Selective laser sintering (SLS)
Selective Laser Sintering (SLS) works by a CO2 (10.6 µm) or Nd: YAG (1.06 µm) laser selectively sintering the contours of a cross section corresponding to the CAD model onto a thin layer of powder spread on the building platform. The laser sinters particles by heating them just enough for their surfaces to soften (melt) and fuse at the point of contact (unlike complete melting where the molten flow of the material can cause deformation). The powder in the build chamber is heated and kept to a temperature just below the glass transition temperature, to reduce thermal distortion and to aid fusion between layers. Un-sintered powder remains in place as support [26, 58]. The processes of sintering have been theoretically and computationally modelled based on current understanding of laser-matter interaction, and reviewed .
In theory any thermoplastic material ground into a powdered form should be suitable for laser sintering but practically this is not the case. A range of factors can prevent production of good quality prototypes such as: different thermal properties of different materials, availability of powdered polymers with suitable particle size and morphology, broad thermal processing window and limitations in laser sintering systems. This has been discussed elsewhere . The density and porosity of the sintered part will depend on the size of the particles and how densely these are packed in the build chamber. Other SLS parameters such as laser wavelength, laser energy, temperature distribution within the build chamber can also affect the mechanical properties of the part such as porosity or anisotropy .
The two main providers of SLS system are 3D Systems Ltd and EOS GmbH Electro Optical Systems (Krailling) . SLS systems differ mainly in the way powder is deposited, either by a roller from feed chambers or by a sieving action. The typical materials in use are polycarbonate (PC), acrylonitrile butadiene styrene (ABS), nylon (PA 12), polyester (PET), polypropane (PP), polyurethane (PU), poly(lactic acid (PLA) and wax.
Despite continuing developments to increase the range of materials polyamide (PA) 12 is still the most commonly used SLS material (constituting ~95 % of all prototypes) as it is easy to process and inexpensive  (Fig. 5).
A distinction can be made between amorphous and semi-crystalline thermoplastics used in SLS. Limited success has been achieved with amorphous polymers such as PC which do not have a clear melting temperature range but have a glass transition temperature at ~100 °C above which the polymer gradually softens becoming rubbery and as temperature increases finally becomes a liquid, without clear transitions. They have a flow and sintering rate less than that of semi-crystalline polymers (such as PA 12) and so are more porous, weaker and less durable. They do however produce more dimensionally accurate parts with good resolution. Semi-crystalline polymers such as PA 12, which have sharp melting points and quickly become viscous liquids, can be sintered to make very dense prototypes with properties comparable to injection-moulded materials. However, they exhibit greater shrinkage (3–4 %) resulting in less accurate parts .
Selective laser sintering (SLS) material studies
Building parameters such as laser power, scanning speed, layer thickness, powder bed temperature, building positions and orientation of parts all contribute to the material characteristics of laser-sintered polymers . The most commonly used material for laser sintering is DuraForm®GF, a glass-filled polyamide 12 by 3D Systems Ltd.
It has been shown that higher energy delivery by laser radiation or increase in powder bed temperatures increase part density, reduce anisotropy, and yield higher tensile strength values, Young’s modulus and elongation at break. This window is limited as at very high energy densities the properties level-off or begin to decrease .
An investigation into the effect of processing parameters on PA 12, using ASTM type 1  samples built with a Sinterstation 25000 plus from 3D Systems Ltd. was conducted by Starr et al. . The laser power varied from 7 to 20 W and the bed temperature was kept at 166 °C. The scan speed ranged from 2.540 to 5.080 mm s−1, and scan spacing from 0.10 to 0.20 mm with layer thickness from 0.10 to 0.15 mm. Samples were built in six orientations to investigate the influence of build orientation. The maximum yield and ultimate tensile strength value recorded for laser sintered DuraForm 12® were found to be similar to injection-moulded PA 12. These values were achieved for all orientations at the highest laser power. At lower power differences in yield stress were observed, particularly for samples built in the z orientation, i.e. vertically .
To save on costs and minimize material consumption, 80–95 % of un-sintered powder left in the build chamber after printing is recycled and a blend of virgin and reused powder are often used. Due to processing conditions such as heating and cooling of the building chamber, un-sintered powder degrades, causing a gradual reduction in quality. Temperature in the bed chamber and duration of the sintering process have major influences on the rate of this process. This also varies according to the location within the build chamber, and it was found that PA 12 powder (PA2200, EOS GmbH) collected and analysed towards the centre and the base of the build chamber had a lower melt flow rate and are therefore less usable . The powder was collected from two SLS machines EOSINT P700 (EOS GmbH) and Sinterstationtm 2500HIQ (3D Systems Ltd). The term “orange peel” has been used to describe the phenomenon which occurs after too many repeated cycles without refreshing with virgin powder where a rough surface resembling the skin of a peeled orange arises .
Cooke et al.  found that in the majority of studies into the anisotropy of SLS objects not all building orientations were included and sample numbers were insufficient to provide statistically significant results. Using 288 DuraForm®GF samples produced using a Sinterstation HIQ (3D Sytems Ltd.) they investigated three different building orientations and the effect of “ageing” (defined as moisture absorption) of samples stored in a non-desiccated environment for 43 days. The samples were found to be transversely isotropic (i.e. isotropic within a layer) as the position of samples within the build chamber greatly affected densification, which could be a result of temperature distribution. Sensitivity of material properties to slight changes in building parameters was also noted .
In contradiction to these findings, Majewski and Hopkinson found that section thickness and build orientations had no significant impact on PA 12 (PA2200, EOS GmbH) laser sintered parts produced on the EOS Formiga P100 machine in their study. However, a decrease in molecular weight between samples produced early in the build and ones produced later was noted, which could be a result of longer exposure to higher temperatures .
Z-Corporation’s 3DP™ process is the fastest RP technology available, and at half the cost of other systems has become very popular (Fig. 6). It was estimated in 2006 to be the third-best seller in RP machines .
3DP is based on ink-jet technology. A thin layer of powdered material is spread on a building piston by a roller from the powder feed piston. Ink-jetting a binder solution onto the powder selectively joins the powdered material. The loose powder around the part stays in place to act as support. Once a cross-section is completed the build piston is lowered and the powder feed piston is raised to allow for another layer of powder to be rolled over ready for the next layer to be bonded [24, 25]. The object is then infiltrated for strength with an epoxy, cyanoacrylate or phenolic resin .
3DP is very versatile as without changing the primary binding material an array of material properties can be achieved by using different powdered materials and adding infiltrants. Z Corporation is the leading provider of 3DP technology. They have developed a composite in powder form, which can be post-cured by spraying with water making it the safest and ‘greenest’ post-curing option. Their machines are also capable of automatically removing and recycling loose powder .
There is also a 3DP™ colour system available where the binder fluid is coloured . This has made 3DP popular amongst architects, artists and within the cultural heritage sector, for printing coloured models and replicas . The ZPrinter® 450 uses a single tri-colour print head, which is quick to change, and costs less than systems with multiple print heads .
3D printing™ material studies
While the more established processes of SLS and SLA received more attention, factors such as low cost and speed make ZCorporation 3D Printing increasingly popular. Until recently it was mainly used for concept modelling rather than for manufacture of end-use products. In 2000 the first commercial colour rapid manufacturing system was launched, the Z402C colour printer . What sets 3D colour printing apart from other RP colour technologies (Laminate Object Manufacturing™ (LOM) and SLS), is that the coloured ink is also the binder. 3DP™ provides high-resolution (up to 600 × 540 dpi) colour prints . However the bonds between particles are not as strong as LOM or SLS manufactured parts where materials are heated to a molten state for bonding and strength of 3DP parts are mainly dependant on infiltration method .
The exact composition of materials are trade secrets, however some information is available [67–70]. The zp™130 powder consists of a plaster containing crystalline silica (50–90 %), a vinyl polymer (2–20 %) and sulfate salt (0–5 %). The binder zb™58 contains glycerol (1–10 %), sorbic acid salt (0–2 %), an unknown surfactant (<1 %), pigment (<20 %) and water (85–95 %). There are various options for the infiltrant: the most popular are cyanoacrylate, epoxy or wax. Z Corporation has also developed a water-cure system using their composite powder, zp™150, which consisting of plaster, a vinyl polymer and carbohydrate (starch). The parts are cured by spraying water and MgSO4 (epsom salt). Rubber-like properties can be achieved with their zp15e powder mixture of cellulose, ‘specialty fibers’, and additives which are capable of absorbing a urethane elastomer, such as Por-A-Mold [30, 68].
Very little research has been conducted into the material properties (thermal, strength or fatigue) of 3DP™ parts and there is a need for research into material degradation and behaviour at different environmental conditions , especially taking into account the growing popularity of this system.
Cyanoacrylate is a popular infiltrant due to ease of use and rapid curing by anionic mechanisms which is initiated in the presence of a weak base such as adsorbed moisture on the surface of substrates. This reaction continues until terminated with an acid. The depth of infiltration can be reduced if the model is not fully dried as reactions take place closer to the surface, block pores and prevent penetration of the infiltrant into the bulk of the material .
In a degradation study of an epoxy infiltrated 3DP™ artworks by Karen Sander, the infiltrant was identified as an aliphatic epoxy resin. Accelerated degradation of reference samples prepared identically to the artwork, using a commercial epoxy (LB Klar, epoxy pre-polymer mix with polyamine hardener) was carried out, and analysis revealed progressive formation of amides. Yellowing was attributed to the formation of quinoid chromophores .
The colour properties and permanence of customised 3D printed colour samples produced on a ZCorp Z510 printer with Zp131 plaster-based powder and Zb60 Cyan, Magenta and Yellow binders were studied by Stanic et al. . Three sets of samples were prepared: untreated, treated with a cyanoacrylate infiltrant (Belinka Kemostik, Slovenia) and treated with a two-part epoxy infiltrant (Selemix 7-410 and Selemix 9-011, Iridia, Italy/PPG Industries, UK). Colourimetry was used after photodegradation in a Xenotest Alpha chamber for 72 h in accordance to lightfastness testing standards [73–75] at 42 W m−2, 300–400 nm, 35 °C, black standard temperature 50 °C, 35 % RH.
Infiltrants not only contribute to mechanical properties but also enhance colour saturation. Cyanoacrylate infiltration contributed to higher chroma and lightness values. The colour stability varied and was found to depend on the binder colour, infiltrant used and the percentage of ink coverage. In both infiltrated and uninfiltrated samples the magenta colour patches showed the biggest total colour change, and in the uninfiltrated samples these were followed by yellow and then cyan. The reverse was observed for infiltrated samples with cyan exhibiting more colour change than yellow. All samples irrespective of the infiltrant became less saturated and faded. All uncoloured samples, infiltrated and non-infiltrated, yellowed during accelerated degradation, but the change was most pronounced in epoxy infiltrated samples .