Step by step guide on how to successfully thermoform translucent film on vinyl
Introduction
To fabricate thermoformed rigid sign faces many companies apply colored vinyl film to a flat plastic sheet then put the resulting laminate through the thermoforming process. Many factors lead to the decision for using vinyl versus, or in combination with, paint to decorate plastic sign faces. In short using vinyl film reduces fabrication costs and improves project cycle time. Perhaps more importantly, using film vastly improves color uniformity especially when the sign is backlit.
There are specific applications where vinyl is the only color element in a sign and others where sprayed paint is used in conjunction with vinyl to add supplemental color. This paper describes the operational elements of using vinyl in a sign fabrication that requires thermoforming. Rather than listing painful effects and their causes and I will follow a basic paint by numbers approach to the process. A fabricator that follows this approach to thermoforming is well on the way to a high-quality sign face with the color already applied.
Preparation of the sign face substrate:
It is absolutely necessary to dehydrate the plastic sheeting before translucent vinyl is laminated prior to the forming process. No matter how it is received or stored acrylic or polycarbonate will be adequately dry if suspended in a vented 175 ° F (78 ° C) oven the night before vinyl installation. Using 180 ° F and twelve hours as a benchmark one can increase or decrease temperature between 150 ° F and 200 ° F (67 ° C ? 93 ° C) if time is adjusted to compensate. Anything below 150 ° F makes the process too lengthy while exceeding 200 ° F can cause the sheet to stretch in the suspending clamps.
It is important to remove any plastic or paper protection film before attempting to dehydrate the sheet. It is also important to use several clamps to suspend the sheet while dehydrating: this spreads the weight of the plastic on the suspension points and eliminates edge distortion which can make lamination of the vinyl difficult. Once removed from the oven a sheet must be laminated within the following day (moisture absorption after twelve hours where ambient moisture is high will be great enough to cause bubbles in the thermoformed article).
Cleaning the sheet prior to lamination
The sign face must be far cleaner when processing a laminated sheet than when applying film or paint after the forming has been done. Any dust or dirt caught between the film and sign face will show as a small spot surrounded by an air bubble.
It is wise to work in a separate space from that which houses other cutting, routing or milling operations. In order to accomplish this some fabricators erect walls while others simply suspend heavy strips of clear PVC from floor to ceiling. Anything that keeps dust and dirt out of the laminating area is a good idea.
One can begin the cleaning of the sheet by using oil free compressed air. Some fabricators go to the extra step of using an air nozzle that ionizes air resulting in a static free sheet as well. Plastic sheet loaded with static electricity is not only a potentially uncomfortable work platform but acts to draw dust from far and wide. Finally a slightly damp, lint free rag is passed over the sheet to dislodge contaminants the compressed air didn?t remove.
Lamination
- For fabricators laminating by hand and using wetting solutions to reduce bubble and wrinkle problems now is the time to lay the premeasured film upside down and strip the release liner away. While plastic or polycoated liners can stand some water spray it is important to keep standard kraft paper liners from getting wet otherwise small particles of the paper coating will be dislodged and contaminate the adhesive coating. The recommended method is to strip the liner from the film on an adjacent (dry) table then spray the adhesive coating with wetting solution in preparation for laminating.
- Treating the solution-sprayed film as though it is dry and tacky, find a position on the edge of the sheet and establish registration on that edge with a firm squeegee pass. Allow the rest of the film to fall freely onto the rest of the sheet. Many installers now use a spray of wetting solution or water on the top side of the film to aid in the use of squeegees without scratching the vinyl film. When working with large areas of vinyl it is good practice to isolate the installation area by using center ?hinging? rather than attempting to use a side or top hinge. This prevents accumulating stretch and wrinkle problems. Once the film has been squeegeed pay special attention to edges. Distortion of the sheet by clamps in the predrying oven or stretching of the vinyl during lamination will have raised small channels that can extend a short distance in from the edge of the sheet. Several passes with the squeegee may be needed to secure the vinyl to the sign face. In any case, be sure the film is flat and completely attached.
- The final step is a clear water rinse of the vinyl to remove all liquid detergent before returning the laminated piece to the drying oven for its final drying before thermoforming. This is simply a damp chamois skin or lint free towel. Soaps contained in the wetting solutions left on a surface cause unsightly stains on the finished thermoformed part.
Post lamination drying
A sheet which has been wet laminated must be re-dried to ensure there is no water left between layers and allow for adhesion build before being subjected to stresses of rapid heating and distortion. Use the same oven and temperatures as when pre-drying but only a couple of hours are needed to achieve adequate post drying.
Note: If one has an automatic laminator and uses no water during this process then returning the laminated sheet to the dryer is not absolutely required but has been found to speed adhesive ?wet-out? and thus reduce pinhole sized bubbles should they be found in the final product.
Tooling
As it is imperative that the tooling not come in contact with vinyl, for first surface applications a male form is used, for second surface a female form is needed.
The tradeoff between ?mark off? due to high texture on the mold and poor draping because of too glassy a surface remains important whether forming laminated or single layer sheets.
An issue that can arise as a result of having the color layer laminated before forming is severe thinning at edges and corners which will show as ?hot spots? in the color. It is therefore additionally important to allow good sheet draping and relatively slow application of vacuum during the forming process.
Edge radii should be no less than three times the thickness of the sheet and draft angles for all walls should be over 93 ° for best distribution of film, minimization of thinning problems ? hot spots? and easy release of the finished part from the mold. Uniform draping of the molten sheet over the mold and slow application of the vacuum will aid in achieving good color consistency across the entire sign face.
Vinyl will stretch up to 400% of its original dimension in any direction but experience has shown that keeping the distortion uniform and at less than 300% at any point will result in the most aesthetically pleasing color. Greater distortion will cause ?hot spots?: where sections of the part become so thin that deep blue becomes sky blue or red becomes pink when the finished sign is backlit.
Heating and Forming
A properly designed cast vinyl film will be stable under all but the highest temperatures needed to thermoform plastics. Vinyl laminated to standard styrene, acrylic, impact modified acrylic and most polycarbonate sheeting which require temperatures up to about 390 ° F (198 ° C) will take the time and heat without smoking, dulling or discoloring. Temperature above 390 ° F for several minutes needed to form very thick polycarbonate will cause some damage to certain colors of vinyl.
For instance some deep red, blue or burgundy shades may transfer pigment or ?bleed? into the sign face causing it to take on a slight tint. This is only important when the vinyl is to be removed to show white background or clear sheeting, which is to be backsprayed later.
Another area of sensitivity is with certain color formulations such as the diffuser films: these can turn from white to straw colored when subject to intense heat. Experienced fabricators use upper and lower heating units and apply most heat to the plastic side of a laminate rather than have such intense heat pass through the vinyl into the plastic sheet. While the ratio varies, fabricators with top and bottom heated thermoforming units usually apply about 75 % of the heat to the sheet side before energizing the heaters on the vinyl side.
Trim and Removal of Waste Material
When trimming unwanted film away from the sheet after thermoforming it is important to use the sharpest of knives. Swivel type razor knives are preferred although a standard X-acto triangular blade attached to the pencil thin holder will suffice.
Once the desired shape or letter outline is scribed into the vinyl it can be lifted away without difficulty. Again properly designed cast vinyl film with appropriate slow build adhesive will make this process easy and the resultant copy will pull away quickly with no adhesive residue while outlines will appear crisp and clean. When possible remove vinyl while it is still warm, this will reduce the tendency to break the film as it is being removed.