Process Description

Molding Cycles. A typical sequence for the injection pressure and screw position are shown in Fig. 8. Prior to filling, the screw rotates in the barrel and the external heaters bring the feedstock to temperature. The dwell time for the feedstock in the barrel must be sufficient to ensure thorough and uniform heating. Then, during molding, the screw plunges forward in a split second. This is traced by the screw displacement curve in Fig. 8. Three pressure curves are included to show that pressure is high at the source (hydraulic pressure) and lags at the cavity. The rapid rise in hydraulic pressure induces feedstock flow into the mold. Once the cavity is filled, the gate freezes and there is little effect of hydraulic pressure on the cavity pressure. The nozzle pressure is intermediate between these two.

Fig. 8 Plots of the screw position and pressures involved in molding

Moldability is a measure of the ease by which a feedstock can be shaped to a given specification. A high moldability is desired from a feedstock. In practice, several combinations of temperature, fill rate, and pressure exist that produce defect-free components. This pressure is generated by the forward motion of the screw in the barrel as a plunger. A low pressure gives incomplete packing with cavities or sink marks on the compact surface. Excessive packing pressure causes the compact to stick in the die, resulting in severe ejection problems.

To compensate for the flow resistance and pressure gradients in the molding system, the screw position and hydraulic system pressure vary during molding in a coordinated manner. Friction along the flow path decreases pressure. After mold filling, pressure is held during the packing stage. The objective is to compensate for the thermal contraction of the feedstock during cooling. Finally, when the binder has sufficiently cooled to hold the component shape and withstand the ejection forces, the die is opened and the component is ejected. The ejector motion can be from the hydraulic system or a separate mechanical motion.

The time for molding relates to the cavity size, filling time, and cooling time. It can be as short as 5 s or as long as 1 min. Initial feedstock flow requires that the molding temperature be higher than the softening point of the binder. This is usually between 50 and 200 °C (122 to 392 °F). A low temperature results in short shots (incomplete mold filling), while a high temperature degrades the binder or causes flashing or powder-binder separation, and requires prolonged cooling. Molding pressure affects the mold filling rate and is usually limited by the machine design. Once the component has cooled, the final step is component ejection. Short shots occur if either the molding temperature or pressure is too low. At the higher pressures and temperatures the components stick to the die walls or open the die along the parting line, giving flash. Voids are captured in a part if the feedstock shoots across the cavity without pushing out all of the air. This is termed jetting and is very undesirable. Jetting occurs with too high a fill rate, so most molding machines are programmed for a fill rate that prevents jetting.

Table 4 gives the range of molding conditions and one example of a specific component and its molding conditions. That component is a steel trigger guard for a rifle formed from carbonyl iron powder and a binder based on paraffin wax, polypropylene, and stearic acid. Once the component is molded, the polymer binder phase is extracted and the compact is sintered to near-full density. These steps are covered in the sintering articles in this Volume.

Table 4 Typical powder injection molding parameters

Parameter

Typical range

Trigger

Barrel temperature, °C

100-200

160

Nozzle temperature, °C

80-200

180

Mold temperature, °C

20-100

40

Screw rotation speed, rpm

35-70

35

Peak injection pressure, MPa

0.1-130

20

Packing pressure, MPa

0-10

8

Fill time, s

0.2-3

0.6

Packing time, s

2-60

3

Cooling time, s

18-45

20

Cycle time, s

8-360

37

Densification. Usually the binder is removed from the component prior to sinter densification. A wide array of options exist for binder extraction. Thermal debinding is the easiest to envision. The component is slowly heated to decompose the binder. As illustrated by Table 5, many variants exist. There are some differences in component precision and debinding rates. For this tabulation, the measured rates are compared on an equivalent basis of 5 /'m steel powder and 10 mm section thickness. The most popular approach is to immerse the component in a solvent that dissolves some binder, leaving some polymer behind to hold the particles in place for subsequent handling. The remaining binder is thermally extracted as part of the sintering cycle. Newer binders are water soluble, so the debinding solvent is water. Major growth is occurring in the use of catalytic phase erosion for debinding. Most of the binder is attacked by a catalytic vapor, and the residual binder is removed during heating to the sintering temperature. This can now be performed as a continuous process at the beginning of sintering. Debinding is highly variable as to binder system, technique, and section thickness. Solvent and catalytic debinding ensure the best dimensional control, because the binder is kept rigid during extraction. For these techniques penetration rates of 1 to 2 mm/h are common.

Table 5 Comparison of debinding techniques and times

Binder system

Debinding technique

Conditions

Time

Wax-polypropylene

Oxidation

Slow heat 150 °C, hold, heat to 600 °C in air

60 h

Wax-polyethylene

Wicking

Slow heat to 250 °C, hold, heat to 750 °C in hydrogen

4 h

Wax-polymer

Supercritical

Heat in freon vapor at 10 °C/min to 600 °C under 10 MPa pressure

6 h

Wax-polyethylene

Vacuum extraction

Slow heat while passing low-pressure gas over compacts, heat to sintering temperature

36 h

Water-gel

Vacuum sublime or freeze dry

Hold in vacuum to extract water vapor from ice

8 h

Oil-polymer

Solvent immersion

Hold in ethylene dichloride at 50 °C

6 h

Water-gel

Air drying

Hold at 60 °C

10 h

Polyacetal-polyethylene

Catalytic debind

Heat in nitric acid vapor at 135 °C

4 h

Note: Section thickness, 10 mm; particle size, 5 /'m; solids loading, 60 vol%

Note: Section thickness, 10 mm; particle size, 5 /'m; solids loading, 60 vol%

The next step is sintering, which can be incorporated directly into a thermal debinding cycle. Sintering bonds the particles together, leading to densification. Often sintering serves the dual role of densification and chemical homogenization. In the latter process, mixed powders are molded and sintering causes them to form homogeneous alloys by long-range atomic motion. Usually sintering shrinkage is uniform and isotropic, so the molded component is oversized to deliver the desired final dimensions. For metals, sintering is performed in a protective atmosphere or vacuum at a peak temperature that causes rapid elimination of the pores that were previously filled with binder. Steels and stainless steels are sintered at 1120 to 1350 °C range for 30 to 120 min, with shrinkages of 12 to 18%. Table 6 details some common sintering cycles, giving the initial green density, heating rate, maximum temperature, hold time, sintering atmosphere, support material, and final density. If the powder structure is formed homogeneously, then sintering is uniform and final dimensions can be held to close tolerances.

Table 6 Sample sintering cycles for powder injection molding materials

Material

D, .'-m

■■'■'g, %

dT/dt, °C/min

T, °C

t, min

Atmosphere

Support

Ag

90

67

1400

900

60

Hydrogen

Stainless

70

Co-50Fe

5

50

50

1250

120

Hydrogen

Alumina

98

Cu

9

70

10

900

60

Hydrogen

Alumina

94

Fe

5

60

10

1200

60

Vacuum

Alumina

100

Fe-49Co-3V

6

58

5

1350

240

Vacuum

Alumina

96

Fe-2Ni

5

64

15

1250

60

Hydrogen

Alumina

99

Fe-2Ni-0.8C

4

58

4

1200

60

Hydrogen

Alumina

97

Fe-50Ni

5

60

10

1250

60

Hydrogen-nitrogen

Alumina

96

Fe-29Ni-17Co

7

60

10

1250

240

Hydrogen-argon

Alumina

97

Fe-3Si

8

60

10

1350

180

Hydrogen

Alumina

97

Mo-15Cu

9

30

10

1400

60

Hydrogen

Alumina

86

NÍ3Ál

14

52

10

1340

60

Hydrogen

Alumina

99

316L stainless

15

62

10

1325

120

Vacuum

Alumina

97

W-10Cu

8

50

10

1350

60

Hydrogen

Alumina

96

W-5Ni-2Fe

2

55

10

1500

30

Hydrogen

Alumina

100

D, particle size; P{ ]. green density; dTldt, heating rate; T. sintering temperature; t. time; Ps. sintered density

D, particle size; P{ ]. green density; dTldt, heating rate; T. sintering temperature; t. time; Ps. sintered density

After sintering, the component has excellent strength, with properties near or even superior to those available from other processing routes. Final densification can be assisted by both hot and cold deformation, including hot isostatic pressing. Other post-sintering steps include coining, drilling, reaming, machining, plating, passivation, and heat treatment. Options in heat treatment include tempering, precipitation hardening, nitriding, and carburization.

A typical PIM component is a trigger guard for a sporting shotgun, the curved piece that surrounds the trigger below the barrel. It is fabricated from a low-alloy iron-nickel steel, usually with a final weight of 40 g. A mixture of 5 /'m Fc and 8 i^m Ni powders are used. These are combined with wax and polyethylene to form feedstock that can be molded at 58 vol% solids. During molding, the nozzle temperature in the molding machine is 175 °C with a die temperature of 40 °C. The maximum pressure applied during mold filling is 20 MPa and a pressure of 8 MPa is held on the feedstock during cooling. The mold filling time is rapid (about 0.5 s), but the mold cooling time is 18 s, with a total cycle time of 37 s between parts.

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