Package making – Closing packages and filled receptacles – By means adapted to engage outturned seam of package cover
Reexamination Certificate
2000-04-19
2004-09-14
Smith, Scott A. (Department: 3721)
Package making
Closing packages and filled receptacles
By means adapted to engage outturned seam of package cover
C053S373800, C053S374800, C053S550000, C053S554000
Reexamination Certificate
active
06789371
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention generally relates to packaging machines, and more particularly to packaging machines having seal bars assemblies using electrical resistance impulse heating to form a bond in polymeric material.
BACKGROUND OF THE INVENTION
Packaging machines are used for sealing a product in an air-tight package. Packages are often constructed of polymeric material. The product to be packaged is placed, by hand or machine, within a bag, a tube or between several sheets of packaging material, then the packaging material openings are sealed closed. Air may or may not be trapped within the package along with the product. A vacuum packaging machine evacuates substantially all the air between the product and the packaging material before sealing. Vacuum packaging is used to reduce package volume and, in the case of perishable product, preserve freshness.
Various techniques are utilized for bonding together layers of polymeric packaging material. One device to effect polymeric bonding is an impulse heating apparatus, or seal bar assembly. The surfaces of polymeric material to be sealed are placed adjacent to each other and in contact with the seal bar assembly. Application of pressure and sufficient heat partially melts and bonds the surfaces of the polymeric material together to form a seal. Cooling stabilizes the bond.
Conventional seal bars are generally constructed of a metallic block having smooth face mating surface. A flat, electrically resistive wire is disposed on a seal bar assembly smooth face surface. Another bar, opposing the seal bar, applies pressure to hold the packaging material in contact with the seal bar. Heat is generated by passing sufficient electricity through the sealing wire for a period of time (dwell time) until the wire reaches a desired temperature. Heat is transmitted from the sealing wire to the packaging material to thermally bond the surfaces of the polymeric material to each other. Electricity is removed from the seal wire and the package is allowed to cool, stabilizing the bond. Contact pressure is relieved and the sealed package is removed from the seal bar to complete a package sealing cycle. The conventional sealing wire is a discrete electrically resistive heating element, but may also be integrally formed onto the surface of the seal bar.
Reducing package sealing cycle time increases efficiency and reduces the cost of packaging. One method to reduce package sealing cycle time is to cool, and thus stabilize, the bond more quickly. Forming the seal using only the minimum heat necessary is desirable, so that time to dissipate heat in excess of bond-forming heat is avoided. However, if too little heat is input, bond quality will decline. Quickly conducting heat away from a completed bond also decreases cool-down time. The seal bar preferably acts as a heat sink to remove heat generated by the sealing wire from the area of the bond after the seal is formed. Therefore, the seal bars are constructed of material possessing efficient thermal conductivity properties.
The conventional seal bar is constructed of aluminum. Aluminum is relatively easy and inexpensive to manufacture. Aluminum possesses desirable heat conduction properties and can be machined to a smooth surface, impervious to moisture and bacteria. Heat is conducted via the seal bar to a cooler ambient environment or to a heat removal fluid (liquid or gas) passed through the seal bar itself.
Conventionally, a layer of insulating material is disposed between the sealing wire and the electrically conductive metallic (i.e., aluminum) seal bar to prevent shorting of the electrical heating circuit. One common method to provide such electrical insulation is to dispose a thin, adhesive-backed TEFLON tape adhered to the metallic seal bar smooth face surface. The sealing wire is subsequently disposed onto the non-adhesive side of the TEFLON tape.
A thin layer of release coating material is conventionally disposed over the outer surface of the sealing wire to prevent the polymers of the polymeric material from sticking to the hot sealing wire, commonly known as “poly prick.” Poly prick decreases the integrity of the polymeric packaging material bond due to the loss of polymeric material extracted by the sealing wire from the joint area.
Additionally, over time, the sealing wire heat transfer capability is reduced in isolated regions along the sealing wire due to the insulating presence of melted polymeric materials adhering to the wire. Bond integrity is subsequently reduced in these regions if insufficient heat is transferred to the packaging material in these areas. Alternatively, if sealing wire temperatures are increased to compensate for the isolated areas of seal wire insulation due to poly prick, un-insulated areas of the seal wire can overheat, burning or otherwise damaging the polymeric material. One conventional method of preventing poly prick is to place another layer of TEFLON tape over the hot sealing wire to form another smooth face surface on the seal bar. The TEFLON is disposed, adhesive side towards the sealing wire, between the sealing wire and the polymeric packaging material to be bonded.
The sealing wire is conventionally constructed from a metal or metallic alloy such as silver or nickel alloy. The long, flat sealing wire thermally expands and contracts as it heats and cools during each impulse heat sealing cycle. The cyclic expansion/contraction stress the sealing wire.
Areas along the sealing wire experiencing differential heat transfer rates due to the presence of polymeric material on the sealing wire from poly prick. Polymeric material on the sealing wire insulates the sealing wire reducing heat transfer from the sealing wire to the packaging material, creating sealing wire hot spots. “Hot” and “cold” regions exist along the sealing wire due to difference in thermal transfer characteristics. Additional thermal stress exists between the “hot” and “cold” regions along the sealing wire.
The thermal expansion/contraction cycles also cause the sealing wire to mechanically creep during each cycle. This sealing wire movement creates wear of the insulating TEFLON tape as the sealing wire moves relative to the TEFLON tape. If the TEFLON tapes wear through, electrical insulation is lost, the sealing wire shorts out and losses ability to generate heat, and thus losses the ability to form a bond in the polymeric material along the entire length of the desired seal.
Along with the mechanical movement of the sealing wire imparted to the TEFLON tapes, the TEFLON tapes also undergo thermal expansion/contraction cycles as heat from the sealing wire is imparted to the layers of TEFLON tape by conduction. The TEFLON tapes frequently bunch during sealing operations using conventional seal bar and seal bar mating block assemblies.
TEFLON tape bunching is deleterious to seal quality. TEFLON tape bunching creates peaks and valleys in the surface of the seal and mating bars, resulting in uneven application of pressure. Once the TEFLON tape bunches, the adhesive-backing releases and the TEFLON tape can overlap itself. Furthermore, conduction of heat from the sealing wire through the TEFLON tape disposed between the sealing wire and the polymeric material varies between the peaks, valleys and increased thickness of overlapped areas of the bunched TEFLON tape. Due to the varying degrees of contact between the polymeric materials and the seal bar, and the varying heat transfer characteristics of the TEFLON tape, bond strength varies along the joint, and quality is thus reduced (seal integrity is only as good as the weakest point along the seal).
Bunching of one TEFLON tape tends to mechanically cause the other TEFLON tapes in close contact to also bunch up. Therefore, bunched TEFLON tapes must be removed and replaced along with the sealing wire disposed therebetween. TEFLON tape replacement requires removing the offending seal bar assembly from service, reducing efficiency and increasing costs. Package machine down time, maintenance labor costs and replacement part cos
Bomgaars Thomas E.
Buysman Jeffrey L.
Desai Hemant M.
Dicke Billig & Czaja, PLLC
Smith Scott A.
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