Assignment 12 19.4.2017

Topic: Moulding and Casting

Learning outcomes:

Design a 3D mould, machine it, and cast parts from it.

Design appropriate objects within the limitations of 3 axis machining.
Demonstrate workflows used in mould design, construction and casting.

Have you:

Explained how you made your files for machining.
Shown how you made your mould and cast the parts.
Described problems and how you fixed them.
Included your design files and ‘hero shot’ photos of the mould and the final object.

What product ?

I decided to make a case for my FAB TinyISP programmer board.
mould s

What kind of mould? Core or not? I modeled my board. Then, I made some modeling of a mould and a mould for this mould. I still needed this board and changed my plan.
I found old broken memory stick and modeled new case for it.

s 1

I used again NX11 CAD/CAM features as my assignment 7. I defined my tools, rough milling by ball head bit Ø3.18 mm (1/8") with free length of 40 mm and fine milling by square bit Ø0.79 mm.This information was important to reserve enough space for milling.
bit 3 mm

The block size of my model is same as Ferris File-A-Wax block, 146.05 x 88.9 x 28.1 mm. In this case, I modeled directly the mould, at first, two cavities for silicon rubber outlines by size is 50 x 35 x 15 mm (WxHxD). Then, I modeled details of the case halves. And, I tried remember they are mirrors of each other as the hole location shows. Upper half include space for the memory stick core. I have plan I put it in place before casting. NX11 owns also mold wizard for mould design, but didn't use it.

stick

Mould design 1.0

I designed two phase milling of the mould. At first, I do rough milling by Cavity mill operation and a Ø3.18 mm bit.
cavity

And then I continue finish milling by Fixed contour operation, and a Ø0.78 mm bit. I limited the operation area by 10 mm offset from mould outlines.

contour

Later, I added also Ø4 mm inlet and Ø2 mm air vent channels to my model. Also, zero XY coordinates must be located at top of workpiece and in one corner, as +Z axis up. Both operations used 250 mm per minute feed rate and 6000 revolutions per minute spindle speed. Final check was that I use millimeter units.

1st m

I made fist cavity mill operation to a Mdf -board by ball head bit, Ø3.18 mm. I used our CNC router machine Excitech E2-1325 for milling.

Mould design 2.0

I inspected results and I changed my milling parameters. The bit will be the same ball head Ø3.18 mm. I lifted up the cavities to minimize milling time and material usage. Now the size is 50 x 35 x 8 mm (WxHxD). Also, I limited stepover to 30% instead of 50%, Depth per cut I set to 2 mm and minimum ramp length to 20%.

non cut

cavity 2 fixed 2

Mould design 3.0

Finally, I read some documentation of NX11 use. And. I changed my plan again. I planned rough mill by square head bit, Ø3.18 mm, but I used finish dimensions. The operation was cavity mill as earlier. Therefore, I got final dimensions to my mould as much as possible. Percent of flat diameter was 50% and maximum depth step 2 mm. Then, the second operation used ball head bit, Ø3.18 mm and I only finalized rounded details. Method was area milling and maximum scallop height 0.02 mm.

I found also new properties of NX11 CAM. If I select 3D Dynamic instead of Replay, I can see dynamically the milling process and finally non-milled material by color codes.

Blue color shows rest material I must mill out. Yellow color shows material I cannot mill out by this operation and settings.

precess rest

Again, I used our CNC router machine Eidtech E2-1325 for milling. And, I used MDF as mould material again. It is cheap and milling quality interested me.

Then, Ivan and Jari helped me in casting practice. I needed a cup, mix stick, gloves and many more. The liquid rubber I used was OOMOO®25 Tin cure silicone rubber, which had a 15 min pot life and 75 min cure time. The two components have one-to-one by volume mix ratio and I used vacuum degassing and release agent. The liquid plastic was Smooth-Cast®310, which had a pot life of 15-20 min and cure time of 3-4 hours. It is two components have also one-to-one by volume mix ratio and didn't require vacuum degassing.

Operation of the mould

Here is my first result, a dummy test casting. It showed my mould works. It had some debris from Mdf board.

2 c r c

The second cast included also my memory stick. I placed it inside the mould by tiny part of paper taped in bottom of the memory stick. Also, I used two sided tape for securing of contacts and it's correct location inside mould. Finally, my memory stick fits and works!

My NX CAD file is here: 3D model.
This also in DXF format is here: 3D model.
And in stl format: FABmoldstick21.stl.

Comments

Ø4 mm inlet and Ø2 mm air vent channels worked well, but next time I do conical inlet. It helps filling the mould.

I filled the mould contacts of the memory stick down and I see the stick lifted up a bit, about 1 mm. The Ø3.5 mm hole I designed as thru hole is as blind one.

Also, I wondered minor grooves on the mould. They copied also to my part. They are parallel and done when finalized my mould. They exist due to incorrect zeroing of the mill bit length. Our calibrator of the router Z axis needs calibration itself.

Earlier during assignment 7 I documented that I needed to edit NX11 postprosessed G-code before use it in our router. Here I found I can use code without editing.

MDF is good mould material.