PLASTICS AND THEIR MANUFACTURE
4.25 Thermoplastic foam casting: through the addition of a foaming agent, a low-density integral skin foam is created. 4.26 Casting: in this pressureless process, the molten material is poured into an open mould and polymerised through the input of energy. 4.27 Foaming: a foaming agent is added to the mass that causes air bubbles to form.
4.28 Integral skin foam has a porous core and a smooth, cell-free surface.
4.26 4.27 Moulding compound
Liquid moulding compound
Openable mould with smooth surface
Heated mould Mould
This process is commonly used for the manufacture of PMMA GS, which as a
are primarily additive fabrication processes in which the three-dimensional body
semi-finished material is less prone to stress cracking than extruded materials
is created through the layer by layer application of a material. An exception is
and is therefore more suitable for reshaping and post-processing. Panel thick-
CNC milling (CNC = Computerised Numerical Control) in which a finished item is
nesses of between 2–250 mm can be poured, but the polymerisation process of
cut or milled from a massive block of material based on data from a digital model.
thick p anels and large items can sometimes take several weeks.
Because the mould used in conventional processes is replaced by a digital model, the production process is more flexible in terms of form, quantity and lead time.
Foaming Foams are materials with a low bulk density and a continuous cellu-
DDM techniques make it possible to realise prototypes and small quantities which
lar structure. Foams may be classified according to their structure as open-cell,
could previously only be produced using moulds specially developed for the respec-
closed-cell and mixed-cell foams. Bubbles of air form in the mass caused by foam-
tive item. The cost-effectiveness of the process depends on the quantity produced
ing agents added to the mixture which vaporise under heat or by gases that result
and should be weighed up against the costs of conventional production processes
from the polymerisation process. The process is divided into stages: prefoaming,
such as injection moulding. With DDM techniques, forms can be designed that do
interim storage and final foaming. In the final stage the prefoamed material is
not need to take de-moulding (mould removal) into account. The ability to directly
passed into a mould and shaped into its end form. Common methods include injec-
test and assess the physical prototype allows one to avoid construction errors in
tion moulding or in-mould skinning for items made of integral skin foam or extru-
the final production. The dimensions of the items that can be produced are still
sion for semi-finished products. Although theoretically all thermoplastics can be
quite limited, but in some cases elements measuring several metres have been
used for foaming, polystyrene and polyurethane are most commonly used.
produced. Depending on the production method used, the strength of the result-
Foams differ from the compact form of the source material primarily in terms of density and a correspondingly much reduced thermal transmittance. For this reason they are ideally suited for use as an insulation material. The stiffness of rigid foams can be even higher than the original material in its compact form. The combustibility of the original material remains unchanged.
ing items is generally lower than those produced using conventional thermoplastic processes. Rapid prototyping techniques make it possible to realise physical three-dimensional prototypes with complex geometries and cavities. The various techniques are divided into laser-based and non-laser-based approaches. Depending on the
Thermoplastic foam casting is a variant of injection moulding in which the addi-
method used, different plastics can be used in liquid or solid form, for example
tion of a foaming agent such as CO2 or nitrogen causes the production of so-called
ABS, polyamide, polycarbonate or photopolymers as well as elastic plastics, paper,
integral skin foam with a lower density. The expansion of the foam is limited by the
wax, ceramics or metal. The combination of different materials is also possible.
dimensions of the mould, which in turn is responsible for the smooth surface of
Some of the most widespread processes are:
the end product. The structure of the end product has a porous foam core and a
Stereolithography (STL) Layer for layer solidification of a liquid photopolymer
smooth cell-free surface.
using a laser beam. Suitable materials are thermosetting resins. Solid Ground Curing (SGC) Layer for layer solidification of a photopolymer using UV
Direct Digital Manufacturing The terms Direct Digital Manufacturing (DDM)
light. Suitable materials are thermosetting resins.
or generative fabrication are used generically for processes such as Rapid Proto-
Selective Laser Sintering, Laser Sintering (SLS, LS) Layer for layer localised sinter-
typing (RP), Rapid Tooling (RT) and Rapid Manufacturing (RM), all of which make it
ing of a source material in powder form. Suitable materials are thermoplastic plas-
possible to generate physical items from digital computer data. DDM techniques
tics, wax and metal.
Published on Oct 1, 2010
This book seeks to fill that gap by providing an introduction to the structural and design possibilities of plastic. It introduces the mater...