Shape selection to optimise stiffness

A common way to increase the bending stiffness of products from melt-inflation processes is to use corrugations. It is not possible to create ribbed shapes, described in the next section, because there is no independent control of the shape of the inner surface of the melt. Corrugations are seen on thermoformed products, blow-moulded bottles and extruded pipes. 

For the shape shown, the stiffening factor is approximately 450. However, the section perpendicular to the pipe length has a low I the corrugations deform like a bellows when the pipe is bent. 

A twin-walled corrugated pipe is shown in Fig. 1.14. The external wall is formed by applying an internal air pressure to the molten tubular extrudate (Fig. 13.2), which expands against pairs of mould sections—these move with the cooling pipe for some distance, then return on a caterpillar track to the die. 

The second, interior wall is extruded, while the inner surface of outer wall is still molten, creating a twin-walled pipe. The weld line between the two layers is visible in Fig. 1.14. Such pipes have a high bending stiffness, both longitudinally and for soil loading. 

The easiest way to stiffen injection mouldings is to use ribs on the hidden surface (Fig. 13.3a). Ribs correspond to slots in the mould, so straight ribs are easier to machine (using a milling wheel) than those with corners. The rib thickness r should be less than 2/3 of the thickness t of the surface that they support, so they complete solidification first, thereby avoiding sink marks appearing on the product surface (Chapter 6). 

---