Seal strength and integrity are important for food safety and shelf life. But, what factors make a strong seal?
Heat, time, and pressure. The perfect combination of these three factors is essential for strong seal integrity and strength in packaging. Maintaining seal integrity throughout the package’s life cycle is important to guarantee high product quality upon opening. For the consumer, there is nothing worse than buying a product, such as a delicious cut of steak, only to find out the product has spoiled because of a seal breakage. Obviously, that reflects poorly upon the meat company.
What exactly is the seal protecting that perfect cut of steak from?
Micro-organisms can contaminate the steak causing the growth of mold or bacteria. Moreover, film and its seal protect the product from heat, moisture, and UV that can cause discoloration, micro bacterial growth that causes spoilage, and other contaminants that can harm the product. Any loss of integrity from the film and the seal means a high potential for the product to be contaminated and lost, especially for sensitive products like steak.
Specifically, products like steak often use MAP (modified atmosphere packaging) to protect from contamination. MAP also protects from discoloration, is used to maintain textural and nutritional appeal, extends shelf life, and preserves the products overall freshness. Depending on the product and MAP replacement practice, mixtures of gases, often involving nitrogen, oxygen and carbon dioxide, replace the oxygen atmosphere in the package. Again, any loss of seal integrity that compromises the MAP atmosphere, will undoubtedly compromise the product as well.
Types of Sealing Methods in thermoforming, traysealing, and flow packaging.
First, it’s important to understand how different sealing methods utilize heat, time, and pressure. Furthermore, product, packaging method, film material, and tray material should be considered when deciding what sealing method would guarantee a strong seal.
- Basic heat sealing is the most common type of sealing method. The process involves the desired amount of heat, pressure, and the perfect amount of time for the plastic to reach its transition temperature. Thus, creating a seal between the film and the tray.
- Impulse heat sealing is primarily used for joining thermoplastics that require a moderate temperature for a strong seal. The process involves a Nichrome alloy placed on either the top or bottom of the sealing bars. The Nichrome alloy is then placed between a heat resistant rubber pad and protective fabric. The jaws then close on the materials by pneumatic or mechanical pressure and an electric current heats the Nichrome for a specific amount of time. The material then cools to fuse fully. Each step requires the perfect amount of heat, pressure and time.
- Ultrasonic heat sealing is when pressure and high-frequency vibration are applied to two pieces of material to establish a seal. This process is often used on rigid plastic. It works when a transducer, or converter, transforms an electrical signal into a vibration. Then, the booster controls the vibration while the sonotrode transmits the vibrations, thus creating the seal.
- Cold Seal Packaging forms a seal with no heat, just pressure. It requires a pressure sensitive coating, or adhesive to form the seal. This method is a faster, cheaper option because of energy costs, and is environmentally friendly. However, it doesn’t offer the same integrity as heat sealing.
Types of Jaw Design for flow pack equipment
Another consideration is the type of jaw design needed for the desired seal method. Jaw design determines the sealing time, one of the three factors needed for a strong seal.
- Rotary jaws are often used for confectionery goods, biscuits and snacks. The jaws meets in the middle to form the seal where sealing time is shortest. This method is high speed but doesn't guarantee a strong seal.
- D-cam jaws are also high speed like rotary jaws. The difference is that the seal time is held a bit longer, making the seal integrity hermetic and strong.
- Long-dwell jaws move with the packaging creating an increased sealing time which means a strong and hermetic seal. Additionally, long-dwell jaws are often used in MAP products.
- Box motion jaws sealing time is increased compared to the above jaws. It creates a strong and hermetic seal that often uses thicker films and supports MAP packaging.
A potential seal complication to be wary of are the byproducts from the product being packaged. Byproducts like liquids from fruit or meat, and crumbs from cakes and cookies, can be left on the part of the tray or film being sealed. During the sealing process, these liquids and crumbs can negatively impact the bond in the sealing zone between the two materials. An impacted seal area can cause a weak seal that degrades over time which may compromise product quality.
How can seal strength be tested?
Exact testing requirements are based on relative industry standards. But, according to ASTM International, formerly known as American Society for Testing and Materials, there are several common seal testing standards which are detailed on their website. The basic practice of physically testing a seal’s integrity and strength has to do with consumer ease of opening and retention of pack integrity. This makes sense because the seal can’t be too strong where the consumer can’t open it.
For some applications, such as containers with flexible lids, the 45-degree angle test is used to simulate a consumer opening the pack. However, there are wide range of standard test methods to test seal strength and integrity based on packaging type.
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