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IMPROVED FABRICATION PROCEDURES
REDUCE COUNTERTOP CRACKING

(Written in January, 1994)

by Jim Heaphy

Although the vast majority of solid surface countertop installations perform very well for many years, occasionally failures do occur. Typically, these failures involve a crack, and are connected with a seam, an inside corner, a built-in or portable heat source, or a combination of more than one of these factors. Such incidents result in unhappy customers and expensive warranty claims. Improving fabrication and installation procedures to reduce the frequency of such cracks has been a major concern of fabricators, distributors and manufacturers in recent years.

Du Pont Corian, the solid surface market leader, last year made major changes in the procedures that it requires of fabricators. These improved procedures address the most common causes of cracks, and, if implemented consistently by all Corian fabricators, should result in a dramatic decline in the incidence of warranty claims in years to come.

Good fabricators understand the importance of creating radiused inside corners where the legs of "L" or "U" or similar shaped countertops join. Stresses tend to concentrate at square inside corners, increasing the chance of a crack starting. Radiused inside corners tend to disperse the stresses, reducing the chance of a crack. As a general rule, the larger the radius at an inside corner, the less likely that a crack will start.

In the past, Du Pont required a minimum radius of 3/16" at these countertop inside corners. This is the radius created by a common 3/8" diameter router bit, even if the router was guided by a template with a square inside corner. However, the minimum radius has now been increased to 1/2". Accordingly, fabricators must now use templates with an appropriate inside corner radius. Remember that built-up edges must have the same minimum 1/2" inside radius through their entire thickness.

Seams are another potential weak point where a crack can start. The seam that connects the legs of the countertop together must be offset at least 1" from the perpendicular front edge, so that the seam does not run directly into the inside corner. Additionally, all seams must now be reinforced with a solid continuous strip of CORIAN applied beneath the countertop. It is no longer acceptable to rely only on a strip of plywood beneath the seam. The reinforcing strip should be the same thickness as the countertop sheet, at least 2" wide, and cut to a length measured from the back surface of the built-up front edge to the back of the countertop. The two long edges of the support strip should be fully beveled at 45o, and the wider surface is then bonded to the underside of the countertop directly below the seam. Clamping is optional but not necessary. For example, if a large countertop is being assembled in its final position, the reinforcing strip can be put in position first, supported by plywood strips so that its top surface is flush with the underside of the countertop, and its front edge will fit against the back of the countertop's built up front edge. When it is time to assemble the seam, fully coat the top and front edge of the support strip with joint adhesive, position the countertop sections above it, and proceed with seam assembly and finishing as normal. A far stronger seam will be the result.

Countertop cracks are often associated with cutouts for drop-in range tops. About four years ago, Du Pont developed a procedure for fabricating high-strength appliance cutouts. This procedure was optional for U.S. fabricators, and relatively few used it with any regularity. However, several years ago, high-strength appliance cutouts became mandatory on all Corian installations in Great Britain. It proved to be very successful there, and warranty claims due to cracks originating at cooktop cutouts declined dramatically. Accordingly, the procedure is now mandatory in the U.S. for all cutouts involving gas or solid-disk electric cooktops.

There are two parts to this procedure. First, 6" X 6" Corian reinforcing blocks are cut from the same thickness material as the countertop sheet. The blocks need not be the same color. Two perpendicular edges of each block are beveled at a 45o angle. The blocks are then fastened to the underside of the countertop using a consistent layer of joint adhesive. This will require one joint adhesive kit for a set of four blocks. These blocks are positioned where each corner of the cooktop cutout corner will be routed later. The beveled edges are positioned to run into the cutout location.

The second part of the procedure is to machine the cutout into a distinctive shape. Each corner of the cutout is flared to the largest possible radius, approaching to within 1/8" of the outside edge of the cooktop flange, and then returning to within 1/8" of the cooktop box, or the portion that passes through the countertop. The size of this radius is dependent on the width of the cooktop flange. However, whether the flange is wide or narrow, this procedure will result in a larger radius that is more crack resistant than previous methods. Combined with the reinforcing blocks, this procedure will produce exceptionally durable cooktop installations consistently.

Drawing #1 illustrates the old method of laying out a cooktop cutout for an appliance with a 3/4" wide flange. Leaving 1/8" minimum clearance between the edge of the cutout and the cooktop box, there is space for a radius of just 3/16" at the corners, and there is a risk that the corner of the cooktop box will nick the edge of the cutout during installation.

Drawing #2 illustrates the current method of laying out the cutout corner shape for the same cooktop. (The reinforcing blocks are not shown here.) By flaring the corner, the radius has quadrupled. The flared corner is less subject to nicking during installation, and is completely concealed by the flange after installation is complete.





Also now required is that you must round over the top and bottom edges of both cooktop and sink cutouts with a 1/16" radius quarter round router bit. The cutouts should then be sanded smooth with 150 grit or finer sandpaper, removing all nicks and tool marks. As in the past, cooktop cutouts must have aluminum heat conductive tape installed properly, completely covering the entire horizontal surface under the cooktop flange, and wrapping down onto the vertical surface of the cutout, with excess tape dangling into the cutout. Do not wrap the tape up underneath the countertop. This is a common installation error.

Another new requirement is that a color matched piece of material from each countertop should be left with your customer for possible future repairs. This can be left in the form of a cutting board or trivet, or it can be stored below a drawer or elsewhere, marked to be saved for repair purposes. This has always been good practice, since the chances for a successful repair are greatly improved if color matched material is available.

Because the Corian warranty program now covers materials and labor when a covered installation fails, Du Pont expects all Corian fabricators to adhere to the highest quality standards to prevent unnecessary warranty claims. These changes are a step forward for the credibility and professionalism of all Corian fabricators.

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