Optimising CAD models for 3D printing in plaster or sandstone

This post is geared towards customers planning to use the Lee 3D online quoting and ordering service. However, it should be useful to anyone modelling for 3D printing using these materials. Please note - optimising is not the same as fixing. If you are considering using an online service it is assumed that your data is print-ready. Optimising is about getting best results.

The Material

Plaster printing using ZPrinter (renamed as Projet x60 series) is sometimes called sandstone, colorstone or similar. Whatever you choose to call it, the material is predominantly plaster of Paris bound with a water based binder in the printer and most commonly hardened afterwards with cyanoacrylate (superglue).

When freshly printed, the models do not have their full strength. We sometimes use the analogy of concrete vs steel to describe the material properties of unfinished and finished parts. Concrete is great under compression but poor under tension, whereas steel is strong under both compression and tension. 

So, parts fresh from the printer have limited strength in tension and therefore thin parts are liable to fail and more especially, thin cantilevering parts are liable to break off. 

Finished parts, hardened with superglue have good all round strength.

Part Orientation

Part orientation affects surface finish and strength of parts.

Strength of parts due to orientation only needs to be considered for thin parts. A thin column printed horizontally is stronger than if printed vertically. Thus, part orientation will affect minimum part size (fig. 4).

Scale

Architectural models rarely get printed at 1:1. If you are modelling in software at 1:1 and you want to print at scale, you need to think in advance about the size of the final model.

If you are printing to scale and want to use an online quoting service you must think about the size of parts as they will be printed. 

Remember that you are making a model. Columns may look weirdly thick in software but when they are printed at 1.2mm diameter they will be thin!

Hollowing

Hollowing parts reduces cost. However, if you hollow parts and leave the powder trapped in the model we will have to charge for the unused powder as we cannot remove it. This may lead to your order being rejected. 

So, consider how unused powder will be removed from voids and remember that powder does not flow out like water - it needs to been blown out with an air brush. Small holes are not adequate for removing large quantities of powder.

It is not easy to model complex structures with consistent wall thicknesses. If you are not careful, weak points can appear as illustrated below (fig.1).  From the outside, these can be difficult to see without viewing the part in section. Thin parts, if they do not break, can become translucent once glued giving the part a patchy finish.

It is usually better to model parts as solid in design software and then use Materialise Magics (or similar) to hollow the part effectively. This will produce a uniform wall thickness which will produce uniform strength and finish. 

fig. 1

Minimum Part Thickness

Everyone asks what the minimum size we can print is, to which I usually answer that it is geometry dependent. It is also usually worth considering the end use of the part. If the part is likely to be handled a lot, then make it strong. If the part needs to fly to the antipodes, make it strong. 

If its useful life is for a single meeting in which you need to make a crucial decision, then perhaps take some risks to make it look great for that meeting. If the model will spend the rest of its days inside of a Perspex display case, make it detailed (it may take us a little longer though).

Below are some rules of thumb to determine minimum thickness for various features:

1.  Freestanding wall features

The fig.2 below shows a fine wall detail well supported by the core of the model which is hollowed to about 3mm. The table shown in fig.3 contains suggested minimum wall thickness for differing wall heights.

fig. 2

fig. 3

2.  Column features supported at both ends

As shown in fig. 4, these features are strongly dependant on print orientation. Some files cannot be oriented so that all fine details are horizontal and in this case all parts need to be thickened for printing horizontally. Table fig.5 shows suggested column heights for parts printed in either orientation.

Please note that free standing columns are much less sturdy than columns supported at both ends and need to be made significantly thicker.

fig. 4

fig. 5

3.  Shadow gap and surface relief size

Fig. 6 shows shadow gaps on the surface of the model. The smallest readable shadow gap is of the order of 0.3mm. Bold shadow gaps should be 0.5mm or greater.

Some thought needs to be given to depth of shadow gaps. It is difficult to remove powder from gaps narrower than 1mm, so there is little to be gained from making these deeper than a couple of mm.

fig. 6

Relief details are readable from 0.2mm at the minimum. To clearly see surface relief, a minimum size of 0.5mm or more is recommended.


4.  Hollowing

Hollowing thickness depends on the size and strength of part required, usually 3mm or more.

Walls need to be of adequate thickness after hollowing to give the part sufficient strength for us to remove it from the printer.  

You cannot make a part the size of the build volume with a thickness of 1.5 or 2mm - it will collapse. We will check every part and will need to modify or reject parts that are too thin. 

fig. 7

For further information about the Lee 3D online quoting and ordering service see Optimising sandstone or plaster models

For more information about Lee 3D go to www.lee3d.co.uk