Die Castings
Aluminum Die Castings, Zinc Die Castings
The die casting die or mold is a closed vessel into which molten metal is
injected under high pressure & temperature, then rapidly cooled until the
solidified part is sufficiently rigid to permit ejection from the mold, usually
within 15 to 25 seconds.
For longevity of operation in this environment the die casting die must be built
from high quality tool steel, heat-treated to the required hardness and
structure, with the dimensions of the die cavity machined to the exact
specification. The two die halves run in a die casting machine that is operated
at the required temperatures and pressures to produce a quality part to net
shape or near net shape customer specifications.
The customers' product design requirements directly affect the size, type,
features and cost of the required tooling. The items involved in the tooling
decision include the number of cavities, number of core or slide requirements,
weight of the die, machining, and finish requirements.
Benefits of Die-Casting
- Lowest per part piece price of all casting processes.
- Thin walls, .080 / .125" thick.
- The ability to cast many features into the casting, eliminating the need for
machining.
- Surface finish as cast 63 to 125 RMS.
- Finishes that can be applied to aluminum, zinc & magnesium die castings are:
paint, powder coating and anodizing.
- Repeatability of dimensions from part to part is .001 / .003
- Cast letters, numbers & logos into parts are possible.
Types of Die Casting Dies
There are various types of die casting dies and each serves a critical need for
the customer. The choice of which type of die casting die the customers requires
is usually determined by the following:
- Size of the part to be cast
- Volume of parts required
- Requirement for "family" sets of parts
- Desirability of core slides
- Requirements for cast-inserts Production Dies
These are the most common types of tools produced. They range from single cavity
die with no slides, to multi-cavity die with any number of slides. The cavities
are made from high quality tool steel, retained in a quality holder block.
Production dies are built to critical dimensions, coring the maximum amount of
stock from the casting and allowing the agreed upon amount of machining. A unit
die is a special type of production die.
Database Guidelines
When databases are utilized, quotations for castings are often based on the
assumption that any CAD databases provided to build tooling and produce parts
are complete, usable & without need of updating. Databases may be deemed
incomplete and unusable if:
- The geometry of the part is not physically moldable.
- The necessary draft & radii are not incorporated.
- Line & surface geometry are not connected within .001".
Note: the database file format may not be compatible with existing capabilities
and may require a translator. STL files are usually only used for creating
prototype parts.
Any necessary database manipulation that is caused by incompleteness as
described could add cost & extended lead time to tooling.
If databases are designed only to nominal dimensions, tool life and casting
tolerances may be adversely impacted.
If solid model databases are used for tool construction, they should be
accompanied by a limited dimension part print (either paper or database) that
contains all tolerancing information and information pertaining to any secondary
machining that is to be performed on the parts. The revision controls for
databases should be as agreed upon between the die caster & customer.
Die Life
H13 tool steel dies generally produce 80,000 to 120,000
shots of aluminum parts & zinc parts usually last from
150,000 to 250,000 shots.
Die casters are frequently asked the questions: " How many shots will I get from
the die before it needs to be replaced?" or " How many shots will you guarantee
the die for?" A better question would be: " What can we do to maximize die life
and to minimize replacement costs?"
Selecting Aluminum Alloys
Aluminum (Al) die casting alloys have a specific gravity of approximately 2.7
g/cc placing them among the lightweight structural metals. The majority of
castings produced worldwide are made from aluminum alloys.
Six major elements constitute the die casting aluminum system: silicon, copper,
magnesium, iron, manganese, and zinc. Each alloy affects the aluminum both
independently & interactively.
This aluminum alloy subsection presents guideline tables
for chemical composition, typical properties, die casting, machining, & finishing
characteristics for 11 aluminum alloys. This data can be used in combination
with design engineering tolerancing guidelines for aluminum die castings.
Alloy A380 (ANSI/AA A380.0) is by far the most widely cast of the
aluminum die casting alloys, offering the best combination of material
properties & ease of production. It may be specified for most product
applications. Some of the uses of this alloy include electronic and
communications equipment, automotive components, engine brackets, transmission &
gear cases, appliances, lawn mower housings, furniture components, hand & power
tools.
Alloy 383 (ANSI/ AA 383) & Alloy 384 (ANSI/AA 384.0) are alternatives to
alloy A380 for intricate components requiring improved die filling
characteristics. Alloy 383 offers improved resistance to hot cracking (strength
at elevated temperatures).
Alloy A360 (ANSI/AA A360.0) offers high corrosion resistance, superior
strength at elevated temperatures, and somewhat better ductility, but is more
difficult to cast.
While not in frequent use, Alloy 43 (ANSI/AA C 443.0) offers the highest
ductility in the aluminum family. It is moderate in corrosion resistance and
often can be used in marine grade applications.
Alloy A13 ( ANSI/AA A413.0) offers excellent pressure tightness making
it a good choice for hydraulic cylinders & pressure vessels. Its casting
characteristics make it useful for intricate components.
Alloy 390 (ANSI/ AA B390.0) was developed for automotive engine blocks.
Its resistance to wear is excellent; its ductility, low. It is used for die cast
valve bodies and compressor housing in pistons.
Alloy 218 (ANSI/ AA 518.0) provides the best combination of strength,
ductility, corrosion, resistance, and finishing qualities, but is more difficult
to die cast.
Machining Characteristics
Machining characteristics vary somewhat among the commercially available
aluminum die casting alloys, but the entire group is superior to iron, steel,
and titanium. The rapid solidification rate associated with the die casting
process makes aluminum alloy die castings somewhat superior to the wrought &
gravity cast alloys of similar chemical composition.
Alloy A380 has better than average machining characteristics. Alloy 218, with
magnesium, the major alloying element, exhibits among the best machinability.
Alloy 390, with the highest silicon content and free silicon constituent,
exhibits the lowest.
Castings can be quoted to print specification by utilizing our in-house
machining capabilities.
Surface Treatment Systems
Surface treatment systems are applied to aluminum die castings to provide
a decorative finish, to form a protective barrier against environmental
exposure, and to improve resistance to wear.
Decorative finishes can be applied to aluminum die castings through: painting,
powder coating, polishing, epoxy finishing, and plating. Aluminum can be plated
by applying an initial immersion zinc coating followed by a conventional
copper-nickel-chromium plating procedure similar to that used for plating zinc
metal/alloys.
Protection against environmental corrosion for aluminum die castings is
achieved through painting, anodizing, chromating, and iridite coatings.
Standard and Precision Tolerances
Seven important sets of tolerance guidelines are presented here as both
"Standard" and "Precision" Tolerances.
- Linear dimensions
- Dimensions across parting lines
- Dimensions formed by moving die components (MDC)
- Angularity
- Draft
- Flatness
- Cored holes for threads
The following features are only specified in Standard Tolerance. Unlike the
features above, parts that exceed the following tolerances will not meet the
requirements of form, function & fit. These features are specified at the
maximum tolerance to meet their requirements. These features include:
- Concentricity
- Parting Line Shift
Standard Tolerances
Standard Tolerances cover expected values consistent with high casting cycle
speeds, uninterrupted production, reasonable die life and die maintenance costs,
as well as normal inspection, packing and shipping costs.
Linear Dimensions: Standard Tolerances
The Standard Tolerance on any of the features labeled in the drawing below
(see bottom of page) dimension "E1" will be the value shown in table S-4A-1 for
dimensions between features formed in the same die part. Tolerance must be
increased for dimensions of features formed by the parting line or by moving die
parts to allow for movement such as parting line shift or the moving components
in the die itself. Linear tolerance is only for fixed components to allow for
growth, shrinkage or minor imperfections in the part. Tolerance precision is the
amount of variaÂtion from the part’s nominal or design feature.
For example, a 5 inch design specification with ±0.010 tolerance does not
require the amount of precision as the same part with a tolerance of ±0.005. The
smaller the tolerance number, the more precise the part must be (the higher the
precision). Normally the higher the precision the more it costs to manufacture
the part because die wear will affect more precise parts sooner. Production runs
will be shorter to allow for increased die maintenance. Therefore the objective
is to have as low precision as possible without affecting form, fit and function
of the part.
Example: An aluminum casting with a 5.00 in. (127 mm) specification in
any dimension shown on the drawing as "E1" can have a standard
tolerance of
±0.010 inch (±0.25 mm) for the first inch (25.4 mm) plus ±0.001 for each
additional inch (plus ±0.025 mm for each additional 25.4 mm). In this example
that is ±0.010 for the first inch plus ±0.001 multiplied by the 4 additional
inches to yield a total tolerance of ±0.014. In metric terms, ±0.25 for the
first 25.4 mm increments plus ±0.025 multiplied by the 4 additional 25.4 mm to
yield a total tolerance of ±0.35 mm for the 127 mm design feature specified as
"E1" on the drawing. Linear dimension tolerance only applies to linear
dimensions formed in the same die half with no moving components.
Casting Sizes
The die casting process is capable of casting parts from
less than an ounce up to 50 pounds or greater in some cases.
Draft
All walls on a die casting perpendicular to the parting plane require draft or
taper. This draft is not always constant as it will vary with the length of pull
or draw. Draft will add metal to the casting, therefore increasing size &
weight. Typical draft on die castings is 1 to 3° per wall or side.
Die Casting Tooling Description
Dies, or die casting tooling, are made of alloy tool steels in at least
two sections, the fixed die half (or cover half), and the ejector die half
to permit removal of castings. Modern dies also may have moveable slides,
cores or other sections to produce holes, threads and other desired shapes
in the casting. Sprue holes in the fixed die half allow molten metal to
enter the die and fill the cavity. The ejector half usually contains the
runners (passageways) and gates (inlets) that route molten metal to the
cavity. Dies also include locking pins to secure the two halves, ejector
pins to help remove the cast part, and openings for coolant and lubricant.
When the die casting machine closes, the two die halves are locked and
held together by the machine's hydraulic pressure. The surface where the
ejector and fixed halves of the die meet and lock is referred to as the "die
parting line." The total projected surface area of the part being cast,
measured at the die parting line, and the pressure required of the machine
to inject metal into the die cavity governs the clamping force of the
machine.
Die Casting Material Properties
Property
|
|
Zinc |
|
Aluminum |
| ZAMAK 3 |
ZAMAK 7 |
ZA-8 |
ZA-12 |
ZA-27 |
A360 |
A380 |
A383 |
| Tensile Strength (psi x 10) |
41 |
41 |
54 |
58 |
61 |
46 |
47 |
45 |
Yield Strength (0.2% offset)
(psi x 10) |
- |
- |
42 |
46 |
53 |
25 |
24 |
22 |
| Elongation (% in 2") |
10 |
13 |
8 |
6 |
2.75 |
5.0 |
3.5 |
3.5 |
| Shear |
31 |
31 |
40 |
43 |
47 |
29 |
30 |
- |
| Strength (psi x 10) |
|
|
|
|
|
|
|
|
| Hardness (Brinell) |
82 |
80 |
105 |
105 |
115 |
75 |
80 |
75 |
| Density (Ib./cu.in) |
0.240 |
0.240 |
0.227 |
0.218 |
0.181 |
0.095 |
0.098 |
0.098 |
| Melting Point (F) |
728 |
727 |
759 |
810 |
903 |
1105 |
1100 |
1080 |
Electrical Conductivity
(% IACS) |
27 |
26 |
28 |
28 |
30 |
30 |
20 |
20 |
Coefficient of
Thermal Expansion
(68-212F) (U in/in/F) |
15.2 |
15.2 |
12.9 |
13.4 |
14.4 |
11.6 |
12.2 |
11.5 |
| Die Shrinkage |
0.005 |
0.005 |
0.007 |
0.007 |
0.008 |
0.005 |
0.005 |
0.005 |
Chemical Specifications (Per ASTM) (% by weight)
| Element |
|
Zinc |
|
Aluminum |
| ZAMAK 3 |
ZAMAK 7 |
ZA-8 |
ZA-12 |
ZA-27 |
A360 |
A380 |
A383 |
| AI |
3.9-4.3 |
3.9-4.3 |
8-8.3 |
10.5-11.5 |
25-28 |
Balance |
Balance |
Balance |
| Mg |
.02-.05 |
.01-.02 |
.015-.030 |
.015-.030 |
.01-.020 |
.6 |
.1 |
.1 |
| Cu |
.10 |
.75-1.25 |
.8-1.3 |
.5-1.25 |
2-2.5 |
.4 Max |
3-4 |
3-4 |
| Fe |
.075 |
.050 |
.075 |
.075 |
.075 |
2 Max |
2 Max |
1.3 Max |
| Ni |
- |
.005-020 |
- |
- |
- |
.5 |
.5 |
.3 |
| Cd |
.003 |
.002 |
.003 |
.003 |
.003 |
- |
- |
- |
| Si |
.003 |
.003 |
.003 |
.003 |
.003 |
9-10 |
7.5-9.5 |
10.5-12 |
| Mn |
- |
- |
- |
- |
- |
.35 |
.5 Max |
.5 Max |
| Pb |
.004 |
.002 |
.004 |
.004 |
.004 |
- |
- |
- |
| Zn |
Balance |
Balance |
Balance |
Balance |
Balance |
.5 Max |
3 Max |
3 Max |
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