Why Can MIM Produce Undercuts But PM Cannot?

One of the most visible differences between MIM parts and conventional PM parts is geometric complexity. MIM parts can have undercuts, side holes, threads, and complex 3D contours. PM parts are limited to pressable shapes that can be ejected from a single-axis die. Why?

The fundamental difference:

Factor	MIM	PM
Material state	Fluid (feedstock heated to 150-200°C)	Dry powder at room temperature
How shape is formed	Injected into a closed mold cavity	Compressed between upper and lower punches in a die
Part removal	Mold opens; core pulls and slides retract; part ejected	Die opens; lower punch pushes part out of die cavity
Side features	Formed by slides that retract before ejection	Not possible — the die cavity must be a straight-walled tube

How PM works geometrically: The die cavity is essentially a tube with a fixed cross-section. The upper punch descends, compressing the powder against the lower punch, then withdraws. The lower punch pushes the green part up and out of the die. Any feature that would prevent the part from being pushed straight out of the die — an undercut, a side hole, a thread — is impossible. How MIM works geometrically: The mold consists of multiple components (cavity block, core block, slides, lifters) that assemble to form the full part geometry. Before ejection, the slides retract, the core pulls withdraw, and the mold opens — leaving the part free to eject with no geometric obstruction. What each can produce:

Feature	MIM	PM
Side hole perpendicular to mold opening	Yes (slide/core pull)	No
Undercut on external surface	Yes (slide)	No
Internal thread	No (must be post-machined)	No
Variable wall thickness	Yes	Limited
Thin wall (<0.5 mm)	Yes	No (minimum 1.5-2.0 mm)
Gear tooth profile	Yes	Yes (if parallel to press direction)

Quick Q: Why can MIM produce undercuts but PM cannot?

MIM uses a multi-component mold with retractable slides and core pulls that can form undercuts, side holes, and 3D contours, then withdraw before ejection. PM uses a simple die cavity with only vertical punch motion — any feature that prevents the part from being pushed straight out of the die is impossible. This is why MIM is chosen for geometrically complex parts and PM for simple pressable shapes.