Uniaxial pressing

Question 1

Why is uniaxial pressing the most widely practiced forming process?

Answer 1

~ close tolerances
~ no drying shrinkage
~ surface relief is possible

Question 2

What is the typical tooling?

Answer 2

~ hardened steel

Question 3

What is the clearance between die and punch for micron-sized powder? Why?

Answer 3

~ 10-25um

~ want air to vent out, but need to avoid particles being trapped

Question 4

How do you facilitate ejection?

Answer 4

~ die walls can be slightly tapered (<10um/cm)

Question 5

What are the types of pressing?

Answer 5

~ single or double action
~ floating die that emulates double action

Question 6

How is granulated powder loaded?

Answer 6

~ via feeding shoe, which facilitates a high and uniform tap density via head of powder in chute and agitation from multiple passes

Question 7

What is the pressing pressure?

Answer 7

~ 20-100 MPa (2,900-14,500 psi)

Question 8

How is lubricant applied? What does it do?

Answer 8

~ lubricant is applied by being coated on granular feed, or sprayed on die/punch walls

~ reduces die wear, increases particle packing uniformity, and reduces ejection pressure

Question 9

What is an example of a lubricant?

Answer 9

~ steric acid (CH3)(CH2)6COOH

~ the carboxyl end is strongly attracted to oxide particles and die wall, but shear resistance between first adsorbed layer and subsequent layers is low

Question 10

Why is good granule flow required?

Answer 10

~ essential for good volumetric filling

~ granule flowability is inferred from angle of repose

Question 11

What is a typical fill/tap density?

Answer 11

~ 25-30 vol%

~ pressing problems (non-uniform density in pressed parts) are reduced with higher fill density

~ in addition to action of loading shoe, vibration of the die can be helpful

Question 12

What is the rate of compaction?

Answer 12

~ rapid until 5-10 MPa, slower thereafter

Question 13

What is the first stage of pressing?

Answer 13

~ granule rearrangement

Question 14

What is the second stage of pressing?

Answer 14

~ granule deformation

~ eliminating intergranular porosity

Question 15

What is the third stage of pressing?

Answer 15

~ granule densification

~ sliding and re-arrangement of individual particles, fracture of aggregates and anisometric particles

~ ejection occurs, particles are elastically compressed, release of elastic strain is “springback”

Question 16

What is springback?

Answer 16

~ release of elastic strain

~ differential springback of <0.75% is desirable for separation of the compact and the punch

~ higher springback can cause end cap and ring cap defects

Question 17

What are lamination defects?

Answer 17

~ results from slip-sticking via die-wall sticking

~ lubrication reduces ejection pressure and tendency for slip-sticking

Question 18

What is pressure transmission?

Answer 18

~ a portion of the applied load is transferred laterally to the die wall during compaction

~ resisting force from die wall friction results in pressure/packing density gradients in the part (pressure and packing density decreasing towards the center and with depth in the part)

Question 19

What is the pressure gradient in a double action or floating die?

Answer 19

~ the minimum pressure and compaction density is at the center of the part

~ this decreases the gradient in pressure/density, and the warping tendencies during fire from the center of the part mutually oppose

Question 20

How do we minimize pressure gradients?

Answer 20

~ press in the thin direction of the part

Question 21

What is surface relief?

Answer 21

~ minor thickness variations in the pressing direction can be accomodated

~ larger variations cause high pressing pressure for thin regions relative to large ones

~ pressing dies may be designed with a split lower punch with spring-loaded regions to equalize pressing pressures