Poisson's Ratio (pronounced "pwason") is used as one of the data inputs for Finite Element Stress Analysis. The values for zinc alloys are:
ZAMAK Alloys 0.27
The two alloys are very similar and are often interchanged. Thus, exactly the same parts, such as car door handles, may be made with #5 alloys in Germany and #3 alloys in UK.
Alloy #3 has slightly better ductility than #5, which is useful for components that have to be riveted or swaged. Alloy #5 has slightly better tensile and creep properties, so may perform better when stresses are high.
Shrinkage occurs when metal changes from liquid to solid as well as when it cools in the die. Quoted values for die cast zinc alloys are as follows:
ZAMAK Alloys 0.007 in./in.
ZA-8 0.007 in./in.
ZA-12 0.0075 in./in.
ZA-27 0.008 in./in.
However, these values may be affected by constraint caused by the die so that some castings or parts of the same casting may shrink differently. Larger parts are often more likely to be constrained than small parts so the designer may use a slightly lower value for large dimensions.
Zinc alloys are non-toxic and zinc is an essential element in the diet. Because food often contains strong acids or alkalis, it is usual to apply an additional surface finish to prevent tarnishing.
However, designers should also be aware that there might be regulations and codes of practice for a particular food or food industry that may limit the choice of materials in contact. For instance, sometimes only stainless steel may be considered acceptable.
Many millions of zinc parts are held together by screws with no problem at all. Because zinc has a low coefficient of friction it is often useful to use either a "star" washer or to serrate the underside of the screw head to prevent it unscrewing, particularly if the parts are subject to vibration.
If the design requires maintaining a minimum tensile stress in the screw and temperatures are likely to be above 150 F, there is a possibility that stress relaxation could occur. Creep data is available to enable the designer to calculate the amount of relaxation.
Zinc die-castings are extensively used for gears varying in size from the tiny pinions used in instruments and switches, to sprockets for chain drives and gear racks for washing machines. In most cases, the teeth are cast to size; however, for special applications they may be machined by hobbing, broaching, etc.
Galvanic corrosion is seldom a factor with ZAMAK and ZA alloys because of the material's inherently good corrosion resistance.
If a small die cast zinc part is electrically connected to a large brass or steel part and immersed in a corrosive electrolyte the rate of corrosion may be increased to the point where there is a problem. This can usually be overcome by either insulating the parts or giving a surface coating to the zinc alloy.
Note that the zinc anodes used to protect boats from corrosion are not ZAMAK or ZA alloys; they are alloys that are specially formulated to corrode, deliberately.
Zinc alloys are generally satisfactory for handling most hydraulic fluids, and are widely used in braking systems, hydraulic jacks etc. The presence of water in the system is a possible source of problems. For more specific details of zinc alloys in contact with hydraulic and other fluids, see the publications NOR-5 and ILZ-4 on this website.
Die cast zinc alloys do not have as good thermal conductivity as extruded aluminum. However, die-casting brings several advantages that can outweigh this fact. e.g.
- More efficient fin design is possible than with extrusions.
- The fins are integral with the chassis resulting in a more efficient heat path, with no joints or interfaces.
- One die-casting acts as chassis, heat sink, and container for both protection and EMI shielding.
It is easy to get customers for your die-castings, just under-quote your competitors. You will have plenty of work but no profits!
Profit margins are much higher for newly designed components than for existing jobs. If you can find new designs, or convince a designer to convert an existing design to using zinc die-castings, you can ensure higher margins and increased sales as the product's market develops.
Concentrate on market development rather than selling. The Interzinc seminars are a very cost-effective way of doing this. It costs only $300 to show your products to audiences of normally 30 - 60 designers and specifiers.
If you want to "go it alone" and demonstrate the capabilities of zinc die-castings as well as your own expertise, Eastern Alloys will help you with both preparation and presentation. We have many prepared materials ready to use.
There are many ways to do this. If you have a new product or even a new story about an old one, contact the technical magazines who are always looking for this sort of "copy". Send them pictures of your product, together with a short, well-written article and there is a good chance they will publish it.
One such publisher is Interzinc, which regularly prints case histories of successfully applied zinc castings. Send your pictures, story and permission to publish to: Sheehan Communications, 1236 Smith Court, Suite C, Rocky River, OH 44116.
Remember that every entry is carefully judged, and often a casting that looks simple to you will grab the judges' attention. Even if a casting does not win an award, it still gives your company some good exposure and therefore free advertising. It is rather like film stars who may not win an Oscar but still gain fame by being nominated for one.
Above all start early, prepare your casting carefully according to all the rules. Also show the judges the finished part or assembly with a clear, interesting, description of the parts functions and attributes.
Now you have gone to all this trouble, also send the same parts etc. to the technical magazines and to Interzinc to publish as a case history.
The specific gravity of a complex shaped part like a die-casting is best measured by the Archimedes technique. This involves weighing the part in air then weighing it immersed in water, which is accomplished by suspending the part on a fine wire or thread in a beaker of water, taking care to get rid of any air bubbles.
Specific Gravity = Weight in Air ¸ (Weight in Air - Weight in Water)
(All weights are in grams)
Conversion: 1 lb/cu. in. = 27.5 g/cc
Specifying porosity levels must take into account the fact that the position and shape of any internal discontinuity is usually more important than the amount. Thus, a crack will often have a very serious impact on the strength of a part but only represent a very small amount of porosity. Small, well-dispersed spherical pores may actually improve the material's strength and fracture toughness.
Where component strength is the issue, it is best to agree on a proof-test, which subjects the parts to loads similar to those encountered in service. Similarly, tests for pressure tightness or visual appearance are used as appropriate.
Weight and density testing may be used to identify sudden changes in process variables but do not usually correlate well with other physical and mechanical properties.
The hardness and other mechanical properties given in the ASTM specifications for die-castings are not part of specification. They are published in order to give designers a means of comparing one alloy with another. The data was generated using specially prepared test pieces made under carefully controlled conditions. One cannot obtain these values on samples cut from die-castings, which inevitably contain porosity, and tiny internal defects that generate false hardness values.
Customers sometimes try to specify a minimum hardness value in the mistaken impression that this will give them the mechanical properties they require. Hardness is an indication of resistance to indentation and scratching but it is not related to either strength or wear-resistance.
Although die castings can sometimes be purchased more cheaply from companies abroad than in North America, these companies often do not have good control over the zinc alloy. Remember that if a part fails in service, your company could lose money, customers and reputation.
Always use the ASTM specifications on drawings and check the analysis of delivered parts against these specifications.
Parts may distort as they solidify and cool in the die, or during ejection from the die. These problems are solvable but usually require discussions between the part designer, the tool designer and the die-casting engineer since they involve increasing draft angles etc.
However, part distortion is quite often caused by mishandling during press trimming, polishing etc. or when the parts are clamped in jigs for machining, plating or painting. These problems are best solved by checking parts after each process.
Blisters are caused when gas is trapped below the surface of the die-casting. As the casting is ejected from the die, the zinc alloy is hot and weak so that the internal pressure causes it to creep and produce the blister.
If the die caster increases the die close (freeze time) part of the cycle, the casting will be cool enough and strong enough to resist this expansive force. However, overtime (or whenever it is heated, for that matter) the casting will still creep under the internal stress and bubble or blister.
To prevent this happening the die caster must eliminate the internal gas porosity by limiting die lubricant, improving die venting and keeping the gate velocity at the correct value. Check parts for possible problems by heating them to 400 or 500 F above the normal baking temperature.
Visual inspection of parts is particularly important if they are to be plated, since the bright surfaces produced by the buffing and plating operations make extremely small defects visible.
First the inspector must have some means of polishing the surface, this is best done with a polishing buff but steel wool can also be used (use a back and forth motion, not a circular one). The next thing you require is really good light level of 2700 lux minimum. This can be achieved by fitting four bare florescent tubes above the inspection bench. A good magnifier or binocular microscope will also help when checking for small defects.
White rust is evidence that corrosion has occurred, usually in a moist environment with either acid or alkali materials present. Obviously, die-castings should be dry before being stored but a common cause is "sweating" or condensation, which occurs when the temperature changes suddenly. The use of paper or cardboard packaging greatly increases the effect since these materials contain acid and they hold the moisture in too.
Open wire mesh containers with plastic separators work best by allowing circulation of air around the parts. The use of "de-watering" solutions or vapor phase inhibitor wrappings is also recommended.
The most common cause of excessive dross formation is that the metal temperature is too high which increases the rate of oxidation. It also increases the rate which aluminum in the alloy combines with the iron of the pot, gooseneck etc. to produce iron-aluminide intermetallic.
The result is excessive dross and excessive wear of the furnace and gooseneck. In extreme cases, this can cause the alloy to go out of specification. Normal operating temperatures are 200 to 800 F above the melting point of the alloy.
Other causes of excessive dross are:
- Remelting thin flash, which oxidizes easily.
- Charging scrap that is wet or oily.
- Charging castings that are plated or contain ferrous inserts.
It is most likely that the metal temperature control is at fault. Check this with a portable immersion pyrometer. First, put the test thermocouple close to the one controlling the bath. If this is OK, move the thermocouple around the bath to see if it is uniform. If is not, you will have to look at the burner position and furnace structure.
If the trouble persists when the temperature is correct, it may be due to the alloy composition. Check this by sending a sample to a qualified lab for analysis.
This is not good practice since there is a risk that the plunger will "seize" in the sleeve. This usually means that the gooseneck has to be removed and the sleeve replaced which is a costly operation and causes considerable downtime.
If the machine is not making castings for 6 hours or more, remove the plunger, clean off the zinc and examine the rings for wear and breakage.
Acid (usually muriatic or hydrochloric) will dissolve zinc quickly from molds, plungers, goosenecks etc. Unfortunately, it also removes the entire protective oxide layer and may even start to dissolve the steel. When the parts are then put back into service they will be attacked even more quickly by the molten zinc.
It is preferable to use caustic soda solution to clean off adhering zinc. This will take longer than acid but will leave the oxide layer intact that discourages soldering and build-up. Caustic soda is probably even more harmful to the human body than acid and suitable safety precautions must be taken.
If the oxide film has been removed from a tool or parts either by polishing and/or the use of acid, it should be heated to 1,0000 F in an air tempering furnace to re-oxidize the surface and give protection from soldering.
Die temperature is one of the most important process variables in die-casting. For zinc die-castings, the normal range is 3500 to 4500 F measured at the surface of the die just prior to injection.
If the die temperature is too low, cold-shuts will be encouraged and the parts are likely to stick in the die and on the cores. If the temperature is too high, there will be an increased likelihood of soldering, burn-on and casting distortion.
Using a hand-held surface contact pyrometer on the die face is a very effective way of checking die temperature. An infrared radiation pyrometer (I.R. "Gun") is a more convenient technique but the emissivity calibration control has to be set accurately. The best way to do this is to measure the die surface temperature with a contact pyrometer then set the emissivity control so that the I.R. "Gun" gives the same reading.
Die and slide surfaces must be clean and free of anything that keeps them from closing. The adage "flash makes flash" is very true. Once a die starts to flash, the damage caused to the tool faces is costly to repair.
The machine lock must be sufficient to withstand the die opening force generated when the metal is injected, therefore the lock-end hydraulic pressure must be sufficient and the die height correctly adjusted so that the toggles lock over hard. Note the hydraulic pressure developed just as the toggles go over center.
Tie bars should be "balanced" to generate equal strain. The best way to check this is to use tie bar strain gauges, which may be built into the machine or clamped magnetically to the tie bars.
Finally, do not resort to making a "soft-shot" by reducing the metal pressure or shot speed to prevent flashing since this will almost certainly produce porous castings or ones likely to blister.
During the past few years, there has been a lot of research that has improved our knowledge on how to design runner systems, which make better castings at higher production rates and with less remelt. It is unlikely that any single toolmaker is aware of all the details of these developments.
You could develop your own rules by sifting through dozens or research papers and hours of video tapes, but modern runner design systems have already incorporated this information and made it easy to use.
There are many such design systems, most are computer based, some will even produce CAD CAM data for direct use by the toolmaker.
Slow stroke, or two-phased injection, has two main functions:
- It gives a ten-fold increase in time for vents (or vacuum) to get rid of gasses from the cavity.
- It reserves the energy in the accumulator until required for cavity fill, thus giving less pressure drop and faster recharging.
Retrofitting a machine that does not have two-phase injection is not complicated or expensive. It will lead to higher integrity castings and sometimes faster cycling.
Whenever you have machines refurbished, you should ask the people responsible to check the machine's final shot-end performance and produce a PQ2 diagram. To do this they will fit the machine with mounting brackets and hydraulic tap-out points, which are also useful when the machine goes into production.
The data, such as dry shot capability, can then be compared with that of other machines and, if you have it, with the machine's capability before rebuild. It is also invaluable for carrying out calculated runner and feed designs. You may wish to specify minimum metal pressures, dry shot speeds etc. before the work is done. The builder will then choose the best combination of valves and pipe sizes to achieve the characteristics you require. This usually does not involve extra cost but will result in a better machine.
Dies, core pins and ejectors are made from tool steel, which has been carefully hardened and tempered to give it resistance to wear and deformation. If these steels are heated above about 1,0000 F they will become soft and weak. The flame temperature of oxy-acetylene burner is high enough to melt steel so that it is very likely that the small cores and ejector will be softened and will either bend or wear.
An air-gas burner will easily melt zinc and are widely used for removing stuck castings, unfreezing nozzles etc. - even this type of burner must be used with discretion to avoid damage to tools.