The technique of vacuum packing food products in plastic trays is an efficient way to increase shelf life, retain nutritional quality, and minimize waste. Using such plastic trays commercially or for retail purposes requires knowledge of the principles of material science and machinery. Unlike flexible vacuum pouches, rigid and semi-rigid plastic trays present distinct structural challenges that demand specialized equipment and methodologies..
The Industrial and Commercial Challenges of Tray Sealing
Plastic food trays, especially when used in retail applications, have been designed mainly for containment rather than high-vacuum removal after their initial use. For businesses and processing facilities looking to preserve food directly within these containers, there are several technical hurdles to be addressed.
1. Trapped Air Pockets and Residual Oxygen
The design of commercial food trays is usually irregular in terms of internal geometry to suit certain pieces of meat, poultry, or other processed foods. This results in formation of micro-voids and deep pockets of air surrounding the food item. The standard vacuum technique fails to evacuate all pockets of air. Any oxygen trapped in these voids promotes oxidation and microbial activity.
2. Structural Failure and the Crush Factor
Vacuum tray sealers used for countertop or commercial sealing work by creating a vacuum within the whole chamber or bag environment, applying pressure differentials between the atmospheric pressure and the container. Exposure to fast vacuum draws makes regular thermoformed plastic trays unable to withstand the pressure from outside. This leads to the occurrence of the “crush factor,” where the tray wall may bend, fold, or even completely collapse.
3. Material Incompatibility and Thermal Sealing Complexities
There are different polymers employed in the fabrication of food trays. For instance, the polymers PET (Polyethylene Terephthalate), CPET (Crystallized PET), HDPE (High Density Polyethylene), and PP (Polypropylene) have different melting points, thermal conductivity, and barrier properties. Employing an inappropriate thermal sealing temperature or dwell time on the standard tray sealer may lead to poor sealing, pinhole leaks, or melting. Delamination or melting of the flange region causes failure in the barrier, allowing air from the surrounding environment to enter.
Equipment Capabilities and Technical Selection
Achieving a repeatable, hermetic seal on a plastic tray requires matching the specific polymer composition of the tray with an appropriately calibrated vacuum system.
Manual and Handheld Extraction Systems
The low-volume manual suction method employs localized suction through a special valve or edge attachment. Although this technique is more manual and does not cause the sudden pressure loss that causes collapse of weaker trays, the process is very labor-intensive and lacks the accuracy needed for commercial purposes and stabilization.
Chamber and Continuous-Run Systems
The industrial chamber system evacuates the air evenly from the ambient atmosphere around the tray, thereby equalizing the pressure between inside and outside the container during the process of vacuum forming. But even after venting, when the air is let back into the chamber for collapsing the film on the tray, the pressure difference might cause distortion in the case of thin gauge plastics.
Industrial-Grade Machinery Integration
For any operation where throughput is constant, standard household or light commercial machines cannot cut it. There are specialist integration providers who are adept at recommending heavy duty tray sealing equipment with capabilities to accommodate varying types of polymers. Roberts Technology Group, Inc. offers expert engineering services to determine that the vacuum settings, the proportion of the gas flush, and heat seal time are perfectly set according to the physical properties of the particular tray being handled.
Methods to Vacuum Seal Food in Plastic Trays
Depending on your production volume, available equipment, and the structural integrity of your packaging materials, select one of the following validated technical protocols.
Method 1: The Product Transfer Protocol (Low-Volume/Non-Rigid Trays)
When dealing with thin-gauge, low-density retail trays that cannot withstand negative pressure differentials, transferring the product to a dedicated vacuum storage medium is the most reliable method to achieve a secure barrier.
- Aseptically Extract the Product: Utilizing clean, sanitized implements, carefully excise the original top-webbing or plastic film from the retail tray. Remove the food item without tearing or puncturing the product surface.
- Reposition into a High-Barrier Medium: The food product should be placed in a multi-layer co-extruded vacuum pouch or a rigid vacuum canister. The product should be kept flat against the bottom of the film in order to avoid the development of air pockets within the pack.
- Execute the Vacuum-Seal Sequence: Position the pouch or canister inside the machine’s interface. Start the vacuum cycle depending on predetermined pressures (usually ranging from 25 to 29 inHg). Wait until the cooling process is completed by the thermal sealing bar before taking out the packaging.
- Label and Archive: Mark the date of processing, the batch number, and the expiry parameters directly on the impermeable surface of the film. Move the enclosed film to the refrigerated storage facility (<4°C or <-18°C).
Method 2: Direct Tray Sealing with Modified Atmosphere Packaging (MAP)
For commercial applications where food items need to be kept in a rigid or semi-rigid tray for purposes of display and preservation, the use of direct sealing through the application of automated machines specifically designed for this purpose becomes necessary.
- Flange Optimization and Inspection: Make sure the sealing flange (the top rim of the plastic tray) is completely dry and free from any moisture, fats, particles, or leftover product. The flange being contaminated is one of the biggest reasons for micro-leaks in thermal seals.
- Tooling Alignment: Place the filled tray in the designated nesting matrix of the tray sealer. Precision tooling stabilizes the side walls and flange base of the tray, making sure that it does not collapse under the force of the vacuum pull.
- Vacuum Evacuation and Gas Injection: The equipment seals the chamber surrounding the tray, thereby removing the oxygen from the surroundings. For sensitive food products that are susceptible to crushing or discoloration, such as fresh red meat and green leafy vegetables, an alternative process called MAP (Modified Atmosphere Packaging) is adopted. Here, the air removed is replaced with a specific blend of gases like nitrogen and carbon dioxide.
- Thermal Die-Cutting and Fusion: The upper barrier film is automatically laid on the heated flange of the tray. Heat and pressure are applied by the system in order to fuse the film with the tray matrix while simultaneously trimming the film to the edges of the tray using built-in knives.
Critical Standards for Preserving Seal Integrity
To maintain an uncompromised food vacuum seal over extended storage periods, strict adherence to quality control parameters is essential.
Puncture Prevention and Film Gauging
Bone-in proteins, frozen seafood, or items with sharp structural profiles present a high risk of piercing the top-webbing or tray liner. Multi-layer co-extruded films, which incorporate a high amount of nylon or Surlyn, offer excellent puncture protection. As a last resort, a protective film should be placed on the product before the sealing process begins.
Headspace Management
Excessive loading of the tray or bag will reduce the performance of the vacuum system and increase the chances of material being pulled into the sealing area. A minimum of 1 inch (25.4 mm) of vertical headspace must be maintained from the top of the food material to the horizontal plane of the sealing flange. This will prevent any liquid from migrating to contaminate the thermal weld process.
Equipment Sanitation and Preventative Maintenance
Vacuum systems are highly susceptible to internal contamination from airborne moisture, volatile organic compounds, and microscopic food particulates. Any build-up of debris on the silicone seals or the heaters interferes with even heat distribution, leading to weak spots in the seal.
- Daily: Wipe down sealing bars, gaskets, and chambers with food-safe isopropyl alcohol solutions.
- Weekly: Inspect vacuum pump oil levels and check sealing elements for signs of degradation, pitting, or carbon buildup.
- Monthly: Validate seal integrity using a non-destructive bubble emission test or a destructive burst test meter to achieve consistency.
Optimize Your Packaging Line
Achieving a flawless, reliable seal on plastic trays requires a precise combination of matching materials, automated control, and rugged machinery design. If you are seeking to expand your operations, end seal failures, or simply make the move into professional tray sealing and MAP processes, Roberts Technology Group, Inc. has the experience and equipment needed to deliver a system that meets all of your packaging needs. Contact us today to get started.