COMPOSITES LAB MANUAL Version 1 Lab 3: Surface Preparation, Wet Layup, and Vacuum Bagging 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 who captured almost all pictures in this manual. This current version (version 1) has been modified and amended by 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. 1
I. Preparing the Layup Surface: The place where you are preparing your tool to layup on should be clean. Any dirt particles or oils can disrupt bonding to the matrix. The ideal layup surface is smooth and has minimized porosity. The way the layup surface looks is exactly how it will appear on the composite. Thus, if a mirror finish is required the tool surface must also have a mirror finish. Example layup surfaces include tempered glass, aluminum sheets, or high density foam coated with Duratek and Bondo. Smooth surfaces can be achieved on some materials by sanding them. Start at a low enough grit to remove the defects in the surface and then continue increasing in grit number until desired surface finish is achieved. Make sure to sand with even pressure, in one direction, making even strokes across the surface. Next, the surface must be cleaned. Use acetone and a microfiber cloth to clean the surface of dirt particles. Use Sealer Release TR-910 to help fill in pores of the mold (this is used only the first time you prepare a new mold, or after a mold was damaged and re-sanded). Ten layers must be applied if using the product shown in Figure 1 (left). Make sure to always read the directions so that you do not ruin your mold. Use Liquid Release TR-930, shown in Figure 1 (right), to prepare the mold every time you lay up. If it is the first time, apply ten layers, after that only 2-3 layers are required each time. If you do not do this properly and let it cure for the full amount of time you will permanently bond your composite to the mold. Trying to release the composite from a mold that has not been properly released could permanently damage the mold beyond repair. Never pry a mold open, always use a proper tool like a release wedge as seen in Figure 2. Prying will damage the mold, most likely break the tool you are using to pry, damage the composite, and possibly hurt the person prying. Whatever your surface looks like, having streaks or scratches, that will appear mirrored on your cured composite when it is finished. This is why it is necessary to take care of all surfaces used for laying up if you intend on using the tool again. Figure 1: Sealer Release TR-910 and Release TR-930 2
Figure 2: Release wedges (Fibreglast.com) Surface Preparation Steps: (1) If necessary sand the layup surface (you may be starting with a scratched metal surface. If this is smooth glass skip step (1) and (2)) i. Start at lowest grit necessary to remove scratches (180-320 grit) (make sure to sand evenly in one direction creating parallel lines. Pick up sand paper to move it over to avoid uneven sanding) ii. Increase grit until a shiny smooth surface is achieved (500-1200 grit) Figure 3: Sanding the layup surface (2) Clean surface with acetone and a clean microfiber cloth Figure 4: Clean surface with acetone 3
(3) Apply the vacuum tape firmly to the perimeter of the layup leaving the wax paper on to protect it from releasing products and resin during layup. Make sure you know where the composite will be laying and give about three inches extra. It s best to have extra space between the vacuum tape and the composite (this is extra important during a wet layup because resin can be messy). Applying vacuum tape now will ensure a better seal than if you wait until later when other chemicals could get on the surface. Figure 5: This is an example of leaving about 1 inch of space between the tape and where you expect the composite to be. It s best to leave more room than this. Figure 6: (left) Mold the tape around corners (don t cut and overlap), (right) Firmly press down on tape to get a good seal (4) Apply mold wax with a microfiber cloth if your surface is brand new or newly repaired. Buff in circles evenly across the surface 4
Figure 7(left) Mirror Glaze Mold Release Wax, (right) Buff small circles evenly across surface (5) Let wax turn opaque (couple minutes) and then buff out with a clean side of the microfiber cloth Figure 8: (left) Wax has turned opaque, (right) Buff out wax with cloth (6) Apply sealer release (TR-910 Multi-Pull Liquid Semi-Permanent Release) if this a new mold or if it has been sanded or repaired to fix possible damages. This is a base coat that seals the molds surface before application of the TR-930. Use a new cloth and apply 10 layers. Wait to air dry 15 minutes between each layer OR use a heat gun to speed up the process. Be cautious using a heat gun as to not damage the mold. If your mold was made using foam or Duratek, check the maximum temperature these products can undergo. 5
Figure 9 (left) Carefully apply release to the end of the cloth, (right) Apply even strokes across the surface Figure 10: Use the heat gun to quickly dry the layers of release (7) Apply liquid release (TR-930 ultra-thin clear release film). Ten layers should be done the first time a mold is prepared, following layups will only require 3-4 layers depending on the state of the mold. Be sure to allow each layer to dry as in step (5). II. Layup Process: Once the layup surface has been prepared, it is ready to be used. It is best to prepare what is known as a kit prior to layup. A kit is pre-cut pieces of pre-preg or composite reinforcement labeled with the fiber orientation of its layer, stacked in the order it will be applied to the tool. Fiber orientations are determined from the warp clock depending on the loading that the part will 6
experience. It is also recommended, especially for longer layups, to keep a written layup schedule with you to check off each layer as you go. Figure 11: (left) Composites scissors, (right) Preparing kit for wet layup Figure 12: Warp Clock Wet Layup Gather three cups, being cautious that the material of the cup will not melt with the resin you are using or acetone. Metal cans (cut in half and washed) or paper cups work well (not red solo cups). Also note that some resins will heat up once mixed and this can also melt the container. One cup will be for the resin, one for the hardener, and then one for mixing both. Have a scale, covered to protect from dripping resin, this will be used to measure the ratios for mixing. Find the ratio of resin to hardener on the technical data sheet for the product you are using. Make sure to have a clock or timer nearby to keep track of the pot life of the resin. Once you start mixing, the pot life begins. Mix small amounts at a time (about 200-300 grams). If your kit is ready, begin measuring and mixing the first batch. Use a brush or foam roller to apply the resin to the tool. The more resin, the better, you want to apply a thick layer. If not enough resin is applied, the fibers will be starved and pores or large defects will appear when the part is done. However, do note that if it is drowned in resin it is possible to change the material properties. The breather cloth 7
should absorb most of the excess resin. Carefully lay down the first layer of cloth (reinforcement material), slowly draping it being mindful to not distort the fibers, and apply more resin. The cloth should look very wet. Once enough resin is applied, use the metal roller to better distribute resin throughout the cloth and to eliminate air bubbles. You should be able to see resin squishing around the roller and hear a sound as it rolls back and forth to ensure enough resin is on the cloth. Lay another layer of cloth, and repeat. When applying the cloth or using the metal roller on it, be cautious as to not distort the cloth. Wet Lay-up Steps: Make sure you have finished surface preparation steps first before proceeding Figure 13: Paper mixing bowls and sticks for each for resin preparation (using sharpie to label which container is for what can be useful) (1) Mix the first batch of resin: i.use the scale to measure out hardener and resin into two separate containers. The correct ratio to measure is provided on the technical data sheet (example: 100 to 27 parts hardener. Do: 100 grams epoxy resin, 27 grams epoxy cure (hardener)). 8
Figure 14: Scale (protected from resin), Epoxy Resin, and Epoxy Cure (hardener) Figure 15: Separately measure out the resin and hardener (2) Combine the two containers into a third container. Start a timer or watch the clock so you know when your resin will gel. Keep the containers separate and do not cross contaminate as you may use them to mix another batch. 9
Figure 16: Combine containers and set aside for mixing more resin later Figure 17: Mixing resin thoroughly (3) Apply the resin to the mold using a foam roller or brush. Use a paper plate to easily apply resin to roller. Make sure that there is a thick enough layer to ensure you get a good surface finish later. 10
Figure 18: (left) Wet roller with resin, (right) Apply even layers of resin to surface Figure 19: Using a brush to apply resin thicker (4) Gently drape the first layer of reinforcement and place it on the mold. Gently press it in place with hands (you are wearing gloves, or should be!) Figure 20: Drape fabric onto surface 11
(5) Use the foam roller or brush to apply resin to the layer reinforcement, again making sure that there is plenty of resin Figure 21: (left) Roll resin on to fabric, (right) Cloth has glossy even surface with enough resin applied (6) Use the metal roller to remove air bubbles that may have accumulated and to even distribute the resin Figure 22: Distribute resin with metal roller (7) Apply the next layer of reinforcement being mindful to apply it in the correct orientation if you have used the warp clock to select the orientation of each layer. 12
Figure 23: Drape on following layer of fabric (8) Continue the resin application as in steps (5)-(7) (9) Move on to the bagging steps in Section VI III. Vacuum Bagging Process: Why should you vacuum bag? Mechanical pressure compacts the laminate, removing air, humidity, excess resin, and volatiles from the resin which can all compromise the curing process and the performance of the finished part. This compaction improves the fiber-to-resin ratio, which is the key to maximizing the strength-to-weight advantages of any composite. Figure 24: Bagging Diagram 13
Bagging Process Steps: *Note: it is best to cut all bagging materials prior to layup so you have them ready to go when you are done with the reinforcement and resin. (1) You should already have the yellow bagging tape applied to your mold. This step is included in the surface preparation steps. However, if you need to apply it now it is still possible. Use acetone to clean the strip where you will apply the tape. If excess reinforcement material is in the way, use scissors to cut it off and immediately place them in a container with acetone so the resin does not permanently adhere to them. Once the area is COMPLETELY clean of release agents and resin, apply the yellow bagging tape. Make sure to go back through and firmly press the bagging tape into the mold (while the wax paper is still on the tape). (2) Cut the release film and breather so that it covers where all of the fibers of the reinforcement are, but so that it does not cover the bagging tape. Make sure it is covering any areas where resin is exposed as well. i. For wet layup: use perforated release film (should see a pattern of tiny holes when looking at the film in light. The holes allow the excess resin to escape to the breather cloth). ii. For pre-preg layup: use non-perforated release film (no holes). Pre-preg already has the perfect amount of resin in the fiber so you do not want it escaping. Figure 25: Cutting the release film using the breather cloth that was already measured as a template 14
(3) Apply the release film attempting to smooth out all wrinkles Figure 26: Pressing release film into fiber and smoothing wrinkles. Cut off any film that may interfere with the bagging tape. (4) Cut the breather cloth, making sure it is the correct size to just cover the release film but not cover the bagging tape. Figure 27: Cutting the breather cloth using the pre-measured composite as a template (5) Apply the breather cloth 15
Figure 28: Applying breather cloth (6) Make sure to insert the vacuum connection at this point, applying it in an area off the composite or on a part where it s okay if an imprint is left. Figure 29: Insert vacuum connection, make sure it s the right one for your pump (7) Cut the bagging film. This time you want the bagging film to be larger than the perimeter of the bagging tape. In fact, make sure to be generous with the bagging film. You may need to apply a pleat if the bagging film isn t perfectly applied. 16
Figure 30: Cut film to size and make sure it covers the tape with a bit of excess material Figure 31: Firmly press the film into the bagging tape (8) Create a pleat when you end up with excess bagging or when you need to help apply pressure on complicated geometries: Figure 32: Cutting a small piece of tape and fold it over on itself 17
Figure 33: Apply the pleat on top of tape and begin smoothing the bagging around the pleat Figure 34: The finished pleat: make sure to check this area well for leaks later (9) Cut an x shaped slit for the vacuum connection: Figure 35: Carefully cut opening for attaching the vacuum (10) Attach vacuum: 18
Figure 36: Carefully twist vacuum connection so that no tears in the bagging occur (11) Check for leaks. Leaks tend to occur where pleats are. The pump should read between 60-80 kpa. (Note: the ideal pressure is 14.5 psi but this is very hard to reach). Leaks can also occur where the vacuum attachment is or occur where the vacuum tape has not been pressed down well enough. Thus, make sure you go over the whole perimeter while firmly pressing down to eliminate all leaks. (12) Removing the part after it has finished curing: (wear gloves to avoid getting splinters) a. Carefully detach the vacuum. b. Remove bagging tape and films carefully, they may be adhered to the part by resin and you do not want to break the part. c. Use a release wedge to remove the part if it is sticking to the mold. Work the wedge under a space and then carefully maneuver it around the perimeter of the piece until it releases. Never pry at the mold with another tool. Be cautious to not get splinters that may be sticking out from the piece. These will need to be cut off or sanded down later. Figure 37: Removing vacuum tape and pump 19
IV. What could go wrong? (1) Resin could get sucked into the pump This can be fixed. Start with a heat gun and acetone, use thick working gloves to protect you from the heat and the metal as it heats up. Separate the breather and connection from the pump as much as possible. The resin should be able to reach glassing temperature so this should be no problem with some patience. Depending on how far the resin traveled you may need a new tube or connection to the tube for the pump. In the image below no new parts needed to be bought and everything came out fine. To prevent this from happening cut a few small extra layers of breather and place them under the pump connection. Figure 38: Resin hardened in the vacuum connection (2) The part could get permanently attached to the mold if the mold is not properly released. The mold in this case is most likely permanently destroyed. (3) The part may not fully cure due to lack of: i. Pressure ii. Adequate temperature iii. Proper ratio of resin to hardener 20