Thanks to its ease of use, to quick cycles and low fusion temperatures (400 to 500° C), zinc alloys (Zamak) are being more and more used in different manufacturing industries: from furniture to ironware, from garments to mechanical components, automotive, electronics, pneumatics, hydraulics etc.
The use of zinc alloy (Zamak) die-casting to replace other materials offers several benefits and advantages, mainly including:
• Speed of production: compared to other processes and materials, Zamak die-casting features definitely higher hourly production rates. This is possible thanks to the significantly reduced cooling time required by Zamak alloys. According to the size of the cast, it is possible to reach double speeds compared to aluminum or plastic.The multi-slide hot chamber technology (DINA 5) developed by MINNITI S.r.l. allows to reach 3500 cycles/hour (dry).
• Energy saving: Relatively low melting temperatures reduce the energy requirement of Zamak processing. The estimated energy savings amount to at least 50% compared to aluminum and 15% compared to magnesium.
• Size precision: Although size precision is mainly determined by manufacturing processes, materials also affect attainable tolerances. Die cast parts can be produced with more strict tolerances compared to other tridimensional processes, with sizes usually 10 times more precise and consistent with respect to extrusion, presswork and lost-wax casting. Die-cast zinc alloys ensure tolerances that would require machining with other materials.
Hundredth tolerances can easily be attained, while aluminum only allows for double tolerance values and magnesium for one and a half tolerances.
Zinc alloys have higher dimensional stability compared to aluminum and magnesium. In addition, they generally require a lower shakeout angle for mold extraction. In fact, it is possible to obtain parallel holes and external walls with a draft of 0.5°, instead of the usual 2° draft. Holes can be made with such a small taper that threading can be tapped directly.
• Easy assembly: Wherever needed, high ductility of zinc alloys (Zamak) allows to insert cast parts that will be folded, hammered or treaded for easy assembly of other adjacent parts.
• Low mold wear: Die-casting molds for zinc alloys (Zamak) normally last over two millions of cycles. Their life is similar to thermoplastic injection molding, i.e. about ten times higher compared to aluminum die-casting, and generally higher also with respect to magnesium die-casting. The same mold can be used to obtain similar items by adding removable and replaceable inserts, in those cases when building a new mold would not be economical.
Each project should be carefully evaluated to ensure the most efficient deploy of equipment, in order to obtain the least unit cost.
• Safety: Unlike aluminum and especially magnesium, Zamak does not pose any fire hazards due to sparking, both during processing and use of die-cast parts.
• Project Complexity: By applying some basic design principles, it is possible to take the best advantage of die-casting.
The following points should not be considered as strict rules, but if used with a grain of salt will help to create a consistent, efficient and cost-effective project. Our extensive experience allows us to suggest minor changes to projects that can significantly affect efficiency and production economics of a specific component.
• Wall thickness
Wall sections should be thinner as possible, while ensuring resistance and stiffness.
The most common size is 1 mm, but lower ones are frequently used up to 0.3 mm.
This helps to minimize required weight of metal and accelerates solidification into mold, thus increasing production speed.
• Corners and joints
Acute angles should be avoided wherever possible.
Internal joints are of a critical importance and it is usually desirable to have a joint at external and internal closed angles. A 0.5 mm joint will have a remarkable reinforcing effect and it will be hardly noticeable, also on external borders.
There are very few geometric shapes that cannot be obtained with die-casting.
However, insertions and other features requiring the use of removable elements into the mold will increase overall costs. Internal indentations are subject to some constraints.
Wide flat surfaces should be avoided as much as possible. If they are required, a slight bending improves appearance and roughness as well as it helps to avoid minor surface defects that may affect the overall look.
External threads are easy to obtain, especially if they occur on the parting line of the mold. Internal threads can also be die-cast but generally, it is more convenient tapping the holes obtained with die-casting. As an alternative, self-tapping screws can also be used.
• Surface coating:
Zinc alloy (Zamak) die-casting is the most direct way, and sometimes the most economical, to manufacture precise and robust components for a wide range of industries. Many items are used in technologically advanced applications that do not require any surface finishing. However, in some cases coating is required to:
- Obtain decorative effects
- Improve corrosion resistance
- Increase wear and abrasion resistance.
The range of zinc alloy (Zamak) die-casting plating is extremely wide and new finishing process are being constantly researched and developed.
• Chemical coating
Chemical coating provides corrosion resistance surface at a low cost. They are obtained by submerging, or in some cases spraying components with a specific solution. There are two main types of chemical finishing – chromate conversion coating and phosphate conversion coating. In addition to corrosion protection, they also provide a basis for further organic coating. Colored coatings also exists that can only be used in mildly aggressive environments and/or if the component is subject to little handling. A special anodizing process turns the zinc surface into a very corrosion and wear resistant film. These coatings only have a functional purpose and usually are olive green matte color.
Despite their name, they do not resemble aluminum anodizing, since the two processes and the resulting coatings have very different features.
• Organic coating
Polished die-casting zinc surfaces have a beautiful appearance that can be maintained with the application of organic coatings. These are acrylic/polyurethane lacquers or polyester powders, both clear and colored. Painted or powder coated die-casting zinc is used in automotive, home appliances and in machine mechanical parts in general.
In electrolytic painting process, the part to coat is immerged as the cathode in a basin (the anode) containing special paints with soluble resins.
Further to the advantage of regularity of the deposed film, this process does not require volatile and flammable organic solvents.
Electrostatic powder coating is increasingly used as a die-casting organic coating method.
After degreasing (and chroming in cases where maximum corrosion resistance is required), parts are electronically sprayed, generally with epoxy or polyester powders, heated to “fluidify” the powders, thus obtaining a solid and adhesive protective film. Epoxy coatings provide the best corrosion resistance, while polyester coatings boast higher hardness.
• Galvanic coatings
Die-casting zinc alloy (Zamak) parts can be finished with several types of galvanic coatings for aesthetic or protective purposes or to obtain special electrical or surface properties. Copper coating is recommended both for aesthetic purposes or to provide a highly-conductive surface for electric and electronic uses and in some cases to facilitate soldering. The copper deposit is also used as a basis for other types of coating. What is generally referred to as “chromium plating” is actually a coating consisting of one or more copper layers, one or more nickel layers, and a final chromium layer. Gold plating and silver plating (always on copper or nickel base) can be used as luxury finishing, when good contact resistance and surface conductivity are required.
Another frequently used surface finishing is nickel-plating.
• Anti-friction properties: Zamak alloys have good self-lubricating features and they are used since years for parts with specific properties. They are a good alternative to brass, especially for parts subject to small loads.
• Anti-vibrating and noise reduction properties: Zamak has a damping capacity at room temperature between 2% and 4%. This value is above 0.5% of aluminum and 1.5% of steel. Only grey cast iron boasts a higher value around 7-15%. Zamak also has a higher absorption capacity compared to aluminum.
• Good machine tool workability:
Zinc alloy (Zamak) die-casting allows to design complicated parts that with different techniques would otherwise require further expensive mechanical processing. However, if any further processing is required, parts can be easily processed using normal tools.
The level of accuracy that is attainable nowadays in zinc alloy die-casting has almost completely eliminated the need for milling or grinding to obtain a flat surface, which can result directly from die-casting. However, in some cases, milling and grinding can be required for recessed parts.
• Good environmental sustainability: zinc alloys are fully recyclable and reusable over time. The use of Zamak is permitted by most strict industry regulations.