Many consumer packaging applications employ bags using recloseable zippers for locking products within the bags. One type of recloseable zipper includes male and female profiles extending along the entire length of the zipper. The male profile is often tree shaped, with an expanded head portion supported by a narrower trunk portion. The female profile is disposed opposite the male profile and is adapted to interlock with the male profile by a pair of legs having locking edges and cornered shoulders. The profiles are interlocked by properly aligning the male and female profiles and pressing them together along the entire length of the zipper. The opposing legs of the female profile slide over the head portion of the male profile, and lock to it by locking the head with the cornered shoulders.
Recloseable zippers are typically formed of a polymeric material by an extrusion process. The zipper is typically attached to a polymeric film used to form the recloseable bag. Each recloseable zipper is normally permanently attached to a polymeric bag or film by a thermal-fusion process in which a heated metal bar is pressed against the zipper while the zipper is in contact with the polymeric bag or film. The attachment is frequently made prior to sealing and cutting of the film's side walls to form the finished bag.
Conventional zippers can be problematic during thermal-fusion because heat sufficient to bond the zipper profiles to the polymeric bag call result in deformation of the male profile, female profile, or both profiles. This thermal deformation can produce a permanent change in the shape of the profiles, and interfere with inter-locking of the profiles. Such interference may result in the profiles not locking, or inadequately locking so that the seal between the profiles permits excess leakage into, or out of, the bag. In addition, in some circumstances, the deformed profile can result in packages that open unexpectedly or open with a lower force than is desirable.
The problems associated with deformation of the zipper profiles are sometimes exasperated in zipper profiles that require more than one type of polymeric material or in closure arrangements having particularly complex profiles or profiles with very low tolerances to deformation. For example, in many applications, the polymeric material forming the recloseable zipper and the polymeric material forming the bag are not the same material. During heat sealing of the zipper to the bag, it is frequently desirable that the temperature of the base portion of the profile in contact with the package film reach a point sufficient to melt, or slightly melt, both the base portion and the package film. However, a polymeric material having a low melting point will sometimes be subject to excessive melting, and this excessive melting can be a problem if the profiles deform during thermal-fusion to the bag. Also, zippers having unusually small elements, or having very close tolerances for forming seals, are also vulnerable to deformation during the thermal-fusion process.
Consequently, a need exists for a zipper profile for a recloseable bag which overcomes the aforementioned shortcomings associated with existing zippers, and which can be thermally-fused to a flexible package with low deformation of the profile members.