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IGMA – August 2016

Watch the cavities

August 4, 2016  By Bill Lingnell

This month we will explore a new document that is presently in committee and being reviewed for future publication by IGMA. This publication will address one of the many technical areas the organization works on to expand the knowledge base and meet the many needs of the insulating glass industry.

It is intended to provide an overview of some of the topics that IGMA encounters with respect to enhancing the important guidelines and informational data on the subject matter. The new document is titled “Voluntary guidelines for insulating glass unit cavity width manufacturing tolerances, TM-4400-xx.” The intent of the document is to present voluntary manufacturing tolerances that relate to the cavity width of an IGU at the time of fabrication of the unit. The guidelines are not intended to address all conditions that exist in an IGU once the unit leaves the manufacturing facility.

It is well known that there are dynamic conditions that exist in IGUs that are determined by environmental factors and changes in the overall window system that will result in conditions that cause a reduction or expansion in the cavity width of the unit. There is a large amount of information about cavity width variability on the IGMA website ( Topics include the effects of altitude differences once the IGU leaves the manufacturing facility and whether the unit is going through a major elevation change from the manufacturer’s plant to the customer’s location and/or a project site. The concerns of temperature and barometric changes will deserve attention and consideration as to the cavity width condition once the IGU is in the final glazed position. There is also information on solar reflectance with discussion of design options to reduce the certain occurrences that naturally occur due to solar conditions.

 The guidelines being prepared for the “Voluntary guidelines for insulating glass unit cavity width manufacturing tolerances” publication are intended to assist in determining the maximum cavity width reduction in the IGU at the initial point of manufacturing. The goal of these guidelines will assist in the evaluation of the manufacturing process of the IGU prior to its being shipped from the manufacturer to the customer for double (single air/gas cavity) and triple (two air/gas cavities) IGU’s. The cavity width is defined as the outside to inside dimension of an IGU, typically the distance between the number two to the number three surfaces or, in other words, the distance between the facing surfaces in an IGU. Cavity width variability is determined by the difference between the nominal cavity width thickness at the IGU edge and the cavity width thickness measured anywhere in the IGU. Normally it would be expected that the greatest cavity variance would occur near the center of the IGU.

Illustrations will be shown to demonstrate the conditions that will be “unrestricted,” “potentially limited” and “limited” to provide guidelines related to the known and anticipated cavity conditions. Supplemental charts will be included to offer guidance for the allowable IGU cavity reductions at the time of manufacture. The charts will be set up to give guidance for cavity width reduction allowances according to nominal cavity widths less than or equal to eight millimeters, greater than eight millimeters, and less than or greater than 13 mm for a series of IGU short-side dimensions. These basic conformance requirements along with additional technical commentary are in the review stage by a task group that consists of industry professionals from primary glass manufacturers, window suppliers, sealant and spacer suppliers, test laboratories, consultants and insulating glass unit manufacturers.

Once the task group is finished with the overall document it will be forwarded on to the technical services and technical policy committees prior to final approval by the board of directors of the IGMA and then on to publication for use in the industry.

Bill Lingnell has over 46 years of experience in the technical field of glass and architectural products. He holds three Masters of Science degrees in engineering: civil, mechanical and engineering science. Lingnell is the technical consultant for the Insulating Glass Manufacturers Alliance.

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