It takes only seconds to mathematically change your model from one size to another, but you may not be happy with the results. Below, you can find the most important elements to consider when scaling a 3D model for 3D printing.
Every model is made up of polygons, or facets. Compare these two models:
You can see that the model on the left is "finer" (more polygons) than the one on the right. But, consider the width of the facet that's highlighted in pink: it is four times as wide as the facet in green. When designing for 3D printing keep in mind the wall thickness limit, but also consider the detail thickness limit.
When designing a model, you must keep the whole object above a certain thickness, but you should also keep the facet size below the "detail limit." If the printer can print a detail at 0.2mm, the large, pink facet is 0.4mm and the finer green one is 0.1mm, then the pink one is going to show up as a flat spot on the model, and won't look curved. The green one will appear as a curve.
Why does this matter when enlarging?
When you take the model on the right with less polygons and scale it up four times, you'll end up exaggerating the flat surface. Try to create the model at 1/2 of the detail limit size, but be aware that means your models won't necessarily still look good when enlarged. The only way you can overcome this is to split every polygon into smaller pieces.
What about scaling down?
Shrinking the size presents a different problem. Again, wall thickness limits come into play. If a segment of the model is 1mm thick, and the overall model is reduced by a factor of 4, that segment is now 0.25mm thick, and it is too thin to be printed. Here, again, you'd have to re-design the model to account for the different thickness limits.
Scaling can be done quickly, but remember the impact of polygons and price. For miniatures, detail is crucial, so while the whole object does have to be above the wall thickness minimum, keep the facet size below the "detail limit." Similarly, when enlarging a model, scaling it up will exaggerate a 3D model's facets.
Visit Shapeways material guidelines to check out the wall thickness and detail limits for yourself, or if you're ready, you can upload your model to check out the sizes and printability in all materials.
*This article was written in collaboration with Stony Smith, the shop owner of Stony Smith Designs, who primarily works with small scale railroad miniatures.