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Hot-wire Foam Cutting

Except for rather small UAVs, we make use of lightweight closed-cell plastic foam to manufacture most of the aerodynamic surfaces of our aircraft. This foam is cheap to buy and available in a wide range of sizes, densities, and colors. We mainly use foam blocks designed for floor insulation in the UK. This has anisotropic properties, see Table 18.3. We cut the foam using hot-wire cutting machines, one of which was manufactured in-house, see Figure 18.7, and a much larger one that was commercially sourced, see Figure 18.8. These machines work by dragging a heated stainless steel wire held in tension through the foam, melting a path just a little wider than the wire as it travels. By using suitable stepper motors to control the end points of the wires, complex aerodynamic shapes can be cut.

When using a hot-wire cutter to make wing parts, it is advisable to design the airfoils to have blunt trailing edges or trailing edges with a definite radius. Sharp trailing edges are difficult to cut, as the hot wire tends to melt fine edges. Holes can be cut for ribs and spars, but they

Table 18.3 Typical properties of closed-cell polyurethane floor insulation foam.

Property

Units

Value

Young’s modulus

MPa

3.64

Tensile strength

kPa

485

Shear modulus

MPa

10.1

Shear strength

kPa

391

Compressive modulus in rise direction

MPa

21.3

Compressive strength in rise direction

kPa

434

Compressive modulus in in-plane direction

MPa

10.4

Compressive strength in in-plane direction

kPa

241

Poisson’s ratio

и 0.2

Mean bending strength - crushing failure (as per BS EN 12089:2013)

kPa

789.4

Mean bending strength - shearing failure (as per BS EN 12089:2013)

kPa

502.5

Density

g/cm3

0.025

Poisson’s ratio

0.4

Direction refers to the orientation during block manufacture.

In-house manufactured hot-wire foam cutting machine. This cuts blocks of foam up to 1400mm X 590mm X 320mm

Figure 18.7 In-house manufactured hot-wire foam cutting machine. This cuts blocks of foam up to 1400mm X 590mm X 320mm.

Large hot-wire foam cutting machine

Figure 18.8 Large hot-wire foam cutting machine.

must be defined so that the wire will cut in from an outer surface. Moreover, creating hollow airfoil sections can be difficult for very thin wings, as the walls of the resulting airfoil sections can be too thin, causing the wire to burn through the section. Foam cutters are also able to cut tapered geometry using different end profiles, but care has to be taken to ensure that the sections at either end line up correctly and are traversed at the correct speeds to make sure that the generated conics are as desired. There are also limitations on the maximum taper that can be cut using the machines. High levels of taper result in the wire being almost stationary at one end during manufacture, which tends to burn holes in the foam. By cutting the blocks in two orthogonal directions before removing the finished parts, all sorts of shapes can be produced. Figure 18.9 shows some typical foam wing parts and the parent blocks.

Hot-wire-cut foam wing parts

Figure 18.9 Hot-wire-cut foam wing parts: (Left) The original material blocks with and without cores removed; (right) with FDM-manufactured ABS joining parts.

 
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