Box shaped enclosure manufacturing

[Dongri Karthik]

I have trying to manufacture box shaped enclosure for a camera. The mold is female mold. I am draping carbon fiber prepreg inside the mold and applying vacuum bagging method. I am not getting proper finish on the part and there are dry spots at corner edges. The  prepreg layers are not compacted properly and gap between layers is observed at corner edges. 

Please suggest a procedure to achieve good finish without any defects in box shaped parts

[John Kimball]

6 hours ago, Dongri Karthik said:

I have trying to manufacture box shaped enclosure for a camera. The mold is female mold. I am draping carbon fiber prepreg inside the mold and applying vacuum bagging method. I am not getting proper finish on the part and there are dry spots at corner edges. The  prepreg layers are not compacted properly and gap between layers is observed at corner edges. 

Please suggest a procedure to achieve good finish without any defects in box shaped parts

Welcome to the forum. Sounds like a great project. We'd love to see your project. Post pictures if you can. 

The most important suggestions I can give when working with prepreg vacuum bagging are as follows:
1. Patience is a virtue.
2. Carbon fiber NEVER stretches.
3. The vacuum bag is not a magic wand for perfect parts.

Having said that, the proper method for your application is centered around getting proper placement of the plies in the corners. Female molds are especially hard to get all the plies snugly placed in the corners without bridging (when plies don't touch each other in the corners and create a bridge or gap in the corner). Every ply needs to be deliberately and carefully placed into each corner. Start in the corners and work the material out to the edges. You are allowed to cut the prepreg and overlap slightly if it won't conform to the mold surface, just try not to have seams on top of seams, try to stagger them from ply to ply.

The next step is to place your vacuum bagging plies as carefully as the prepreg plies.
1. Peelply - Peelply will not stretch, so place it the same way you place the prepreg, cutting the material where it won't conform.
2. Release Film - The release film can be placed without cutting it by folding the material on top of it's self, but if you need to cut it, use flash tape to hold it in place.
3. Breather fabric - It will need the same attention as the peelply, but it does stretch a little bit, so you can less careful. 

The most important step is the vacuum bag. You will need a much larger bag than the part to allow the bag to get into all the corners. The bag will most likely have wrinkles in it, and that's OK. After assuring that you have a leak free bag, let some air back in the bag and then carefully work the bag into the corners using your fingers or a dull pusher stick to push the bag into the corners as tight as possible. Attach the vacuum again and then check the corners for bridging. If you see bridging, repeat the process of letting air back in the bag and pushing the bag in the corner. Having too much bag is a blessing, and having a crease in the corner will allow for bridge free corners.

You can watch the video below to see some of the bagging techniques. You can see how placement of the breather is important.

Here are some other pointers that may help:

1. Air leaks in the vacuum bag will reduce the amount of compaction the bag provides. Always find and seal any leaks in the sealant or vacuum bag. A simple crease in the bag on the sealant tape will cause a leak. Also, pin holes in the bag will cause leaks.
2. NEVER place the vacuum port directly on top of the part. Always place it off to the side, but provide a vacuum path from the part to the vacuum port.
3. Always place the prepreg or wet layup materials in the mold as tight to the surface as possible, especially in the corners.
4. If you have thick laminates with more than 8 plies, a debulk operation may be required. Debulk is an intermediary step of vacuum bagging the part to compress the plies before adding more plies. In most cases, a debulk can be added every 4 plies if the part is very thick.
5. Darting or cutting the prepreg to get it to conform to the surface is acceptable, but keep it to a minimum. ¼” to ½” overlaps at the seams is preferred. Try to not place seams on top of each other ( from one ply to the next). Think of brick a wall. The bricks are staggered to prevent cracking at the seams.
6. NEVER assume that the vacuum bag will stretch the material or bag into corners. It won’t. Ever. Wrinkles in the vacuum bag are acceptable.
7. Larger tools may require more vacuum ports to provide enough compaction for the entire part. About 10 ft2 per vacuum port is a good standard to use.
8. A release agent on the tool (mold) surface is required. If not, you will bond the material to the surface. Sometimes permanently. Chemical release agents work best. 
9. A vacuum bag/Oven only cure will provide about 12-14 psi of equivalent pressure to the part surface. The addition of an autoclave can add an additional 40-150 psi to the surface. The more pressure you can get on the surface, the better the part will be structurally and aesthetically.
10. It is also acceptable to press mold prepregs, but the thickness must be controlled to ensure that the resin is not pressed out of the material, and the press must be heated to cure the prepreg.
     

 

[Dongri Karthik]

15 hours ago, John Kimball said:

Welcome to the forum. Sounds like a great project. We'd love to see your project. Post pictures if you can. 

The most important suggestions I can give when working with prepreg vacuum bagging are as follows:
1. Patience is a virtue.
2. Carbon fiber NEVER stretches.
3. The vacuum bag is not a magic wand for perfect parts.

Having said that, the proper method for your application is centered around getting proper placement of the plies in the corners. Female molds are especially hard to get all the plies snugly placed in the corners without bridging (when plies don't touch each other in the corners and create a bridge or gap in the corner). Every ply needs to be deliberately and carefully placed into each corner. Start in the corners and work the material out to the edges. You are allowed to cut the prepreg and overlap slightly if it won't conform to the mold surface, just try not to have seams on top of seams, try to stagger them from ply to ply.

The next step is to place your vacuum bagging plies as carefully as the prepreg plies.
1. Peelply - Peelply will not stretch, so place it the same way you place the prepreg, cutting the material where it won't conform.
2. Release Film - The release film can be placed without cutting it by folding the material on top of it's self, but if you need to cut it, use flash tape to hold it in place.
3. Breather fabric - It will need the same attention as the peelply, but it does stretch a little bit, so you can less careful. 

The most important step is the vacuum bag. You will need a much larger bag than the part to allow the bag to get into all the corners. The bag will most likely have wrinkles in it, and that's OK. After assuring that you have a leak free bag, let some air back in the bag and then carefully work the bag into the corners using your fingers or a dull pusher stick to push the bag into the corners as tight as possible. Attach the vacuum again and then check the corners for bridging. If you see bridging, repeat the process of letting air back in the bag and pushing the bag in the corner. Having too much bag is a blessing, and having a crease in the corner will allow for bridge free corners.

You can watch the video below to see some of the bagging techniques. You can see how placement of the breather is important.

Here are some other pointers that may help:

1. Air leaks in the vacuum bag will reduce the amount of compaction the bag provides. Always find and seal any leaks in the sealant or vacuum bag. A simple crease in the bag on the sealant tape will cause a leak. Also, pin holes in the bag will cause leaks.
2. NEVER place the vacuum port directly on top of the part. Always place it off to the side, but provide a vacuum path from the part to the vacuum port.
3. Always place the prepreg or wet layup materials in the mold as tight to the surface as possible, especially in the corners.
4. If you have thick laminates with more than 8 plies, a debulk operation may be required. Debulk is an intermediary step of vacuum bagging the part to compress the plies before adding more plies. In most cases, a debulk can be added every 4 plies if the part is very thick.
5. Darting or cutting the prepreg to get it to conform to the surface is acceptable, but keep it to a minimum. ¼” to ½” overlaps at the seams is preferred. Try to not place seams on top of each other ( from one ply to the next). Think of brick a wall. The bricks are staggered to prevent cracking at the seams.
6. NEVER assume that the vacuum bag will stretch the material or bag into corners. It won’t. Ever. Wrinkles in the vacuum bag are acceptable.
7. Larger tools may require more vacuum ports to provide enough compaction for the entire part. About 10 ft2 per vacuum port is a good standard to use.
8. A release agent on the tool (mold) surface is required. If not, you will bond the material to the surface. Sometimes permanently. Chemical release agents work best. 
9. A vacuum bag/Oven only cure will provide about 12-14 psi of equivalent pressure to the part surface. The addition of an autoclave can add an additional 40-150 psi to the surface. The more pressure you can get on the surface, the better the part will be structurally and aesthetically.
10. It is also acceptable to press mold prepregs, but the thickness must be controlled to ensure that the resin is not pressed out of the material, and the press must be heated to cure the prepreg.
     

 

Thank you so much for your suggestions. I will implement your suggestions and try to manufacture again. Here are the pictures of the manufactured CFRP part

[John Kimball]

7 hours ago, Dongri Karthik said:

Thank you so much for your suggestions. I will implement your suggestions and try to manufacture again. Here are the pictures of the manufactured CFRP part

This looks like a great start.

My 3 observations:
1. As mentioned, you need to make sure the fiber is deliberately placed into the corners, one ply at a time. There is definitely bridging in all the corners.
2. Your vacuum port appears to be isolated from the breather on the part. You must have a vacuum path from the port to the breather on the part. This will help with the porosity on the surface and may help in the corners a bit too. Just add a strip of breather from the port to the part to achieve full vacuum on the part. Otherwise, the vacuum will seal itself off from the port and not pull vacuum on the part. In the video link above, you will notice that there is always a vacuum path the part.
3. It appears that you have enough bag and it's tucked in pretty good, but you may want to tailor fit the breather so there isn't too much overlap. too much breather can result in less vacuum pressure on the part. You could also use the the envelope bag method (from the video link) to use less bag and make the bagging easier.

[Dongri Karthik]

2 hours ago, John Kimball said:

This looks like a great start.

My 3 observations:
1. As mentioned, you need to make sure the fiber is deliberately placed into the corners, one ply at a time. There is definitely bridging in all the corners.
2. Your vacuum port appears to be isolated from the breather on the part. You must have a vacuum path from the port to the breather on the part. This will help with the porosity on the surface and may help in the corners a bit too. Just add a strip of breather from the port to the part to achieve full vacuum on the part. Otherwise, the vacuum will seal itself off from the port and not pull vacuum on the part. In the video link above, you will notice that there is always a vacuum path the part.
3. It appears that you have enough bag and it's tucked in pretty good, but you may want to tailor fit the breather so there isn't too much overlap. too much breather can result in less vacuum pressure on the part. You could also use the the envelope bag method (from the video link) to use less bag and make the bagging easier.

Thank You Sir.

[Dongri Karthik]

Hi John,

I implemented your suggestions and tried manufacturing. I could eliminate bridging at corners and pinholes but resin is bleeding out of prepreg and the part is having rough surface on inside and outside. I am following the curing cycle recommended by the manufacturer. I am using single layer of bleeder cloth and the vacuum port is connected to the breather on the part. How to minimize resin bleed out during curing?

[John Kimball]

9 hours ago, Dongri Karthik said:

Hi John,

I implemented your suggestions and tried manufacturing. I could eliminate bridging at corners and pinholes but resin is bleeding out of prepreg and the part is having rough surface on inside and outside. I am following the curing cycle recommended by the manufacturer. I am using single layer of bleeder cloth and the vacuum port is connected to the breather on the part. How to minimize resin bleed out during curing?

Make sure you are using the correct materials in the correct order.
1. Carbon fiber layup
2. Peel ply
3. Release film
4. Breather cloth
5. Vacuum bag

The peel ply will help to evacuate any trapped air in the part.
The release film will control the resin bleed and prevent the breather from sticking to the peel ply.
The breather allows the vacuum to stay constant throughout the cure cycle.

A general rule of thumb for me is that every layer of consumable vacuum system will be slightly larger than the previous layer. For example, if your layup is 12" x 12", then your peelply will be 13" x 13", Release film 14" x 14", etc.

Without the proper materials and placement, the resin can bleed out too much and cause problems. And I can't stress enough the need for a leak proof bag.