PLASTICS

The use of plastics in the rapid prototyping industry is, broadly speaking, broken into two categories: (1) reactive polymer systems such as the photopolymers used in stereolithography equipment, and (2) nonreactive polymers used, for example, in the sinter-based systems. Research and development in these areas have been equally effective in moving the materials closer to the property performances of engineering plastics.

In terms of market share, the North American rapid prototyping materials market is dominated by Ciba Geigy's epoxy line of resins (SR 5170, 5180, and 5190), and by DTM's powder systems, including nylon, wax, and polycarbonate.

Reactive Polymer Systems

The polymers of this phylum are thermoset in character: that is, the polymer chains grow and cross-link during curing. The cure system is initiated by energy input to the resin, and in rapid prototyping systems, most generally by light (usually ultraviolet). The basics of these reactions are amply covered in Johnson (1994) and Pang (1995) .

To understand the Japanese market, readers should be aware of the terminology in use. In rapid prototyping, resin systems are categorized in Japan as acrylate chemistry and epoxy chemistry types. The acrylate system uses a free radical method of polymerization, whereas the epoxy system uses a cationic method of polymerization. There are also thermally initiated polymerizations, principally in the epoxy chemistries.

The cationic systems (commercialized in North America by Ciba Geigy, DuPont, and Allied Signal), and the epoxy systems in particular, show high accuracy and low distortion performance, and have markedly better properties than the earlier resins. The initial commercialization of Ciba Geigy's resin 5170 was achieved in July 1993 (Pang 1995). Fig. 5.2 shows the relative performance of representatives of the current set of North American photopolymers.


Fig. 5.2. Modulus vs. strength plot of extant photopolymers in North American market (see also Table 5.1).

Table 5.1 shows some of the particulars of these same resins. The polystyrene row gives representative values for the material of comparison used in the North American market. ABS is the representative material in the Japanese market.

In this gross summary of properties, some of the tensile strength and modulus properties of photopolymers are now approaching those of some commodity materials. This is born out by inspection of other properties as well.

Table 5.1
Modulus and Strength Values, Extant Photopolymers in N. American Market

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Published: March 1997; WTEC Hyper-Librarian