Modeling a phone case

Hi everyone!

I have just started taking industrial design classes and one of my classes is Intro to Model Making. I have designed a phone case that I really want to model but I have no idea how to make it so that it fits perfectly on the phone. The case has two pieces… 1 silicone and 1 plastic. It’s nothing special… just a simple snap-on case with 2 pieces. I know I’ll have to create 2 molds for the pieces but I don’t know how to create a mold of the case. Do I first have to model the two pieces of the case with clay and then make a mold for that?

Thanks in advance!


Well, the TAP demo was interesting, but a bit incomplete. Go get a cup of coffee, and have a seat, this is going to take a minute or two.

Do I first have to model the two pieces of the case with clay and then make a mold for that?

Yes, you do need to make a model, or pattern as it is called, to make a mold. But NOT out of modeling clay. Industrial oil-based modeling clays like [u]Chavant[/u] (as used to sculpt automobile bodies) contain sulfur which inhibits the cure of silicone RTV (room temperature vulcanizing) mold making materials. [u]Plasticine Modeling Clay[/u] (which actually has no mineral clay in it) will not inhibit RTV but is extremely soft and malleable at room temperature - making it worthless for dimensional modeling.

But why do you need to make a mold? Generally, molds are only made after the design has been verified, and when many more identical “models” are needed for testing, user group studies, etc.

A model of your phone project might be an SLA “pattern” generated from your computer modeling program. But since you are taking an “intro to model making” class it could also be hand fabricated from sheet Plexiglas, ABS plastic, aluminum, steel, etc. (whatever is appropriate for the purpose), using hand tools, power band saws, drill presses, milling machines, lathes, etc. to shape the various elements of the design and then assemble it using adhesives, bondo, etc.

All details of the design are required in any model, be it computer-generated, or hand fabricated; material thicknesses, draft angles (if the product is to be molded), mounting bosses, webs, surface textures, etc. It is precision work intended to “prove” the design.

As to my comment that the TAP video is incomplete. RTV molding compounds are very thick, thicker than most pudding; but at the same time are capable of reproducing minute detail on the surface of the pattern onto which they are poured. i.e. If you leave fingerprints on the surface of the pattern, the RTV will pick them up, and the subsequent “model” cast from that mold will have fingerprints on them.

Since this material is so thick, it will trap air bubbles in it as you stir it up in the cup. If poured directly out of the cup onto the pattern, bubbles will be entrapped within the mold and will later cause problems with the parts cast out of it, and most likely will shorten the usable life of the mold. Typically a bubble will find it’s way to the surface of the pattern leaving a microscopically thin layer of RTV against the surface. The first part out of the mold will rip this thin layer off exposing the bubble cavity (which will fill with material each time the mold is used). To eliminate this problem the ideal solution is to mix the RTV in a paper cup, and after it has been thoroughly been mixed place the cup in a vacuum chamber and “de-air” it (the vacuum draws the bubbles to the top of the cup). The final step is to cut a “mouse hole” shaped door in the side of the bottom of the cup with an Xacto knife, to allow the material to flow out of the bottom (farthest from any remaining air bubbles). Pour the material in a corner away from the pattern, not directly onto it. The material will slowly fill up the flask (the box around the pattern) and cover the pattern. Pouring directly onto the pattern can generate bubbles. … which you just took extreme pains to eliminate.

Here’s a few more “youtube” demos. Spend some time looking at what is being used to set up the molds, what the patterns are fabricated out of, what tools are being used, etc. There is lots to learn, and you aren’t going to learn it all from this class; I’ve been doing it for thirty years, and still find new tricks all the time. Youtube is a great teaching tool, I wish it had been around when I had these questions to ask. I used “mold making” in the search engine and was surprised to see how many there were. Unfortunately, thanks to the English language, if you try to use the words, “model making” or “pattern making” you will find vids about putting model airplanes together, and how to make shirts, and dresses… . … There have been quite a few discussion in the [u]Materials & Processes[/u] discussion group over the years so you might want to do a search there as well. The Core77 archives would be another source to search. - Lew

Mold Making - Casting Thin-Walled Parts (Preview) - YouTube (this one is especially applicable to you)

Freeman Supply Co. Video Library:


Have you thought about just having it printed somewhere, with all that time and energy and etc with the DIY you may just find it being cheaper and easier having it done with a 3-d printer.

Just a thought.

You can check it out here

The idea of DIY is good for a learning exp but other than that, maybe take too long and too much money!

“Super Sculpey” (google it) is great for making pattern for a mould. You can quickly sculpt the general form, then bake it for 10mins at alow temp. After this it becomes really solid, chip resistant, you can sand it, machine it and polish it to get a really high quality finish. Plus it’s pretty cheap.

Apparently the class is named “Intro to model making”. You should stick to it. Meaning keep it simple.

That is avoid complex processes like molding. As lmo pointed out molding is used in modelmaking to duplicate parts. There you just have two different parts. So no need for a mold.
It can also be used to change a material. That is you build a solid part then mold it and pour a transparent or rubberlike resin to change the material. Useful for your silicon part but again seems too much trouble for me if you are a newbie. Simply paint the “rubber” part matt, take a studio picture and everyone will assume its rubber if you say so. There are soft touch paints that even do the trick when manipulating the model but not sure you can buy some easily.

Don’t use clay neither. Making a perfectly clean clay model isn’t something you easily succeed at the first time. You will probably end with the asymmetrical potato version of the iPhone.

So here is an old school way of doing a simple model :
First material. Back then at school we used machining board. Nicknamed “lab”, was a common brand name there. They are resin (polyurethane mostly) mixed with wood powder or similar to create woodlike boards but totally homogeneous, without the grain. They generally are offered in a variety of grades (densities), exactly like polyurethane (yellow) foam.
Don’t use foam its too soft for a cell phone sized model.
There an exemple I quickly grabed on the net : Prototyping & Tooling Materials | Curbell Plastics

Then process
So :
Step 1 Make an Illustrator blueprint of your project. Or a 3D model if you know 3D. By the way the goal its learning modelmaking so having it 3D printed won’t learn you much imho. Make a 2D blue print of the 3D model
Step 2 Print it (on paper). Double check size is really 100%
Step 3 Glue it on a small block of the forementioned resin board.
Step 4 Bore the hollow parts using a manual machining…er…machine (english is not my language…)
Step 5 Cut or machine or simply file the external surface details. Do not make step 5 then step 4 ! How would you then grab tightly your nice external surfaces without damaging them ?
Step 6 Sand then paint it. Glue the 2 parts together. Its better to use double sided tape to allow for dissassembly.

If there is time left you can try to make a silicon part from your hard part.

Seek advice at your school there is certainly a teacher, or better still someone responsible for the modelmaking workshop, who can give more detailled info based on what you have or have not on that particulatr workshop.

Keep us informed !

Thanks for the suggestions everyone! I’m busy with other projects right now so haven’t had time to work on this one. Besides, I don’t think this project is a beginner level project so I’d rather get some experience before I start working on this one. :slight_smile: