Multi-layered heat sealed lidding for rigid food containers
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Optimized tool designs and careful control of conduction-based direct contact constant heat sealing are not enough. Lidding materials, especially the heat seal layer, must be appropriate for the container and the food inside.
Find out more 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. Soft Dairy and Spoonable Foods Lidding Sealing Challenges examines the challenges to getting consistently better and safer lidding seals.
Rigid packaging lidding requires multiple laminated or coated layers. Each of these layers has a specific purpose. These layers consist of foil, paper, plastic or metalized plastic.
Soft dairy and spoonable food lidding must be relatively easy to open without shredding, especially for single-serve, ready-to-use foods like yogurt. In addition to being peelable, this lidding needs to provide a protective barrier and a strong seal that is tight enough to prevent leaks.
The simplest die-cut and rollstock lidding must provide these three essential functions:
The most basic type of lidding for ready-to-use soft dairy and spoonable foods has these layers:
It is not unusual for soft dairy and spoonable food lidding films to have more than three layers. Each added layer serves a different purpose. Adhesive tie-layers combine two or more layers that would not otherwise adhere well to each other.
Typical 3-layer lidding film with an adhesive tie layer
Substrate layers are the foundation of lidding films
Substrate layers are the mechanical foundation for lidding. Other lidding layers bond to the substrate layer. Substrate layers are usually suitable for printing. But, if they aren't printable, they are bonded to a printable layer directly above them.
Lidding uses one of three different substrate materials. Any all-plastic lidding film uses a plastic substrate.
Basic types of lidding substrate:
Physical characteristics considered for lidding design and material choices:
Service conditions that need to be considered when choosing lidding:
All food lidding film and die-cut lidding require barrier layers
Good seals and an attractively printed, tough substrate are not enough. As mentioned earlier, heat sealable die-cut lidding and lidding films for soft dairy and related foods are multi-layer. Lidding always includes one or more barrier layers.
Barrier lidding layers for soft dairy and spoonable foods
Food products need to be protected from contamination to keep them fresh, tasty and safe to eat. While good lidding seals maintain product quality and protect consumers, the barrier layer extends shelf life and protects the sensory quality of food products.
What are lidding barrier layers?
Barrier layers in food packaging prevent the transmission of oxygen, moisture and other contaminants. These environmental barriers help keep food fresh for longer periods and extend product shelf life. Blocking oxygen is especially important for protecting live culture products like yogurt.
Essential lidding barrier layer functions:
Cups, pots, tubs and their lidding provide barriers to:
Barrier layers also keep desirable odors and flavors in and undesirable aromas and flavors out.
What lidding heat seal layers provide for both die-cut and plastic lidding film:
What is the heat seal layer initiation temperature?
Two temperature specifications apply to all lidding heat seal layers. One is the seal initiation temperature (SIT), while the other is the heat seal operating window. The SIT temperature is the lowest temperature where the sealing layer creates a strong seal against the container rim.
The heat seal operating window is the temperature range between the SIT on the low end and the highest temperature the heat seal layer can withstand and still form a strong seal.
Sealant layers are coated onto, laminated onto or extruded onto lidding and are the final or bottom layer of multi-layer lidding. Sealant layers allow the lidding to be heat sealed to the rims of preformed and thermoformed cups, pots, tubs and trays.
There are two basic types of heat seal layers used for soft dairy and spoonable food lidding:
Heat seal layers are polymer-based heat-activated adhesives (hot melts). They are usually added using extruded or laminated film layers or solvent-based liquid coatings (lacquers). In general, the mechanics of the heat sealing process for both types are the same.
Beyond the two basic types of heat seal layers, there are other issues to consider when selecting a lidding and its heat seal layer:
For soft dairy and similar foods, these are service conditions that need to be considered when choosing the heat seal coating or layer and the seal it creates:
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Soft dairy packaging uses lidding that comes as either loose pre-cut die-cut lids or continuous lidding rolls called rollstock. Both types of rigid packaging lidding are designed to be heat sealed onto the rim of the food containers.
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Rollstock lidding and the lidding rolls used to create die-cut lids are types of flexible packaging. Companies that make multi-layer flexible packaging, including lidding, are known as converters.
Converters are experts at modifying and combining basic materials to generate wide, long, and massively big rolls of flexible film and lidding. The most common production methods they use are coating, laminating, and printing. They use paper, polymers (plastics), metalized plastic or foil for the different lidding layers.
After producing these large industrial-sized rolls, they are slit, printed and rolled into more manageable sizes for automated food processing equipment. These smaller, narrower rolls or rollstock are used directly on packaging machinery or pre-cut into single die-cut lids.
Aluminum foil layers are common in die-cut lidding used for soft dairy and similar meals. This popularity is because it is an excellent barrier to moisture, light and oxygen.
The foil used in lidding must be coated or laminated on both sides using adhesive tie layers to bond to the other lidding layers. The foil is protected from punctures and tears by a top or outward-facing layer. This protective layer is usually printed or bonded to a printable layer above it.
Foil die-cut lidding
The simplest foil die-cut lids are made from aluminum with a heat-activated lacquer coating, or plastic layer added that is heat sealed to the container rim.
Plastic die-cut lidding
Plastic die-cut lids are composed of multi-layer plastic, like flexible packaging film rollstock. Polyester (PET) plastic is the most typically used plastic for die-cut lidding.
Paper die-cut lidding
Paper die-cut lids are made from a combination of paper-laminated aluminum and plastic film.
Plastic lidding film advantages versus foil lidding:
Foil lidding has exceptional built-in barrier properties, but plastic-only lidding films have distinct and attractive benefits.
Here are some of the benefits of using multi-layer plastic lidding films instead of die-cut foil lidding:
Peelability
What is peelability?
One of the most critical factors in choosing lidding heat seal layers for their performance is the peel seal or peelability desired in the packaged product. The lidding on single-use dairy packages is designed to peel away from cup rims without shredding or damaging the container.
Peelability requires matching the proper lidding seal layer to the cup's material, as well as a well-controlled seal initiation temperature
The peelable lidding design challenge
One of the key challenges of designing peelable lidding seals is that it needs to maintain its seal even as the packages move through the food supply chain.
General ways you can categorize lidding peelability or peel strength:
Soft, spoonable dairy and related food products tend to be packaged with medium to strong peel strength. This level of peelability protects them during shipping and handling but provides practical ease of use for consumers.
Why is peelability important?
Peelability necessitates matching the lidding and heat seal layer to the container plastic and controlling the heat seal process. The ultimate result is a peelable seal that isn't too weak to fail and isn't welded and bother customers.
A recent issue affecting the design, production and use of lidding films is recyclability. As more manufacturers work to reduce their waste stream and increase their efficiency, market pressures increase the demand for recyclable lidding film solutions.
A growing trend in consumer-packaged dairy products is a mono-plastic, mono-polymer or mono-material lidding film and cup combination.
What is mono-material packaging?
It is important to note that increasingly soft and spoonable dairy packaged products use mono-material or mono-polymer flexible plastic lidding films. Mono-material films are films where all the layers of the lidding film are the same plastic-type as the cups they are sealing. This use of mono-materials makes the packaging easier to recycle. Easier to recycle means the cup and lid are recycle-ready for existing single-stream recycling.
What is single-stream recycling?
Mono-material rigid plastic and lidding combinations fit more readily into single-stream recycling systems. Single-stream recycling is a simple recyclable material collection process.
Single-stream recycling allows consumers to use one container to collect and recyclable materials. It makes recycling collection much simpler and more manageable.
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Processing soft dairy and spoonable foods require choosing the right lidding and rigid container materials. In addition, food safety and quality require sturdy, protective seals between the lidding and the cup rims.
All of this comes together during the heat seal process. Optimizing tool design, control and sealing parameters provides consistent seals. The results are lower, predictable production costs and minimal scrap while maintaining production line speeds.
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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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