Maintaining the right heat seal parameters requires careful setup and management
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Optimized tool design and control allows conductive direct contact heat sealing machinery to create consistent, high-quality seals.
Soft, spoonable dairy and similar foods require processing and packaging machinery that reliably seals a peelable, flexible lid to the rim of rigid plastic containers.
Optimizing processing for a particular production rate is not cost-effective without also minimizing scrap rates, improving processing machinery reliability, and providing consistently strong seals.
Conduction-based heat sealing is the most common method used for sealing food packaging. This process involves monitoring and managing heat sources and temperature control sensors built into the body of the tooling mounts. Conduction transmits the necessary heat to the sealing surfaces of the tooling faces. This heat activates the lidding’s seal layer, which creates a bond between the lidding and the container.
Soft Dairy and Spoonable Foods Heat Seal Overview looks at what it takes to package ready-to-eat soft dairy and spoonable foods in plastic cups.
Heat Seal Lidding Materials for Soft Dairy and Spoonable Foods considers material choices for effective lidding sealing.
Even with different lidding design options, effective, safe and durable seals still require careful control of the sealing parameters at the tooling surfaces where the package seals are created.
Control of sealing drums, sealing heads and platen seal heads includes controlling the pressure and dwell time as well as temperature. The right lidding and rigid packaging material choices plus the right tooling design and sealing process must work together to make consistent, robust, safe, and user-friendly seals possible.
Die-cut lidding and rollstock lidding processing machinery sub-systems
Machinery manufacturers and designers usually offer sub-system options for either die-cut or rollstock lidding. The decision of which type of integrated lidding sub-system to use is important and is made early in the planning stages for each processing and packaging line.
Long-term management of soft dairy processing and packaging lines requires fine-tuning, refreshing and optimizing the design and control of heat seal tooling. Partnering with a tooling supplier with the engineering capabilities and experience to develop and deploy the best tooling solutions ensures good product seals.
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Consumers expect easy-to-peel lidding
For soft dairy and spoonable food rigid packaging, peelability means the separation or peeling away of the lidding away from the cup or rigid container. The intent is that single-use dairy packaging lidding pulls away from cup rims without shredding or damaging the container.
Getting peelability right requires a suitable lidding seal layer and container material match, plus a carefully controlled heat seal. Correctly optimized heat sealing is critical to creating peelable seals between the lidding and the edge of the container. It does not weld the lidding to the plastic rim.
Hot tack strength is the strength of the seal formed between the heat seal layer and the rim of the container immediately after making the seal but before it cools down to room temperature.
Why is hot tack strength necessary?
Because hot tack measures the strength of a heat seal before it cools down, it matters during processing. Hot tack strength is significant for bagged food. ASTM F / FM - 12() is the standard used for testing these packages.
Because seal strength and hot tack strength are strongly correlated, hot tack strength is essential for rigid packaging lidding. Testing hot tack strength for flexible and semi-rigid packaging seals requires heat seal strength and hot tack testing equipment to conform to ASTM F.
Mechanical heat seal strength
For peelable lidding, heat seal strength may be described as mechanical peel strength. In general, seal strength must be within a range that protects the quality of the food inside. It must also be consistent. Consistent seal strength can indicate whether the food filling and sealing is well-controlled and reliably producing optimally sealed packages.
Seal layer initiation temperatures and operating windows
The heat seal layer of all lidding has two related temperatures. One is the seal initiation temperature (SIT), and the other is the seal temperature operating window. SIT is the temperature at which the sealing layer creates a seal to the container rim that is strong enough to be useful.
What is the heat seal layer operating window?
The operating window is pretty much what you would expect. It is the temperature range starting with the SIT on the low end. The temperature at the high end of the operating window can create good seals. But temperatures higher than this are too hot.
Seal layers exposed to seal temperatures that are too high cause noticeably distortion. This distortion also warps the lidding creating poor seals. This distortion also warps the lidding creating poor seals.
Mono-material lidding films and production sealing modifications
An important issue with using mono-materials for plastic lidding films is appropriately adjusting the production sealing parameters. Mono-material lidding films tend to be more heat-sensitive, so they usually require modifications to the production process. These modifications may include sealing parameters (temperature, dwell-time and pressure), heat seal tooling design, and processing optimization.
Simply put, excellent seal quality using mono-material packaging materials requires more stringent control of lidding heat sealing. Material choices, including the sealing layer choice for multi-layer lidding, can have a significant impact on sealing parameters, tooling surface design and the quality of the final seal. Find out more about lidding materials.
Time and the challenges of a good package seal
The longer a heat seal tool like a seal head, roller sealer or platen head is in contact with the lidding film, the more heat reaches the sealant layer. A heating time or dwell time that is long enough but not too long creates a good seal. This dwell time ensures that the sealing layer reaches the necessary seal initiation temperature (SIT) but stays within the sealing layer operating window.
Increased processing and packaging machinery speeds create heat seal challenges
Fully automated processing and packaging machinery lines have become faster because faster processing is more cost-effective.
One consequence of these higher processing speeds is that optimized timing (dwell time), temperatures and pressures required to seal lidding or lidding films to rigid containers have become even more critical. Whether packaging sealing is semi-automatic or fully automated, fine-tuning the lidding sealing sub-systems is now vital to performance and profitability.
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The goal of designers and operators is to meet line speed requirements using a blend of machinery design, packaging material choices and heat seal tooling design. All three work together well when followed up by good machinery setup, monitoring and maintenance.
Machinery design has a significant impact on providing sufficient dwell times at higher line speeds. Generally, outside of managing heat seal tooling temperatures, an essential point to keep in mind regarding dwell time is that adjustments and fine-tuning become even more critical.
Higher line speeds amplify problems as line speeds increase.
A changeover is a switch in machine tooling, lidding or rigid container for a food packaging processing line. Changeovers often accommodate different packaging sizes or foods being processed. An excellent example of this is a tooling changeover from one cup size to another.
Why changeovers matter
Changeovers often mean different timing for the machinery sub-systems and sometimes even different container and lidding materials. If the temperature settings of heat seal tooling are not adjusted to compensate for line speed and material changes, sealing temperatures may become too high or too low. Temperatures that are too high affect seal quality by reducing hot tack strength. They also can cause lidding film distortion, fracturing or even melting. Temperatures that are too low mean unsealed products or products with seals that are too weak.
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Pressure and the challenges of a good package seal
Heat sealing causes melting of lidding seal layers:
During the sealing process, heat seal tooling applies pressure where lidding contacts the package rim. A mating tooling support fixture supports the container's rim.
The lidding sealant layer melts. It then spreads out (wetting) along the packaging seal contact surfaces. Molecules of the melted sealant layer then move into the plastic of the cup sealing surface or rim (diffusion). The sealant must also stick to itself both when it melts and after it cools down (adhesion).
Applied pressure during heat sealing is essential because it needs to provide a firm enough contact to cause the sealing layer to melt evenly to create a good seal. Pressure adjustments for optimizing heat seal tooling can be challenging to control. Often, operators base their control decisions on experience plus some experimentation.
If there is too little pressure, the seal will be poor. Too heavy a pressure usually creates an uneven pressure that can crack or split the lidding. Using a conforming (compliant or flexible), highly heat conductive material for the heat seal tooling face and support fixture helps avoid this. Conforming materials like high-temperature resistant silicone provide more uniform pressures and more predictable results.
Seal heads and platen head mounts include springs in their mechanisms. Springs work in combination with conformable heat seal tooling surfaces to improve the quality of the applied pressure. Better quality pressure is more uniform. More uniform pressure applied to sealing surfaces also allows heat seal tooling to compensate for potential contamination of the sealing surfaces. Improving the applied pressure quality plus good tooling surface design also helps direct force to the specific part of the seal that needs it most.
Temperature and the challenges of a good seal
Heat seal tooling materials and design can promote or inhibit their ability to conduct heat.
The seal layers of die-cut lidding and lidding film are heat-activated. Good package seals require the right initiation temperature and keeping the tooling sealing surface temperature within the sealant layer's functional temperature range.
Heat seal tooling not temperature-optimized can lead to:
Excessive heat at the sealing surface causes:
Insufficient heat at the sealing surfaces causes:
It is essential to recognize the real challenges of consistently achieving the required seal layer initiation temperatures and maintaining tooling surface temperature within the seal layer's operating window (functional temperature range). The sealing head, roller sealer or platen head sealing surfaces need to be at the correct temperature. The proper sealing heat must also be conducted through the lidding film itself to the heat seal layer.
What complicates optimizing the sealing process is that temperatures at the heat seal layer will be different than the temperature setting of the heat seal tooling. This difference is one of the reasons the sealing heat control is more complex than merely adjusting controller temperature settings.
Heating control, heat conduction and tooling design all work together to allow heat seal tooling to produce good package seals. Tooling design requires optimization for:
What is heat seal tool recovery time?
Recovery time is the time it takes the heat seal tooling surface to return to the desired temperature. Fully automated, higher-speed packaging lines require faster heat seal tooling recovery times.
Proper engineering design ensures there is enough heating power and thermal mass (ability to store heat) in the heat seal tooling and mount. Thermal mass and enough heating power provide good heat seal head recovery times at the tool sealing surfaces. If the tooling has a compliant sealing surface, this needs to have good heat conductivity also.
Summing it all up
Starting with the correct lidding and rigid container materials is essential for competitive, safe, and secure soft dairy and spoonable food packaging. This combination only works when the automated processing machinery is optimized to produce strong and consistent packaging seals.
Automated machinery meets production line goals by taking advantage of optimized tool design, control, and sealing characteristics. Optimization reduces production costs, produces safe food, and presents it in appealing packages with a minimum of waste.
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Selected Reference Resources
Our blog post series about heat sealing packaging for soft dairy and spoonable foods
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