We are often asked to fabricate topographic map (topo maps) and architectural models using different type of materials, from EPS (Expanded Polystyrene) to EPP (Expanded Polypropylene) to Sintra (also known as PVC), XPS (Extruded Polystyren) and XPE (Cross Linked Polyethylene).
In this post, we will explore the difference in surface finish of each material when being machined on our CNC router, focusing on the ease or difficulty of fabrication.
In order to be able to compare surface finish between each of the materials, we machined the same Mt. Rainier model, using a 0.25" flat end mill. The model was divided into quarters, where each quarter represented a different level of detail achieved. For the majority of the materials especially the harder more dense ones we first used a 5/8" endmill to rough out the shape of the mountain milling the bulk of the material away before using the smaller endmill to bring the model to final depth and size. This allows us to take deeper more agressive cuts as well as saves wear on the endmill
The main factor that determines the level of detail retained in a machined part is the size of the step over, often measured in percentage of the end mills diameter. Stepover is defined as the distance the cutter moves horizontally from one cutting pass to the next. A smaller stepover results in more detail and a better surface finish.
We explore this idea by using three of our quadrant each with a step over of 15%, 35%, and 65% for a fine, medium, and rough finish respectively.
The final quadrant shows a 2.5D topographic outline of the mountain. This is akin to the topographic maps that one might use while hiking, where each new step in elevation represents a determined amount of elevation gain. For our tests we are using a 0.25” step. Which would translate to about 1000’ of actual elevation change on the mountain.
EPS (Expanded Polystyrene / Styrofoam):
EPS is one of the easiest materials to machine, sculpt and work with. It is often the cheapest as well, making it is the common choice for most art and non-engineering specific projects.
Due to its light density it is very forgiving on the machines, allowing us to take full depth cuts up to 8” at a time.
EPS comes in 4 common densities of 1lb, 1.5lbs, 2lbs and 3lbs. EPS is 90% air, with the other 10% being small spheres tightly packed together. The heavier the variety of EPS the denser the spheres will be packed resulting in a better surface finish.
This machined model shows all 4 surface finishes for comparison, using EPS.
Here are photos showing the different machining finishes. Hover over each picture for more information: Top left: Fine Finish
Top Right: Rough poor finish
Bottom Left: Medium finish
Bottom Right: 2.5D Topographic map
Video of EPS being machined
EPP (Expanded Polypropylene):
EPP, Expanded Polypropylene is a closed-cell, beaded foam. It is from the same family as EPS (Expanded Polystyrene), the main difference being the polymer that binds the material together.
It comes in 1.3lbs, 1.9lbs, 2.5lbs and 3.7lbs density options, it is a light, flexible, and durable material. It demonstrates high energy absorption and it is a great thermal insulator with high water and chemical resistance. It can be saw-cut, water-cut, die-cut, hot-wire cut, abrasive-wire cut and machined.
The beads that compose EPP are much more substantial than those of EPS resulting in only a subtle difference in the surface details between the medium and fine finishes.
It could even be argued that for most uses the medium finish is most desirable as the fine finish can leave behind small hair like imperfections from the numerous passes it must make.
This machined model shows all 4 surface finishes for comparison, using the EPP material
Here are photos showing the different machining finishes:
Top left: 2.5D outline
Top Right: Rough finish
Bottom Left: Medium finish
Bottom Right: Fine finish
Here is a video of the machining process, using the EPP material.
Sintra (Expanded PVC):
Sintra is a lightweight yet rigid and durable expanded closed-cell polyvinyl chloride, also known as PVC. It is dent, scratch, heat, chemical and even electrical resistant and can easily be fabricated to form almost any shape.
Sintra can be cut, machined, drilled and bent quite easily using ordinary tools, and can leave good surface finishes with certain cuts. In the photos below you can see multiple different shades of black resulting for the material being cut at different rates and from different directions.
Take a look at this model showing all 4 surface finishes using Sintra (PVC)
Here are photos showing the different machining finishes:
Top left: 2.5D outline
Top Right: Fine finish
Bottom Left: Medium finish
Bottom Right: Rough finish
Here is the video showing machining Mt Rainier model using Sintra material.
Extruded Polystyrene (XPS):
Extruded polystyrene or XPS is a rigid closed-cell foam. It is most commonly used as insulation in buildings. The pink material is dimensionally stable, air tight, resistsant to heat transfer and has impressive compressive strength and is mostly known for its resistance to moisture absorption.
Due to its manufacturing process it is prone to warp when machined into long thin parts. The material has a consistent density throughout, resulting in an even surface finish. However it is still prone to small hair like imperfections on the very fine finish.
Here are photos showing the different machining finishes:
Top Right: 2.5D outline
Top Left: Medium finish
Bottom Right: Fine finish
Bottom Left: Rough finish
And hee is a model showing all 4 surface finishes using XPS pink insulation material
Here is a video of the machining process, using the EPP material.
Cross Linked Polyethylene (XLPE):
XLPE is a soft rubber feeling foam, very similar to EVA, made from Polyethylene resin. It comes in many densities and is commonly used for material packaging due to its impact resistance and high density.
Due to its rubber-like characteristics it can be hard to machine when taking larger cuts, as chips do not break away as easily. This can be seen in the rough finish on Mt Rainier model.
The soft rubber consistency leaves lots of a hair, stricks and residue excess material when being machined.
Here are photos showing the different machining finishes:
Top Right: Fine Finish
Top Left: Rough finish
Bottom Right: 2.5D outline
Bottom Left: Medium finish
Here is a video of the machining process, using the EPP material.
To learn more about all the products we fabricate and our foam cutting and machining processes visit us at WeCutFoam