Post on 28-Apr-2023
POWDER INJECTION MOLDING
A COMPARISON OF TWO
DEBINDING PROCEDURES BASED
ON SOLVENT EXTRACTION
Paulo A. P. Wendhausen; Márcio C. Fredel;
Rubens M. do Nascimento; Júnior G. Justino;
Luis Mauricio de Resende and Aloísio N. Klein
Universidade Federal de Santa Catarina
Laboratório de Materiais - LabMat
E-mail: paulo@labmat.ufsc.br
ABSTRACT
We present a comparison between two debinding
procedures based on solvent extraction. The solvent
debinding was carried out by: (i) immersing the
samples in boiling hexane; (ii) exposing the samples to
hexane vapour. In order to evaluate the debinding rate,
samples were taken at different exposure times and the
mass loss was measured. The pore structure
evolvement was then evaluated by mercury
porosimetry and scanning electron microscopy.
Although the mass loss under condensing vapour
conditions at the beginning (t < 2h) was lower than in
the case of immersion extraction, for both procedures it
tends to level off at 4.5 wt. % when time approaches
4h. From the mass loss and DSC analyses it can be
concluded that at this stage most of the PW and EVA
was extracted. SEM observations show that after
solvent debinding the powder particles are held
together by a web-like structure of PP.
INTRODUCTION
Powder injection moulding (PIM) is a technology based on the
injection moulding of a polymer binder mixed with a high content of
solid particles (e.g. metal and ceramic powders). The development
of suitable techniques to extract the binder without disrupting the
particles or distorting the part was a key step to the evolution of the
PIM technology. A subsequent sintering step is necessary to
provide a part with sufficient density and strength. PIM is an
expanding technology because the sintered part combines the
desirable complex shape and high precision of plastic injection
moulding while achieving performance levels unattainable with
pure or filled polymers. These features make PIM competitive with
other forming technologies(1, 2). Because of the high amount of
organics in the injected parts, a crucial point for the development
of PIM concerns the removal of the binder without loss of integrity
of the product. As a consequence, binder removal from injected
parts is a very time consuming step, which is a serious drawback
of this technology. Taking this into consideration care should be
taken to choose the organic system to be used. Therefore
debinding has been profusely studied in order to find out a way to
extract the binder from the PIM-parts as fast as possible(3, 4, 5). One
interesting strategy is the use of the so called multi-component
binders, which are constituted of components with different
stabilities. For these binders a low stability component is removed
while another, more stable, remains in the compact to hold the
particles in place.
We present here a comparison of debinding under hexane vapour (3) and liquid hexane(4), based on mercury porosimetry, scanning
electron microscopy (SEM) and mass loss analyses. The
subsequent treatments (thermal debinding and sintering) were
carried out under the same conditions in order to evaluate the
effect of the two different procedures on the overall processing.
POWDER INJECTION MOULDING
Schematic Representation
Injection
Powder Base Polymer Aditives
Mixing Peletizing
Debinding Sintering
Focus of this work
EXPERIMENTAL PROCEDURE
INJECTION 38 MPa
SOLVENT
DEBINDING
(Hexane)
69oC 240 min
Liquid Vapour
THERMAL
DEBINDING
Hydrogen Atmosphere
3oC/min 450oC
180 min
SINTERING Hydrogen
Atmosphere 1250oC 75 min
175oC
Condensationcolumn
Sample
Heating Plate
Liquid GlycerineLiquid Hexane
Solvent Debinding Apparatus
Materials Characteristics wt. %
Iron Powder* Carbonyl (d 5m) bal.
Nickel Powder Carbonyl (d <10m) 2%
Polypropylene Pellets 3,3%
Paraffin Wax Powder 2,8%
Ethylene Vinyl
Acetate)
Pellets 1,7%
*Carbon content of 0.8 wt. %.
FEEDSTOCK
0 30 60 90 120 150 180 210 240-5
-4
-3
-2
-1
0
Boiling Beginning
liquid hexane
condensed hexane
m
/mi (
%)
Time (min)
Complete extraction of paraffin and EVA
after 240 min
Fastest extraction rate is observed with
liquid hexane
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
cum
ula
ted v
olu
me (m
l/g)
liquid hexane
120 minutes
poro
sity
(%
)
Pore Diameter (m)
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
condensed hexane vapor
240 minutes
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
green sample
0
5
10
15
20
25
RESULTS AND DISCUSSION
Mass Loss due to
Binder Extraction
Pore Size
Distribution
green sample
120 minutes
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06C
um
ula
ted V
olu
me (m
l/g)
30 minutes
poro
sity
(%
)
Pore Diameter (m)
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
60 minutes
0.001 0.01 0.1 1 10 100 10000.00
0.01
0.02
0.03
0.04
0.05
0.06
240 minutes
0
5
10
15
20
25
15 minutes
0 30 60 90 120 150 180 210 240 270
16
17
18
19
20
21
22
23
24
25
from mercury porosimetry
from mass loss
Poro
sity
(%)
Time (min)
Monotonical evolution of porosity, typical from
diffusion controlled processes
Good match of the curves. Difference can be accounted to the initial inherent porosity of the green samples
Percentage of small pores increases with time of solvent debinding
90% of porosity lies between 0.1 and 0.5 m
Porosity as a function
of debinding time
Porosity distribution
as a function of
debinding time
SINTERING
0 20 40 60 80
l/l
0
Time (min)
0 200 400 600 800 1000 1200-7
-6
-5
-4
-3
-2
-1
0
1
Temperature (oC)
Dilatometric curve of
sample heated under
H2-atmosphere
Microstructure of a
part sintered at
1250oC during 75 min
50 m
Because fine powders (< 10 m) are used an
appreciable shrinkage is observed above 600oC
The well known change in shrinkage rate associated to the a g -transformation takes place at a lower temperature (800oC) due to the presence of Ni
CONCLUSIONS
Solvent debinding using hexane results in a
network of interconected pores within the
injected part, mainly caused by the extraction of
PW and EVA as demonstrated by mercury
porosimetry, SEM and DSC.
The extraction rate is slightly lower for
condensed vapour debinding when compared to
straight liquid extraction.
The evolution of the chemical debinding
promotes the growth of pore channels in its
length but does not change pore diameter
significantly.
Systematic studies of the different debinding
processes on samples with complex shapes are
necessary to evaluate their performance
regarding distortion problems
[1] Gummeson, P.U. The Metal Injection Moulding Opportunity - A Critical View.
The International Journal of Powder Metallurgy. Vol. 25, no. 3. 1989.
[2] German, R. M. and Cornwall, R. G. Worldwide Market And Technology for
Powder Injection Moulding. PMI, Vol. 33, no. 4. 1997. p. 23-27.
[3] Lin, S. T. and German, R. M. Extraction Debinding of Injection Molded Parts
by Condensed Solvent. Powder Metallurgy International. Vol. 21, no. 5, 1989.
[4] Hwang, K. S. and Hsieh, Y. M. Comparative Study of Pore Structure
Evolution During Solvent and Thermal Debinding of Powder Injection
Molded Parts. Metallurgical and Materials Transactions A.
Vol. 27 A, Feb. 1996. p. 245-253.
[5] Ebenhoch, J. Trübenbach, P. and Weinand, D. Process Parameters for
a Fast Catalytic Debinding System. In: Powder Injection Moulding
Symposium. Metal Powder Industries. 1992. p. 385-392.
REFERENCES
ACKNOWLEDGEMENTS
This work was funded by the Brazilian Science and
Technology Ministry - MCT under the FINEP-Project No.
41960853-00