Uniform wall thickness is critical. Non-uniform walls may cause distortion,
internal stresses, voids, cracking, and sink marks. Variations in wall
thickness also cause variations in shrinkage during sintering, making
dimensional control difficult. Examples
of designing for uniform wall thickness are shown in Figure 1.
One method used to attain uniform wall thickness is coring, some examples of
which are shown in Figure 1.
Coring can also reduce cost by reducing material
and processing times. In some parts, coring can easily be achieved
by adding holes that are formed by pins protruding into the mold cavity.
Through holes are easier to mold than blind holes, because the core pin can
be supported at both ends. Blind holes are formed by pins supported at
only one end and can be off-center due to deflection of the pin by the flow of
feedstock into the cavity. Therefore, the depth of a blind hole is generally
limited to twice the diameter of the core pin. Holes perpendicular to one
another cause special problems of sealing-off or closing-off in the mold. By
redesigning one hole to a “D” shape, the tooling will function better, be stronger,
and minimize flashing. An example of this construction is shown in Figure 2.
Ribs and Strengthening Members
For parts designed with thin walls, or parts with cross-sections reduced to promote
uniformity, reinforcing ribs are an effective way to improve rigidity and
strength Examples of ribs, or webs, are shown on schematics of parts produced
for Remington Arms Company in Figure 3. The thickness or width of a rib should
exceed the thickness of the wall to which it is joined, with the principle of
uniform wall thickness being maintained whenever possible. While ribs
can increase part strength, improve material flow, and prevent distortion
during processing, they may also produce warpage, sink marks, and stress
concentrations. Ribs should be added to a part design cautiously, and it is
often better to wait for an evaluation of the initial tool samples.
Wall Thickness Transition
In some parts, different wall thicknesses cannot be avoided.
A gradual transition from one thickness to another reduces
stress concentrations and poor surface appearance (flow lines).
recommended ratio for transitions is shown in Figure 4.
In addition, the mold should be gated at the heavier section
to insure proper packing of the feedstock.
Feedstock enters the mold cavity
through an opening called a “gate”. In general, gate
locations should permit the feedstock to flow from thick
to thin sections as it enters the mold cavity. Ideally,
the flow path from the gate should impinge on the wall
of the cavity or a core pin, as shown in Figure 5. A
flow path of thin to thick,
Generally, will cause voids, sink marks, stress concentrations, and flow lines on
the part surface. Many MIM components are produced using multiple cavity tooling,
where each cavity must be identical to the others. To assure part reproducibility,
the gate and runner system to each cavity must be carefully sized and located to
insure that each cavity will be filled with the identical amount of feedstock at a
balanced fill rate. Since the gate will leave a mark or impression, its location must
be carefully selected with regard to part function and Our specialists will work with
you to select the best gate selection.
Draft and Knock-out Pins
Draft, or a slight taper, may be required for the ejection of parts from the mold cavity.
This is particularly true for core pins, and the need increases with the depth of the hole or
recess being formed. When draft is required, an angle from .5° to 2° is generally sufficient.
Knock-out (ejector) pins are usually required for removing parts from the mold.
Pin placement is important to prevent part distortion during ejection. An adequate pin
surface is also needed to prevent puncturing the part. Good design of the knock-out pins
is critical to minimizing flash and marking of the parts. The need for draft or knock-out
pins is generally left to the discretion of the producer, and is based on the ability to
eject the component from the mold.
Reducing Stress on Centrations: Fillets and Radii
There is an abrupt increase in stress at sharp corners, the magnitude of
which depends on the corner radius and part geometry. Stress concentrations
can be dramatically reduced through the use of generous fillets or radii,
which will also improve flow of the feedstock during molding and assist in
ejection of the part from the cavity. Both inside and outside corners should
have radii as large as possible, typically not less than 0.15 inches (0.4mm).
Fillets and inside corner radii should be .015 to .030 inches (0.4 to 0.8mm).
When required, external and internal threads can be automatically molded into the
part, eliminating the need for mechanical thread-forming operations. External threads
can be molded in two ways, the least expensive being to locate the parting line on the
centerline of the thread as shown in Figure 6. There will actually be two parting lines,
180° apart, which might
increase the thread diameter up to .004. To hold a tight
tolerance on a thread diameter, specify narrow flats on the parting line surfaces. If
this is not
acceptable, or if the axis of the thread is in the direction of the mold-
opening, the alternative is to equip the mold with an external, thread-screwing device.
This method increases both mold cost and size, and significantly decreases molding rates.
Internal threads are typically molded in parts by using automatic unscrewing devices.
Experience to date indicates that this approach may not be the most cost-effective and
a subsequent tapping operation should be considered.
Parting lines are formed by the
opposing faces of the mold, in the plane where the mold
halves are separated to permit removal of the part as was
shown in Figure 6. With molds of normal construction, this
feature is transferred as lines or witness marks onto the
surface of the part. These lines can sometimes be made less
noticeable by designing the mold to separate along
inconspicuous edges of the part. Since molds will wear more
at these surfaces, flashing may occur and the section chosen
should not be a critical one.
Undercuts, classified as internal and external, are often
required for part function. Undercuts may increase tooling
costs and lengthen cycles, but this is dependent on the type
and location of the undercuts on the part. External
undercuts, often specified on MIM parts for “o”-ring
seating, can be formed by using a split cavity mold as was
shown in Figure 6. As with the threaded component, there
will be two parting lines 180° apart on the surface of the
undercut, which may be objectionable in an “o”-ring groove.
Internal undercuts can be formed by using collapsible cores.
However most MIM parts are relatively small and cannot
accommodate this approach. Designing MIM parts with internal
undercuts or recesses are not recommended.
Wall Thickness and Coring