Mold Surface Finish and part release

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#1

Hi,

My mold is done - it’s a a double mold of 7075-T651, the part is an elongated hex shape about 1" effective diameter, and 36" long. Both halves are identical as far as the part shape is concerned. This is a bladder mold.

The cavity still has cutter marks from the ballnose end mill. Since the part is a handle, some surface roughness is desired. I’d really like to have the existing finish transferred to the part, but I’m worried about getting the part out. Wall thickness will be between 0.030" and 0.050" so the part will flex some. I have heater bands on the outside of the mold and it will get to about 250*F during curing.

Question is, especially given the shape of the molds as I have them, will the very nice and consistent cutter marks hold the part in so I can’t get it out?

What’s the general difficulty in removing a cylindrical part when the surface is rough vs. buffed (dull) vs. highly polished?

When I do the next mold, I’ll tilt the shape to allow easier exit, but for now I have to make the most of what I have.

Thanks,

Ron

#2

Try a small piece first :slight_smile:

As long as the surface isn’t porous, and the surface isn’t too rough, you shouldn’t have problems. I’ve used moulds with anti slip powder coating on large areas, it released without problems after a few parts, the first where a bit sticky. It wears, so it wouldn’t be my choice on moulds though.
From the pic it looks quite shiny and smooth, I dont think it will stick.

#3

Hey Ron,
The cutter marks will not hold your part in. Be sure to use a release agent (I use Safelease #30 or Freekote). Machined tools like that are very common in the industry and I wouldn’t be concerned about the part removal. Any marks you have on the tool will transfer over to the part (assuming your bladder holds pressure!).

Good luck!

#4

Thanks,

Since this is for prototyping, I’ll polish one cavity and leave the other. I can always re-dress the surfaces if I really like the cutter-mark texture.

The next challenge is capping the ends; I’m having a ‘plug’ made for each end, one end I’ll drill and tap for a fitting, the other end I’m looking at making a vent arrangement to allow trapped air to escape.

I read that trapped air can cause bubbles, and that some drill vent holes right into the mold…how does this affect part surface (using twill) and release?

After that, I evaluate the overall mold thickness…looks to be a bit much to heat up and cool down, according to some tests with my heater strips and ramp/soak controller…

Thanks,

Ron

#5

I agree - your tools look a little bit thick. That’s a lot of metal to heat up and cool down.

It’s odd, but in my experience I’ve found less trapped air pockets in tools with visible machining marks. Highly polished tools can sometimes trap the air to the surface because there is no channel for the air to escape. The machining marks seem to provide enough space for air to escape the tool.

Try both and let us know how it goes!

#6

Mixed results after first shaft…

I used Expancel as the inflating agent. Part was made of several layers of prepregged CF/Kevlar and Kevlar 2D braided sleeve. Bladder was Stretchlon 800 cut and welded into a tube just a wee bit larger than the ID (I didn’t want the Expancel to fight the Stretchlon).

I placed the layup directly into the as-machined (with cutter marks) mold with no additional resin. I did put a high temp mold release in but probably not enough.

The end of the mold were covered with gasketed aluminum plates. I tried to vent the mold from one end plate but apparently not successfully.

The shaft was a PITA to get out. Thank goodness I put jackscrews in the mold! I reckon either more mold release or I’ll have to polish the cutter marks out…or both.

When I did get it out, the center 1/3 did not even touch the mold; it’s noticeably smaller in diameter. This may be due to: 1) a huge air pocket, or 2) I didn’t expand the layup well enough and it didn’t have freedom to expand, or 3) I didn’t distribute the Expancel well enough in the bladder. Will have to do another few to test.

The shaft is also a little warped which could be from the odd resulting shape or maybe uneven heating. Regarding heating, each mold half has two 12" long heater bands bolted & glued to the backside. I staggered the top and bottom to try to reduce hot spots. Cure time was 270* F for 4 hrs per the prepreggers recommendation.

Fabric “print through” is also evident. Also, there are a lot of fiber whiskers that may cause the occasional splinter…I tried lightly sanding and spraying with clear enamel but this didn’t totally eliminate them.

I’m guessing that a series of 5 or so holes on each mold half will help vent the air. I was reluctant to drill holes in the mold before I even made a part! Or, maybe applying a vacuum to one end?

The shaft is somewhat usable, is light and seems strong. Flex is hard to determine since the reduced diameter makes my bench-test measured results meaningless.

Your thoughts, ideas, and help to improve my next effort are most welcome!