3D printing: the future of manufacturing- Part 2

In the previous post the various types of 3D printing techniques have been discussed. If you have not gone through that, read it here. In this post I shall deal with the steps involved in manufacturing a part from the concept state to the finished state.

Process sequence steps in 3D printing
The basic process sequence in all the 3D printing technologies are the same. It involves:

1. Creation of a CAD model of the part to be made
2. Conversion of the CAD part to STL format [1]
3. Slicing of the part into a number of layers
4. Generation of scan path
5. Manufacturing the part layer by layer

They only differ in the last step, i.e the way the individual layers are put together, details of which has already being dealt with in the previous post. 

1. Creation of CAD model

This is the first step and is very simple and self-explanatory. The solid model of a component to be produced is created in any CAD environment like Solidworks, Autocad, etc and the file is then prepared to be sent to the next module.

2. Conversion of the CAD model to STL format

STL is the de facto standard in the rapid prototyping industry and is therefore widely used. The process involves the approximation of the model surface with triangles. With the help of the triangles it is possible to represent any surface. The triangles are represented by the co-ordinates of the three vertices along with a unit normal vector. The syntax for an ASCII STL file is as follows:

The purpose of the normal is to indicate the direction in which the material is present. The figure below shows how a sphere is represented in the STL format (increasing the number of triangles means increasing the accuracy).

3. Slicing of the model

Slicing involves the intersection of the part (STL file) by a set of horizontal planes [2]. This process is very important as it determines the quality of the finished part. In general, more the number of layers [3], less is the surface roughness of the finished part. But increase in the number of layers results in an increase in the build time. 

4. Generation of scan path

The scan path denotes the path that the laser head/extrusion nozzle/... follows as it scans each layer. The three main scanning configurations are commonly referred to as stripe, checkerboard and islands patterns [5] as shown in figure below:

5. Manufacturing the part layer by layer

This is the final stage wherein the layers are deposited by the printer head (or depending on the process of deposition) following the scan path specified in step 4. The printer bed then moves down by an amount equal to that of the layer thickness which is calculated in step 2 and the next layer is deposited following the same method and the process is repeated.

P.S:

[1] STL stands for Standard Tessellation Language and was developed by 3D systems. Do note that the conversion of the CAD part to STL file format is not mandatory and slicing can be done on the CAD file directly. However it is the popular choice in the industry.

[2] Here it is assumed that the Z-axis represent the build direction.

[3] The locus of the points of intersection of the planes with the part is a closed polygon because the curved surfaces are approximated by triangles in a faceted model. The distance between any two successive planes is known as the layer thickness. Depending upon whether the layer thickness is constant or variable throughout the entire length of the build axis [4] the slicing process may be either uniform or adaptive. In uniform slicing the layer thickness is constant and result in a number of errors and therefore must be avoided. Adaptive slicing takes into the account the variation of the part profile along the build direction and adjusts the layer thickness accordingly. The detailed discussion of the slicing procedure is beyond the scope of this article.

[4] The direction normal to the cutting planes is known as the build axis. Here it is assumed that the Z axis is the build axis. It is same as the build direction.

[5] Island pattern is a random version of the checkerboard pattern