oil canning and chill markd or vaccume trap?

[rwolf]

i am having some problems with a sign face. the plastic is clear 3/16" polycarbonite with back graphics 3m colered vinyl. the mold is made out of aluminum the head is an 1" deep draw and the outer pan is 2 3/8" deep. the problem i am having im not sure what to call it, or how to fix it. if you look at the face from the side it is bowed out the compitetions face is nice and flat. there are also waves in the part of the face to the left of the head? is this oil canning? and last there is an almost semi circle line that goes from the right top of his head around to his right shoulder is this vaccume trap? i am geting ready to tear this mold apart and redo it due to the head embossment is the wrong size i could use some imput and ideas into what the problems are and what my solutions might be. thanks rwolf i will post more pics in a response.

[rwolf]

here are some more pics.

[neilw20]

Is the mold strong enough to walk on?
Would it deform if you did that? Probably.
You can get pressures approaching 15PSI with the vacuum.

Work that out and I suspect your mold is deflecting.
As the finish is optical, even a small deflection will be easy to see.

Turn it over and cast about 3-4" of concrete, but it would be best to calculate how thick it really needs to be.

[rwolf]

the top layer of the mold is a 1/4" solid aluminum sheet sitting on top of an 3/4" rib structure that sits on another 1/4" base plate that is welded to a 2"x1" aluminum thick wall waccume box. so yes i can walk on it i think i could build a house on it.

[metalworkz]

Hello,
You are calling the mold pan a draw, and if the metal was formed into this shape from a single piece over a die etc. I would think that oil canning is very possible. I know nothing about this molding process but have worked sheetmetal for many years, and in order to get that large inside area to be flat it may be better to do a weldment which would not cause oil canning of the metal. Sometimes the metal can be relieved along the sides that are connected to the canned spots before forming or the metal can be oven flattened after punching or forming, and the type of aluminum used can make a big difference in the finished formed part. If it is canned now you will not be able to remove the bow or deformities without heat treating as far as I know, or you may be able to weld something across the back side in the bowed areas to draw the surface back to a flat state. Perhaps the competitor has made his from a weldment and that will explain the very flat inside surface?

[trophy]

Can I ask as to why you have used polycarbonate?
Acrylics form much easier.
Are you sure your competitor is using polycarbonate.
Polycarbs can be difficult to form and hold a good shape.

[rwolf]

the YUM! cut sheet that gives me my design limits and that requires me to use polycarbonite. the mold surface is perfectly flat i can put a straight edge on it and it is true. i have heard from some other sources that this might have somthing to do with having an non tempture controlled mold? any ideas.??

[trophy]

I find it hard to see your probs from the pics, I assume you only have vac holes around the edges.
I think from what I can understand from your description that you are getting air trapped in the face due to insufficiant vac holes.
Lexan does have a habbit of doing this and I do not know why.
You need to put some holes in those areas to evacuate the entrapped air
They need to be small enough not to imprint on the face of the sign.
The man with all the know how is Spektr hope he chips in.

[rwolf]

there are vaccume holes throughout the entire surdface of the mold. i dont think that that is my problem. the holes are not leaving any marks in the face. i learned that lesson the hard way once. we are tearing apart the mold soon and i will post some more pics of the inside strcture and i will post some better pics soon.

[spektr]

Your mold is way too polished. The lack of smoothness in the plastic is in all probability caused by the plastic "shutting off" in local areas leaving a series of non connected bubbles. This makes the part come out non-flat. this is a common error made by machinists making molds. Now the easy part. Lightly sand the mold with 36 grit sandpaper to put small grooves into the mold surface. The scratches will vent the flat tool face but be small enough hnot to print thru into the part......

When this works, I'll let you send me a hi quality local beer for my collection.....
a nice Hefewiezen would probably hit the spot.....

Scott.