mold 1st contact point marks

[nbedard]

anyone here has a solution for this ?

When thermoforming ABS, a line appears through the plastic where the mold first touched the plastic. See attached picture.

We are using a Vacuum box or draw box from above to pre-stretch the sheet upward, then the male mold lifts in the sheet from below.

I tried more/less heat on the mold, more/less heat on the sheets, teflon mold release, with no luck.

[DieGuy]

So you are pulling vacumm on the non finish side? I built fridge liner vacumm molds a few years back and we pulled the vacumm on the finish side.

[johnd]

Male molds will produce flow lines. Some things that will help reduce this tendency is to heat the tool. This reduces the tendency of the material to grab and slide. You can't do much with an epoxy mold which I assume you are using. An aluminum tool would be preferable. A female mold would be preferable.
Another thing you can do is prebellow the sheet from the mold side. As the sheet ballons the mold is moved in. Then vaccum is applied and the plastic is snapped to the mold.
Last thing are you drying your sheet prior to forming? ABS is hydroscopic. A high moisture content will contribute to flow lines.

JohnD