Batching and Materials Handling / Forming and Finishing

Designing Flexible Tools

June 1, 2011
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Dry bag isostatic presses are most effective when the proper tooling designs are considered.



Dry bag isostatic press.

Dry bag isostatic pressing techniques are being applied to a wider range of powders and components due to their ability to produce parts with consistent pressed density and reasonably tight tolerances. This can result in fewer sintering defects, which reduces costly machining in the sintered state and eliminates total scrap.

Many types of equipment are available, all of which require their own tooling design considerations. Outlined here is one procedure for designing molding bags to press high L-to-D ratio components in presses working on the top fill-bottom eject principle. The molding bag simply transmits the fluid pressure to the powder, with little or no loss, while also imparting the required shape to the final pressed part.

When the pressure is applied to the powder charge, the powder is compacted to some higher value of density. This density is termed the "pressed density," and its value depends on the powder characteristics and the level of applied pressure. If we expand this over a range of applied pressures, we can develop a characteristic curve such as the one shown in Figure 1.

Molding bags.

Powder Densities

We will consider two values of powder density. The first is the bulk density (Db), or free-pour density, which is measured by allowing powder to pour freely into a container of known volume and then weighing the powder. The goal is to find the lowest density of free powder, so care must be taken to prevent excessive "packing" into the container.

The second density is known as the tap density (Dt), which is obtained in the same way as bulk density except that the powder is vibrated or tamped into the container. This method enables more powder to be packed into the container, thus producing a higher density value.

The relationship is between the two is represented as:

Bulk Density (Db) < Tap Density (Dt)

Figure 1. Pressed density depends on the powder characteristics and the level of applied pressure.

VCR vs. Applied Pressure Curves

Isostatic pressing compacts powders to a higher density state. For a fixed mass, the density varies in proportion to the volume. The ratio of fill volume to pressed volume is called the volumetric compaction ratio (VCR). This ratio is far more useful than the often-used linear compaction ratio, which relates linear dimensions and is thus restricted to individual configurations of parts.

A VCR figure can be applied to any configuration of compact, and the necessary fill dimensions can be calculated. If we consider a new powder for which no practical pressing data is available, an estimate of the VCR will have to be made. Thus, it would be convenient if characteristic curves could be produced that would assist in this estimate.

For a fixed mass, the ratio of fill volume to pressed volume can also be expressed as the ratio of pressed density to fill density. The fill density can be either the bulk density or the tap density, or anywhere in between. Knowing that pressed density varies with pressing pressure, we can similarly produce characteristic curves of VCR vs. pressing pressure (see Figure 2).

Figure 2. Volumetric compaction ratio (VCR) vs. pressing pressure.

Control of Powder Properties

So far we have seen how parameters such as powder densities and fill density affect the VCR and this, in turn, affects the design of the molding bag dimensions. It is necessary to control these powder parameters if accurate control of pressed dimensions is required. The technology of powder preparation is outside the scope of this work, but the control of pressed dimensions from batch to batch can be improved by controlling properties such as particle distribution, bulk density and moisture levels.

Part Particulars

Before the VCR can be used, we must establish the section dimensions of the part to be pressed. Depending on the finished part tolerances, required various allowances must be added to the finished sintered section to establish the required as-pressed condition. These include:
  • Sintered machining allowance (allows for machining in the sintered state, which may be required if final tolerances are very tight)
  • Sintered shrinkage allowance (expected shrinkage associated with sintering of the part)
  • As-pressed machining allowance (an allowance for machining before the part is sintered). It can often make sintered machining unnecessary, but this depends on the consistency of the sintering process.
After the necessary allowances have been added, we reach the as-pressed section sizes. It is to these that the VCR will be applied.

Designing the Molding Bag

Variations in component design features are potentially endless, and the consideration of each of these variations cannot be given in the length of this article. When considering molding bag design features, however, we must consider the change of volume that is taking place when pressure is applied. Additional factors that need to be considered include:
  • Changes in pressed part section
  • Powder entry
  • Part extraction
  • Molding bag manufacturing technique
The most common technique used for manufacturing molding bags is generally gravity casting, typically using polyurethane materials. The manufacturing technique for molding bags should always be kept in mind during their design to ensure that the final quality of the molding bag is up to the required standard. It is often worthwhile to refer to the manufacturer for advice on best practices.

Press Capabilities

In dry bag isostatic pressing, the molding bag is isolated from the hydraulic iso-fluid, which helps enable desirable productivity rates. All press equipment has capability limitations that affect the range of parts that can be processed through any given press model. Early consultation with the chosen press supplier is recommended to ensure that a suitable press model is readily available.

For more information, contact Mark Thomason at Simac Ltd., a Gasbarre Products, Inc. company, 590 Division St., DuBois, PA 15801; email mthomason@gasbarre.com.

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