Continuous roll forming produces 'knobby' sheet.

A new continuous roll-forming technology has come to North American sheet extrusion. The process produces "knobby sheet" - a waffle-patterned HDPE product that acts as a load-bearing barrier protecting building foundations and roofs against water and/or soil. Instead of using normal cooling rolls, the sheet is extruded over a continuously rotating drum whose surface is stippled with raised knobs over which the hot melt is formed by vacuum. The supplier of the production system - Welex Inc., Blue Bell, Pa. - calls it a rotary thermoforming (RTF) system.

Welex's RTF lines have been used in Germany, Norway, and Switzerland to make the barrier sheet for the last six years. The sheet is widely used in Europe as an alternative to tar for protecting building roofs and foundations. Knobby-sheet production began in North America about six months ago, when one Canadian building-products company purchased a number of RTF lines. The company, which asked not to be identified so as not to alert potential competitors, is making coextruded HDPE knobby sheet averaging 24 mils thick that's said to have a compressive strength of 5200 lb/sq ft.

HOW IT'S MADE 

The rotary drum-forming process produces sheet that's typically 3 meters (approx. 10 ft) wide and 15-20 mils thick from a fractional-melt HDPE resin. The surface is formed into a series of raised knobs or truncated cones of equal shape and height. These knobs give the sheet its load-bearing capacity and create an air space between the sheet and the foundation, keeping the foundation dry. The edges of the sheet are left flat so that it can be fastened flush against a building foundation to prevent soil and water from slipping behind or beneath it.

The drum surface typically has 10,800 knobs, each one 0.250 to 0.375 in. high. The shape and spacing between knobs are designed to provide the desired load-bearing strength.

Welex president Frank Nissel says the best way to make the sheet is to form it in-line with extrusion. He says reheating and post-forming such wide sheet involves additional cost and handling difficulties (e.g., sagging).

A conventional extruder and flat die are used to produce the web - what's special is the forming drum. Welex currently makes that drum in one size of 40 in. diam. x 126 in. wide. The drum's circumference is divided into 15 parallel aluminum sections or "slats" that contain the forming knobs. Each knob has a vacuum slit at its base, and each slat has its own vacuum hoses. A disk-like rotary commutator on the end of the drum turns the vacuum on and off for each slat as it rotates into position to receive the web of melt from the die. The commutator is vital to the process, Nissel says. Without control of the vacuum, the sheet would wrap entirely around the drum. Adjustable controls are used to set the vacuum of each slat.

Processors typically hold the vacuum for 270 [degrees] of the drum's circumference in order to give the sheet sufficient cooling time, Nissel says. The drum has a variable-speed d-c drive.

Water-cooling and vacuum passages are cross-drilled inside the drum. The RTF system also has specially designed pull rollers with raised strips that pull the sheet without flattening the raised knobs. Welex offers a jumbo roll-winding system, as well as in-line slitters for making smaller rolls.

MONO- OR COEXTRUSION

Using a Welex 4.5-in. 30:1 L/D extruder, the RTF reportedly makes 3-meter-wide sheet at rates around 30 ft/min or 1200 lb/hr. Nissel says some processors are now making two-layer sheet that's black on one side and brown on the other. One processor reportedly runs at 2400 lb/hr using two 4.5-in. extruders with a coex feedblock.

The RTF can be operated by Welex's Ultima III controls. A complete system costs $1-1.5 million.

Nissel says Welex is working on a number of RTF modifications - including different knob shapes and deeper draws. Welex is even redesigning the system to replace the continuous forming roll with a reciprocating former that operates like a thermoforming trim press, giving it the ability to create other products like container lids.