Peel Rate

Theller Engineering 707-762-3820

HOT TACK HEATSEALER
PEEL RATE

Peel rate is controlled by the value that the operator types for "Clamp Separation Rate", which can be varied from 24 to 425 cm/min. Dynisco/Theller uses a default rate of 200 cm/min. This translates to a peel rate of 100 cm/min, which we have calculated (see below) would be about the rate at which a heatseal with marginal hot tack strength might fail in a packaging machine.

The peel rate used in the test is important, since it directly affects the measured value of hot-tack force. It is well known that the faster a seal is peeled apart, the greater is the force that is required. It is important to choose the peel rate for the laboratory test to be in the general range of rates that seals fail in packaging machines . The test values of seal strength will then be at about the same level as the forces that must be resisted in the real world.

In a conventional form-fill-seal operation, the web of packaging material is drawn into the machine, and first formed and then sealed into a vertical tube, open at the top and closed at the bottom by the horizontal sealing jaws. While the sealing jaws are closed, the product being packaged flows into the bag through the open top of the tube. During the end-sealing cycle, the jaws simultaneously form the bottom seal of the bag being filled and the top seal of the preceding bag, and sever the material between the seals -- which permits the lower bag to be carried away on a conveyor belt when the jaws subsequently open.

Until the sealing jaws open, the weight of the upper bag contents is carried by the jaws and grippers. When they open, the weight is then carried by the hot seal, which must be strong enough at that point in time to resist being peeled open. That situation persists during one complete cycle of the machine, until the top seal is made on the next cycle and the filled, sealed bag is cut free and transferred to the conveyor.

At a production rate of 150 bags/min, one cycle requires 400 ms, which is the approximate length of time the weight of the product would be stressing the seal (at that production rate).

If the strength of the hot seal is marginal, the first evidence is usually "mooning" -- where semi-circular regions of failure appear on the edge of the seal being stressed. Bottom seals are typically about 10 mm (3/8") wide - more or less. For the seal to fail completely, one or more of the failure areas must advance across its full width; however, to maintain a reasonable margin of safety it would be undesirable to accept more than a 2mm depth of mooning (but preferably none at all, of course).

In this example, the force due to the weight of the product would peel a barely acceptable seal at a rate of 2 mm per 400 ms, or 30 cm/min (equivalent to a grip separation rate of 60 cm/min on a tensile tester).

A clearly unacceptable material, when subject to the same force, would peel at a rate of 10 mm per 400 ms, or 150 cm/min (equivalent to 300 cm/min grip separation rate).

Dynisco/Theller has set 200 cm/min as the default rate of grip separation, to be in the same range as calculated for the packaging machine. The testing rate is variable, however, depending on the application.

It is important to recognize that the other hot-tack machines, with single-stage withdrawal and pull, typically pull at a grip rate of about 1200 cm/min - very much faster than calculated above. This artificially high rate is dictated by the need to pull the slack out of the sealed strip fast in order to get the hot-tack measurement made before 500 ms. Dynisco/Theller uses high-speed air cylinders to withdraw the seal and remove slack - so the rate of the motor-driven peel process can be adjusted to conform to what happens in real life. The high-speed pull of the single-stage machines leads to much higher values of measured seal strength than when pulling at the lower rate of the Dynisco/Theller machine.