Basic and advanced materials for selective laser sintering Rapid Prototyping Technology

Basic and advanced materials for selectivelaser sintering, Rapid Prototyping Technology

P. S. Panchal1, N. R. Patel2, H. J. Patel3

1 P.G. Student Sardar Patel Institute of Technology, Piludara2 P.G. Student Sardar Patel Institute of Technology, Piludara

3 Asst. Prof. Mechanical Engineering Department, SPIT, Piludara.

Abstract: Selective Laser Sintering is one of therapid prototyping process that fabricates threedimensional parts by means of Laser to selectivelysinter (heat and fuse) a powdered materials.

In this paper, we emphasize on basic and advancedmaterials used for realization of parts by SLS.Generally SLS materials are available in powderform. SLS machines can fabricate objects in a widerange of materials, such as plastics, glass,ceramics and metals.

SLS basic materials are Carbon-Fiber, Glass FilledPolyamide, Nylon 11 derivative, Fine Polyamide,Nylon 12, Alumina-ammonium phosphate. Metal objectscan be fabricated by Direct Metal Laser Sinteringand materials are Aluminum, Cobalt Chrome Alloy,Nickel Alloy, Maraging Steel, Stainless Steel, andTitanium Alloy for variety of structural,electroceramics and bioceramics applications.

Keywords: Selective Laser Sintering, RapidPrototyping, Rapid Prototyping Materials.

1.0 INTRODUCTION

Rapid Prototyping (RP) can be defined as a group of techniques

which is used to quickly fabricate a scale model of a part or assembly

using three-dimensional computer aided design (CAD) data.In 1987, Carl

Deckard at University of Texas found that polymer powders can be

selectively sintered using a laser beam to create complex solid objects.

Also the development of computers and CAD systems in the past decade

made this technology commercially viable and in the recent past

researchers found this technology suitable for any type of material

which can be pulverized in the form of powders [1]. Due to the varied

material capabilities, Selective Laser Sintering (SLS) process now

stands as an alternative to conventional manufacturing techniques.

Because of the time compression between product conceptualization to

realization, these technologies are sometimes referred to as Rapid

Manufacturing [2]. Because of the wide range of materials it can

process, SLS is superior to other Rapid Manufacturing techniques . The

materials include wax, cermet, ceramics, nylon/glass composite, metal-

polymer powders, metals, alloys, steels and polymers [3].

Initially Polycarbonate powders (Bisphenol-A polycarbonate) were

used. Later Nylon and nylon composites have become industry standards

for prototypes and functional models due to high wear and chemical

resistance [5]. Researchers tested the use of a sacrificial polymer

binder and found that any material can be combined with a low-melting-

point material which will serve as glue/binder in SLS. Metal systems

were studied for laser sintering since rapid tooling needed accurate

metal dies and moulds. Recent research efforts showed the capability to

process high temperature, high performance materials, making this

process comparable to conventional manufacturing techniques in producing

metal components with almost same mechanical properties by successfully

processing nickel base superalloys, titanium alloys and superalloy

cermets into functional components for automotive and aerospace

applications.SLS also processes bio-materials for fabricating scaffolds

in tissue engineering scaffolds. Layer-bylayer additive fabrication in

SLS allows construction of scaffolds with complex internal and external

geometries. Moreover, virtually any powdered biomaterial that will fuse

but not decompose under a laser beam can be used to fabricate scaffolds.

SLS enables fabrication of anatomically shaped scaffolds with varying

internal architectures, thereby allowing precise control over pore size,

porosity, permeability, and stiffness. Control over these

characteristics may enhance cell infiltration and mass transport of

nutrients and metabolic waste throughout the scaffold. SLS also allows

for the fabrication of biphasic scaffolds that incorporate multiple

geometries into a single scaffold, allowing for ingrowth of multiple

tissues into a single scaffold structure. Recent advances focus on

processing of Polycaprolactone, hydroxyapatite by SLS for bone and

cartilage tissue engineering [6]. This paper presents advances in above

mentioned fields and the paper is organized into different sections

based on the different materials. These materials are namely polymers,

wax, cermets, ceramics, nylon/glass composite, metal-polymer powders,

metals, alloys. Research issues in processing of bio-materials and

functionally graded material (FGM) for bio –medical applications have

been dealt in last section.

Figure 1 Selective Laser Sintering (SLS)

2.0 MATERIALS FOR POWDER BASED SLS RP

SLS can be used to process almost any material, provided it is

available as powder and that the powder particles tend to fuse or sinter

when heat is applied.

Figure 2 Materials for SLS

Powders that depict low fusion or sintering properties can be

laser sintered by adding a low melting temperature binder material

(typically a polymer binder) to the basic powder. Figure 1 shows the

wide range of materials SLS can process.

2.1 Polymers

The initial materials used in SLS are polymers which are materials

made up of long-chain molecules formed primarily by carbon-to-carbon

bonds. Generally, thermoplastic polymers can be classified into two

types: amorphous and crystalline. Amorphous material has chain molecules

arranged in a random manner like in polycarbonate (PC). Crystalline

material has chain molecules arranged in an orderly structure like in

nylon. Amorphous polymers are able to produce parts with very good

dimensional accuracy, feature resolution and surface finish (depending

on the grain size). However, they are only partially dense parts. As a

consequence, these parts are only useful for applications that do not

require part strength and durability. Typical applications are SLS

masters used for manufacturing silicone rubber and cast epoxy moulds

[7].The first sintering model developed for processing of polycarbonate

shows the effect of energy density on the sinterability of polycarbonate

powder beds [8]. Also the accuracy of parts depends mostly on the

process parameters as well [9]. Semi-crystalline polymers on the

contrary, can be sintered to fully dense parts with mechanical

properties comparable to injection moulded parts [4].

Prototypes made by these materials widely employed where strength

and wear resistance is the main consideration. Typical applications of

these materials are fully functional prototypes and sometimes as the

final product. Figure 2

shows some of the nylon

parts.

Figure 3 Some polyamide parts produced by SLS

Shrinkage of these semi-crystalline polymers during processing is

typically 3-4 per cent [22] and depends on the process parameters, which

complicates production of accurate parts. New grades of nylon powders

(i.e. Duraform PA12, Fine Polyamide, PA2200) even yield a resolution and

surface roughness close to those of PC, making PA also suited for

casting silicone rubber and epoxy moulds. Other polymer-based materials

available commercially are acrylic styrene for investment casting and an

elastomer for rubber-like applications [10]. Shi et al. [11] studied the

relationship between the crystallinity of the polymer material (Nylon

12) and the accuracy of the SLS part. They found the crystallization

rate, which is closely correlated with crystallinity, greatly affects

the accuracy and precision of the SLS part. Tontowi and Childs [12]

measured density of commercially supplied powders, known as Duraform

(nylon-12_ & Protoform (glass filled nylon-11) and studied the effect of

varying bed temperature on the density of sintered parts produced by the

SLS process.

2.2 Reinforced and Filled Polymers

Polymer powders can be easily reinforced with other materials in

order to further improve their mechanical and thermal properties.

Several grades of glass fibre reinforced PA powders are readily

available the market . The part fabricated from glass filled polyamide

(PA3200 GF) has excellent mechanical properties and high accuracy.

Typical applications of these materials are housings and thermally

stressed parts. Childs and Tontowi [13] measured density of glass filled

nylon-11 and simulated the effect of varying bed temperature on the

density of sintered parts. DTM Corporation (Austin, USA) introduced in

mid-1998, copper polyamide, which is a thermally conductive composite of

copper and plastic and can be used to create tooling for short runs of

production equivalent plastic parts. Copper polyamide is suitable for

injection moulded inserts to mould around 100–400 parts in polyethylene

(PE), polypropylene (PP), glass filled PP, polystyrene, ABS, PC/ABS, and

other common plastics. Lower material strengths are the limitation in

application of Copper polyamide moulds. Recently, Windform XT is

introduced into commercial market which is based on a carbon-filled

polyamide and produces black parts with a smooth finish and a sparkling

appearance [30]. It has a low density and a high tensile strength and

tensile modulus.

2.3 Metals and Alloys

In usual practice, SLS allows producing metallic parts using some

kind of sacrificial polymer binder. Nowadays, direct sintering of

metallic powders without the use of a polymer binder is also

investigated. This further enlarges the range of powders used in SLS.

Early attempts [14] to SLS process metallic powders and powder blends of

copper, lead, tin, and zinc proved to be unsuccessful because of

balling. As the increase in energy density causes a larger degree of

melting, causing material to form spherical balls whose diameters tend

to increase with further increase in energy density as shown in figure

5. Since the molten metal is fully contained by loose powder rather than

a fully dense material, the tensile traction on the melt is not

sufficient to confine it to a layer wise geometry. A two-phase powder

approach was used to overcome balling effects [15]. This was achieved

using a pre-alloyed single phase powder system in which melting occurs

over a range of temperature, or a powder blend of two phases with

different melting temperatures. In the former case, laser processing

parameters are manipulated so that only partial melting occurs.

Figure 4 Balling effect found on Ni alloy on quartz

substrate [15]

DTM Corporation has developed a process that applies polymer-

coated steel powders (1080 Steel, 316 or 420 Stainless Steel particles

coated/mixed with a thermoplastic /thermoset material) for the SLS of

metal parts. During laser sintering, the polymer melts and acts as a

binder for the steel

particles. This binder

needs to be debinded

to get the green part.

After debinding, the porous steel part is infiltrated with copper or

bronze.

Figure 5 Direct laser sintered 3D metal parts [30].

LaserForm ST-100 (420 Stainless Steel based powder), is the latest

tooling material system offered to replace RapidSteel 2.0 and Copper

Polyamide powders. LaserForm ST-100 tooling is reported to be fully

dense after LS with surface roughness of 5 μm Ra. RapidTool moulds have

been successfully employed in both plastic and wax injection moulding

[16]. EOS avoids the use of polymer binder and uses direct sintering of

metal powders with a low melting point, i.e. bronzenickel based powders

(EOS-Cu 3201 containing Cu- Sn, Cu-P and Ni particles) developed by

Electrolux Co. [17]. After SLS, the part is infiltrated with epoxy resin

to fill in the pores. Hence the final part is a bronze-epoxy composite,

rather than a plain metallic part and its mechanical and thermal

properties are limited. The direct metal laser sintering (DMLS) process

and a new powder (EOS-DMLS Steel 50-V1 containing steel, Cu-P and Ni

particles) yielding improved mechanical properties was introduced in the

market by EOS [18]. Some of the parts produced by DMLS is shown in

figure 6.Studies show that the average interaction time from the laser

beam with the particles is too much short to initiate sintering. In this

direction, Schueren and Kruth [19] examined different metal powder

mixtures of (Cu, Fe, Sn) for sinterability. The best results are

obtained with a mixture of Fe and Cu powders. Zhu et al. [20]

demonstrated the feasibility of producing Cu-based metal parts directly

by SLS using various metal systems such as Cu–Sn, bronze–Ni, Cu-solder.

2.4 Ceramics

Ceramics are hard, brittle, very high melting points with low

electrical & thermal conductivity, good chemical and thermal stability,

and high compressive strength. They exhibit both ionic and covalent

bonding. The most common ceramics used in RP are Al2O3, SiO2, and ZrO2 .

Alumina parts were made using the laser sintering followed by an

infiltration step using an alumina colloid. After sintering maximum

strengths obtained were around 14 MPa due to the low sintered densities

of about 55% [21]. SLS has been used to produce ceramic investment

casting molds. Partially stabilized Zirconia molds for Titanium casting

were made by SLS of stabilized Zirconia which was then infiltrated with

unstabilized Zirconia before being sintered [22]. Aluminum with SiC is

light weight, high conductivity and strength, low thermal expansion

coefficient and sufficiently high wear resistance. Thermal conductivity,

as the next important property, can be changed within a wide range by

addition of different amounts of SiC particles to the starting powder

mixture. Because of these beneficial properties, several parts are

produced from Al–SiC composites, mainly for the automobile industry, and

for electronic packaging applications [23].

2.5 Foundry Sand

Now sand powders are commercially available that can be laser

sintered to produce foundry sand moulds. DTM offers Zirconium and

Silicon sand commercial name SandForm ZrII and SandForm Si. SandForm Si,

used for Al castings, is based on silica, and has a low density.

SandForm ZrII is used for Al and Fe castings and its binder system

matches silica [4]. The LASERCON coated sand offers by EOS have a

composition of 96.8% quartz sand and 3.2 % resin.

Figure 6 Left cylinder head of V6-valve car Sand Moulding was Core

Produced on EOSINT S 700 Direct Croning System [17]

2.6 Functionally graded materials

Functionally graded material (FGM), also called heterogeneous materials,

are a new generation of engineering materials wherein the micro

structural details are spatially varied through non-uniform distribution

of the reinforcement phase(s), by using reinforcement with different

properties, sizes and shapes.

Figure 7 Heterogeneous primitives [32].

SLS can fabricate such heterogeneous objects. The material

deposited can be varied continuously to yield a functionally graded

material object with varying material distribution. Some of the studies

in processing of polymer composites by SLS have been reported by Zhou et

al. [24]. Das and Chung [25] discussed the fabrication of FGMs by SLS of

Nylon- 11 composites. They built one dimensional FGM with varying

compositions of glass bead on nylon 11. They investigated processing of

Nylon-based composites with different volume fractions of glass fiber

and glass bead reinforcements. They also reported previous attempts of

one dimensional FGM part processed by using blend of tungsten carbide &

cobalt powders and H-13 tool steel & copper powders.

2.7 Biomaterials

As the powders are subjected to low compaction forces during their

deposition to form new layers, SLS-fabricated objects are usually

porous. This interconnected porosity is a key property requirement in

biomedical applications, including artificial bones and tissue

engineering scaffolds. Figure 9 shows some of the complex 3D Scaffold

designs. The nature and extent of this interconnected porosity can be

tailored and controlled effectively to meet different application

criteria through material selection and physical design, and owing to

the additive nature of the SLS process, control over internal structure

is possible.

Figure 8 complex 3D Scaffold designs [31]

The porosity also offers an opportunity during postprocessing to

introduce additional materials into the object to alter material

composition as well as help to control part stability. Polymethyl

methacrylatecoated calcium phosphate powders have been successfully

processed via SLS and subsequent postprocessing enables to produce

strong porous structures [26]. Tan et al. and Chua et al. [27] found

micropores formed within the scaffold structure produced via SLS from

physically blended Hydroxyapatite (HA)/polyetheretherketone and

HA/polyvinyl alcohol composites. Internal porosity with 150 mm average

pore size in the SLS-fabricated HA/poly(L-lacide) specimens are also

reported [28] . Das et al. [29] investigated the development of optimal

SLS processing parameters for CAPA®6501 polycaprolactone powder using

systematic factorial design of experiments. The test scaffolds with

designed porous channels were able to achieve a dimensional accuracy to

within 3%–8% of design specifications and densities approximately 94%

relative to full density.

3.0 CONCLUSIONS

Current state of the art in processing of different materials

through SLS is presented though this paper. Studies involving developing

new materials and improving the existing materials were discussed.

Although many materials have been developed, there is still a need for

research into new materials for better results. It should be noted that

the SLS process is still a relatively new process and therefore

continued development of the technology and understanding of process

fundamentals is needed to carry the technique forward. The addition of a

secondary material to modify the mechanical properties of polymers is

common practice, to ensure materials meet design requirements and are

suitable for a wide range of applications. Addition of rigid particles

and clay to polymers can produce a number of desirable effects on the

mechanical properties of parts. The knowledge of existing materials and

the nature of complexity in processing them by laser will be helpful in

achieving functional requirements of parts for present and future

applications. The future of bio-manufacturing which combines principles

of RP and Bio-science can form complicated bio tissue scaffolds, is a

potential technology to make artificial organs and complex parts for

industrial applications.

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