How to use the 3D printer

The Gensys-XS 3D printer (from Stratasys) has a printing volume of 12” x 8” x 8”, and can print arbitrary geometries out of white thermoplastic polyester compound (P1500). There are three basic steps in printing a part:

Prepare and verify a geometric specification
Setup the printer for printing
Preprocess and dispatch the file

Preparing a geometric specification

The standard format used to specify the geometric shape of the part to be printed is called STL (Stereo lithography). This is a very simple format that essentially describes the boundary of the part as a list of triangles with normal vectors pointing to the exterior side. Typically only the pure part geometry is described – without printing supports, bases and other auxiliary structures that are usually added at the preprocessing stage. Note that an STL file has no units. Most STL processors assume by default the file to be in either millimeters or inches, but the final units of measurements can be user specified. There are three ways to generate the file:

Using a CAD system
From a modeling library
Directly

Generating STL files on a CAD system

Typically a design is manually created using solid-modeling CAD software, and then exported in STL format. Most commercial CAD systems have an “export” or “save as” option for this purpose; for example both SolidWorks (we have a 30 day traial version which has not been installed yet) and Parametrics ProE (SGI/NT) support STL export. A critical parameter in generation of an STL file is the resolution in which the part surface is facetted. The resolution is typically measured as the maximum deviation permitted from the true surface of the model. The maximum deviation should be, say, an order of magnitude finer that the printer resolution. The Genesys-XS resolution is approx 0.2 mm, so a good faceting resolution is 0.05mm.

Using a modeling library

In some cases it is easier to generate part specification from within a program. In this case, use a library such as Unigraphics Solutions' Parasolid or Spatial Technology's ACIS to generate the solid definition of the object, and then use geometry inquiry functions to obtain the precise boundary and generate an STL file. Some libraries might provide an explicit STL Export function. In the Demo Lab we have purchased the Parasolid solid modeling C library for NT, for which I have written a short subroutine that uses the built-in graphic renderer to extract a faceted description of the geometry and save it in STL format. We have been offered (Nov-10-1999) the ACIS C/Scheme/Lisp Library for unix/linux free of charge if there is interest.

Direct generation

It is always possible to generate an STL file directly. The STL file can be generated in ASCII or in binary form. The precise file specification can be viewed here. A brief description follows:

BINARY STL file format is accessed by byte. The format is as follows: the first 80 bytes are used for description, and the next 4 bytes represents the total number of the facets(Long Int), followed by the facet information (normal and 3 vertices), the normal and vertices are stored in floating point format, each occupying 4 bytes. At the end of each facet information section, there are two bytes spaces, then the next facet is repeated till the end of the file. When BINARY format is used to describe STL file, the data size is much smaller than ASCII format, so most STL files available now use BINARY format.

Verifying the specification

It is strongly advised that the geometry be inspected using a third party software before printing. Although the Printer’s software does verify and display the STL file, it is often hard to see details, and impossible to perform measurements or see internal/hidden geometries. There are several STL viewers around. At the Demo Lab we have installed Actify’s 3DView on the NT machines as well as Solid Concepts' SolidView. 3Dview is an excellent viewer (free for academic use) that not only renders the part from STL, but also allows making various measurements, changing transparency setting to make hidden parts visible, as well as cross sectioning. It also has an option for computing volume – this option is useful for estimating print time (about 1 hour per cubic inch) and for checking for “leaks” in the boundary definition.

Setting up the printer

Before dispatching a print file the printer must be setup and turned on-line. The printer is essentially a stand-alone Linux box connected directly to the network. After switching on, it typically takes a few minutes to boot, and then can be operated from the small 4-line LCD and keys on the front panel.

Routine Setup

Typically, after switching on it should be made certain that the printing platen is clear of previously printed parts or leftovers. Especially make sure there are no small strands of plastics left along the right edge of the platen, where the print heads performs a print extrusion test before every print job. Once the platen is clear, use the up/down and enter keys to select “Clear platen and home”. This operation might take several minutes as the platen needs to warm up. When complete, select “Put printer Online”.

Routine Shutdown

After printing is completed, remove the part from the platen using the plastic spatula provided with the printer (be careful not to scratch the printing surface). Once the platen is clear, select “Shutdown”, wait 30 seconds while the system shuts down and then turn off the power.

Loading Material

Print cartridges are loaded into the machine through the top door. Open the door to reveal 10 cartridge slots. Some of the slots may be vacant, while others might contain cartridges, either with material or empty. New cartridges simply insert and click in. Remove empty cartridges by pressing them inwards, lifting slightly and pulling out. A single cartridge supports approx 10 hours of print (equal to approximately 10 cubic inches printing volume) and costs $90. Make sure you have enough printing material before you start, although additional material can be loaded in mid print by pausing the print job.
Important: When inserting a new cartridge, make sure the plastic wafers are vertical and flush with the cartridge edge. Usually the wafers tend to tilt forwards so that their lower edge is not flush with cartridge end, and a "stapling error" will occur later. To align the wafers, push and release them against the spring several times before you insert the cartridge.

Maintenance

In some instances it is necessary to perform additional printer maintenance activity.

Clean the print platen with alcohol.
Replace a printer tip when it becomes clogged or printing is uneven (or when you get too many “stapling” problems?) – See user guide for instructions.
Recalibrating plastic pump extrusion and suck-back rates– See user guide for instructions.

Stapling Error

A stapling error occurs when the wafers do not feed well into the pump. Typically this happens because the wafers are not exactly vertical in the cartridge. When inserting a new cartridge, make sure the plastic wafers are vertical and flush with the cartridge edge they are pushing aginst. Usually the wafers tend to tilt forwards so that their lower edge is not flush anymore. To realign the wafers, extract the cartridge, then push and release them against the spring several times.

Preprocessing and dispatching files

Once an STL file has been prepared and the printer is online, the file can be preprocessed and dispatched. The preprocessing software used with the Genysis printer is called Autogen, and is installed on the NT boxes. Versions are also available for the SGI’s. Basically, the program loads an STL file, generates supports and auxiliary structures, prepares a printer instruction file (CMB file) and dispatches it to the printer. Operation of the software is straightforward, but several options should be noted:

Supports

The programs generates print supports by default, these are usually good. For particularly difficult geometries with severely overhanging heavy volumes, set the support option to “maximum”. If you are unsatisfied with the supports, select “none”, and generate your own support structures as part of the STL file.

Print quality

By default, Autogen fills volumes with solid plastic. A “sparse” filling option can be selected – where solid volumes are filled with a sparse structure. The part will look the same, will print faster, use less material, but will be more fragile. Use for visualization models.

Units

Be sure to select appropriate units – millimeters or inches. The model can also be scaled arbitrarily.

Manipulation and packing

The models' scale, orientation, and position on the print platen can be specified directly.

Printing

When the STL file is loaded and ready for print, select the “Print” command. Select the “preprocess only” option to prepare the CMB file but NOT to dispatch it. This is useful to get an estimate of print time before printing, or to preview the print. The estimated print time is correct to +/- 15% for 85% of the models. The part can be printed or “packed”, in which case it will not be printed now, but stored until it can be printed along with additional parts on the same platter and print job. The nesting of geometris in the volume is not controllable by the user.

Print Preview

At our request, Stratasys has provided us with a simple “Print Preview”. This enables loading a CMB file and seeing the exact print trajectories to be taken by the printer head, as well as the generated support structures ahead of time. The program is called “pcview” and is installed on the NT machines.

Hod Lipson, November 1999