Materials and Equipment

Feedstock. PIM begins with the mixing of selected powders and binders. The particles are small to aid sintering, usually between 0.1 and 20 /m with near-spherical shapes. For example, a 5 /'m carbonyl iron powder is widely used in the PIM process, as is a -16 /(m gas-atomized stainless steel powder. Most common engineering alloys are used, including various steels, tool steels, and stainless steels. Likewise, ceramics, refractory metals, and cemented carbides are processed in a similar manner. The binder is based on a common thermoplastic such as wax or polyethylene or wax-polypropylene, but food-grade polymers, polyacetal, cellulose, gels, silanes, water, and various inorganic substances are also in use. Usually the binder system consists of two or three components. An example binder, which is molten at 150 °C, consists of 65% paraffin wax, 30% polypropylene, and 5% stearic acid. A typical binder content is near 40 vol% of the mixture; for steel that corresponds to about 6 wt% binder. A few other binder systems are:

• 90% polyacetal, 10% polyethylene

• 69% paraffin wax, 20% polypropylene, 10% carnauba wax, 1% stearic acid

• 50% carnauba wax, 50% polyethylene

• 55% paraffin wax, 35% polyethylene, 10% stearic acid

Feedstock is a term for the mixture of powder and binder. Many types of powders can be used, but great differences exist in mixing and molding, especially if the particle shape is nonspherical. The formulation of a successful feedstock balances several considerations. Sufficient binder is needed to fill all voids between particles and to lubricate particle sliding during molding. A viscosity similar to that of toothpaste is generally most desirable. Mixing is best achieved using a continuous twin screw compounder. Actually the viscosity depends on several factors. At too high a powder-to-binder ratio there is a high viscosity and insufficient binder to fill all void space between the particles. Consequently, it is hard to mold such a feedstock. Alternatively, too much binder is undesirable because component shape is lost during debinding. Inhomogeneities in the feedstock lead to defects in molding; thus, a high shear is required in mixing to force the binder among all particles. Consequently, special mixing practices are required to compound feedstock for most applications. The final step in feedstock preparation is to form pellets that are easily transported to the molding machine. Figure 3 shows both worms and pellets formed for molding. An important evolution in the technology has been the preparation of feedstock by major chemical companies, removing some of the licensing and technological barriers, allowing rapid growth in the field. Table 1 details the composition of a few common injection molding feedstocks, showing the binder, powder, and formulation details.

Table 1 Examples of powder injection molding feedstock

Powder

Binder, wt%

Solids loading, vol %

Density, g/cm3

Molding temperature, °C

Viscosity, Pa ■ s

Strength, MPa

4 ■ m Fe

60PW-40PE

58

4.90

120

35

5

4 ■ m Fe

55PW-45PP-5SA

61

5.12

150

19

22

4 ■ m Fe-2Ni

90PA-10PE

58

4.52

180

190

20

2.5 ■ m Mo

60PW-35PP-5SA

58

5.97

113

200

7

10 .'''m stainless

55PW-45PP-5SA

67

5.60

130

100

15

15 .''m stainless

90PA-10PE

62

5.33

190

80

20

12 ■''m tool steel

90PA-10PE

62

5.33

190

180

20

8 ■ m W

65PW-30PP-5SA

56

11.22

142

5

1 ■ m W-10Cu

60PW-35PP-5SA

64

11.41

135

55

6

PA, polyacetal; PE, polyethylene; PP, polypropylene; PW, paraffin wax; SA, stearic acid

PA, polyacetal; PE, polyethylene; PP, polypropylene; PW, paraffin wax; SA, stearic acid

Fig. 3 Feedstock pellets and worms for molding

Pelletized feedstock is injection molded into the desired shape by heating it in the molding machine and hot ramming it under pressure into the tool cavity. By virtue of the binder, the feedstock becomes low enough in viscosity that it can flow into the die cavity under pressure. Cooling channels in the die extract heat and solidify the polymer to preserve the molded shape. The shaping equipment is the same as that used for plastic injection molding. It consists of a die filled through a sprue, runner, and gate from a heated barrel. Most popular is molding in a reciprocating screw machine. Here the screw in the barrel stirs the feedstock while it is melting and acts as a plunger to generate the pressure needed to fill the die. In the actual molding stroke, the molten feedstock is rammed forward to fill the cold die in a split second. Molding pressures depend on several parameters, but might be 60 MPa or more. Pressure is maintained on the feedstock during cooling until the gate freezes to reduce the formation of sink marks and shrinkage voids. After cooling in the die, the component is ejected and the cycle is repeated.

Tooling. The tool materials used in PIM are similar to those encountered in many metal working, plastic injection molding, and powder metallurgy operations. Table 2 identifies some of the common tooling materials. The tool material choice depends on the anticipated number of molding cycles and the required wear resistance. On the one hand, machining difficulty and material costs need to be considered. For molding tools, P-20 is the most common material, because of the combination of strength and cost. Yet wear concerns with PIM make the selection of higher-hardness tool steels most common. Rapid prototype tool materials, including epoxy, have been used in pilot production. Soft alloys of aluminum, zinc, or bismuth are used during tool development because of easy machining. Cemented carbides are useful where wear is a primary concern, but tool fabrication is expensive and tool damage is a problem because of the low toughness. Material cost varies by a factor of ten between these materials. Tool steels are best because of the combined strength, toughness, hardness, and machinability.

Table 2 Construction materials for injection molding tools

Material

Composition

Hardness, HRC

Suggested applications

420 stainless

Fe-14Cr-1Si-1Mn-0.3C

50

Corrosion-resistant cavities, cores, inserts

440C stainless

Fe-18Cr-1Si-1Mn-1C

57

Wear-resistant, small inserts, cavities, cores

H13 tool

Fe-5Cr-1.5Mo-1Si-1V-0.4Mn-0.4C

50

Larger or intricate cavities, high toughness, low wear

M2 tool

Fe-6W-5Mo-4Cr-2V-0.3Mn-0.8C

61

Core and ejector pins

P20 steel

Fe-1.7Cr-0.8Mn-0.5Mo-0.4V-0.35C

30

General purpose, hot runner, large cavities

Cemented carbide

WC-10Co

80

High wear, compressively loaded small inserts

Tool fabrication occurs in a machining center via progressive removal of material from an initially oversized block of material. Most machining is computerized, but there is still the necessity to hand-finish critical components or dimensions in the tool set. A final surface roughness of 0.2 /'m (8 /'in.) is typical, but smoother finishes are used in selected applications. The desired tool hardness is typically more than 30 HRC, which is satisfied by many heat-treated stainless steels or tool steels. Under normal conditions, an injection molding tool set can mold up to one million parts. With soft tool materials like aluminum, the life is less, at 1,000 to 10,000 cycles.

The tool set has the cavity and further consists of the pathway for filling the cavity with ejectors for extracting the component from the cavity. In most instances, the tool set consists of a single cavity. This cavity captures the component shape, and it is oversized to allow for component shrinkage during sintering. Around the cavity are several tool parts needed for opening and closing the cavity, ejecting the component, aligning the die sections, moving inserts, cooling the component, and locating the sprue, runner, and gate. Figure 4 is a sketch of a molding tool set with ejector pins, ejector plate, and keyed slides to ensure proper closure of internal die components. Many operations use a three-plate mold for automated removal of the gate on mold opening.

Fig. 4 A sketch of the tool set for powder injection molding, showing major components

A primary concern in designing injection molding tooling is component shrinkage. On a volume basis, the typical feedstock contains 60% solid and 40% binder. To attain the desired final component properties, the linear shrinkage during sintering may be 15%. The shrinkage in dimensions is known as the shrinkage factor Y, calculated from the solids loading, and the sintered fractional density, P/PT:

where Pis the final density and rT is the theoretical density for the material. This assumes isotropic shrinkage in sintering. For example, if a 13.8 mm dimension shrinks to 12 mm, then the shrinkage factor is 0.13 or 13%, calculated as the change in a dimension divided by the original dimension. Because the target is the final component size, each dimension of the tool cavity is oversized to accommodate shrinkage. If the desired final dimension is Zf at a fractional density P/PT from a feedstock with a fractional solids content of 0, then the initial dimension of the tooling is given in terms of the tool cavity expansion factor, Z:

As an example, if the shrinkage factor Y is 0.15 (15% shrinkage), then the tool expansion factor Z is 1.1764. Thus, to obtain a 12 mm final dimension requires a tool dimension of 14.11 mm (12 x 1.1764). Note that the shrinkage from 14.11 mm to 12 mm is 2.11 mm, so the measured shrinkage factor is 2.1 ^ 14.11 mm, or 0.15 (15%).

At the end of the runner is the gate leading into the die cavity. It is a small opening designed to freeze before the cavity, runner, or sprue freezes. A solidified gate allows removal of the pressure at the machine while the mass in the cavity cools. Gates are usually near 3 mm in diameter. Actual gate size is determined by two factors: the filling shear strain rate and the section thickness. For the gate to freeze before the compact requires a thickness between 10 and 50% of the compact thickness.

In the typical tool set, the feedstock flow path is from the molding machine nozzle into the sprue, along the runner, through the gate, and into the mold cavity, as evident in the test geometry shown in Fig. 5. This five-step geometry has two gates fed by the runner system from the sprue. This flow path is surrounded by various clamping plates, alignment and locating pins, and ejector components. Alignment of the components and their proper sequencing and smooth motions are important to successful PIM. Most of the tool components are available as premachined packages, so only the cavity needs to be custom machined. Also, within this geometry are the cooling or heating networks designed to control mold temperature. One key advantage of injection molding is the ability to fabricate complex shapes that cannot be produced by alternative techniques. Accordingly, complex tool designs are a necessary aspect of injection molding.

Fig. 5 A five-step test geometry with dual gates attached runner and sprue

The number of cavities in the tool set depends on the number of components to be fabricated, the shot capacity of the molding machine, tool fabrication costs, and the available clamping force. Tool sets with up to 40 cavities have been used for high-volume production. Most injection-molded steel components are generated in tool sets with 1 to 16 cavities. A single cavity tool set is satisfactory for low production quantities, below about 200,000 parts. As higher productivity is required, more cavities may be justified, because production increases are gained without the purchase of new molding machines. As the number of cavities increases, the cost of manufacturing the tool set increases, but the net production cost per component decreases. Because tool cost is distributed over the production quantity, there is a minimum total cost that depends on the total production quantity increases.

After molding, the component is cooled in the die cavity. Cooling causes the binder to contract and this results in a progressively lower pressure, eventually allowing ejection of the part. The ejection force depends on the component shape and feedstock. To accomplish ejection, pins in the die body move forward with the ejector plate and push the component from the cavity. If inserts, internal cores, or threads are put into the shape, these must be retracted (possibly by motorized motions) to allow free ejection. Sometimes core pins, inserts, or other features are placed in the cavity before filling and these items become encapsulated in the part, ejecting out on each cycle. Ejector pins blemish the component, because they concentrate ejection force on a soft material. Some of the blemishes associated with molding include ejector pin marks, parting lines, and gate impressions.

For ejection, pins move from flush positions on the tool walls and push against the component to extract it from the cavity. To build in undercuts or holes perpendicular to the molding direction, the tooling must contain side-actuated cores or inserts. Using rotating cores to create threads or rifling patterns without a parting line is also possible. Unlike die compaction, in injection molding it is possible to design into the tooling perpendicular holes, undercuts, and indents using side cores that are mechanically actuated on mold opening and closing. A tool set can contain several such cores, which may be actuated using hydraulic pistons, electric motors, or mechanical motions.

Molding Machines. The three most common molding machines are reciprocating screw, hydraulic plunger, and pneumatic. Table 3 summarizes the attributes of a few such molding machines. Pneumatic machines simply apply gas pressure to move heated feedstock into the mold. They are inexpensive and effective for small components where internal flaws are not objectionable. However, voids form because the feedstock is under low pressure that fails to compensate for shrinkage on cooling. In a hydraulic molding machine, a plunger rams heated feedstock into the mold. Excess pressure is generated to compensate for the volume contraction normally encountered by feedstock cooling. This pressurization is important to forming defect-free compacts, but the control systems usually are not suitable for forming complicated shapes.

Table 3 Sample attributes of powder injection molding machines

Attribute

Low pressure

Moderate tonnage

Intermediate tonnage

High tonnage

Type

Pneumatic

Reciprocating screw

Reciprocating screw

Reciprocating screw

Clamping force, ton

0.7

25

75

200

Clamp type

Pneumatic

Hydraulic

Toggle

Hydraulic

Platen size, mm

480

260

530

900

Drive motor, kW

2

5.5

15

30

Screw diameter, mm

None

18

28

50

Injection volume, cm3

3000

35

70

350

Fill rate, cm3/s

0.5

46

66

100

Injection stroke, cm

9

11

14

Plastication capacity, kg/h

12

20

40

Screw speed, mm/s

110

300

110

Molding pressure, MPa

0.5

190

230

200

Shortest cycle time, s

4.8

2

1.2

3.6

Maximum temperature, °C

150

200

200

200

Control

Open-loop

Adaptive

Closed-loop

Adaptive

High-volume injection molding uses a horizontal reciprocating screw located inside a heated barrel. The screw is designed to compress and transport the feedstock to the die, and it becomes a plunger during mold filling. Figure 6 is a typical layout of a horizontal machine and Fig. 7 is a picture of a contemporary molding machine with vision system, robot, process controller, and data acquisition computer. The tooling is clamped in the center of the machine. The granulated or pelletized powder-binder feedstock is placed in the hopper for metering into the injection barrel. This is the beginning of the molding operation.

Die and

Die and

Collection Conlrols Hydraulics bin

Fig. 6 Overview of a horizontal injection molding machine and key components

Fig. 6 Overview of a horizontal injection molding machine and key components

Fig. 7 Picture of a research injection molding machine with attached vision system, robot, data acquisition computer, and control computer

The heart of molding lies with the motions of the reciprocating screw. This needs to be wear resistant to withstand abrasion by the particles. It has a helical pitch, the design of which is adjusted for the viscosity of the feedstock, but generally it consists of gradual section changes along its length. Screw rotation is controlled via a hydraulic motor and heat is supplied by external heaters on the barrel.

An important role of the screw rotation is to de-air the feedstock and prepare the next charge for injection. This is termed metering, where the screw rotates to pressurize feedstock to the nozzle. During metering the screw acts as a mixer to ensure uniform powder-binder distribution and uniform heating. The screw has a check ring behind the tip that acts as a valve that allows feedstock flow into the front of the barrel. This ring seals on the screw during mold filling and forces molten feedstock to flow into the die cavity through the nozzle at the end of the barrel. Effectively, the screw becomes a plunger during mold filling. Control of the screw rotation, displacement, and pressure is important to the fabrication of precise components by injection molding.

During the molding cycle, the screw initially rotates, compressing the feedstock. Then, during the injection step, the screw moves forward, closing the check ring, and the shot is injected into the mold. A closed-loop control system with a quick response servohydraulic valve is required for screw position and pressure control. During the fill stroke, the volume of feedstock injected into the mold depends on the cross-sectional area of the screw and on the stroke length. A typical screw diameter is 22 mm, but it might range from 15 to 40 mm (0.6 to 1.6 in.), depending on the machine capacity.

Feedstock flow in molding depends on the applied pressure and viscosity. For a cylindrical runner, the volumetric feedstock flow rate, Q, varies with the runner diameter, D, to the fourth power according to Poiseuille's equation:

m til

where P is the applied pressure on the feedstock, L is the runner length, and '/is the feedstock viscosity. The rate of mold filling is very sensitive to the injection pressure and runner diameter. Usually a high feedstock flow rate is needed in order to fill the die before the feedstock cools.

The barrel holds the rotating screw and is surrounded by heaters that control the mixture temperature. There are often multiple heater zones to ensure temperature control during mold filling. Because cold feedstock is abrasive, the first heater zone is geared to rapidly heat the feedstock, and subsequent zones might be at lower temperatures. The materials used in constructing the screw and barrel are critical to long service without contamination. Hard materials and close tolerances are required to reduce abrasive wear. Tool steels containing vanadium carbide and boride-clad tool steels prove most durable. Similarly, other machine components in the flow path can exhibit considerable wear, resulting in a loss of machine control.

All of the molding steps are controlled by a computer that might even correct errors during molding. Besides the molding machine, coordination is required with the peripheral operations needed in automation schemes. For example, robots are used to stage the compacts for debinding. Other automation features include conveyor systems, parts and tooling storage with automated retrieval, and continuous feedstock preparation and component debinding steps.

Powder Injection Molding

Randall M. German, The Pennsylvania State University

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