Tooling Design

Traditionally, P/M tooling was designed on the basis of production experience. In simple parts, such as single-level class I and II parts, these determinations proved successful. As state-of-the-art materials and presses advanced to the production of complex, multilevel parts, the "cut-and-try" method of tool design became obsolete. The high cost of complex tooling and adapters, plus downtime to redesign and rebuild tooling, requires the partmaking system, including the press, to be carefully analyzed in terms of load, stress, and deflection.

Tooling layout is required to design a suitable set of tools and to determine the physical dimensions (length and thickness) of tooling members. A preliminary layout helps to determine fill, pressing, and ejection positions and to eliminate interference at these positions.

The die space drawing supplied with every compacting press, which usually starts with the ejection position, is the basis of the tooling assembly layout. Generally, tooling members are never closer than in the ejection position, which constitutes the minimum space available to contain all components and their adapters.

Die Design. Dies are commonly constructed by using inserts that are held in the die case by shrink fitting. The amount of interference between the insert and the die case depends on the inside and outside diameter of each member and on the compacting pressure used. The powder can be considered a fluid in a closed container that transmits the compacting pressure in all directions; therefore, the die must be designed as though it were a pressure vessel with internal pressure.

In actual practice, radial pressure on the die walls due to compacting rarely exceeds 50% of the compacting pressure. The interference fit of the die case and die insert should be such that the stress on the insert always remains in compression for round dies. However, for shaped dies such as gears, cams, and levers, the use of finite element analysis is the best method for accurately determining stress and deflection.

In P/M tooling, the die normally controls the outer peripheral shape and size of the piece part. Typically, it is constructed from materials such as tungsten carbide or high alloy tool steels, such as T15, D2, CPM-10V, or CPM-15V with high hardness and good wear resistance. Dies are usually constructed in one or more sections and compressed into a retaining ring made of a low-alloy steel, such as AISI 4340 or 6150.

Considerations in die design and material selection include initial tool cost, shear strength of the die material, and die shape. A large die may require tungsten carbide, which costs ten times as much as tool steel materials. Tungsten carbide may be the best material for a set of gear tools with a relatively steep helical angle. Sectional die construction may be required for specific shapes such as sharp corners or projections into the die cavity.

Die Wall Thickness. An exact calculation of the stress on die walls is almost impossible from a practical point of view because stress distributions in the compact are extremely complicated and include variables such as part shape, particle size distribution, and other factors that affect transmission of compressive stress in the lateral direction (Ref 2). The vertical axial load can exert a horizontal force after a certain degree of consolidation has been attained. For example, when a simple shape is compacted at 400 MPa, as much as 120 MPa pressure can be exerted radially against the die walls.

If for purposes of simplification, the internal pressure is considered strictly hydrostatic in nature and the confined material is an incompressible liquid, then the die wall thickness for a cylindrical die could be determined by using Lame's formula:

where S is the maximum allowable fiber stress for the material of the die, D is the outer diameter of the die, d is the compact diameter, and p is the radial stress acting on the die wall. This is a simplification because during metal powder compaction the pressure is not hydrostatic and the material is not incompressible. Initially, the powder is compressed with a consequent reduction in the vertical height of the space filled by the powder. The compressed material begins to resemble a solid after a certain degree of compaction has been reached.

Poisson's ratio is 0.3 for fully dense and isotropic steel. While this wrought form value cannot apply to powder metal, it is assumed to be applicable in the fully compacted condition. Thus, the Poisson's ratio is introduced into the previous equation, and the following modified Lame's formula is used for estimating the die wall thickness for metal powder compaction.

where fl= Poisson's ratio = 0.3. This formula, however, does not take into consideration that the internal pressure acting over the length of the compact is balanced by the strength of the die having a larger length. The formula does address the friction at the tooling/powder interfaces resulting in nonuniform pressure distribution in the compact.

Generally speaking, the formula produces more conservative results than are necessary. The interference fit between the shrink ring and the die insert should be such that the stress on the insert always remains compressive for round dies. For shaped dies such as those used for production of gears and cams, the use of finite element analysis is the best method for accurately determining the stress and deflection.

Core Rods. Basically, the core rod is an extension of the die that controls the inner peripheral shape and size of the piece part. Tungsten carbide and M2 or M4 high-speed steels are the most common materials used for core rods. Primary factors in materials selection include wear resistance and hardness, which enable the core rod to resist the high compressive force exerted during compaction and the abrasive action sustained during part ejection. Core rods >25 mm (1 in.) in diameter or area are held to a base by mechanical means, such as a screw, while smaller core rods are held by means of silver solder or braze.

Punches can perform the function of a die or a core rod and carry the full load of the compressive force required to compact the P/M part. Wear resistance and toughness are the most important factors in materials selection. The most commonly used materials are A2, D2, S7, and H13 tool steels. Dimensional control, especially in areas such as concentricity and hole-to-hole location, depends on the amount of clearance that can be maintained between the punches, die, and core rods. Clearance should be calculated for each specific range and size of part. It is important to note that thermal size changes occur during operation, primarily because of the friction created by stripping the compacted part and the speed of the pressing cycle.

Punch Component Stress. Compacting powder causes compressive stress in the punch. This stress must be below the yield strength of the punch material. Calculation of buckling stability should be made for long, thin-walled punches.

Figure 14 shows the effect of axial compressive force on a tubular punch. A tubular punch is subjected to internal pressure during compacting of multilevel parts. In this case, the resulting circumferential tensile stress in the punch wall should be calculated. If the stress and accompanying deflection is excessive, tooling clearances should be designed so that when the outer punch wall expands, it is supported by the die wall before the stress reaches the yield limit (Fig. 15).

Fig. 14 Effect of compressive stress on tubular punch

Fig. 15 Tensile stresses in a tubular punch during compacting. Large arrows indicate action of powder on walls of punch.

During ejection, the punch is subjected to compressive stresses by resisting the stripping action of the die and to tensile stresses from the stripping action of punch. These stresses normally are lower than compacting stresses. Components of the punch subjected to stress include the punch clamp ring and bolts, which should resist the ejection of the punch without permanent deformation. Punch adapters are subjected to bending loads that create a tensile stress around the center hole during compacting. This stress should not exceed the fatigue limit of the adapter material.

Tubular adapters must have sufficient cross-sectional area to withstand the pressing load without permanent deformation. A stepped core rod, or a core rod forming a blind hole, must not buckle during compacting. The base of the core rod must resist, without permanent deformation, whatever ejection loads are imposed on the core rod.

The core rod clamp ring and retaining bolts should be sized to withstand the ejection force on the core rod without permanent deformation. The core rod adapter generally is strong enough to resist both pressing and ejection loads, due to the size of the adapter when space is provided for clamp ring fasteners.

Deflection Analysis. Pressing of P/M parts at pressures >690 MPa (50 tsi) presents unique considerations for size and tolerance in multilevel parts. A variety of tool members should be utilized to establish proper fill ratios, and deflection and springback can occur. Deflection occurs because of the column loading effect on the compacting tools during the briquetting cycle. For column load consideration, the bottom section of the lower punch is considered fixed, while the top section or working end of the lower punch can be considered free to rotate. The amount of deflection on the tool member will be determined by the column slenderness ratio of the punch and the adapter. When the column load is released after the press goes through the bottom dead center compaction point, the deflected punches will return to their original lengths, if their elastic material property limits have not been exceeded. This return movement is generally called springback and can be deleterious to the green part, depending on the fragileness of the green part section geometry involved.

Deflection can be minimized by strengthening the various tool members through changes in physical size or shape and/or by changes in material selection. The most common method of minimizing deflection effects is to equalize deflection using tool members and adapters that are designed to match the deflection characteristics of the most critical member. The ability of the tool designer to find the proper balance is paramount for production of crack-free parts.

When designing tools for production of parts other than single-level class I or II parts, deflection analysis of the tooling, tooling adapters, and press is desirable. These members are essentially stiff springs, each with a different spring rate or modulus. When the compacting load is applied, the parts deflect. When the load is released, they return to their original length. If the press contains two or more separate lower punches, the total deflection of each punch and the supporting members must be the same. Otherwise, the compacted part will move with the punch that has the greatest total deflection, leaving a portion of the part unsupported. This condition is likely to cause cracking during part ejection.

A punch under load normally is in pure compression and therefore will follow Hooke's law. If the punch has varying cross-sectional areas, each length having the same cross-sectional area is calculated individually. The total punch compression is the sum of these calculations. For a long, thin-walled punch, local buckling of the punch wall under load should be investigated. Compression of punches and their supporting members may be calculated using the equation given in Fig. 16.

Y= — AE

Fig. 16 Punch compression. P is total punch load, L is length, Y is deflection, A is area of punch, and E is Young's modulus.

Adapter Bending. The adapter, on which the punch is mounted, usually is a flat plate with the punch and the load positioned at the center, around a hole through which either another punch or a core rod passes. This plate, if supported at the outer edge, is subjected to the pressing load around the center hole. Two forms of deflection--bending and shearing-occur in this area. Adapter deflection is linearly proportional to force. Calculated adapter stress should be compared with the allowable adapter material stress to evaluate design suitability.

Press Deflection. Like the tooling and support, the press is subject to deflection. This tendency is considerably less than that of punch compression or adapter bending, but it must be considered in total tool design. Data regarding press deflection should be obtained from the press manufacturer. Deflection is linearly proportional to the amount of force exerted:

0 0

Post a comment