Replicas

Fig. 7 An outline of the soft tooling process

Recent developments directly use a CAD file stored in the computer to be used as the master model (Ref 14). Using the master model, a soft tool is fabricated from procedures such as casting, lamination, and computer-aided buildup.* The master is separated from the cured tool and is used to make replicas by a forming technique of choice.

Several rapid-prototyping techniques for fabrication of plastic objects have been covered elsewhere in this Volume. Casting is briefly described in this section. Most thermosetting materials used in casting are also excellent adhesives. Therefore, a release or parting agent must be applied to the master model before application of the tooling resin. Particulate fillers such as aluminum, silicon carbide, and sand are often added to the resin in order to increase thermal conductivity, improve surface hardness, and reduce costs, respectively. The high viscosity of filled suspensions makes it important to ensure the removal of air bubbles entrapped during the mixing or pouring stages (Ref 15). In order to fabricate large tools, the soft tool can be cast onto a core of materials such as stainless steel, aluminum, kirksite, or resin-fiber glass laminates in a procedure known as backing (Ref 9). More recently, an additional stage known as fronting has been developed in order to create a thin metal film on the surface of the soft tool in order to improve thermal properties and hardness (Ref 16).

Attributes and Applications. Complex shapes can be cast or laminated to final dimensions using soft-tooling techniques. Lower costs can be achieved compared to conventional tooling because fabrication times are short and expensive machining is often unnecessary. The time for fabrication is on the order of days as opposed to weeks or months. There are two main limitations of soft tooling. The poorer thermal properties result in an increase in molding cycle times and impose restrictions on the maximum processing temperature. The inferior mechanical properties, in general, reduce the lifetime of the tooling to the order of 10 to 1000 parts. Table 3 summarizes the comparisons between the attributes of soft tooling and conventional hard tooling.

Table 3 A Comparison of attributes of metals and polymers for tooling

Comparison variable

Result (for polymers)

Capital investment

Favorable

Labor cost

Favorable

Lead time

Favorable

Large tool cost

Favorable

Tool weight

Favorable

Repair and design changes

Favorable

Short runs and prototypes

Favorable

Ease of duplication

Favorable

Corrosion resistance

Favorable

Surface finish

Favorable

Skilled supervision

Approximately equal

Material cost

Approximately equal

Thermal conductivity

Unfavorable

High-temperature strength

Unfavorable

Long production runs

Unfavorable

Source: Ref 9

From the above discussion, it is evident that polymers are most useful for the quick and inexpensive fabrication of tooling for low-volume prototyping runs. Further, the tooling is useful when a high level of surface finish is required. Soft tooling is applicable to binder-assisted P/M forming techniques such as powder injection molding and LPM.

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