc alloy cannot directly compete with steel in terms of stiffness and toughness this ability to add material locally means that diecastings can often compete functionally with pressed steel assemblies and often at lower process and tooling costs.

Zinc Alloy Diecastings vs Magnesium Diecastings

• Lower process costs
• Lower draft angles
• Better precision
• Superior stiffness
• Superior tensile strength and elongation
• Better corrosion resistance
• Superior formability
• Longer tool life
• More finishing options
• Non sparking

Magnesium is notable for its very low density. Magnesium diecasting alloys can be hot chamber diecast using specialist machines, hence cycle times are faster than for aluminium which can only be pressure diecast using the cold chamber process. Material price per unit volume is similar to aluminium; hence for identical designs magnesium alloy diecastings are competitive in pure material cost terms with both zinc and aluminium diecastings. However when compared in terms of yield strength to cost ratio magnesium alloys are not so attractive and when rigidity to cost ratio is considered, magnesium’s attraction reduces further. The properties of zinc diecasting alloys are superior to those of magnesium’s in almost every respect other than weight. Hence it is only where weight is crucial that magnesium tends to be specified.

Zinc Alloy Diecastings vs Plastic Injection Mouldings

• Vastly superior stiffness
• More consistent properties
• Better precision
• Much lower process costs for thicker section components
• Far superior thermal conductivity
• Electrical conductivity
• EMI shielding

A wide range of polymers and polymer alloys are available and each of these has a range of properties dependent on such as factors as the degree of polymerisation and type and quantity of fillers and pigments. Compared to metals therefore, the properties of plastics are much more dependent on their source.

Plastic injection moulding is the most commonly used production process for complex shapes that will only be subjected to very low stresses in service. The main problem with subjecting plastic mouldings to more elevated stresses is their relatively low elastic modulus. Even glass fibre filled plastic injection mouldings have much lower elastic moduli than metal diecastings. Hence, for parts demanding even moderate rigidity, plastic mouldings must inevitably be much thicker than metal parts. Increasing the wall section of a plastic injection moulding not only increases the material content and its attendant cost but also it dramatically increases the time required to mould the part. Process costs increase in line with cycle time, hence plastic mouldings become increasingly uncompetitive as rigidity requirements increase.

Reference 2

The information on the plastics materials in the table above is drawn from a large number of sources and as can be seen they show very wide variations. In the following graphical presentation of the information the mid point between the maximum and minimum quoted is used for each property. This has lead to some anomalies, for instance the yield strength of glass filled PA66 is shown as being considerably higher than its tensile strength, obviously a practical impossibility. This situation has probably arisen because the data set containing the lowest tensile strength figure did not include a figure for yield strength