Honeycomb Core Benefits Custom Door

FENSTÜR Windows & Doors switches from fiberboard to composite honeycomb cores for custom-built doors

by: Peggy Malnati

Duncan (Vancouver Island), B.C., Canada ― Fenstür Windows and Doors, a manufacturer and direct retailer of high-end custom-built residential wooden doors and windows, has begun replacing fiberboard cores in its doors with a unique composite core produced by RhinoKore Composite Solutions (Armstrong, B.C., Canada). The composite cores offer several benefits, the most important of which is nearly halving door weight, which reduces shipping costs and eases handling and installation of the door at the construction site, plus brings the carry-over benefits of mass decompounding, such as being able to use smaller and lighter hardware to hang the doors. Additionally, the new cores improve sound damping (for quieter opening/closing) and thermal insulation (R value is 3.15 to 3.19 per 1 inch/2.5 centimeter thickness), and they also proved to be a drop-in replacement during the assembly process without a need to modify anything, including the adhesive.

Innovative Solutions

The collaboration began at the recommendation of a local builder who was working on a residential construction project where Fenstür was asked to produce a custom interior door for a walk-in cooler. The homeowner, an avid hunter, wanted a cold room with a hidden door to hold game, food, and wine. The unique oversize door is designed to disappear into the wall when closed, so it features concealed hinges as no hardware is visible when it is closed ― not even a handle. Instead, a removable spindle is used when the owner wants to open the cooler door. The side of the door facing the inside of the cooler is finished in birch wood to match the cooler interior, while the outside (exercise room-facing side) of the door is covered with hackwood to match floors and walls in that room. Fenstür Switches to RhinoKore Composite Cores on Custom-Built Doors.

Completely Customized Product

The door was built before the walls were paneled. Wood for each side of the door was brought to Fenstür's shop and laid out across the door and marked. Board offcuts were then made and labeled so they could be attached to the walls adjacent to matching boards on the interior and exterior sides of the door. This extra effort allowed the team to "board match" paneling on both sides of the door so the grain lines up and continues onto the walls, helping the door blend even better into the rooms.

Lighter Core Material

Due to the significant differences in both temperature and humidity anticipated between the cold/dry cooler-facing side of the door and the warm/humid exercise room-facing side, the Fenstür team knew the door would need to be thicker than normal (2.25 inches/5.72 centimeters) to prevent warpage. However, a door that thick and large (42 by 84 inches/107 by 213 centimeters) using a conventional fiberboard core was also going to be very heavy, which would bring numerous challenges. That's when the local builder mentioned RhinoKore's unique composite core as a possible solution to replace the heavy fiberboard.

Synthetic Core Manufacturing Process

The patented honeycomb core is made from a tough, heavy-duty fabric woven with nylon (polyamide) and polyester fibers. The fabric is fed into a custom-built "honeycomb" machine where it is quickly exposed to hot air (427°C/801°F), which heats and softens the fibers and allows them to be manipulated and formed easily. The fabric is then pressed into sheets of conjoined hexagonal cells (13 mm/0.5 inch in diameter with 1.1 mm/0.04 inch walls) and cooled. Next, the solidified honeycomb sheets (122 centimeters/4 feet wide by 244 centimeters/8 feet long by 152 mm/6 inches thick) are removed and placed in a mold, where a two-part, closed-cell polyurethane foam is injected into all the cells and allowed to cure in a heated press. The finished core is then sliced to thicknesses ranging from 0.25 to 6.0 inches/0.64 to 15.2 centimeters and then can be bonded to faceskins of thermoplastic or thermoset composite, metal, wood, or even concrete. The honeycomb core has a very-good stiffness- and strength-to-weight ratio and helps improve the structural integrity ― especially torsional rigidity ― of any structure to which it is bonded. It is thermally and dimensionally stable and provides very-good compression and shear strength, as well as thermal and acoustical insulation. At any thickness, the core offers excellent impact strength (energy absorption/damage tolerance). And unlike fiberboard, it is impervious to water and will not trap moisture or rot. It also is much lower in density than fiberboard, which helps reduce part mass significantly.

Dimensionally Stable Core

"We ended up trying the RhinoKore product on the cooler door for a couple of reasons," explains Braden Gierc, Fenstür general manager. "We'd had some issues with the overly large front door to this house, so we opted to use the lighter core for the cooler door, which was also oversize. The new core gave us added dimensional stability and resistance to warpage and cracking, which we felt was really important given the significant temperature and humidity differences on either side of the door. The cold/dry side of the door wants to shrink, while the warm/humid side is in a different dynamic equilibrium. And the fact that the core is insulated is a bonus, as it helps make the cooler and its door more energy efficient."

Easy to Work With

The Fenstür team built the door using the composite core and found that it was a drop-in replacement for the fiberboard they'd previously used. Nothing had to be changed in the production process ― even the adhesive (polyvinyl acetate (PVA, also known as white carpenter's glue)) they had previously used to bond fiberboard to wood faceskins could be used with the honeycomb core. The final door consisted of 0.75 inch/1.9 centimeter-thick layer of hackwood paneling on the outside, a 1 inch/2.5 centimeter-thick composite core, and a 0.5 inch/1.3 centimeter-thick layer of birch paneling on the inside. Once the new door was assembled, Fenstür's carpenters and finishing specialists were amazed to see that it was roughly half the weight it would have been with a fiberboard core. It also was quieter and took less effort to open and close. The team knew that that would not only make it less costly to ship to the work site, but would make it easier for the local builder's construction crew to hang and true the door.

Significant Weight Reduction

"The weight reduction is really significant," notes Gierc. "On a conventional 50 by 100 inch [125 by 250 centimeter] door that can weigh 400 to 500 pounds [181 to 227 kilograms] with a fiberboard core, we can now reduce the weight to 150 pounds [68 kilograms]. That's a benefit to anyone who touches the door and is definitely going to make the installer's job a lot more pleasant. Not only does it reduce the risk of ending up with a door that lists to one side once hung, but it also reduces the chance of a door getting dings and dents as it's moved around, and it reduces warping and cracking too. It gives everyone some added peace of mind."

Customers Approve New Core

Owing to a positive experience with the new core on the cooler door, the Fenstür team built several prototypes. They then offered the new core as an upgrade at no additional cost to the first few customers to see how well it would be received. "To date, no one has turned us down," says Gierc. "In fact, we've already sold four doors and have another nine doors on order using this core."

The only downside Fenstür has seen is that because the core makes doors significantly lighter and also provides sound damping, it eliminates the heavy clunk traditionally associated with a door closing and with someone knocking. "We've even tried bonding quarter-inch [6.35 mm] plywood between the core and the skins to solve the sound issue, but the door still stayed very quiet," adds Gierc. "Some customers don't like the change, but others could care less."

Doors Are Stronger at the Core

Going forward, the company plans to offer the composite core as a premium upgrade for a modest cost increase and will begin experimenting to see if it can be used for windows as well. Access to the technology has made company management more confident in bidding on very large doors, which could open a new market segment for them. "You could say that now that we have access to this core," explains Gierc, "it's given us new bragging rights because it allows us to build oversize doors and still maintain dimensional stability without losing strength. In fact, we're gaining strength and we're able to achieve something that previously was very challenging. This means we're now better equipped to address the creative projects architects and homeowners imagine. That makes it a win-win situation."

"The new business with Fenstür Windows and Doors is a welcome addition to the RhinoKore Composite Solutions family," notes Robin Arnold, RhinoKore president. "The foundation of our company was built on the principle of creating innovative solutions for specific needs. Our work with Fenstür is a perfect example of this principle in action."

Fenstür Windows and Doors was established in Duncan (Vancouver Island), B.C. in 2014 with the goal of building superior, custom-made wooden doors and windows that combine the best of traditional woodworking techniques and modern technologies sourced from around the world. The family owned and operated business is dedicated to supporting local economies and sustainable development by using local labor and materials whenever possible. With 150 years of combined forestry and engineering experience, the Fenstür team includes civil engineers, foresters, carpenters, and finishing specialists who search for the best products and newest technologies, and watch the latest trends in order to craft dependable, highly customizable, and high-performance products. Fenstür sells finished doors and windows direct to architects, builders, and homeowners. For more information, see http://www.fensturwindows.com.

Since 2007, RhinoKore Composite Solutions (Armstrong, B.C., Canada) has produced a unique honeycomb core for sandwich panel composites originally developed for the oil and gas exploration industry. The core is produced from a tough nylon/polyester fabric that is formed (under heat and pressure) into a series of conjoined hexagonal cells, which after cooling are filled with a two-part closed-cell polyurethane foam. The core can be bonded to a wide variety of materials ranging from thermoset and thermoplastic composite skins to metal, wood, or even concrete, forming a lightweight but stiff and strong sandwich structure that also provides good thermal and acoustical insulation as well as impact strength (damage tolerance). The core is impervious to water and will not trap moisture or rot like fiberboard cores can. While not as stiff as paper/phenolic cores, it is far more durable (impact resistant). And unlike balsa wood cores (a rainforest product with inconsistent properties and limited supplies), the honeycomb core is tougher and stiffer. To date the core has been used for crane pads, rig mats, insulated water tanks, load floors on commercial buses and long-haul trucks, and now as door cores. For more information, see http://www.rhinokore.com.