Die Cutting Fome-Cor® Material
One of the most common applications for the Fome-Cor® products is die cutting.
Die cutting with steel rule dies allows for the rapid production of flat shapes or cutouts. Typical applications would include the die cutting of letters and shapes, openings in a sheet used as part of an assembly, puzzle pieces, and 3D assemblies. This process can utilize one of the unique features of genuine Fome-Cor® , edge pillowing. The key elements to consider when die cutting are the press, the steel rule, the ejection rubber and the substrate. Each of these elements must be selected properly to yield satisfactory results.
The polystyrene foam core can compress during die cutting. Fome-Cor® has the unique property that the crushed foam will remain crushed. The foam does not tend to assume its original thickness. This results in the die cut edges remaining closed (about 40 to 60 mil residual foam thickness). This provides a rounded effect at die cut edges, called pillowing. This process can also be utilized to produce alternating raised and lowered areas (embossed).
Fome-Cor® is most commonly die cut on flat bed presses. The presses can be either moving platen type or "clam shell" type. Either type of press can be utilized without affecting the quality of the die cut. The key press consideration is proper "make ready". Make ready is the preparation of the press bed (anvil) to assure that the steel rule cuts evenly through the Fome-Cor® without dulling the steel rules. Typically Fome-Cor® is cut on a “hard anvil” to give a pillowed edge. Make ready for this type of die cutting utilizes carbon paper. The press is lowered to the point where the steel rule just touches the anvil. The places where the rule fails to touch the anvil are built up with 1 mil thick shim tape. This process is repeated until a complete imprint of the steel rule is apparent. Make ready is very important because the platen of the press does not necessarily close evenly. This can be caused by misalignment, uneven cutting loads or by deflection of the platen. As a rule of thumb, a 4 post press will deflect 1 mil per ft. Steel rules, that have been dulled by improper make ready will cut poorly, have increased cutting loads and can contribute to liner cracking problems.
Definition of Steel Rule Die Cutting
Steel rule dies work basically the same way as a cookie cutter. They are made of a 1"-wide strip steel with one pre-sharpened edge. The cut strips are called "knives." The strip steel is typically made in a thickness range of .014"–.166". To specify thickness, the term "point" is used. The strips are bent to the shape of the trim line and held in place by a block called a "die body." In order to facilitate ejection of the part, strips of a compressible material such as neoprene are glued along the perimeter and protrude above the cutting edge of the rule.
During die cutting, the steel rule die (SRD) assembly is fixed under the top platen, and the sheet material is placed on a steel bottom platen. Pressure is applied to force the knives of the SRD through the (often preheated) sheet material. The platens are then opened and the parts removed. In some cases, additional work such as finishing the cut edge might be required.
There are three types of steel rule dies that can be used to cut Fome-Cor® material, Cutting rules, scoring rules, and serrated rules. The most common is a cutting rule. The type of cut needed will determine which rule is used.
Cutting rules are the most common when die cutting Fome-Cor® . These rules are used to both cut and pillow the edge. There are three types of bevels: A center bevel, inside bevel, and outside bevel.
A center bevel is "V" shaped, i.e., honed from both sides. A center bevel is used when both the inside and the outside of a cut have to be saved, e.g., as in a puzzle. In this case, the cut is wedge-shaped so that the cut face on the periphery is sloping away from the inside and the cut on the inside piece is sloping away from the outside. The longer the bevel and the narrower the thickness (point) of the rule, the straighter the cut.
An inside bevel has the straight unhoned side of the rule on the outside of the cut and the beveled side on the inside of the cut. The rule of thumb is that the beveled side is always towards the scrap.
An outside bevel has the straight unhoned side on the inside of the cut and the beveled side on the outside. An outside bevel is used if the inside piece must be saved.
Scoring or creasing rules are used to create a fold line. Scoring rules are shorter than standard rules. These rules cut through the top liner but leave the bottom liner intact. This technique is also referred to as slit scoring or "short knifing". This method is often used when additional materials are laminated to the Fome-Cor® products. Unique to Fome-Cor® , because of its foam structure, is the capability to crease cleanly.
Serrate rules are used to cut Fome-Cor® without closing the edge. The best serrated edge rules to use with Fome-Cor® have 12 -20 teeth per inch and a side bevel rule. The non-beveled side should be placed toward the good piece. When using a 12-tooth rule, the cut piece will have an unclosed edge with fine serration marks. A 20-tooth rule will slightly indent the top liner but the serration marks will be almost invisible. The side toward the bevel (bleed off area) will be partially closed with more noticeable serration marks. Serrated rules must always be used with a soft anvil because the teeth must penetrate completely through the bottom liner.
Stripping and Ejection Rubbers
Stripping rubber is essential when cutting Fome-Cor® . It serves two purposes. This is used to assist with edge pillowing and to prevent cracking in the die cut piece. Liner cracking problems need to be considered whenever coated products such as Fome-Cor® are die cut.
Rubber used to merely remove the part from the die is called ejection rubber. Rubber is rated by its hardness. Typical Shore "00" Udometer hardness ratings are: Soft 20 – 40, Medium 30 – 50, and Hard 40 – 60. The proper techniques for the use of ejection rubber with Fome-Cor® include:
- The ejection rubber should be at least the height of the steel rule and preferably 1/16” - 1/8" higher than the rule.
- The ejection rubber should not touch the steel rule. This prevents dragging on the blade or getting the rubber cut by the rule. This also prevents the liner from being pulled away from the rule by the distorting rubber.
- Medium to hard rubber is most commonly used. It is best to completely rubber the rules to prevent cracking and achieve uniform pillowing.
- On critical areas, soft rubber can be used for additional build-up to prevent cracking. Critical areas are usually near tight bends or sharp points. For these areas, the stripping rubber can be built up as much as 1/4" above the rule height.
- The rubber should be selected to allow it to fully compressed between the steel rules without crushing the Fome-Cor® or bending the rules when the press is fully closed.
- A more gradual pillowing effect can be obtained by using wider, soft rubber stripping. Stripping rubber from 1-2" wide is commonly used.
Fome-Cor can be bent while attempting to die cut a small piece from a large sheet. To prevent bending, the dieboard should be lined with a sheet of soft, low density (2-3 lbs/ft3) polyurethane foam about 3/4" thick.
Die Cutting 3/8” Fome-Cor®
Die cutting 3/8” Fome-Cor® with pillowed edges can be successfully done. This type of die cutting has somewhat different requirements than for cutting thinner Fome-Cor®. The dieboard should be 1/2 inch thick and the rule 1.125" high. A 3- pt, long, side bevel, coated rule works best. The rule should be continuously lined inside and out with medium to hard ejection rubber. The ejection rubber should be placed 1/16" away from the rule on both sides rule. A continuous sheet of soft rubber can also be used in place of the strips of ejection rubber. If cracking results with the continuous rubber, the outer 1/2" of the soft continuous rubber should be replaced with strips of medium rubber. When designing the figure to be die cut, it is best to avoid sharp corners and narrow spaces. A minimum distance of 1 inch is recommended between pieces. When sharp corners cannot be avoided, additional, very soft, foam avoid localized cracking.