csun composites lab manual

California State University, Northridge (CSUN) Composites Lab manual

College of Engineering and Computer Science (CECS) Version 1 (2016)

Mechanical Engineering (ME) Department Dr. Peter L. Bishay

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COMPOSITES LAB

MANUAL Version 1

Lab 5: Prepreg layup

The original version of this manual was a one–student senior design project written by

Katherine White, the Composite lab assistant, during Spring 2016 semester under supervision of

Dr. Peter L. Bishay who captured almost all pictures in this manual.

This current version (version 1) has been modified and amended by Dr. Peter L. Bishay

during Fall 2016 semester in order to be used by students enrolled in ME436/L course: Mechanics

and Design with Composite Materials, as well as Senior Design students.




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Pre-preg Layup

A pre-preg is a composite fabric that already has resin pre-impregnated into it. Pre-pregs are stored

in freezers to prevent the pre-impregnated resin from curing. In industry, the “out time” is kept on

a log because pre-pregs can expire. The less time the composites is out of the freezer the better.

You want to cut your kit efficiently in a cool room to prevent it from curing. Also, before cutting

the kit you want to let the pre-preg thaw to room temperature in a sealed bag to prevent

condensation on the composite. When you are finished cutting you want to seal the composite back

into the bag with the least amount of air possible and return it to the freezer immediately.

Once the surface has been prepped and you have made your kit, you are ready to layup. There are

two films on a pre-preg to protect it. Here we speak exclusively of the ones that are on the uni-

directional carbon fiber pre-preg we have at CSUN. One side is blue and thin, while the other is

brown, slightly thicker than the blue, and is like wax paper. You will need to take off one of these

films to be able to put your composites on the mold you are using (this process is like very carefully

applying a sticker). You want to avoid wrinkles and bubbles when applying the composite.

Pre-preg Layup Steps:

(1) Take out the pre-preg (kept sealed) to thaw to room temperature. Leave out for at least an

hour before unsealing the container. (In industry allowing to thaw for 12 hours is more

acceptable. However, note that the composites we have are already expiring and an hour

will work).

(2) Complete the surface preparation Section in Lab 3 before beginning. A good time to do

this is when the pre-preg is thawing

Figure 1: Preparing surface

(3) Cut out all of the pieces you will be needing. It is best to use sharp Stanley blades, a cutting

mat, and a T-square. Carefully measure dimensions you will be needing with a measuring

tape and mark out with a sharpie.




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Figure 2: (left) Use T-square to cut straight, (right) Layup kit prepared and labeled

(4) Remove one side of the film from the pre-preg (a blade will help snag the film). You will

find it easier to work with one side depending on the shape of the mold or personal

preference. Experience will let you know which you will choose. Pull ONLY in direction

of the fibers, do not pull up, try to give it more of a “shear force” so the fibers do not pull

up.

Figure 3: Use a blade to help separate the film from the composite

(5) Use a plastic squeegee and your hands (with gloves) to apply the pre-preg. Start with one

corner, and slowly apply one side (This corner will be your datum for all other layers). Lay

down the rest of the layer putting pressure from the middle out attempting to not cause

wrinkles, like applying a sticker.




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Figure 4: Apply your datum corner, press down this edge. Press from middle to left then middle

to right. Doing this prevents you from moving bubbles across the entire layer.

(6) Peel off the second layer of film once the pre-preg is applied and inspect the surface. Lay

the film back down and smooth out bubbles or other defects that you observe. When done

remove the film for good (only smooth in direction of fibers. Otherwise they can separate).

Figure 5: Use blade to help peel off film. Peel off in direction of fibers.

Figure 6: The blue film is best for seeing where wrinkles and bubbles are




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(7) Continue this process by applying the rest of the layers in the same fashion being mindful

of your layup direction, datum corner, and alignment of each layer.

Figure 7: Apply remaining layer

(8) Move on to bagging process in Lab 3 (wait to connect vacuum until in oven)

Figure 8: Apply release film and smooth all wrinkles out




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Figure 9: Make sure to firmly press bagging into vacuum tape to prevent leaks and do not forget

to put in the vacuum connection.

(9) Put piece in the oven and THEN connect the vacuum connection

Figure 10: Carefully transport glass and place in oven. Attach vacuum.




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Figure 11: Vacuum pump outside of oven fed through opening. Be sure to plug this opening with

the grey plug once the pump is connected into the bagging.

(10) Select ‘STOP’ on the top left of the screen. Program should be on 3-hour panel. If not select

‘PGM. No.’ and type in the corresponding program number for 3-hour panel (for curing

uni-directional pre-preg). To find the number select “Program Setup” from first screen and

search for the correct program name and number. Select ‘START’ in Program Mode once

the correct program is selected. You will hear the oven start. If you want to monitor the

oven select the “Monitor” box on the first screen. Going through the menus on your own

will help teach you how to use these menus.




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Figure 12: (left) Main oven screen (right) Program run and selection screen

Figure 13: The oven is running, and the vacuum pump is connected to the layup through the side

opening

csun composites lab manual

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