By Edward E. Ball
Manager, Applications Technology, Appliance Business Group
Bayer Corp., Polyurethanes Div.
Pittsburgh, Pa.
Edited by Amy Higgins
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When a cabinet is filling with polyurethane foam, many physical and chemical reactions happen at once. Polymer formation reactions take place. The reaction of diisocyanate and water generates carbon dioxide and heat. The physical blowing agent then gets vaporized, and bubbles form in the liquid. Gases within the bubbles expand from heating, and air in the cavity gets displaced as the foam expands. The foam becomes more viscous as reactions progress. |
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Shown is a typical pour hole and cabinet vent arrangement. Designing additional venting in the top breaker area can improve filling. |
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The sideview of this refrigerator cabinet helps illustrate that when liquid gets properly distributed, a regular foam flow pattern is established. Ideally, once the top fills, the foam moves unidirectionally and uniformly toward the vents. |
Ever heard the old saying, "stuck between a rock and a hard place?" That's where refrigeration designers and manufacturers now find themselves. Consider that last year the federal government said all household refrigerators and freezers in the U.S. must use 30% less energy than stringent standards already set in 1993 (See MACHINE DESIGN April 5, 2001, pg. 50.). As of Jan. 1, 2003, however, manufacturers will no longer be able to use HCFC 141b; the primary blowing agent for insulating polyurethane foams. Why? It, like its predecessor, CFC 11, is regulated as an Ozone-Depleting Substance. Manufacturers now must find an alternative. Problem is, approved replacements have poorer insulating properties and generally, less energy efficiency.
Newfangled refrigerator and freezer designs also have features and compartments that take up insulation space between the unit's outer case and inner liner. These "hidden" items can get in the way when filling the cabinet with foam. Though seemingly a manufacturing problem, blocked foam flow may keep the machine from meeting energy standards. This is why it pays to incorporate "polyurethane foamfriendly" features into new product designs.
FILL ‘ER UPWhen insulating refrigerator cabinets and doors, a chemical reaction takes place as liquid polyurethane raw materials are combined and poured into the cabinet. This chemical reaction creates foam that fills the space between the exterior cabinet and interior liner. In most cases, the process takes less than 30 sec.
The most common cabinet orientation for foam filling is the "breakers up" position because it offers manufacturing advantages and shorter foam-flow paths. The composition of the foaming material and the configuration of the cavity both have a lot to do with what happens during pour and fill. The better an article fills, the better the foam performs.
One step in designing refrigerator cabinets for better filling is to keep the injection path clear. For example, the pour hole at the bottom of the refrigerator cabinet should let poured liquid miss the food liner. When pouring, the bulk of the liquid should be near the cabinet top and the liquid puddle should be V-shaped and symmetrical about the cabinet centerline. The centerline should be kept clear of liner supports, wires, tubes, and other objects that might keep liquid from reaching the cabinet top. Once the top is filled, the foam moves in one direction, uniformly toward the vents. The vents should be the last place to fill with foam.
Where vent holes sit in the refrigerator cabinet is an important consideration. The foam needs to displace several cubic feet of air and the off-gases of the rising foam to prevent air pockets or voids from forming in the cabinet walls. Vents should be large enough and distributed so that venting gases can escape freely without generating back-pressure on the rising foam. For instance, it might be wise to place small vents in problem areas where gases get trapped.
THE PATH OF LEAST RESISTANCE
Obviously support parts should be kept out of the foam-flow path between the inner and outer case. Also to be avoided are dramatic changes in wall thickness and sharp turns that might impede flow. Think of water flowing through a pipe. It takes more pressure to move water through piping with elbows and valves than it does through straight pipe. The same is true when polyurethane foam fills a refrigerator cabinet. In this case, however, the pumping system for the foam is the heat of reaction.
This heat builds gas pressure inside the bubbles that make up the foam, expanding them. If the foam hits an obstacle that has an associated pressure drop exceeding that exerted by the foam, flow stops until the heat of reaction again builds enough pressure to drive the foam forward. But there is a loss of valuable filling time as enough pressure rebuilds. The foam loses steam and at the next obstacle, the cavity may not get filled.
The foam takes the path of least resistance like any flowing liquid. Obstructions may send foam in multiple directions, causing knit lines and trapped gas voids where flow fronts meet. These defects hurt energy performance. They may indirectly boost manufacturing costs by forcing the use of more foam or electromechanical devices with an efficiency higher than would be needed otherwise. All in all, it pays to spend time improving cabinet filling by redesigning parts, keeping obstructions close to the wall, and laying wires flat rather than bundling.
THERMAL WARP
Thermal warp is a special consideration when designing refrigerators with plastic liners. This deformation or "bowing" of the steel, foam, and plasticcomposite panels of a finished refrigerator, is often attributed to inadequate foam insulation. The real problem, however, comes when designers don't take into account the physics of large panels with dissimilar facers. Consider a steel cabinet and a plastic liner; each has a different coefficient of linear thermal expansion (CLTE). Plastics used for liners have a larger CLTE than metals and thus move more in response to temperature change. This is particularly noticeable in a side-by-side refrigerator's exterior freezer wall or both walls of a vertical freezer that haven't been adequately designed to handle this problem. Thermal warp may lead to door seal gaps, cause shelves to fall, and make it difficult to move the refrigerator out from its surrounding cabinetry.
Designers can calculate thermal warp of composite panels. When a panel has thin, flat faces and a thick core with a high shear modulus, as found in typical refrigerators and freezers, use this equation:
B = C2(a1T1 – a2T2)/ 8D
where B = bow of the panel; C = panel length; a1 and a2 = facer CLTEs; T1 and T2 = temperature changes of the faces; and D = distance in inches between the neutral axis of the two faces.
Designers can compensate for thermal warp by making walls thicker. However, this approach is not without problems: It can lead to longer mold residence times, added foam weight, and less room inside the refrigerator. Another possibility is cutting the effective length of the panel by designing a corrugated liner that acts like a series of small panels rather than one large panel.