Extrusion Laboratory

Report

Abstract

In this experiment, lead billets (which were in the warm working states because of its soft

metal characteristics) were extruded with the observed variables being extrusion

direction, die geometry, and use of lubrication. The objectives of the experiment

included determining the significance of each variable in the extrusion process, both

individually and in combination with each other. In addition, the presence of dead metal

zones and piping were examined. Eight trials were run with each combination of the

variables, and data of the force required and displacement during extrusion was captured

using a dynamometer and LVDT connected to a data acquisition computer. Two level

factorials, a cube plot, and a probability curve were used to determine significance of the

effects. From our data analysis, it was determined that using a reverse extrusion process

will require less force than a comparable direct extrusion process, due to less friction of

the billet in the chamber. A square die, which reduces the surface area compared to

rounded dies, will also reduce the break through force required for the extrusion.

Lubrication reduces overall friction and therefore reduces the force necessary for the

extrusion process, although a greater change in force is observed when using lubrication

with direct extrusion. The use of a square die will generally result in larger dead metal

zones, and piping will also occur when the extrusion uses a square die. Piping and dead

metal zones are usually dependant upon die geometry, but independent of the direction of

extrusion.

Table of Contents

Title Page 1 Abstract 2 Table of Contents 3 Equipment Used 4 Analysis 6 Conclusions 15 Appendix 16

Equipment Used

Prior to the extrusion itself, red layout dye was painted on the inside split of the billets and a height gauge was used to scratch lines every 0.100 inches both along the axis of the split billets, and perpendicular to the axis. This would allow a visualization of the flow characteristics after the extrusion.

Two different extrusion dies were used in the experiment, one was a direct extruder, and the other was a reverse extruder. The direct extrusion die stays stationary while the force is applied, while in the reverse extrusion process, the force is applied to the die, which applies pressure to the billet to form the extrusion, as seen in Figure 1 below.

The machine used to force the lead through the die was an American Machine and Metals, Inc. tensile test machine; model M20101-R, serial number R-44237. The hydraulic pressure created by the machine placed a force, in this case, compressing two large plates together. When the extruding dies and billet setup were placed between the plates and the machine was engaged, the extrusion was performed.

Direct Extrusion

F

FF

Reverse Extrusion

Die (square)

Die (square)

F

Figure 1. – Extrusion Processes

Figure 2. – Extrusion Data Gathering Schematic

A linearly variable differential transducer (LVDT) measured the displacement from the initial position using a coil transformer. The input for the LVDT was the displacement of the extrusion equipment (the length of the extrusion, the basis for the displacement values in Figure 3 and all charts) in inches, and the output was millivolts.

The Burr-Brown PCI Systems ordinary amplifier took the output of the LVDT in millivolts, and amplified it to volts. This made the signal more readable and usable by the analog to digital converter, which followed the ordinary amp.

A Kistler 9071dynamometer, serial # 301891, was used to measure the amount of force exerted on the billet as it was extruded using a piezoelectric crystal to convert force [lbf] to an electric charge [pico-Coulombs, pC]. Different stages of the extrusion process resulted in distinct forces and changes in forces, which were accurately measured by the dynamometer.

A Dual Mode Amplifier Kistler, model 5004 and serial # 170005, charge amplifier amplified the signal of the dynamometer. The input is a charge (in pico-coulombs) and is amplified as a voltage [volts] before being sent to the analog to digital converter.

Receiving analog voltage signals from the ordinary and charge amplifiers, the analog to digital converter translated the analog signals to a system that could be read and understood by the computer software. Digital signals, also known as binary data, or bits were outputted by the A/D converter and inputted by the Labview 7.1 software on the computer. Labview takes the bits of data and records and plots the data in tables in computer units (See Appendix for data).

Billet

Computer with Labview 7.1 Software

ORDINARY AMP

CHARGE AMP

A/D CONVERTER IN COMPUTER

[pC]

[V][V]

[digital signal]

LVDT

Tensile Test Machine

[mV]

Extrusion

Dynamometer

Analysis

Figure 3. – Initial Extrusion Plot1. Figure 3 shows the force versus displacement plot for the initial conditions of

the experiment (forward extrusion with a square die without using any lubrication). Data points were recorded every four seconds. As can be seen in the plot, four distinct regions exist, corresponding to three stages of the extrusion process.

Area I represents the large force needed to overcome the static flow restriction (flow stress) of the metal prior to extrusion. At the last point in Area I (same as the first point in Area II), the maximum force is seen. This is the point at which the static flow restriction has been overcome and the extrusion actually begins, and is known as the ‘breakthrough force.’

The extrusion begins to form in Area II. There is a large change in the slope of the plot in Area II. Instead of increasing forces like in Area I, the slope decreases throughout Area II. Two reasons exist for this phenomenon. The first is that the static flow restriction (breakthrough force) has been overcome, so the force no longer rises. The rationale

S.O. #1 (Forward, Square, No Lube)

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of the decrease of the force, rather than a constant force, is that the reduction of the length of the billet in the chamber (as the test proceeds) reduces the friction, and therefore, the overall force decreases as the displacement increases (until Area III). The horizontal line at approximately 7100 lbf represents the separation of force due to deformation and friction. The force below the line represents the force due to deformation and the area above the line represents force due to friction. When doing indirect extrusion, the area above line is greatly reduced (See Appendix for other charts).

At Area III, the force required for displacement once again increases. This is due to the small amount of billet left in the chamber. The vast majority of the billet has already been extruded, leaving only the dead metal zone in the chamber. The force required to compress this dead metal zone is quite high due to its already compact state, and the sharp angle it would need to take to leave the chamber with the extrusion. The extrusion is usually stopped at or before Area III because the majority of extrusion has been completed and the remaining extruded part that is comes out in Area III usually has imperfections in its structure.

There is usually a region before Area I in which the slope of the curve is less than the slope of Area I. The above graph does not illustrate this, but most of the graphs in the appendix do. This region is associated with the billet taking shape of the chamber and the air being pushed out.

Using the breakthrough forces of the eight trials, the contrasts and effects could be found. A sample calculation of the contrast and effect can be found in the appendix.

Figure 4. – Effects Statistical Table

2. From the probability plot (See Appendix), it can be seen that the direction (D), geometry (G), lubrication (L), interaction of direction and lubrication (DL) are significant. This is known because their respective effects are outside of the 95% range on the probability plot, which corresponds to ± 400. Those numbers that lie between plus and minus 400 are not considered significant for our experiment.

Using reverse extrusion, rather than direct (forward) extrusion, will require an average of 1397 lbf less of force than using the direct extrusion technique. The reason for this is the difference in the process fundamentals of the two techniques. When direct extrusion is used, the entire surface area of the billet is always moving against the walls of the chamber. When identical settings are used for indirect extrusion, it can be seen that the billet does not move along the chamber wall, but on the portion of the billet moving through the die is experiencing friction with the die or chamber. Moreover, it is not just that the surface area of the billet is touching the walls of the chamber and moving with respect to them, but when an immense force (several thousand pounds) is pushing against the billet to extrude it, some of this force is transferred to a force pushing out against the walls, further increasing the friction force.

When geometry is considered, it can be seen that when a rounded rather than square die is used, the required force is on average 483 lbf higher. This may not seem logical at first glance, as a rounded die is smoother and would seem to cause less friction. However, the main factor to consider when comparing the geometry is the surface area of the die. A square die has linear walls; a rounded die has linear walls for most of the length, but curves out near the end, increasing surface area, as seen in Figure 5. More surface area leads to more friction, and therefore higher required forces for extrusion.

Square die

Round die – more surface area

Figure 5 – Die Geometry

Lubrication was the most significant factor in this experiment, which was evident especially when looking at the break through force of forward extrusion. When using lubrication, there was an average of 1004 lbf less force needed. This correlation is fairly obvious. If lubrication is added to the exterior of the billet prior to extrusion, friction between the billet and the die and walls will be reduced, and therefore less force will be needed to extrude the metal.

The interaction of extrusion direction and the use of lubrication were also significant. This is logical, as the two were both quite significant as independent main effects. Since the billet does not move in the indirect extrusion process, the frictional forces on the wall do not need to be overcome. However, when we did use lubrication during the reverse extrusion, we did see a reduction of 303 lbf in break trough force. Since lubrication was added to the outside of the billet, and force was applied to the billet by the die, the only place the lubrication could leave the chamber was through the die. The break through force was less but not nearly as significant as it was in direct extrusion. When we used lubrication on forward extrusion, the required forces were 1705 lbf less than without using lubrication. The use of lubrication has a greater effect on the direct extrusion process because the frictional force caused by the billet pressing against the chamber wall is greatly reduced.

4.

Figure 6. – Cube Plot of Effects

Table 1- Break Through Forces

  S.O.1 S.O.2 S.O.3 S.O.4 S.O.5 S.O.6 S.O.7 S.O.8Break Through Force

(lbs)Lab Estimates 9312 8076 9956 8398 7646 7729 8213 8140

Breakthrough Force (lbf)

D

G

L -

+

+

+

9312 8076

9956 8398

7646 7729

8140 8213

5.

Figure 7- Extruded Samples

Upon inspection of the velocity profiles of the extrusions, several similarities and differences arise. To a certain extent, all of the profiles look similar in that the center has been extruded more than the outside of the billet. This is due to the fact that friction is present in the walls of the die and the chamber, and also the fact that the center of the billet merely has to be extruded in a straight line out the die, while material further from the center has to be forced toward the center before being extruded. All of the extrusions show this trend. A difference is seen between the samples that used lubrication and those that did not. Those that used lubrication (samples 5-8) have less severe ‘bowing’ of the transverse scratches in the velocity profile than those samples that did not use lubrication (Samples 1-4)

Another difference seen in the velocity profiles is that when the billet is extruded in reverse (even numbered samples), the profiles seem to be smoother and the extrusion longer than the directly (forward) extruded samples of equivalent conditions. The smoothness of the profile represents less internal friction and stresses twisting and pushing the metal, which allows it to be extruded in a state more similar its original state.

Also visible is the contrast of the velocity profiles of the square die extrusions and the rounded die extrusions. The square die extrusions generally have a smoother profile than the rounded die extrusions, once again due to reduced friction (and reduce force applied) due to reduced surface area. When looking at the 8 samples, the two smoothest profiles are sample 2 and sample 6, the extrusions that used reverse extrusion and square dies, which has the least surface area and friction of all the samples.

Table 2 – Dead Metal Zones and Piping Effects

  S.O.1 S.O.2 S.O.3 S.O.4 S.O.5 S.O.6 S.O.7 S.O.8

Order 1 2 3 4 5 6 7 8

Direction Forward Reverse Forward Reverse Forward Reverse Forward Reverse

Die Geometry Square Square Round Round Square Square Round Round

Lubrication NO NO NO NO YES YES YES YESDead Metal

Zone Size L M M N L M M N

Amount of Piping M M N N L L N N

6. The dead metal zones are located next to the die, but on the outside edge of the chamber (See Figure 8). From Table 2, it can be seen that generally the square dies will produce larger dead metal zones. Samples 1 and 5 all had large dead metal zones, and samples 2 and 6 had moderate side dead metal zones. All of these samples used square dies. This agrees with the theory and mechanics of the die. For metal along the outer edge of the chamber, it has to make its way to the center of the chamber, and then make a 90 degree turn to be extruded. This is difficult to do, so a dead metal zone results. For rounded dies, the metal along the wall of the chamber has a smoother path to be extruded (no sharp bends), so usually minimal or no dead metal zones result. When using a round die and a forward extrusion process, frictional forces are present between the billet and the chamber wall, so it is harder for the material on the outside of the billet to make it through the die. Dead metal zones were present in this extrusion, but not to the extent when using a square die.

Figure 8- Dead Metal Zones

7. Piping, or a pipe defect, is where the far edge of the billet begins to be pulled into the center near the end of the extrusion process (See Figure 9). It is fairly easy to tell when piping has occurred because the top (not yet extruded portion) of the billet is not flat as normal, but has formed a concave shape following the extruded portion. The occurrence of piping occurred primarily when extrusion was with a square die, with either lubrication or no lubrication. Theory would suggest that using square dies would

lead to more piping in the samples (which agrees with the presented results), as the sharp angle needed to be taken by the metal to be extruded will cause the far edge of the billet to be pulled toward the center. At this point it is easier for the center of the top of the billet to move in the direction of the extruded portion (causing piping) than for the outside of the billet to translate to the center of the chamber and make a sharp turn forming the extruded portion. The rounded die did not form piping because a smoother path can be taken to the exit of the die (it is easier for metal near the outside of the billet to move to form the extruded portion, even though there is more friction with the increased surface area of the rounded die).

Figure 9- Piping Defect

Conclusion

By looking at breakthrough forces of the trials, several conclusions can be drawn. Using a reverse extrusion process will require less force than a comparable direct extrusion process, due to less friction of the billet in the chamber. Square dies reduce the surface area versus rounded dies, and therefore also reduce the force required for extrusion. Lubrication reduces overall friction and therefore reduces the force necessary for the extrusion process. The use of a square die will generally result in larger dead metal zones, and piping will occur when extrusion is done with a square die.

One possible source of error in this experiment is the fact that the tensile test machine may not have been operated in exactly the same manner for all specimens. The knob was supposed to be placed at a scratch representing 3.5 on the machine, but some groups may have been slightly higher or lower, resulting in faster or slower extrusion, which will affect the strain rate of the lead. We also saw metal extruded between the chamber wall and the ram, which would lead to a lower recorded break through force than actual.

Suggestion for future labs would be to combine the data from all of the labs so that there is more data. The analysis of one sections data is not enough to fully conclude what should be correct. If one group in our section did the extrusion wrong (e.g. had the dial at 4.5 instead of 3.5), the data for our section would be incorrect but since we only have a sample size of 1, we have to assume that is correct. The sample sizes of each extrusion process should be larger. The operator should have less control over the process. The dial should be automated and should only have to press one button to start the test. By having a manual dial, and the process of pushing two buttons at once, there are at least two more variables being added to each extrusion. Since we had 8 different samples, there were most likely 8 different operators, which means 8 more variables and possible sources of error. However, we assumed that none of these variable existed, which is not entirely correct, and could have led to inaccurate data by fault of no one.

Appendix

Sample Calculations:

Contrast of D: (-9312+8076-9956+8398-7646+7729-8213+8140) = -2784

Effects of D:N= 8 (number of Samples)(Contrast of D)/(N/2) = -696/(8/2) = -696

P(i)= 100*(i-.5)/Ni = ascending order number associated with each effect (lowest to highest)

OrderEffects Value P(I)

L -1003.5 7.14D -696 21.43

DG -119.5 35.71GL 3 50

DGL 41.5 64.29G 486 78.6DL 701 92.86

S.O. #2 (Reverse, Square, No Lube)

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S.O. #8 (Reverse, Round, Lube)

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