Open cell vs. closed cell
When backer rod compared to the closed cell, the open cell is much more flexible. The open cell is dependent on its physical and chemical make up in order to attain its rigidity. However, closed cell rigidity is merely the result of the gases trapped inside the material. You cannot often see in commercial building construction that joint widths conform to its exact design. For example, a one inch joint will usually vary from as little as a quarter of an inch or lesser. It is really impossible to manufacture or put up large building panels that can accommodate the precise dimensions between joints in order for the panels to stay close to each other.
In order for the backer rod to be easily inserted between the building panels, it has to be very flexible. Companies manufacturing closed cell backer rods suggest to their customers that the product should only be compressed up to at 20 percent of the rods’ diameter. It is also recommended that the applicator should change the sizes frequently. Closed cell rods can be used in varying width joints while open cell rods have to exceed the 25 percent compression limit. Unlike the closed cell rod, the open cell rod cannot be hurt even if you compress it as much as 75 percent of its diameter. This allows the worker to use fewer sizes in various construction applications.
There are three primary purposes of the backer rod. First, the rod forms a base on which the sealant is applies to. The rod will therefore determine how thick the sealant bead is. The backer rod forces the uncured sealant to come out laterally under pressure. This then results in 100 percent contact of the sealant to the substrate. The lines made are called bond lines. The backer rods dictate the cross section of the sealant bead. The flexibility of the backer rods play a very crucial role in tooling the sealant beads that are newly applied. The backer rod is secondary to the sealant which is the primary product. The desired result is an excellent seal that will meet or even exceed its specifications. The cell backer rods which are of the close type are able to withstand much heavier pressure. The open cell rod is much flexible and it is because of this feature that the caulker is recommended to use a much lighter tooling pressure in order to minimize spring back.