PTM&W Laminating Systems FAQs
- What is the proper resin/fiber ratio for a laminate?
- Can a laminate be too wet?
- Can a laminate be too dry?
- What causes delaminations?
- What causes trapped air in a laminate?
- What is the recommended laminating procedure?
- What causes a laminate to warp?
- What causes a laminate to shrink?
- Why vacuum bag laminates?
- What are the factors to consider when vacuum-bagging?
- Can wet lay-up tools be compressed in an autoclave?
- What causes the back of a laminate to be sometimes tacky?
- Can laminating resins be thinned by using solvents?
- How critical are times and temperatures of cure cycles?
- How do you stop vacuum leaks in high-temperature tools?
What is the proper resin/fiber ratio for a laminate?
For every combination of resin/hardener and for each style of fabric, there is an optimum resin to fiber ratio. Tooling fabrics, because of their coarse open weaves, have higher resin ratios than production fabrics do. Laminates made by the contact lay-up method have higher ratios than laminates made by vacuum bagging.
The contact lay-up method of laminating, using tooling fabrics is primarily used for room-temperature service tools, i.e., holding, drilling and routing fixtures. These resin/fiber ratios are not critical, but should be kept around 50-55%. If resin is applied too liberally, excessive exotherm could cause problems. A contact laminate made by a skilled toolmaker who carefully wets-out the cloth should have a finished resin content between 45-50% resin.
It is recommended to start at 50% resin content when making vacuum-bag laminates for tooling. The excess resin is then pulled from the laminate by the bagging process. The ideal finished resin/fiber ratio for high-temperature tooling is 38-42% resin, depending upon the style of cloth being used.
Can a laminate be too wet?
Yes. High resin/fiber ratios can cause excessive exothermic heat buildup during the laminating process and be dimensionally unstable. Also high-temperature laminates with high resin ratios will develop excessive shrinkage during post-cure cycles. Most people will use too much resin because it makes it easy to wet-out the cloth. Getting the lower ratios means more time and effort working the resin out of the cloth.
Can a laminate be too dry?
Yes. Low resin/fiber ratios can cause delaminations between plies. Dry laminates create leak paths in tools requiring vacuum integrity.
Dry tools are caused by using improper laminating techniques such as not adequately wetting out the fabric with resin. Another cause of dryness is from excessive pressure being applied, if the tool is given an autoclave cure. Excessive pressure forces too much of the liquid resin out of the fabric.
What causes delaminations?
Low-resin content, improper compression, trapped air, improper surface preparation, and/or contamination result in delaminations between plies of a laminate.
Dry plies lack enough resin for the plies to get an adequate bond to each other. These plies will delaminate as the tool is worked or heat cycled.
Improper compression comes from either poor laminating technique or loss of vacuum-bag pressure before the resin gels. This results in the plies not being forced into contact with each other.
Air bubbles between plies of high-temperature tools expand and contract during heat cycles. These voids create starting points for delaminations.
If the resin in a laminate is allowed to cure hard before a subsequent ply is applied, the new ply will not properly bond. Before the new ply is laminated, the cured surface must be roughed up by sanding. A release ply or peel ply can be applied to the back of a laminate, if it is going to cure hard before the laminate can be continued. Once removed, the peel ply creates a good bonding surface for the remaining plies.
Airborne oil or mold release contaminates can settle on laminates while they are being made and cause a weak bond from one ply to another, which results in delamination.
What causes trapped air in a laminate?
Poor laminating technique is the primary reason for trapped air in a laminate. Other possible reasons are whipping too much air into the resin system while mixing, dry areas in the laminate, and bridging plies of fabric on inside corners. Vacuum bagging can minimize or eliminate these problems.
What is the recommended laminating procedure?
There are a number of acceptable methods of laminating. The procedure we recommend that has been proven to control resin/fiber ratios, control warpage and minimize trapped air is outlined here.
If resin content is critical, like for high-temperature close-tolerance tools, we recommend precutting the plies of fabric and weighing them. Then weigh out the total amount of laminating resin and hardener in batch sizes appropriate for the working time of the system. Make sure the total weight of the resin/hardener is the same as the cloth weight. During the laminating process, monitor the number of plies vs. the weight of laminating resin used. For example: If you have laminated 3 plies of a 12-ply laminate, only ¼ of the total laminating resin should have been used.
We recommend rotating the ply orientation, typically 0°, +45°, -45°, and 90°. This is especially important if the fabric is not a balanced weave. Also, never roll out full lengths of cloth on large laminates. Cut the lengths into smaller squares appropriate to the size of the tool. The first 3-4 layers should be butted together. Subsequent plies can have their edges overlapped, if desired.
Test have proven that less air is trapped while laminating if the resin is applied first and dry cloth applied to the wet resin. This procedure allows the resin to wick up into the cloth, pushing the air ahead of it. If resin is applied to dry fabric, the air has to pass through the resin to escape and tends to trap more air. Another method is to wet-out the cloth on a table (in effect pre-preging the cloth) and then place the wet plies on the work. This method is used for large jobs where one person is impregnating the cloth and others laminating. This method also forces air to pass through the resin and is easily trapped in the laminate.
On flat surfaces use a squeegee to apply resin. Squeegees and/or rollers can be used to compress and wet-out the cloth. For tools with contours and detail, a stiff brush (PTM&W Tooling Brushes) should be used to spread the resin and to compress and wet-out the fabric by stippling.
What causes a laminate to warp?
Laminate warpage is caused by unequal shrinkage where one area shrinks more than another, resulting in a build up of stress. This stress is relieved as the tool warps.
Laminates can warp when constructed with long lengths of cloth. It is best to use smaller lengths that can move easier when being compressed. Balanced ply orientation is important to minimize warpage.
A tool can also warp by using back-up structure that gives inadequate support.
If a tool is heated substantially above its Tg or HDT, it can warp. If heated too hot, the resin matrix can soften to the point where it does not have enough strength to support the shape of the tool.
What causes a laminate to shrink?
Compared to polyester or vinyl ester systems, epoxies have very low shrinkage. The amount of shrinkage is controlled by resin content, hardener speed, and the post-cure cycle.
All epoxy resin systems shrink to some degree. The higher the resin content of a laminate, the more shrinkage occurs.
There are two types of shrinkage, both caused by heat; primary and secondary. Primary shrinkage is caused by excessive exothermic heat from using a hardener that is too fast for either the mass of material being used or the ambient temperature of the shop. This shrinkage can be observed after a room temperature cure. Primary shrinkage can be controlled by the correct choice of hardeners for the conditions.
Secondary shrinkage is caused by heat from an external source applied during post-cure cycles and is observed after the final cure. Fast heat-up rates and short dwell times during step cures will cause more secondary shrinkage than if the same tool were step cured with slow heat-up rates and long dwell times.
Shrinkage is rarely equal in all parts of the tool. Unequal shrinkage directly leads to warpage
Why vacuum bag laminates?
Vacuum bagging is used to control resin content, remove trapped air and increase contact between plies of a laminate. At sea level, a laminate bagged under full vacuum will be compressed under 14.7 pounds per square inch. If proper bleeder and breather materials are used, vacuum bagging will remove excess resin from the laminate, along with any trapped air. This compression also forces each ply into intimate contact, for good interlaminar bonding.
Vacuum bagging is less effective when done at higher elevations because pressure decreases as altitude increases.
It is a common misconception that vacuum bagging can “bleed” a laminate too dry. Vacuum-bag pressure alone cannot stave a laminate. When additional pressure is added, such as during an autoclave cycle, it is possible to remove too much resin if excessive pressure is used.
What are the factors to consider when vacuum-bagging?
It is important to verify that the pattern used for making the tool is vacuum tight before laying up the laminate. A leak in the pattern or the bagging film will reduce the vacuum level inside the bag and cut the effective pressure being applied to the laminate. In addition, leaks can induce air into the laminate.
Vacuum bagging is only effective if done while the resin is still fluid enough to be moved out of the laminate into the bleeder material. As epoxies cure they become thicker and thicker until the point where they gel. At some point during this cycle, the resin becomes so thick it will not move under the pressure of the bag and the suction of the vacuum. This point is considered the working life of the material. If vacuum is applied after this time, it is useless.
The bag needs to be applied well within the working life of the resin that was laminated into the first plies of the laminate. A common mistake is to use a hardener with a working time that is too fast for the job. In this case, if vacuum is applied, it just pulls the excess resin and air from the last few plies of the laminate that is still liquid. This is material that has been freshly mixed and applied, and has no effect on the most critical first plies of the laminate.
Always allow plenty of time to get the job bagged because often times leaks occur in the bag and it takes time to resolve them.
One method to keep track of the working time is to make a small 6” X 6” laminate off to the side of the actual tool. For every ply of cloth applied to the tool apply one to the small laminate. As the laminate continues, check the first plies of the small laminate to make sure the resin is still fluid. When these plies indicate that the resin is thickening, it is time to get the tool under vacuum. Another way is to leave a small amount of resin from the first mix in a cup and watch it. If it starts to thicken, it is time to bag the tool.
If the entire thickness of the tool cannot be made within the working time of the resin, make only the number of plies consistent with that time. Apply vacuum until the resin gels. Then, remove the bag and start again on the remaining plies.
The first few plies of a high-temperature tool are the most critical for removing air. It is best to take extra care while laminating the first 2-4 plies and then bag these plies overnight. Subsequent plies are less critical, so more can be laminated before bagging.
Can wet lay-up tools be compressed in an autoclave?
Autoclave-cured wet lay-up tools can be made to the same quality levels as autoclave-cured prepreg tools. Care should be used because a liquid resin system will flow under pressure much easier than prepreg resins. It is easy to remove too much resin under autoclave pressures. Normally 30-40 psi is sufficient to tightly compress a wet lay-up laminate. It is important to install a trap between the laminate and the vacuum pump, as the liquid resin can be sucked into the pump.
The autoclave is used to pressurize the laminate until the resin gels. The tool can then be given a heat-set and post-cured in a conventional oven, thus minimizing expensive autoclave time.
As when vacuum-bagging, an autoclave-cured laminate must be under pressure within the working time of the resin system.
What causes the back of a laminate to be sometimes tacky?
If laminates are made with amine-based epoxy hardeners in high humidity conditions, the back surface of the laminate will sometimes be tacky to the touch. In some cases, it will be hard to sand and actually gum-up the sandpaper. This condition is known as “amine blush”. It is a natural reaction between the moisture in the air and the amine chemicals in the hardener.
Amine blush is strictly a surface phenomenon and is no indicator of how cured the laminate is. It also has no effect on the cured properties of the laminate. This tacky film can be washed off with soap and water or solvents.
Can laminating resins be thinned by using solvents?
Thinning epoxy resin systems with solvents is not recommended. Solvents can be trapped in the laminate as it cures, creating low mechanical properties and porosity. It is better to choose a lower-viscosity resin system to start with than try to thin an existing system.
How critical are times and temperatures of cure cycles?
Cure times and temperature cycles are suggested on the individual product data sheets. These are only guidelines. Tool thickness, contour, dimensional tolerance, resin systems and use temperature are factors which determine optimum times and temperatures. It is always safer to use slower heat-up rates, more dwell temperatures, and longer dwell times than to rush through cure cycles. The few hours saved by speeding through a cure cycle is often paid for by excessive shrinkage and warpage of the tool.
Published cure temperatures are recommended for the actual temperature of the tool. A common mistake is to just go by oven temperature. A thick cross section takes more time for the heat to soak through and reach the center than if is was thin. Once a tool has stabilized at a temperature, it should dwell there for enough time for crosslinking to occur.
If a tool is being cured on plastic-faced plaster, more time should be allowed for the heat to transfer from the back of the laminate all the way through to the face. The PFP takes a long time to heat, so the heat usually comes from the backside of the laminate.
Some resin systems are designed as “room-temp set, high-temp use”. Once a tool has cured at room temperature, it can be removed from the master and be given either a post-cure or placed in service. Hardeners for these types of systems are hybrids containing some components that cure at room temperature and others that cure at high temperature.
Even though the tool cures hard at room temperature, it will soften the first time it sees heat until the high-temperature portion of the hardener crosslinks. That is why we recommend a post cure before putting the tool into service. If placed in service without a post cure, like for example a vacuum-forming tool, it is best to give it a few production heat cycles without forming parts. This allows the resin to at least partially crosslink at higher temperatures.
If there are questions regarding a cure cycle, please call and ask for our technical service department.
How do you stop vacuum leaks in high-temperature tools?
If a tool has to be vacuum-tight for oven or autoclave use, it is best to make the tool with vacuum-bag or autoclave construction instead of contact lay-up methods. Properly done, these tools should not leak.
Never let production workers cut prepreg on composite tool surfaces.
If there are leaks in high-temperature tools they can be stopped by using the following method:
Lay a couple of plies of dry fiberglass cloth on the working surface of the tool, then bag the tool while placing a vacuum gage either on the tool surface or somewhere in the line between the tool and the vacuum pump shut-off valve. Pull vacuum on the bag, then close the valve and measure the vacuum drop on the gage. Mix PTM&W PT2050 with PH3210 hardener (100:30 pbw) and brush coat the entire surface of the tool that is outside the bag. Do this while the bag is under vacuum with the pump on. As this low-viscosity epoxy is applied, it will suck into any leak path, damming it up. As the leaks are plugged, the vacuum gage should start to climb. Allow the resin system to cure. To determine when all the leaks are filled, pull a vacuum on the bag, then turn off the pump and close the valve. The gage will not drop when the leaks are sealed.