Most packaging films are heat sealed as an intermediate or final step to create a finished package. Achieving an integral heat seal is the goal for: vertical form/fill/seal VFFS vertical baggers, L bar / I bar sealing, horizontal form/fill/seal HFFS , flexible laminations, stand up pouches, USPS automatable polywrap, IQF frozen foods or heat shrink film.
How the interdependent factors of heat,pressure and dwell time affect heat sealing of plastic films.
To make a heat seal, the film must be melted together then allowed to cool down enough to become what is called “crystalline”, or solid. The three interdependent elements to heat-sealing are:
Heat Pressure Dwell Time
Think of it like moving around inside a triangle:
|If more pressure is applied, less heat and dwell time are needed.||Less heat and pressure are necessary if dwell time is increased.|
|If less pressure is applied, more dwell time and heat are needed.||More dwell time is required if heat and pressure are decreased.|
For optimum results, the film must be in a relaxed state while cooling down or gaps and pinholes will result due to stretching the plastic film while in a molten state. There is a good reason bag making machines are designed to seal in a relaxed state and allow a few seconds for cool-down. The combinations of these factors are measured in milliseconds on vertical form/fill/seal VFFS machines.
Pinholes in VFFS packages often result from dropping the package while it is still molten. This is easily solved by adding a chute like a playground slide to gently slide the nascent bag onto the conveyor belt.
There are two methods to measure the melting point of a film. They are Vicat softening point and DSC melting point. These measure the temperature at which a polymer begins to soften. They are different test methods, so they yield different results.
The softening point should not be confused with the seal initiation temperature (SIT) which is measured by a number of methods.
Copolymers such as PEVA lower the melting point and seal initiation temperature. Some but not all metallocene resins melt and seal much faster than PEVA.
A common mistake is to use too much heat and overcook the film if a seal is not easily achieved. Films with a low melting point/seal initiation temperature seal better with more cooling time, more pressure and less dwell time. The result is usually faster cycle speeds.
The enemies of good seals besides machine settings are contaminants inevitably encountered when heat sealing vertical form fill seal VFFS. While water does not usually interfere, blood, fat and especially powdered milk and cocoa powders in complete baking mixes can be frustrating.
To caulk and seal through these contaminants, a cheap general purpose homopolymer polyethylene just won't do. We provide forgiving sealing layers by including three or more resins with different sealing temperatures in films such as our 622 and IQF grades.
Bottom gussets which on printed bags must be sealed " treat-to-treat" have always been problem children. Watch this video to learn how to go from good to great bottom gusset seals.