Zinc alloy and mazak diecastings
Zinc alloys are sometimes referred to as mazak and are strong and tough. Rarely beaten for mechanical properties by other cast materials such as aluminium, magnesium, brass and plastics, they perform well under conditions of high impact and impulse loading with excellent wear characteristics. Their high ductility allows zinc diecastings to be formed after casting and allows for easy joining operations. They have good thermal and electrical conductivity and will accept a wide range of plating finishes with ease. Zinc alloy is also fully recyclable.
The benefits of using zinc diecastings include:
Tooling can produce zinc diecastings with accuracy comparable to machining tolerances. Typical precision of 0.1% of the dimension is possible, i.e. .025mm over 25mm. Also, once tolerances have been achieved, they are set for life, giving long-term reliability and consistency from the tooling. Due to the precise nature of casting in zinc many machining operations are eliminated. However, where internal threads and undercuts are required to lower tooling costs these are easily machined and the physical properties can be pre-determined.
Zinc alloys have excellent casting properties, which means that thin walled cross-sections are possible. Zinc diecastings are often cast to a thickness of 1mm, with thinner sections possible depending on the flow of material to that area. Thin walled casting has a superior strength to weight ratio, thus strong and light zinc diecastings can be designed using the minimum of metal. This also improves the production rate and lowers cost.
With a relatively low melting point of 380 – 405°C zinc alloys are less aggressive to dies than the higher melting point, lower ductility alloys. This offers long tooling life, typically six times longer than the casting process for aluminium. Zinc diecast tooling will comfortably produce 500,000 shots with little maintenance and will often last up to several million shots with routine maintenance.
Joining & Threading
Zinc can be joined or fixed to other assemblies through a variety of methods at no extra cost. Due to its high ductility, zinc can be deformed to offer riveting, staking and crimping, permanently fastening the casting by displacing the metal at a clamping point. External parts can also be fitted into a zinc diecasting using an interference fit, which takes advantage of the dimensional accuracy of cast holes.
Zinc Alloy Threads
Most research into zinc alloy threads has concentrated on steel fasteners acting in an internally threaded zinc die casting and no results are available for the properties of zinc alloy external threads. However, it can be reasoned that their properties would be similar, but slightly weaker. The shear forces involved should be exactly the same whether the zinc diecasting is the male or female part, and the only difference is the tensile forces. The external thread of a zinc die cast screw will be in tension, and thus slightly weaker compared to the compression of an internal thread.
The retained load a zinc thread can carry will be much lower than the initial preloading. Zinc alloys tend to cold flow, even at room temperature, relaxing the thread stresses and reducing the clamping force. This relaxation is a function of retained load over time and it is essential to provide for this relaxation.
To improve the retained loads increase the thread engagement length. Increasing the length from 10mm to 15mm can increase the thread load by 50%. A coarser thread pitch gives minor improvements to retained load and so does the quality of the thread. Machined threads give superior performance to roll-threaded castings.
The following equations can be used as a guide to determine the allowable load and torque on a thread. This is dependent upon the diameter of the bolt (D)mm and the engagement length of the thread (L)mm. These equations are applicable to a steel bolt in an internal zinc thread, and only within the parameters of the test.
D=Bolt Diameter (mm)
L=Engaged Length (mm)
The load required to pull a steel bolt out of a threaded ZL3 zinc diecasting is approximate to F(max)=90pDL
For thread diameters 3mm-6mm, and thread lengths 1mm-6mm.
The torque necessary for a zinc plated steel bolt to strip the threads out of a ZL3 zinc die casting is approximate to the following relationships:-
Tapped Threads: T(max)=0.08D²L
Self Tapping Screws T(max)=0.12D²L
For thread diameters 3mm-6mm, and thread lengths 1mm-6mm.
Zinc and Corrosion
Zinc has excellent resistance to corrosion in many environments. There are many applications that require zinc’s strength and hardness and zinc die castings can be used as cast without a protective surface finish.
Zinc alloy has very good atmospheric corrosion resistance. Typical rates for pure zinc are shown, however zinc alloy has a greater resistance than pure zinc.
Rural 0.1 – 2.0 µm pa
Urban 2.0 – 10 µm pa
Industrial 4.0 – 20 µm pa
The dulling and greying of zinc diecastings is normal. It is an oxidation of the surface of the zinc diecasting that also acts as further protection.
Fresh Water – Typical corrosion rates vary between a hard water minimum of 1.5 µm pa, up to 15 µm pa possible in salt water. In hard waters, a protective layer of scale will form on the zinc die casting, giving even better corrosion resistance. At higher temperatures the scale has less adhesion to the zinc, reducing the extra protection.
Salt Water – Permanent immersion the corrosion rates are as follows:-
Temperate seas – 12 µm pa. Tropical seas – 25 µm pa.
In tidal areas with twice daily immersion and exposure to atmosphere, the figures above should be doubled.
Zinc diecastings perform well in weak alkaline and acidic conditions. With a practical pH range between 5 and 11.5 pH, widening under scale forming conditions, it is suitable for use with many commercial detergents and cleaners.
This is the phenomenon that occurs when two different metals are in contact with each other in the presence of an electrolyte, such as salt water. Zinc can behave as a base metal in certain situations, where it will be worn away to become plated onto a more cathodic material. An insulating layer, such as a non-conductive finish can prevent galvanic corrosion.