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“Handle with care” is a particularly relevant caution for light metal extruders, since scrap loss due to damage to the product after it leaves the die is an avoidable cost that extruders need no longer accept.

Why use felt?  Felt is formed by matting or pressing fibres randomly together.  Its extraordinary strength comes from the essential cohesion of the fibres themselves, rather than from weaving them in only two directions.  The self-healing property of felt considerably extends its useful life.  The weakness of a woven fabric, for example, is that if a thread breaks, the fabric will usually begin to disintegrate from that point.  A small cut in felt, however is soon integrated into the felt surface.

Refractory felt roller covers, pads and belts specifically developed for aluminum extruders by Telon-Yida, and available from Castool, feature quality, strength, and durability never before achieved on the runout.

Telon-Yida felts are a proven investment in both quality and productivity with a worthwhile return.  They are an investment that is now very affordable.

Castool is pleased to announce very impressive pricing of all Telon-Yida refractory felt products.

Contact us for an immediate quotation.

What is Coring?

Coring is an extrusion defect involving an internal discontinuity, relating to reverse inflow of billet surface into the interior (or “core”) of an extrusion shape.

See Figure below – the effect can be extremely distinct and can significantly influence integrity of the material and in-service performance .

During extrusion, the billet surface and near surface layers can flow either onto the extrusion surface or into the extrusion interior.  Billet surface consists of the heavily oxidized as-cast surface, segregated regions near the billet surface which contain material of higher alloy composition and surface casting defects such as cold shuts and blister.  In addition, contaminants picked up on the billet surface during transportation through and from the billet heater into the press, may also contribute to coring and its severity.

Before focusing on the coring type defect resulting from billet surface flow into an extrusion interior, let’s first consider the difference between “forward” and “reverse”  inflow.

Illustrated below are sections through long butts showing Type 1 “forward” and Type 2 “reverse” inflow of billet surface.

Type 2 reverse flow occurs toward the end of extrusion and involves accumulation of some of the billet surface material.  This accumulation collects toward the back of the billet and if sufficient of the billet is extruded, the accumulation of billet surface will flow into the extrusion interior.  This Type 2 flow effect creates coring -  or back end defect .

At what %age of billet extrusion does coring start to flow into the extrusion, and what if any process parameters influence it?  Can it be controlled?

The following figure shows the onset of Type 2 flow as measured when two different process parameters are changed.  The vertical axis shows the amount of billet extruded when Type 2 flow started.  The first parameter relates to the die aperture, or the die “edge distance” -i.e. the distance a port or pocket extremity, or in the case of flat face dies the actual aperture itself, is located from the container wall.  This parameter is more critical for Type 1 “forward” inflow, and the lines on the chart below, show little difference in the onset of coring for differing die edge distances.  The second parameter is shown on the horizontal axis and relates to the billet surface depth – i.e. the depth of the segregated layer, including any cast surface finish irregularities.   This is expressed as a percentage of container diameter.

In reality, billet surface layer thickness also includes the thickness of alloy skin left remaining in the container after any billet is extruded.  The container skin thickness is a function of the dummy block diameter in relation to the container diameter, and how well the dummy block performs (expands under extrusion pressure and relaxes on release of pressure).  In addition, a container will expand somewhat under extrusion pressure.  The net effect of container liner expansion and dummy block expansion equals the skin thickness (or skull) left inside the container.  The skull thickness should be added to the cast billet segregation depth (plus cast surface effects) to reach an effective billet surface layer thickness.   Between 1.5 and 2% of container diameter are typical surface layer thicknesses in commercial extrusion, but obviously this can be reduced with improved billet quality and reduced container skin thickness.

From the figure above it can be seen that the onset of coring at surface layer thickness levels of around 1.5 -2%, occurs after 85-87% of the billet is extruded.  This equates to a 13-15% butt length, where butt length is measured as a percentage of crushed billet length, not original billet length.

What other process conditions influence coring?

Obviously, billet quality is of prime importance, and reducing the depth of segregation layers, and improving cast billet quality, reduces the effective surface layer thickness.

Reducing the difference between expanded dummy block diameter and container liner can help also, by reducing the skin (or skull) thickness left in the container.  This relates to dummy block design and performance, but also to the container design itself, and how effective the container mantel is at controlling liner deflection during extrusion.

Increasing container temperature can help.  This helps discourage the reverse type flow of billet skin, but by nature encourages forward Type 1 flow.  As a consequence, extrusion surface is likely to worsen with this practice.  A similar effect can be achieved by using colder dummy blocks.  Older practices where loose dummy blocks were used and often 2 or 3 blocks in rotation were beneficial in delaying the onset of coring.

Some work has been done relating extruded shape effects (shape factor) to coring, but the results are inconclusive.

Summary:

If elimination of coring type defect is necessary, then an extruder may have to use butt lengths around 13-15% of the crushed billet length are necessary.  Some gains can be realized by tuning the expanded dummy block design to be a closer fit the container liner.  Use of good quality cast billet with minimal inverse segregation depth and good cast surface quality is beneficial.

(Long butt lengths can create problems with container return hydraulics, and in these instances it may be necessary for the extruder to extrude to a shorter butt, and consider scrapping sufficient from the back end of each extruded length to remove any coring defect.  Note: As coring tends to apply to structural products often in the harder 6061/6082 type alloys, extruding additional back end scrap can be advantageous in ensuring all “coring-free” material is pushed into the quench and does not suffer tensile property problems associated with quench delay effects during dead cycles.)

Bill Dixon

QED Extrusion Developments Inc.

3/14/2010.

In theory, for any profile and alloy being run on a particular press, there could be an operating formula specifying all critical temperatures and speeds, that would result in optimum productivity.  In fact, because light metal extrusion is a system in which components interact and temperatures constantly change, the number of permutations and combinations that may occur at any point in time is virtually infinite, and in any event critival temperatures and speeds could never be accurately determined simultaneously.

Recent advances in the technology of both pyrometry and measurement, however, have made it possible for Castool to develop a new monitoring method aimed at optimizing key areas of the extrusion production process.  The new Castool Visual Optimizing Method allows the extruder to tell at a glance if all critical temperatures and speeds are at or close to the desired levels in real time, as it is happening.  Also, if at a particular point in time the extruder wants to record the combination of temperatures and speeds that is producing a new level of productivity to be used as a base for further improvement on the next run,  a “Freeze-Frame” can be taken to record all the information being monitored at that precise instant,

Here’s how the Castool Visual Optimizing Method works.

At the operators post, above the press a large back-lit monitor screen will show the actual as well as the desired temperature or speed at each point being monitored.  If the actual is on target, it will appear in green.  If not, it will be shown in red. The operator will then be able to tell at a glance how close he is to target at each point being monitored, and take whatever action is required to bring the system back on track.  The Castool Visual Optimizing method is customized to fit each customer’s needs, and budget, but it will typically include ram speed, dead cycle time, billet temperature, container liner temperature top and bottom at both entrance and exit, die temperature, profile exit temperature, quench rate, and also the temperature status of the die in each single cell die oven.

The Castool Visual Optimizing Method makes it possible for any experienced and capable extruder to make every repeat run closer to optimum productivity.

A large back-lit monitor screen, prominently mounted above the operator’s post, allows the operator to determine at a glance which critical temperatures and speeds are currently operating at predetermined targets.  Also, by using the “Freeze Frame” feature, all actual temperatures and speeds being monitored, that occurred simultaneously at a specific point in time, are recorded.   These then act as benchmarks when preparing formulas, and setting targets to improve productivity.