Radius all corners where possible. Resistance to metal flow is decreased through radii as is the tendency to drag and bend at sharp
corners. The outer skin is the hardest (strongest) casting area, and by radiusing, the skin remains intact.
Allow maximum draft where
practical. This assists casting release from the mold, and as with radiusing, bending and tearing are minimized.
Keep wall thicknesses
as uniform as possible. This enhances metal flow by avoiding pressure drops - going from thick to thin to thick surfaces - during
the injection cycle. Pressure drops inhibit metal flow, promote backfilling, and result in cold shut and porosity.
Casting design must
be kept practical with the casting process. Dimensions across the parting line and dimensions across moving die members (cam actions,
hydraulic pulls, etc.) must be specified given that such areas are the most difficult to maintain in die casting. Proper engineering
and die construction minimizes problems associated with moving die members.
Keep tolerances practical and essential as is necessary.
Keep
machining stock to a minimum in order to avoid porosity openings.
Cored Holes
- Aluminum: min. .080
- Zinc: .040
Cast Threads
- Keep in class A
- Allow flats at parting line if possible
To cast pressure tight components, give special considerations to all details of the molding process. Depending on pressure tight
requirements, the cost of impregnation can be avoided by following the basic design guidelines of radii, draft, uniform sections,
etc. Take particular care not to break the casting skin. The following are also recommended:
- Incorporate a vacuum system into the die design. This need not to be complicated, and will assist in removing gasses from the die
surface that could "trap" during injection.
- Pay close attention to fillets and metal savers.
- Avoid heavy sections. Heavy sections promote porosity due to heat concentration.
- Core holes as opposed to machining. Machining exposes porosity.
- Similarly, avoid the removal of large amounts of stock when machining.
- It is recommended that the die caster pressure test a percentage of the castings. Imperfections not visible by physical inspection
do exist. The cost associated with pressure testing is minimal compared to impregnation.
In certain applications, it is desirable to place an insert into the die and cast the molten metal around it. Inserts are used where
demands on the component go beyond the capabilities of the zinc and aluminum alloys.
Where a steel thread is required for strength,
for example, an insert can be placed in the appropriate area of the mold and metal diecast around it. The casting now conforms to
the requisite engineering needs.
The same applies to heater elements. These can also be inserted, with the casting providing the housing
for the element as well as the housing for the liquid to be heated.
Copyright (C) 2011 Bardane Inc. All Rights Reserved
GENERAL DESIGN CONSIDERATIONS
Pressure Tight Castings
Bardane Manufacturing Company
Insert Castings
Wall Thickness
Ribs
Gusset Type Ribs
Round vs. Square Corners
Holes
Knurls and Teeth
Overly thick walls slow production, invite porosity, and are expensive. Recommended maximum wall thickness for zinc die castings
is .380", and for aluminum die castings, .250". Nonuniform wall thickness and unnecessary material bulk should also be avoided
because of differential contraction in cooling, often causing cracks.
Part strength should be increased
by adding gusset-type ribs rather than by increasing wall thickness.
Ribs that strengthen flat surfaces
should be narrower than the main wall of the diecasting and should be offset to avoid metal bulk at intersections. Perpendicular intersections
are less bulky than angular ones.
Minimum practical hole diameter depends on metal characteristics, and
maximum hole depth depends on hole diameter. For zinc die castings, recommendations are: minimum diameter, .040", maximum through-hole
depth, 8-diameters; and maximum blind-hole depth, 3-diameters. For aluminum die castings: minimum hole diameter, .080"; maximum through-hole
depth, 4-diameters; and maximum blind-hole depth, 2-diameters. In some cases, the holes can be cast from two sides. Large holes in
thinwall sections of the diecasting should be reinforced with flanges to ensure that the area remains flat.
Exceptions
to the general rule that edges of ribs and walls should be rounded are edges formed by meeting tool members. Such edges should be
sharp, making the tool stronger and longer lasting. Similarly, feather edges should be avoided. A small land -.030" to .040" - is
recommended.
Die castings can incorporate internal
or external teeth, knurling, or other features for easy gripping. Only straight knurling is practical (diamond knurls require more
complex tooling). Recommended pitch is at least .080", and knurl depth should be shallow. In addition to knurling, other possible
gripping aids are small ribs, grooves, splines, and flats.