| DEFINITION |
HOT TACK: WHAT IS IT?
"Hot tack" is defined as the strength of a hot seal measured at a specified time interval after completion of the sealing cycle but prior to the temperature of the seal reaching ambient. |
COMMERCIAL
APPLICATION |
Hot tack is important in form-fill-seal packaging applications, where the critical window of time is when the seal is in the packaging machine and must resist the stresses of the process. The time interval starts the instant the sealing jaws of the machine open, and continues until the sealed package is delivered from the machine to the conveyor belt. It is about a second in duration - more or less - depending on machine speed. During this period, the seal is cooling and its strength is increasing. |
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HOT TACK: HOW IT'S MEASURED
A plot of the strength of the hot seal versus time, for the first few hundred milliseconds of the seal's life, describes the hot tack behavior of a heatseal. This plot is the cooling curve of the seal, and is the physical basis for hot tack measurement by all present-day electronic testing machines - although the Dynisco/Theller machine has the additional capability to generate a plot of the complete curve from each test cycle.
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COOLING CURVE
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| NUMERICAL VALUE |
Since a numerical value of hot tack is needed to compare heatseals and materials, the computer of the Dynisco/Theller machine reads the cooling curve and prints out the value of strength (hot tack) at any cooling time specified by the operator. 250 ms cooling time is widely used as the default time for comparing hot-tack results, but any other time can be specified and used, depending on the application of the data.
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Machines that cannot plot the cooling curve, and that report only one hot-tack value per cycle, peel the seal after an initial delay time and identify the maximum force encountered - which is one point on the cooling curve -- as the hot tack strength for that delay time. This procedure lacks the physical significance inherent in identifying the hot tack value with the actual cooling time at which it was measured, and consequently yields a less-precise value of the hot tack of the seal.
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COMPLETELY
AUTOMATIC |
THE HOT TACK TEST
Operation of the Dynisco/Theller hot-tack machine is automatic from start to finish. Once the sample strip is clamped in the machine and the operator presses "start", the computer takes over. It positions the sample between the dies, closes the dies on the sample for a dwell time set in milliseconds (applying pre-set temperature and pressure) and removes the sealed strip instantaneously upon opening of the hot dies. The strip is transferred to an adjacent position where the seal is enveloped in low-pressure air of ambient temperature during the peeling (and cooling) phase. The computer starts the peeling process immediately (no "delay time" is used).
Peel rate is usually set to pull the seal apart in a little less than 1 second, although other rates can be selected. During the peel the computer measures and records the peeling force continuously, which is then plotted on the monitor as the cooling curve.
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| APPROVED BY ASTM |
The Dynisco/Theller hot tack heatsealer and the testing procedure described above are approved for hot-tack testing under ASTM F1921, " Standard Test Method for Hot Seal Strength (Hot Tack) of Thermoplastic Polymers and Blends Comprising the Sealing Surfaces of Flexible Webs".
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| FAST COOLING |
ULTIMATE STRENGTH MODE
"Ultimate Strength" means strength of a heatseal after it has cooled to ambient temperature and achieved strength stability. In this mode, the machine operates exactly the same as in the hot-tack test until the end of the sealing cycle is reached and the hot seal has been withdrawn from between the dies. At this point, instead of peeling the seal apart, it is cooled to ambient temperature in a few seconds by the machine's automatic air-cooling system. The machine then peels the cooled seal, generating a plot of the ultimate-strength seal profile, or it can be removed intact from the machine for a conditioning period
prior to measuring its strength. |
PROFILE OF STRENGTH
ACROSS SEAL WIDTH
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| MANUAL OPERATION |
HEATSEALING MODE
This is manual heatsealing, where the operator manually inserts the test specimen between the dies. The seal is then made under computer control to the set conditions, and then manually withdrawn when the jaws open. The specimen may be a strip of the desired width for subsequent strength testing, or it may be full length of the jaws.
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TENSILE
TESTER |
PEEL STRENGTH MODE
The machine in this mode functions purely as a tensile tester to peel pre-formed heatseals and measure their strength -- plus generate the graphic of the seal profile.
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PRECISION AND ACCURACY
The high performance of Dynisco/Theller heatsealers is a result of experience and the technology developed by Theller over the past 20 years.
Performance depends ultimately on high precision and accuracy of control of the two most important variables in heatsealing and hot-tack measurement: temperature and time. We measure and control these variables as follows:
- Die temperature is measured to 0.1° resolution and
0.2°C
accuracy with individually-calibrated platinum RTD sensors. The control band is ± 0.2 °C.
- The timing circuits in Theller hot-tack machines are
accurate to ± 2 milliseconds. This includes measurement of dwell time during the sealing cycle, cooling time during hot tack measurement, and control timing of the various operations comprising specimen insertion and withdrawal. Most other electronic hot-tack machines measure and control only to ± 25 ms.
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REPRODUCIBILITY
Heatsealability testing -- both for ultimate seal strength and hot tack -- has always been notorious for poor reproducibility of test results and dependence on operator technique. But now, Theller's patented technology has achieved outstanding reproducibility, both within and between laboratories -- made possible by its high precision and accuracy of control of test conditions.
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| REPRODUCIBILITY |
Test results of hot-tack peel strength from any Dynisco/Theller machine, when configured and calibrated and operated as recommended, are normally within 5%* of the average of test results from all similar machines, when the test is conducted under conditions that minimize variance from sources other than the machine and operator.
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In situations where a customer has several machines on which materials must be tested interchangeably, we can set up those machines to agree within closer than normal tolerances.
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AUTOMATIC CALIBRATION
- The temperature circuit for each sealing jaw individually is automatically calibrated by the computer every 3 minutes.
- Seal strength is measured electronically to a precision of ± 3 gm. The circuit is calibrated by the computer during the first few milliseconds at the beginning of each test cycle, to ensure continued maintenance of accuracy.
- The sealing-pressure circuit is likewise automatically calibrated by the computer at the outset of each test cycle.
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EXTRAPOLATION
TO ZERO TIME |
HOT TACK DATA IMMEDIATELY AFTER
THE SEALING JAWS OPEN
Theller's patented two-stage specimen-withdrawal
technology permits the machine to start measuring the strength of the hot seal within 100 milliseconds after the end of the sealing cycle. This compares with the 250+ ms typically required by the single-stage "delay time" machines. But in addition to displaying hard data starting at 100 ms, the software does a least-squares extrapolation of the cooling curve back to zero cooling time and calculates the hot tack of the seal at the instant the jaws opened.
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COOLING CURVE
SHOWING EXTRAPOLATION
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CONSTANT
CONDITIONS |
CONTROL OF COOLING CONDITIONS
The Dynisco/Theller hot-tack machine is unique in controlling the conditions of cooling while strength of the seal is being measured during the critical first second of its life. Unlike any other machine, the seal remains in one location where its
environment can be controlled during the entire peeling and strength-measurement process. The seal is normally maintained under slightly elevated pressure of air at ambient temperature. This function can be on or off - at the operator's option.
Users have employed this property of the
machine's design to study effect of controlled cooling condition on the
cooling curve.
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COMPARISON OF MATERIALS
When materials are being compared for hot tack performance, the comparison is usually based on the hot-tack curve, which is a plot of hot-tack strength at some specified cooling time vs sealing temperature.
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HOT TACK CURVE
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The Dynisco/Theller machine generates the hot-tack curve for materials tested, and also produces a data file that can be read by any spreadsheet program to prepare presentation-quality tables and graphics.
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| COMMERCIAL SIZE |
SEALING DIES FOR SIMULATION
OF COMMERCIAL MACHINES
All Dynisco/Theller heatsealers are equipped with commercial-size
sealing dies. The dies are shorter than on most commercial packaging machines, but they are in the same range of dimensions with respect to width and height, and can be used for studies of the effect of die configuration on seal strength.
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| SIMULATION |
Copper and aluminum dies are available for technical work (measuring the heatsealability of materials). But if the testing objective is to determine how well a particular material will run on some specific commercial packaging machine, it is necessary to set up the laboratory machine with steel serrated jaws of the same type as on the field machine.
We build steel serrated dies in both vertical and horizontal style, and in serration angles of 60, 90, and 120 degrees. The serration pattern is diamond-ground into the hardened surface of an insert that fits into a keyway in the die body. Inserts are interchangeable, so only the insert - not the complete die - is changed when switching serration angles or horizontal/vertical styles.
Our aluminum dies are made in the insert type. We also offer die blanks, which the customer can have finished to proprietary designs by others. All Dynisco/Theller dies are quick-change design.
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PATENTS
The machine is covered by US Patents 5,331,858 and 5,847,284.
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ADDITIONAL INFORMATION: |
