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PLASTICS AND THEIR MANUFACTURE

THERMOPLASTICS

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.

4.25

Foaming

4.26 4.27 Moulding compound

Prefoaming

Interim storage

Foam block

4.28

Polymer

Liquid moulding compound

Openable mould with smooth surface

Heated mould Mould

Foaming agent

Pressure application

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.

4.27

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

4.25

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,

4.28

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.

43

PLASTICS in Architecture and Construction  

This book seeks to fill that gap by providing an introduction to the structural and design possibilities of plastic. It introduces the mater...

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