Zinc alloys are characterized by low melting points and excellent flowability. Hot-chamber die casting produces exceptionally complex and precise parts with excellent surface finish. These are suitable for very demanding tasks such. Electronic housings or precision mechanics for electronic, optical or mechatronic systems. Due to the good castability of the material, very small wall thicknesses are already realized up to clearly less than one millimeter. However, many designers lose their courage because of the fear that they would not be able to master the process with sufficient safety even with significantly lower wall thicknesses.
"We took a close look at the housing of a modern-day smartphone," says Martin Schlotterbeck, Head of Training and Process Consulting at Oskar Frech GmbH + Co. KG in Schorndorf (Germany). It was found that, for example, the elaborately designed smartphone base shell, with dimensions of approx. 60 x 120 x 5 mm, is made from a solid piece of aluminum. The entire inner structure was created by cutting from the solid, the remaining wall thickness is only about 0.65 - 0.69 mm. The inner structure is quite demanding with numerous smaller and larger projections, threaded blind holes, bearing surfaces, openings, brackets and undercuts on the side walls. The cutting volume is around 85%, with high demands on accuracy. The processed visible surfaces require a high surface quality. For the production, numerous different milling and drilling tools must be used, which requires frequent tool changes and correspondingly long processing times. In comparison with the possibilities of a precision molding process such as die casting, such a production method naturally causes exorbitant high costs. Such high cutting rates are therefore usually found only in products that are particularly expensive and are needed in relatively small quantities, e.g. in racing or in aerospace engineering.
Comparison criteria: costs, optics ...
"Compared to this costly full-time milling process, hot-chamber die casting of zinc is an exceptionally cost-effective process," adds M. Schlotterbeck. Among the highest cost factors in die casting are the permanent metal molds made from tempered hot-work steel, whose production costs are only calculated if the series is large enough. However, this is guaranteed from the outset for mass-produced products such as smartphones or other IT enclosures. Due to the low melting temperatures of common zinc alloys (ZL5 approx. 420-430 ° C), the molds are hardly stressed and often achieve service lives of the order of magnitude of several million casting processes. The excellent flowability of zinc allows an exceptional detail in the imaging of even very filigree structures, and the desired dimensions can be met with very tight tolerances. For zinc die-cast parts, the amount of rework required is therefore very low, and is often limited to the removal of burrs and the insertion of threaded holes. In addition, zinc has other important advantages: it provides excellent protection against radio interference, and can be painted, patterned or electroplated using most common processes, giving the designer numerous stylistic devices for a decorative, refined, high-quality product. On the whole, zinc die casting thus offers a much more cost-effective alternative to cutting aluminum from solid.
... and weight
"However, if zinc is to prevail in the market, must also overcome a handicap," reveals M. Schlotterbeck. This is about the weight aspect. While aluminum, with its density of only 2.7 kg / dm3, is one of the lightest metals, zinc components with the same volume weigh around 2.6 times as much. Especially with "mobile" applications, however, a low device weight is a crucial aspect. To compensate for the weight disadvantage, the zinc die casters must produce components with much thinner walls. In the specific case of the smartphone housing, this would correspond to a wall thickness requirement of only about 0.25-0.27 mm. However, when using commercially available zinc alloys and casting technologies, about 0.6 mm has been the bottom limit. Underlying wall thicknesses of up to approx. 0.4 mm could usually only be produced with great effort and non-standard "high fluidity" alloys, so that this was mostly restricted to niche applications. But is this really the minimum thickness attainable? In view of the large market volumes involved in modern IT applications, Frech decided to thoroughly investigate the question of which minimum wall thicknesses can actually be achieved under which conditions.
What is possible with conventional foundry equipment?
"We were not concerned with academic records, but with practice, i. E. the question of what foundries with standard equipment can achieve," says M. Schlotterbeck. The desired target was a wall thickness of 0.15 mm. The main focus, however, was on the question of how far one could get with the usual equipment of a normal die casting foundry. Therefore, a standard high-tech hot-chamber machine type W80 Zn-RC in the Frech technical center was used for the casting trials. This had the standard full equipment, but without special optional accessories. The test alloy was commercially available ZL 5, for comparison purposes also HF-Alloy was used. No vacuum support was used, rather a passive venting system with a double "washboard" that was made of steel rather than CuBe. For the tests with wall thicknesses of 0.28 mm, 0.2 mm and 0.15 mm, a 15 year old modular training form was used. With this, one could present different wall thicknesses by means of appropriate inserts and test the effect of different pouring channel and gating systems, cross sections as well as vents. The tempering channels of the mold were designed according to the usual calculations. In order to be able to achieve higher mold temperatures for certain experiments, a heating / cooling system for magnesium was used, which was able to reach media temperatures of up to 300 ° C. For the different wall thicknesses, varying mold inserts were used or connected. Limits due to positional tolerances were already encountered at 0.2 mm with the existing mold. Wall thickness values between 0.13 and 0.22 mm were measured when examining a corresponding cast part. These wall thickness differences were adjusted by post-treatment.
"Altogether 25 test series were carried out with this equipment", reports M. Schlotterbeck. Essential process parameters such as pressure, piston velocity, starting points, priming, deceleration, mold temperatures, spray techniques, release agents, runner, sprue and vent systems were varied. Castings that appeared externally completely free of defects were extensively studied. In addition to the casting process, the main criteria were the dimensional stability of the casting, the tightness or pore content, and the surface quality both in the raw state and after galvanic coating.
From Table 1 in the gallery, it can be seen that it is possible to produce large-area zinc components from ZL5 up to a wall thickness of 0.2 mm with sufficient process reliability with the equipment used. A special machine is not required, however, in certain areas such as e.g. to recommend special equipment to the mold spraying. Special attention should be paid to the qualifications of the operating staff. This should have good process know-how and be able to precisely balance and keep the essential parameters within narrow limits. The use of HF-Alloy brings advantages only with wall thicknesses of 0.15 mm, whereby inferior mechanical properties, surface problems, and cracking must be accepted.
A very interesting market
"In conclusion, it is possible to use zinc die casting to produce much thinner-walled parts than before and thus tackle highly interesting, new high-volume market segments," concludes M. Schlotterbeck. This includes almost the entire range of mobile electronics systems such as smartphones, tablets, cameras or connectors, as well as scientific equipment and measuring instruments. Thanks to the extensive testing, the required know-how has been gained. This knowledge is made available to customers in the context of consulting and training services.