BMW Concept Car Inspired Pencil Holder

In this instructable I will guide you through the steps necessary to build a pencil holder with a unique look.
I designed this pencil holder as a tribute to the newly released concept car BMW M Next Vision. This is a very beautiful futuristic looking car, and BMW released a 3D printable model of it.

You can see photos/renders of it as well as get the original 3D model on the website of BMW: LINK. I linked the german website but look for "bmw m next vision" in your favourite search engine and you should find it quite easily in your language.

However, the model is not perfectly fit for FDM printing (Overhang sometimes >45° and roof almost flat builds very visible print lines). Anyway, I decided to design the pot as a tribute to the design of this car. I didn't use the 3D model, but modeled it from scratch.

Credits to BMW for the partial images I used to explain the inspiration sources/design elements I re-used.

I used (or tried to use) various specific aspects such as:

• the rear light

• the "gills" on the rear window

• the design pattern similar to BMW logo on the front grill. Those I modeled correctly, but the precision of FDM 3D printing was not good enough with such fine details, and once casted with concrete (and since the concrete I used wasn't fine enough) those details were unfortunately almost invisible.
• Color and overall form / style

Before we get started, I want to indicate, that the original design was made to have LED lights in it, in order to have a glowing red BMW logo as rear light (just as in the concept car). However, I used black PLA which is quite opaque. It is possible to have light shining through it if you have a very thin layer (through polishing).

This is however in my experience only achievable when printing flat on the bed (this way the layer can be quite thin).
In my case, I decided to not go for LED lighting. You could if you wanted to, either:

• try to polish a thin enough layer of PLA to see through
• choose a printing Material slightly less opaque

The 3D designed parts allow to build the lighting if you want to.

To do this project you will need following components/material:

• Concrete or cement (see chapter about casting for more details)

• PLA or similar printing filament

• Primer coating (optional)

• Red spray paint (I used Tamiya bright red PS-34)

• A spoonfull of vegetal oil or similar (used as release agent in the mold)

Tools:

• 3D Printer (or access to one)

• Abeaker/cup for mixing the concrete

• A spatula / Stick of some sort for mixing the concrete (I used a kitchen spoon)

• Sanding paper of various grain size (I used 100, 400, 600 and 1200)

• Some metal clips for holding the mold under pressure while curing

• Double side gluing tape or hot glue

The first thing to do is to print the mold. I designed it for printing with as few support as possible.
It is composed of 2 external walls, that fit in together (with a small adjusting pin) and define the outside of the pencil holder, as well as an internal part, that keeps concrete from filling the "hole" of the pencil holder. I built those three parts, so that they fit together on their own.

I printed with .2 mm layer height (in order to shorten the printing time), however if you have the chance to I would recomment .1 mm. This should give you better details.

Print the parts as shown in the screenshot.

You can then put the external walls together (check that adjust pin fit in the little holes), secure this with the metal clips, and then fit the inside part in as shown in the pictures. It should fit just fine if your printer did well. You now have a stable mold for casting your concrete into.

Files can be found also on my thingiverse: Link. Including the 3D model of the pot itself (and not only the mold).

Now this part is the tricky part, and I could have done it better. I tried a first time with rapid hardening cement, which was a catastrophy. The problem is, that pouring half liquid cement in such thin walls (of the pencl holder) is not easy, therefore takes time and the cement started hardening before I could get half of the mold full.
So my recommandations are following:

• first of all oil your mold on all surfaces that come in contact with cement

• find a cement with the finest grains possible, but if possible not a fast-hardening one

• If possible get a cement, that is quite fluid during processing but still cures well (some floor covering cement are very liquid but can cure much better than ours trapped inside the mold)

• Mix your cement with water according to the instructions

• Very important and I did this on my final cast which helped a lot, start casting without the central part (the inside 3d printed part of the mold that comes on top). Pour the first 1-2 cm first, then push the top part of the mold inside. This way cement will already be at the bottom of the mold which might be difficult otherwise and lead to important air bubbles formation.

• then pour the rest of the cement in the walls until the height of the mold where the walls build an angle (see picture)

• To get rid of the bubbles you can either induce vibrations inside the surface on which your mold is (for example a hammer or similar on a table) or stir the cement with a thin metal rod ot similar (or both).

Once this is done you have to wait for the concrete to cure. Depending on the material you use this might vary. However with our very enclosed mold, do not hesitate to wait a bit more before opening it. I waited about 2 days which was maybe not quite enough and I generated a crack on one wall (which I repaired with the fast hardening cement and a small spatula + sanding paper)

Once the cast is fully dry, you can equalize the top of the pot with sanding paper.

Now we can print the cover parts that gives to the pencil holder this specific look.
As I said earlier, I planed for LED lighting and the parts are built so that you could implement it (placeholder for 2 AAA batteries, little channel for cables and a spot for an on/off switch).

I let the design in this state, since it doesn't bother us. Those changes are not visible on the finished product (or almost) and this way you could try and get the light through LED to work.

Print the parts with the Orientation shown in the screenshot for as little support as possible.

Once the parts are printed, you need to polish them (for a cleaner look after Spray painting them).
I used progressively (only on visible surfaces) Sand paper with grain of 100, 400, 600 and finally 1200. On the Rear Light (the part that you won't paint if you used black PLA) I used some polishing paste and a dremel with polishing tool to get a shiny darker look.

I then apply about 3-4 layers of primer, and then 3-4 layers of paint, then let it dry.

Once all parts are ready, glue them/tape them on to the concrete pencil holder.

And voila, you have your pencil case.

I have to say, the overall looks is simewhat similar to the iron man helm. One line is similar to the cheeks of the helm, and the light looks like the eye flipped upside down.

I hope you enjoyed this instructables.