Solid Freeform Fabrication

Abstract

This research document mentions the commonly used techniques for Solid Freeform Fabrication or Rapid Prototyping. It discusses comparison of solid freeform fabrications methods or techniques. It discusses the basic principles of working of different SFF techniques. The research document enlightens the capabilities of 3D printing. It also enlightens the products of 3D printing in international market and the different types of applications of 3D printing.

Introduction

Solid freeform fabrication (SFF) is a technique for manufacturing solid objects by the sequential delivery of energy and/or material to specified points in space to produce that solid. SFF is sometimes referred to as rapid prototyping, rapid manufacturing, layered manufacturing and additive fabrication.

Prototypes are an essential part of the research and development process for engineers. They allow pre-production testing and serve to highlight potential problems within a design. In the past, producing a prototype model has been a very expensive process, relying on traditional machining processes more suited to producing large numbers of a single product. The use of these processes increases the time taken for an original idea to reach the consumer as a finished product.

The speed of the production of prototypes was greatly increased by the introduction of Computer Numerically Controlled (CNC) Machining, reducing waiting time for a prototype to a few days or weeks.

More recently, Rapid Prototyping systems have been developed which are capable of producing a part in as little as a few hours.

The Basic Principles of Solid Freeform Fabrication

In order to create a part by any Rapid Prototyping process, it is first necessary to create a 3D drawing of the object using CAD software. The computer then divides the drawing into layers and gives the RP machine a description of each individual layer.

A piece of artwork created by Solid Freeform Fabrication

The machine then creates these layers, one on top of the other, to produce a whole part. In some cases it is then necessary to finish the part by hand (i.e. by sanding of painting it).

There are various types of Rapid Prototyping, several of which are described in greater detail below. The main difference between these systems is the method they use to lay down or create the layers.

The systems also use different types of materials to produce the models. Materials include thermoplastics, wax, ceramics, metals and, most commonly, polymers. In stereolithography liquid acrylic polymer is used to form models, while laminated object manufacturing uses paper or plastic sheets.

Techniques

Fused Deposition Modeling

Figure 1 The FDM Extrusion Head

This system builds up models made from wax or plastic, which is liquefied in the extrusion head then passed through the tip. The head moves over a path defined by a CAD design in order to produce a prototype part. A single layer is extruded, and then the table is dropped so that the next layer can be extruded on top of the first.

Before production can begin, a base must be laid down for the part to be constructed on. This is done using the same method as that used for the production of the actual model, but the material extruded for this purpose is less dense, and so can be easily separated from the finished piece. Also, for more complicated structures, support material is required to maintain the shape of the piece until the liquid has cooled and solidified completely. These support structures are extruded in the same way as the base

Thermoplastics and wax are the most common materials used in this process. Production of a model by Fused Deposition Modeling may take anything from a few minutes to a few hours, depending on the size of the part and the material used.

Laminated Object Manufacturing

This is an unusual Rapid Prototyping method, in that its raw material comes in solid form. Sheets of paper or plastic are laid down one at a time with heat activated adhesive between the layers. After each layer is applied it is cut to shape with a laser. Objects manufactured in this way look like softwood.

Excess material must be removed after production, but the parts are easy to finish and coat. The machines are cheap to buy and are sometimes called "desktop Rapid Prototyping machines"; however they are also small so the size of model that can be produced is limited.

Ð'* Tolerances are similar to those in Stereolithography, at around 0.0125mm.

Ð'* Raw materials are paper and plastics.

Ð'* Parts are useful only as visualization aids, since their size and strength are limited.

Selective Laser Sintering

Figure 2 Schemetic of Selective Laser Sintering

Selective Laser Sintering is a procedure by which (usually) non-metallic powders are sintered into the shape of the required prototype.

The powder delivery piston moves up and the roller spreads the exposed powder over the top of the build cylinder. The laser then sinters the powder to produce a layer of the part according to the CAD pattern. The build cylinder then drops to allow another cycle to take place. The final part produced is surrounded by non-sintered powder at the end of production. No further processing of the part is necessary.

The most commonly used materials for this process are polymers, though wax and ceramics are also used. This system is put to best use producing parts which cannot be easily formed by machining or where higher heat resistance is required.

Stereolithography

Figure 3 Laser being used for Stereolithography

This was the first RP process to become available in the commercial market. The system involves using a UV beam to harden liquid acrylic polymer into the desired shape. The liquid contains acrylic monomers and photo initiator.

The container in which the process takes place is filled with the liquid polymer and contains a platform whose height is variable. Initially, the platform is raised to the highest position, so that it