Design of Die-Castings
Sales and Marketing
Quality Control
Die-Casting Technology
The following is a list of frequently asked questions. Click on any question to
see the answer, or click here to show all
the answers.
Design of Die-Castings
Q:
What is the Poisson's ratio of zinc alloys?
A: 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
ZA-8
0.29
ZA-12
0.30
ZA-27
0.32
Q:
How does Zinc alloy #3 compare with alloy #5?
A: 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.
Q:
What are the die shrinkage factors for zinc alloys?
A: 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.
Q:
Are zinc alloys safe in contact with food?
A: 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.
Q:
Will screws come loose when used to fasten zinc die-castings?
A: 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 1500 F, there is a possibility
that stress relaxation could occur. Creep data is available to enable the
designer to calculate the amount of relaxation.
Q:
Are zinc alloys suitable for gears?
A: 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.
Q:
Is there a problem with galvanic corrosion of zinc alloys?
A: 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.
Q:
Is zinc OK in contact with hydraulic fluid?
A: Zinc alloys are generally satisfactory
for handling most hydraulic fluids, and are used widely 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.
Q:
Are zinc die-castings as good as extruded aluminum for heat sinks?
A: 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.
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Sales and Marketing
Q:
Where do I get more customers?
A: 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
Q:
How can I get some free advertising for my company?
A: 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.
Q:
I would like to enter the NADCA Die Casting Competition but don't have an
outstanding die casting at the moment.
A: 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.
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Quality Control
Q:
How do I measure density or specific gravity of a die cast part?
A: 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
Q:
How do I specify porosity levels?
A: 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.
Q:
What minimum hardness should I specify? I cannot get the hardness value
stated in the book. What do I do?
A: 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.
Q:
My Company buys zinc die-casting overseas, which is cheaper. Is this
wise?
A: 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.
Q:
What causes part distortion?
A: 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.
Q:
I got rid of die casting blisters by increasing the freeze time but now they
are being caused by the powder coat people.
A: 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.
Q:
How do I inspect castings for surface finish?
A: 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.
Q:
My customer is getting white rust on stored parts despite careful
packing. What is the answer?
A: 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.
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Diecasting Technology
Q: I get a lot of dross when melting zinc alloy. What is the
cause?
A: 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.
Q:
My pots keep freezing up. Should I increase the bath temperature?
A: 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.
Q:
We leave the machine plunger in all weekend. Is this OK?
A: 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.
Q:
The tool room used acid to clean the dies of build-up but we get a lot of
burn-on.
A: 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.
Q:
Is die temperature important and how can I measure it?
A: 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.
Q:
How do I reduce flash?
A: 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.
Q:
My tooling engineer has 30 years experience. Why do I need a runner
design system?
A: 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.
Q:
My machines do not have slow-stroke capability. Does this matter?
A: 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.
Q:
I have had some die-castings machines completely refurbished but they do not
seem to have the power of the new ones in the shop.
A: 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.
Q:
I have heard I should not use any acetylene burns for removing stuck casting
but my operators say it is impossible with other torches.
A: 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.
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