Prototype molding

Ok, so I am trying to get a prototype of my tip design molded for my stylus project. The reason I need it molded is because the material I am using is only available in pellet form, no solid blocks.

Now I’m not quite sure how to go about this in the most economical way. I really dont want to have to pay a mold fee for each iteration. Granted I pretty much have it nailed down, I just want to confirm that it works with the material.

I’ve thought of a couple options:

  1. Just pay the mold fee for each iteration.
  2. Try and mold a block, then have it CNC’d.
  3. Use an insert for the cavity so that any mold changes would just require a new insert, not a completely new mold.

Any suggestions?

I’m still waiting on a reply from my prototyper so see what the mold fee would be. She said it would be cheap, but that’s a pretty relative term…

Without seeing the design, all 3 are potential options.

Option 1 will be the most expensive and least preferred.

Option 2 may be possible, depending on the material. When you say “stylus” what material is being used? Certain materials, especially those with additives are too brittle to properly CNC when molding, and molding a large block can be challenging due to how the block cools and air bubbles that can potentially get trapped in the material.

Option 3 is probably the best solution. If you can isolate the areas of change to a specific part then put that part on an insert and then it’s more easily modified. Just be prepared to understand where your witness line will be.

Im using a conductive plastic for the stylus tip (it is about as big as as the sharpened part of a pencil). It is PA66 (polyamide 66) with stainless steel for conductivity.

I figured option 2 would have cooling problems…

I think I will go with option 3 once I hear back about the mold fee. Since it is a relatively small part a revision shouldn’t cost much if I am using inserts.

Hey Greg,

Is the conductivity necessary for the prototype? I had to source a prototype material in similar situation and ended up finding a 3D printed material with similar properties. A urethane for RTV molding might be useful too.

Try Matweb’s property search: Search Engineering Material by Property Value

Maybe im not thinking this throughly. But for a prototype?

Could you just made a 3D Print. Silicone mold and then pour the plastic? (Might melt mold?*)

Plaster Mold (melt out 3d print) Repeat process?

After that go for a much more polished model?

At this point the conductivity is necessary. I have gotten past the form models and want to confirm that the tip will in fact work with a touch screen. I attempted to use conductive ABS filament and 3D print it, but the filament was not quite conductive enough and did not give reliable results (it is significantly less conductive than the PA66/SS).

I tried molding, but only with the conductive filament I previously mentioned. I had trouble with melting the plastic down to a molten state so I could pour it. I wil probably try again when I get the official material.

Any suggestions on melting the plastic? When I tried melting the filament I just put it in a small metal bowl and stuck it on my grill for a while. It all just stuck together. FWIW, the PA66/SS has a melt temp of 530-550 F (according to the mold parameters tech sheet).

From what I have read, unless the plastic is something like a urethane that is liquidy, it is VERY hard to pour into a mold without proper equipment.

As I’m understanding it, the pellets contain chopped SS wire to be injected along with the melted plastic, correct? If so, I’m worried there could be serious QC issues when mass producing, as the wire could orient semi-randomly and not necessary make an electrical connection, even if your prototype is functional. Has this material been used in a similar application before?

For a part this size, tooling will be inexpensive by any metric. The best way in my opinion would be to mold the part. You will get all off the character, surface finish, slide, material flow, etc. CNC machining metal filled plastics might have a lot of different surface character. You can make twenty for the same price as one, cost will be mounting into the mold and turning on the machine and all of the material in the screw.

Starting with the smallest iterations you can always use the electrode to cut larger interations into the same space. With a mold insert into a mold base, you could no doubt mold four or five different versions in the same go, right from the start. Your plan would be to make one test mold with multiple variations, adapt the variations and re-shoot if needed, finalize and make the final mold insert into the mold base.

I dont think the electrical connection will be an issue. I was able to get a sample plate from the company to test if the material would actually work, but not in the form I want. The plate was injection molded so I dont think it will be an issue. I guess I will find out for sure when I get it molded.

You have tested the sample material and found it conductive, it is likely it will be conductive in the form you want.

You have finalized the form with a non-conductive plastic.

Why would marrying the two need mold changes?

Why do you need iterations of the final form?

If you really need the prototype to be conductive, why not machine some aluminum?

I just went through this on a project, so I understand the pain and requirements.

The issue isn’t conductive or not conductive, it’s how much.

There is a reason almost every stylus you see has a very fat, often squishy tip - because certain touch panels will not be triggered unless they see a certain level of capacitance. This means if you design a hard tip that is 2mm in diameter, even if it’s conductive there may not be enough surface area to work on certain touch screens. In my case, we were able to get a reference design from a touch panel vendor but it was made easier by the fact the stylus is only working on a single device/touch panel.

Machining it out of a different material, and trying to cast it are not going to work. Even if you get a satisfactory result, the magic is in the material and the material takes on very specific properties when molded.

Cut the tool, and like Shaw mentioned you can always make tool safe modifications to GROW the part, but not shrink it. Isolate what you can to an insert or slide if needed and you’ll be good.

Thanks for the explanation, I did not know.

And yes, a metal-safe tool. I always would rather burn another 0.002 than buy a new insert. But in reality, a new insert is pretty cheap, especially since this can be a mud tool.

Thanks Cyber! This is probably what I should have explained to start off with.

Interesting, good question and comments.

I have attempted molding stainless steel fiber filled plastics for EMC, to replace expensive, ugly, and inconsistent spray coating, for very demanding, safety certification.

The surface conductivity was very inconsistent, depending on random SS fiber distribution. Also, molding process parameters of heating the tool results in a “skin” effect of neat resin with the SS fibers slightly below. Again end result is inconsistent surface continuity. SS fiber filled plastic injection molded housings of an electronic device failed near and far field EMC testing; copper painted interior, carbon spray coated exterior housings passed all tests, except visual aesthetic.

The injection molders disliked the material intensely. The SS fibers are magnetic and stuck to the screw barrel, machine cleaning was a huge effort. They are also very abrasive causing visible damage to machine parts in one sample run.

If I were to try again, I would try these nickel coated carbon fiber filled plastics.