Volumes of production

Most suited to batch production, rather than high-volume mass-production.

Unit price vs. capital investment

Low capital investment and moderate unit cost when compared with similar methods (see below).

Speed

Dependent upon the shape or product required—for example, in the case of a batch of wall panels, the skins and frames can be preassembled. The foam injection is a quick process, although the assembly can remain clamped in its retaining jig for up to eight hours to cure the foam. Output would therefore be accelerated with the use of multiple retaining jigs.

Types/complexity of shape

Panels can be made either flat on one side and undulating on the other, or with two undulating surfaces that mirror each other. Because the plywood skins can be bent before the foam is injected, the process does not need to be restricted to linear or parallel boards. Solid inserts can be installed during production for “hard points,” for example to enable the attachment of legs or fittings, or to join sections together.

Scale

The scale is restricted by predetermined plywood stock sheet sizes. It has the potential for use in furniture, but there is almost certainly the possibility of a myriad of sculptural and spatial applications for architectural and interior design purposes.

Tolerances

Working with a natural material under pressure to freely form “unnatural” three-dimensional shapes is not an exact process. Initially, there was difficulty in predicting the outcome of the curves, and the results were sometimes unexpected. However, when pressure points have been positioned and temperature and foam quantities are constant, visually similar results have been achieved.

Relevant materials

Polyurethane expanding foam (there are variations with fire-retardant additives and versions without free icocyanates). Birch-faced aero-ply ranging in thickness from 1/30 to 1/8 inch.

Similar methods

Deep three-dimensional forming in plywood (p.83) and foam molding into plywood shells (p.181).

Sustainability issues

Plywood is manufactured from very thin slices of natural wood, a renewable and sustainable source. The expansive nature of foam, which creates hollow air-filled beads, also means that a small amount of material is consumed in relation to its overall size. The foam-injection phase limits waste as the flow is stopped when the shell is filled to the desired shape and size. However, pressure buildup sometimes causes the plywood to explode, creating an excess of material that cannot be reworked back into the process.

Further information

www.curvycomposites.co.uk

Forging

with open- and closed-die (drop), press, and upset forging

Product	raw, semifinished spanner
Materials	steel
Manufacturer	original manufacturer undisclosed, but finished by King Dick Tools in the UK
Country	Germany and UK

This unfinished ring spanner is the result of the closed-die forging process and is seen here before it is finished by drilling with a pilot hole and broached with a serrated tool to obtain the twelve points.

Forging is a major process in metal forming, sometimes utilizing architectural-scale machines for pounding metals into shape. It is not only a method of forming metal, it also produces a change in its physical properties, resulting in enhanced strength and ductility. In its simplest manual form—open-die forging—it involves a chunk of metal being heated to just above its recrystallization temperature and then being formed into shape by repeated blows with a hammer, as performed by a traditional blacksmith. Movement of the work piece is the key to this method. In its more industrial incarnation, it incorporates several variations, including hot and cold forgings.

Closed-die (or drop) forging involves a very similar process to that of the open-die method described above. In this instance, however, the shaped hammer is held in a machine and repeatedly dropped onto the metal, which sits in a shaped die. The shape of the two parts determines the formed shape. Drop forging can be either hot or cold. The hot form involves the blank metal being heated, and results in stronger components due to the realignment of the grain.

Press forging involves a heated bar being slowly squeezed between two rollers, which form the metal as it is fed through. Upset forging is used for shaping the ends of the rods by compressing them as they are held in the die. Typical products produced by upset forging are nails or bolts.

–  One of the main reasons for choosing forging is for the control it gives over the grain structure in the metal. It allows for the grain flow to be aligned to specific shapes, making the part stronger and more ductile.

–  No gaps or voids occur in the metal, as they can in die casting (see p.219) and sand casting (see p.228).

–  Less waste than with runners and sprues.

–  Forged parts often require machining to remove the excess metal that is left when the two halves of the die are brought together.