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

30 min in liquid hexane

30 min in vapour

hexane

240 min in liquid/vapour

hexane

MICROSTRUCTURE

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