Notes on the cost of 3D printing

This post sets out to address the cost of plaster 3D prints - also variously known as ZCorp, Sandstone and Colorstone. 

All 3D print technologies have different pricing parameters so the points below do not necessarily translate to other processes. Plaster printing does not need supports to be printed as the part is fully supported in the bed of plaster powder as the print progresses. Unlike the SLS process which is also powder based, all of the unused powder can be reused in the next print.  

In addition to the powder used to make the part, a greater expense in binder fluid, cleaning fluid and printheads needs to be factored into the material cost of the print. Despite this it is usual (but not universal) to charge per cubic cm (£/cc) of material used.

For many parts including most architectural massing models consideration needs to be given to whether or not a part can be hollowed out to reduce the amount of material used and ultimately to save cost.

Parts need to be cost effective for the long term success of a bureau and hollowing parts is key to this.

For many heavier 3D printed parts there are 3 possible outcomes to treating a 3D print:

• Print solid
• Print hollow and leave unused powder trapped inside the part
• Print hollow and leave an opening to remove unused powder

Buildings usually sit on the earth and are consequently not viewed from the underside. This makes it possible to hollow most building massing models leaving the underside open to remove unused powder.

Hollowed 3D print

Solid models may be needed for vac forming etc and in some parts just have the wrong geometry for hollowing and need to be made solid.

Other parts can be hollowed but a hole or other opening is unwanted and then unused powder may be left trapped inside the part. Consideration needs to be given in such circumstances as to whether it would be desirable for powder to leak out if a model were broken. 

So let us take an example of a cube measuring 100 x 100 x 100mm.

• Solid this occupies 1000cc
• Hollowed with a 4mm wall thickness it occupies 221cc
• Hollowed with a 3mm wall thickness it occupies 169cc

This sheds some light on those stories of horrendously expensive prices sometimes quoted (and sometimes paid) for 3D printed parts. Be careful of this when getting quotes for parts online, software is not likely to pick up on the fact you are asking to print a lump of material that could do with being hollowed. A human is usually better able to spot this kind of thing.

In the case of our 100 x 100 x 100mm cube it is worth noting that the difference in volume between the 4mm wall thickness and a 3mm wall thickness is 52cc. Which priced on a cubic cm basis could be a 23% difference in price. 

Choosing an appropriate wall thickness depends on various things. The size and strength of the part, its purpose, whether it needs to travel and even on very tight deadlines the time available to remove it from the machine and get it to the customer.

Generally we hollow parts for customers as part of our file optimisation service at Lee 3D. To do this we use Magics RP, the industry standard software for preparing models for 3D print. This allows us to hollow complex parts with a uniform thickness.

For more information about 3D printing at Lee 3D please visit www.lee3d.co.uk

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