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Dialing in the right seal temperature

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Q. Trying a new film. How can you speed the process of finding the correct sealing temperature?

A. Unfortunately, there are many variables involved in sealing a film. Some of those are the film or laminate’s thickness, dwell time, pressure, heat transfer and even the type of sealing mechanism being used as well as the configuration (or pattern) of any sealing devices. As a result, there is not an easy answer to your question. There are, however, some ways in which you might be able to narrow it down to a good starting point and then adjust your sealing temperatures from there.

One way would be to have the film supplier provide a seal strength vs. temperature curve for the type of polymer you will be working with. Basically, the supplier will generate this curve by starting at a low temperature point where the film is not yet sealing and go up in fixed increments. As the temperature increases, the seal curve will show the temperature at which the seal starts to initiate, where the seal is at its maximum strength, where it plateaus, and where it eventually begins to drop off and burn through occurs.

If your film supplier is not able to provide you with the curve, then the next best thing would be to use the polymer’s melting point as your starting seal temperature. From there, you can move up in temperature until you reach your desired seal strength.

Here are the melting points as measured by a DSC (Differential Scanning Calorimeter) for some of the more common polymers being used in our industry today:

28% EVA
160 °F18% EVA
185 °F12% EVA
199 °FPlastomer Metallocenes
176 – 221 °F4% EVA
225 °F
LDPE
230 °FMDPE
239 °FOctene
244 °FButene
250 °F
Hexene
253 °FMLLDPE
239 – 257 °FLMDPE
257 °FHDPE
275 °F

These temperatures can be valuable in providing guidance, particularly when changing from one film type to another. For example, if you are running an LDPE film at sealing temperatures of 325 °F for good seals, a Butene product would have to be run at least 20 °F hotter based on the melt points (230 °F vs. 250 °F DSC melt points).

Polymers can differ tremendously on their seal performance. So the temperature that gives you a good seal for one film may not be adequate for another, as you can see above. In addition, the sealing window or the temperature range for acceptable seals varies. Some can be very narrow, maybe 20 °F wide. Other more moderate seal windows may be 40-50 °F wide and yet, others can have a very broad range up towards 100 °F wide.

Another important point is that the film manufacturer can modify the composition of the sealant film to affect the seal temperature and range. It is common practice to blend several of the resin types listed above to move the seal range up or down or to increase its width.

The best advice we can give you is to work closely with your polyethylene supplier. They should be able to help guide your efforts by providing you with some of the information mentioned above.