Eq 2

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:

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