Steel Mold UD Prepreg Problem

[jokete]

Hello friends. I'm new among you.

I make products for various R&D projects. I produce the products in molds made of aluminum and steel. I use blow molding technique in production.
The firm I worked for asked me to make a hollow bar with UD on the outermost surface. However, I am having the problem appearing in the pictures. Has anyone encountered this problem before?
I would be grateful if you could help me with how I can fix it.

My technique;

40 °C --> 4 Bar pressure
60 °C --> 6 Bar pressure
80 °C --> 8 Bar pressure
100 °C --> 10 Bar pressure

I turn off the heating at 135 °C degrees.
20 minutes later; I take it to the cooling pool.

 

[Compositepro]

You appear to have used a plastic release film between the part and your mold. Plastic has a much higher CTE than steel or carbon fiber. When the plastic film expanded due to heating during cure it buckled away from the mold surface and created those dimples in the surface of your part.

Use a release agent on your mold or a Teflon impregnated fiberglass film. The fiberglass restrains the thermal expansion of the Teflon.

 

[jokete]

Thanks for answer.

Mold release frekote 770 nc
Mold sealer frekote b-15
I use their products

Is there a problem with these products?

 

[Compositepro]

The dimples in your part appear to have a very smooth surface finish which typically indicates they cured while in contact with another smooth surface. The only smooth surface that causes this is usually a release film, which I have seen many times. If these dimples did not contact a smooth surface during cure, the surface finish will be determined by the surface tension of the resin and how it interacts with the carbon fiber. Then there will typically be a sharp ridge where the resin transitions from being in contact with the mold surface to no contact. A fingernail or other probe will will catch on this ridge so you can feel it. If this is the case those dimples are fiber puckers.

Puckers are usually caused by a release liner on the unidirectional carbon-fiber tape shrinking after the liner and prepreg have been laminated together. When the liner shrinks the carbon fibers are compressed axially and they will become wavy in the plane of the prepreg. You can see the waviness in the fibers in your part by the way they reflect light with a tiger-stripe pattern. These waves behave as springs in storing mechanical energy. Ten small waves in one centimeter requires more energy than one wave. The smaller in-plane waves (called squiggles) form first because the visco-elastic resin restrains the displacement of the fibers. But with time the many small waves will relax into one larger wave. This wave can be in the plane of the prepreg or out-of-plane depending on how the fibers are restrained. A vacuum-bag or pressure bladder does not provide much restraint against out-of-plane fiber buckling, which is what those dimples are.

Once the carbon fiber alignment becomes distorted there is little that can be done to correct the problem. If you leave a sheet of prepreg like this flat on a table over night you will see puckers the next day. They look like pimples on the flat sheet. If you warm the prepreg to about 100F the puckers will form in a few minutes.