PTM&W Surface Coats FAQs
- How do you properly mix surface coats?
- What is the correct thickness?
- How should surface coats be applied?
- Can epoxy surface coats be sprayed?
- Should surface coats be applied in one or two coats?
- What causes soft spots in surface coats?
- What causes “fish-eyes”?
- Why are surface coats sometimes rubbery?
- How do you get rid of porosity in surface coats?
- What causes blisters in high-temperature surface coats?
- What causes cracking in high-temperature surface coats?
- What causes surface coats to separate from the laminate?
- Why do external corners of a tool easily chip?
- Why does a surface coat change color when exposed to high temperatures?
- What causes “print-through” and “mark-off”?
How do you properly mix surface coats?
Proper mixing of surface coats is critical because soft spots or uncured material in a tool face can require rework or scrapping of the tool. Refer to FAQ “General Information” for proper weighing methods.
When mixing surface coats, always “double-cup”. This simply means to weigh the resin and hardener into one container and thoroughly mix by scraping the sides and bottom of the container. Transfer the mixed material into a second container and remix in the same manner. This procedure eliminates the possibility of applying unmixed material.
Never mix in only one container, as there is a good chance that either the resin or hardener will cling to the sides, bottom or corners of the container and not be properly blended. Also, never scrape the sides of the container when transferring the material to the working surface.
What is the correct thickness?
For low-temperature applications, surface coat thickness is not critical. It should be applied just thick enough to get the job done. If the tool surface needs to be high-wear, or requires sanding to control finish or contour, then apply a thicker coating. Thicknesses greater than 0.080-inches are not usually recommended.
High-temperature surface coats should always be kept as thin as possible. The reason is there are large differentials in coefficients of thermal expansion (CTE) between surface coats and laminates. As the tool heats and cools a surface coat will expand and contract at a much greater rate than a laminate. A thick surface coat will build up more stress than a thin coat, which results in cracking.
Ideally, a high-temperature surface coat will have cloth worked into the back surface. This is not usually practical because any scribing or sanding will expose the cloth and compromise vacuum integrity. Thicknesses in the 0.015-0.030 inch range work well for high-temperature applications.
Be aware of allowing surface coats to drain off of vertical surfaces and mass on inside corners of the pattern. These areas can build up quite thick, which become the external corners of the tools. These are the areas that are most prone to cracking and chipping.
How should surface coats be applied?
Epoxy surface coats are best applied by brushing or spreading with a rubber squeegee. For paste surface coats, use a short-bristle brush, like the PTM&W Tooling Brush, or cut the bristles of a normal brush to ¾ - 1 inch long. For softer, lower-viscosity surface coats, regular length bristles are adequate. Brushing is the best method when the pattern has a lot of contour or detail.
Spreading surface coat with a squeegee is the preferred method when large, low contour, smooth surfaces are to be covered. Use a paint thickness gauge to control application thickness.
Can epoxy surface coats be sprayed?
Unlike polyester gel coats, epoxy surface coats are generally not sprayed. Epoxies are usually thicker and have higher surface tension, which makes spraying difficult. When spraying is attempted, epoxies tend to “fish-eye” and not leave a smooth solid surface.
Some of the lower-viscosity epoxy surface coats have been successfully sprayed by using a stucco gun, which splatters the material on the pattern surface. This is just a method of getting the surface coat evenly distributed on the pattern surface. The sprayed material then has to be brushed smooth.
Do not use solvents to thin epoxy surface coats to make them more sprayable.
Should surface coats be applied in one or two coats?
Both methods are acceptable. The advantage of one coat is it is easier to keep the surface coat thin. The drawback is that it is easy to push cloth through the surface coat if applied too soon. With two surface coats, the first coat can be allowed to gel to the point where cloth will not push through.
Always allow the first coat to gel to the “finger-print-stage” where you can leave a finger print on the back surface and it will not come off on your finger. This is the proper time to start laminating or apply the second surface coat.
One method used for high-temperature surface coats is to work the first ply of laminate into the second surface coat while it is wet. If the laminate is going to be vacuum-bagged, the second surface coat can be mixed with the same long pot-life hardener as the laminating resin allowing the second coat to stay liquid while being bagged. This allows the first layer of cloth to be pressed into the surface coat.
If this process is used, some toolmakers will mix the second surface coat and the laminating resin with the same long pot-life hardener. The two are then blended at a ratio (usually 50:50) to get the desired consistency. This method changes the viscosity of the second coat to somewhere between the surface coat and the laminating resin, which helps in wetting-out the first layer of cloth.
What causes soft spots in surface coats?
Soft spots in cured epoxy surface coats are always caused by improper mixing. Use the “double-cup” method.
What causes “fish-eyes”?
“Fish-eyes” are areas in a freshly applied surface coat that tends to thin or open up on their own accord. Sometimes, these spots can be re-brushed and still continue to open. This problem is caused by the high surface tension of the epoxy applied on a slippery surface, such as a silicone-based mold release. Some surface coats, usually high-temperature, have more of a tendency to fish-eye than others do.
The best solution is to use a less-slippery mold release, such as a high-temperature paste wax.
Why are surface coats sometimes rubbery?
If the entire surface coat is rubbery, it is do to either not being fully cured or an improper mix ratio. If not cured, the solution is to use a faster setting hardener, or allow more time to cure, or apply heat to increase the cure rate.
Mixing a surface coat off-ratio, with either too much or too little hardener, will cause the material to stay rubbery and not fully cure. If the surface coat is cured hard in some areas and rubbery in others, improper mixing procedures were used.
How do you get rid of porosity in surface coats?
Porosity in surface coats is caused by air induced during the mixing process and/or the application process being trapped when the system cures. One or more of the following may minimize porosity:
- Mix in less air by gently stirring the resin and hardener together instead of vigorously mixing.
- Vacuum de-gassing the surface coat before application.
- Brushing a thin coat to allow air to escape.
- Gently applying heat from a hot-air gun on the back surface of the material. This lowers the viscosity of the system and causes the air bubbles to increase in volume and rise to the surface and pop.
- Using a lower viscosity (thinner) surface coat
What causes blisters in high-temperature surface coats?
Blisters are caused by air trapped in the surface coat (porosity) or behind it. When heated, this air will expand and take the path of least resistance, usually blistering out the face of the tool.
The solution is to minimize porosity (see above question) and eliminate air trapped between the surface coat and laminate. Poor laminating technique causes trapped air. This problem can be minimized by:
- Spending more time and extra care while laminating the first few plies behind the surface coat.
- Applying dry cloth to wet resin, which allows the air to pass through the cloth, instead of wet resin to dry cloth, which forces air to pass through both the resin and the cloth.
- Using a stiff brush to stipple the resin and completely wet-out the cloth.
- Using a ribbed-roller to wet-out the cloth in flat areas.
- Vacuum-bagging the laminate to remove air and excess resin.
What causes cracking in high-temperature surface coats?
Surface coat cracking is caused by it being applied too thick. See above FAQ “What is the correct thickness?”
What causes surface coats to separate from the laminate?
If an epoxy surface coat is allowed to cure to a hard, glassy stage before laminating, it will not properly bond to the laminate. Always start laminating behind a surface coat when the material has reached the “finger-print” stage. If it has cured hard, the back of the surface coat must by sanded to remove the glazed surface.
Why do external corners of a tool easily chip?
Surface coats tend to be hard, brittle non-reinforced materials. They get their strength from the laminate backing them up. If they are thin, they tend to be tough, if thick they tend to chip easily. External corners of tools are made from the internal corners of the pattern. If not carefully applied, surface coats will slide down vertical surfaces and build up in the bottom corners, which causes the cured surface coat to be too thick.
Also, it is sometimes difficult to get the cloth to lie down tight on inside corners. It is easy for the cloth to bridge the corner and create a poor bond between the surface coat and laminate. This makes it easy for the surface coat to chip or break away from the laminate.
Why does a surface coat change color when exposed to high temperatures?
High-temperature hardeners are made up of various components including amines. Certain varieties of this chemical will darken or turn a reddish color when exposed to heat. This change in color does not indicate a problem; it is just a natural occurrence.
What causes “print-through” and “mark-off”?
Print-through is the cloth pattern from the backup laminate showing through the surface coat. Usually this is a cosmetic problem and has no effect on tool performance. It is caused by resin shrinkage in the laminate. When measured in the “Z” direction, there is less resin where there is a strand of fiber and more resin in the areas between the “X & Y” fibers. The thicker areas shrink more than the thinner. This shrinkage transfers through to the face of the tool and shows the cloth pattern. It happens during the post-cure cycle and it only happens once, so if it is sanded and polished out, it does not return.
If no print-through is allowed, the only solution is to use very fine fabric for the first few plies and give the tool a slow heat-up rate to minimize shrinkage.
Mark-off is caused by the surface coat softening and allowing the production part to create a permanent mark on the surface. This commonly occurs during autoclave cycles where there is a combination of heat and pressure. The solution is to use a higher temperature rated surface coat and give it a proper post-cure before use.