Design
General Comments
The single most important person involved in the production of a diecasting is the designer. He or she has a critical influence on the performance, appearance and cost of the assembly incorporating the diecasting. A good designer considers every aspect of the function and production of the finished assembly. Of course the designer cannot be expected to be an expert in every facet and ideally he/she should work as the leader of a team incorporating specialists who will have important contributions to make. These specialists should include representatives drawn from companies who will be involved in the supply chain, such that at the end of the design process all involved jointly “own” the design and have an extra incentive to make it work. In the case of a zinc diecasting the suppliers who should be involved are the raw material producer, the diecaster, the toolmaker, plus representatives of companies performing any significant secondary operations.
In the real world such teams are only justified for major projects. For more minor projects and at the concept stage of major projects the designer will usually rely on his own experience and those of his in house colleagues. It is important therefore that he/she should have a good basic understanding of the material and the process such that the fundamental requirements of the concept are achievable in practice.
Zinc alloy diecastings typically weigh anything from a fraction of a gramme up to about a kilogramme, and have a maximum dimension of around 300mm. Much bigger castings are possible where required. The preponderance of smallish castings is explained by the increasing cost advantages of zinc as casting size decreases and production volumes increase. This in turn has produced a preponderance of small diecasting machines within the zinc diecasting industry. However it should not be forgotten that the special characteristics of zinc diecastings can sometimes be used for much larger castings to good effect.
The basic shapes that can be achieved by pressure diecasting are limited by the fact that the die is made of rigid parts that must be capable of separation from the casting that it forms. In practice very complex shapes can be produced, and in zinc the minimal draft angle required allows net shape forming on frequent occasions.
The form of a component should be determined by its function and not by the history of its production. Many products reach the production stage still embodying characteristics that betray the fact that they are conversions to die casting from other processes. In particular, as many diecastings are converted from sand casting, a common shortcoming is superfluous bulk.
One of the major functions of a component is the ability to withstand operational stresses with a margin for safety. As the economy of zinc lies in the reduction in weight of an object, it is clearly crucial to decide what the safety margin should be. Rather than over-design, it is suggested that the item be designed simply to withstand the operational stresses because the thickening of walls, etc, can easily be made throughout the pre-production run of the die. For this reason it is better to under-design initially and test to destruction the first few castings produced. When designing diecastings it must be remembered that it is necessary to take into account the ease of filling the die cavity with molten metal and also of ejecting the component without distortion. However, designers are strongly advised to get in touch with their die caster at this stage.
Levers, links and cranks are often obviously designed by ‘eye’ from proportions determined by their visual relationship with the assembly as a whole, rather than the function, performance or stresses encountered. It is not uncommon to see all the elements of the mechanism scaled up when a new and larger model is being designed, even though some of these components are performing exactly the same function under the same conditions as they did before.
As with other casting and moulding processes, the designer should attempt to keep wall sections fairly even and avoid sudden changes in section. Failure to do so does not necessarily mean that the casting cannot be made, but the difficulty (and hence cost) will be increased. For components with complex geometry, computer solidification simulation can prove very helpful in optimising component geometry and die cooling placement, hence avoiding casting defects at critical positions.
The minimum practical wall section depends on its distance from the ingate ie the position where molten metal enters the die cavity proper. As a rule of thumb for short distances, say less than 50mm, it is about 0.5mm ranging up to 2.0mm at about 200mm. Application of computer aided runner and gating techniques give a better indication of minimum achievable wall sections where this aspect is critical.
The split line of the die is dictated largely by the design of the component. The capability of producing a flash free component is affected not only by the quality of the toolmaking but also by the ease of manufacture and maintenance of the tool, which benefits from careful thought at the casting design stage. Sometimes it is better to produce a deliberate flash area on the casting that can subsequently be positively removed by press trimming, this avoids small areas of unintentional flash which because of their flexibility can be difficult to remove reliably. Where not necessary or desirable through holes and windows in a casting are usually best avoided since they simply increase the length of the parting line.
When considering the split line of the component, the position and length of the ingate should be born in mind. If the only available position does not give the opportunity for metal to flow directly into all parts of the casting then good surface finish and low porosity will be more difficult to achieve. Changing the configuration of the component will often overcome this problem. It should also be remembered that adequate gate area will be needed and if the gate length is restricted the gate will need to be thick with possible consequent trimming problems and a heavy witness mark. Again computer optimisation of runner and gate will prove extremely helpful in determining what gate length and thickness is needed to produce good casting quality.
For reliable automatic production of diecastings it is critical that both the casting and the runner system are retained firmly on the moving die half when the mould opens. Any tendency for it to stick on the fixed half must be positively counteracted. This is achieved by ensuring that the combination of gripping due to thermal shrinkage of the casting/runner and retention by any sliding members of the tool serve to pull it squarely away from the fixed half. Sometimes depressed ejector pins with an undercut on the end, so called snatch pins or Z pins are also used. The design of the component is affected by this requirement. One consequence is for through holes to be either completely cored from the moving die half or 2/3 from the moving half, 1/3 from the fixed half.
Once the casting has been separated from the fixed half the sliding members, if any, will move back. At this stage the casting will be very soft, and it will distort if it is not positively retained against any gripping on the moving members. Such retention is usually provided by die design features but these often have an effect on the component design, varying from the position of parting line witnesses to specially cast in features.
The final stage of casting is ejection from the moving die half. The ejection must ensure that the casting and runner are moved squarely and does not distort the component. In general terms an ejector pin must be provided close to every position where the casting will tend to grip the tool and positioned such that any strain that the ejector produces in the casting will tend to loosen it from the tool. This is largely the province of the diecaster and toolmaker to decide but casting design can make the job easy or difficult or impossible.