Why Choose A Hard Lunch Box Over Plastic Containers

304 stainless steel resists dents/scratches better than flimsy plastic, keeping meals fresh 6+ hours vs. 2-3. BPA-free variants avoid chemical leaching when heated; rigid lids seal tighter, preventing leaks.

Health and Safety

Research indicates that when temperatures exceed 70°C, the migration rate of harmful substances like Bisphenol A (BPA) from ordinary plastic containers increases exponentially. A study conducted by Harvard University found that participants who used polycarbonate plastic bottles (containing BPA) for cold drinks for one week straight saw their urinary BPA levels surge by 69%.

Temperature is the Critical Factor

Lab data shows that braised pork at approximately 85°C placed in an ordinary plastic container can lead to the leaching of phthalate plasticizers within 10 minutes, reaching concentrations of up to 0.05 mg/kg. This is not an isolated case; tests on 50 commercially available plastic lunch boxes found that after immersion in a 70°C hot oil simulant for 30 minutes, over one-third of the products exceeded the EU standard limits for harmful substance migration.

1. The Danger Line on the Thermometer

The safety threshold for plastic is lower than we think. Common polypropylene (PP) lunch boxes, although marketed as microwave-safe, typically have a maximum continuous use temperature of 110°C. This number is deceptive: microwave heating involves instantaneous high temperatures. A refrigerated dish heated for 3 minutes in an 800-watt microwave can reach a center temperature of over 95°C, while edges and oily parts, absorbing heat faster, can momentarily exceed the 110°C safety line. This is even more true for cheap plastic containers of unknown material, whose heat resistance might only be 60-70°C; pouring freshly boiled water (100°C) into them is enough to cause deformation and significant chemical release.

2. The Secret of Leaching Rate

The leaching of harmful substances is not linear but exponential. Studies show that for every 10°C increase in temperature, the migration rate of chemicals increases by approximately 1.5 to 2 times. When the ambient temperature rises from 25°C (room temperature) to 75°C (hot food temperature), the 50°C difference is enough to accelerate the leaching speed of some plasticizers by over 30 times. This is because high temperature gives plastic molecular chains greater kinetic energy, increasing their activity and widening molecular gaps, like loosening a tight net, allowing small molecule additives (like plasticizers, stabilizers) originally “locked” in the polymer structure to escape into the food.

3. Oil and Acidity

If your food is oily or acidic, the risk doubles again. Oil is the “best solvent” for most chemical plasticizers. Experimental data shows that for the same plastic sample at 95°C, the migration of harmful substances into fatty food is 5-8 times higher than into neutral aqueous solutions. For example, the risk of carrying a spicy hot pot red oil broth is much higher than a clear broth. Similarly, acidic foods with a pH below 5 (like scrambled eggs with tomatoes, sweet and sour pork ribs) can accelerate plastic degradation, acting like a weak “corrosion,” further damaging material stability and facilitating chemical leaching.

4. Invisible Wear and Tear

Each high-temperature heating causes irreversible damage to the plastic lunch box. Scanning electron microscopy reveals that after 20 microwave heating cycles, the inner wall of a PP plastic lunch box develops numerous micrometer-level (1-10μm) scratches and pores. These damages are not only breeding grounds for bacteria but, more critically, they significantly increase the surface area of plastic contact with food, providing more “launch points” for chemical leaching during the next heating. This means an old lunch box that looks intact may pose a risk several times higher than a new one.

5. Data Comparison

Unlike plastic’s “chemical defense,” stainless steel and glass rely on physical barrier. The melting point of 304 or 316 medical-grade stainless steel exceeds 1400°C, making our daily 100°C heat negligible. Its dense metal crystal structure fundamentally prevents substance migration. Borosilicate glass, composed of a silicon dioxide (SiO₂) network, is extremely chemically stable, with a heat resistance up to 400°C, and its smooth, non-porous surface doesn’t absorb grease or retain odors. Under harsh test conditions of 121°C for 30 minutes, no substance migration from glass into food simulants has ever been detected. This “zero migration” safety guarantee is unattainable by any plastic material.

Material Showdown

The EU regulation for food contact materials sets the Specific Migration Limit (SML) for Bisphenol A (BPA) in plastics at 0.6 mg/kg. Behind this seemingly safe number lies the premise that “migration is inevitable.”

In contrast, the requirement for stainless steel is a limit on heavy metal migration, requiring no detection of any intentional additives or migration of non-glass substances.

1. Stainless Steel

The sense of security from stainless steel comes from its “metal lattice” structure. Taking the most common 304 food-grade stainless steel as an example, its composition requires Chromium (Cr) content ≥ 18% and Nickel (Ni) content ≥ 8%. Chromium reacts with oxygen to form an extremely dense chromium oxide passivation film on the surface, only 1-3 nanometers thick. This film can self-repair scratches shallower than 200 micrometers, meaning daily washing abrasion is almost impossible to destroy its protective layer. It maintains structural stability below 850°C, far exceeding the 200-300°C range of cooking and heating. Testing agencies immerse it in a 4% acetic acid solution at 100°C for 30 minutes, then detect the leaching of heavy metals like lead, cadmium, and arsenic. The pass standard is below extremely stringent limit values. This is a form of “passive safety,” not reliant on user caution.

2. Glass

Borosilicate glass (heat-resistant glass) shares the same material genes as laboratory glassware. Its thermal expansion coefficient is about 3.3 x 10⁻⁶/K, meaning its volume expansion is negligible per degree of temperature increase, allowing it to withstand instantaneous temperature differences from -30°C (freezer) to 400°C (oven) without cracking. Its chemical stability stems from the continuous three-dimensional network structure formed by silicon dioxide (SiO₂), with very high chemical bond energy, reacting with almost no acids or bases at room temperature. During testing, it is subjected to autoclave conditions of 121°C for 30 minutes to observe ion leaching in acidic, alkaline, and aqueous solutions. The standard is usually that leachates are undetectable or the total amount is below the instrument’s detection limit. The safety of glass is “inherent safety”; its composition means it has no questionable chemical additives that can migrate out.

3. Plastic’s “Formula Black Box”

The core safety issue with plastic lies in the complexity of its “compound formulation.” In a plastic lunch box material, the polymer resin (e.g., PP) itself may only account for 70-80%, while the remaining 20-30% consists of various additives like plasticizers (for flexibility), heat stabilizers (to prevent thermal degradation), antioxidants (to prevent aging), and colorants. “BPA-Free” might only mean using substitutes with similar chemical structures like Bisphenol S (BPS) or Bisphenol F (BPF). Research published in 2020 in a Cell journal indicated that BPS can disrupt the neuroendocrine system of zebrafish at concentrations as low as 1 micromolar, and its potential risks are not fully understood. Furthermore, plastics generate microplastic particles during aging (e.g., exposure to UV light, repeated heating). One study estimated that opening a plastic package or tearing cling film can release 1000-10,000 microplastic particles per action. These particles can adsorb environmental pollutants and enter the human body with food.

4. Wear and Lifespan

The long-term safety of a material depends on its wear resistance. After 100 cycles of simulated wear testing, observation under an electron microscope shows stainless steel surfaces with only slight scratches less than 0.1 micrometers deep, not affecting the passivation film integrity; glass surfaces remain smooth; while plastic surfaces are covered with grooves up to 10-50 micrometers deep. These scratches not only harbor microorganisms like E. coli and Staphylococcus aureus (bacterial counts can be 10 times higher than on smooth surfaces) but also greatly increase the surface area for additive migration into food. A plastic lunch box used for over six months may have a chemical migration rate 20%-50% faster than a new one, unbeknownst to the user.

Long-Term Cost of Use

A plastic lunch box priced at 25 RMB, if replaced according to the suggested cycle of 3-6 months (taking the median 4.5 months), used for 5 years, totals (60 months / 4.5 months) x 25 RMB ≈ 333 RMB. A 304 stainless steel lunch box priced at 180 RMB, conservatively used for 5 years, has an annual cost of only 36 RMB. This initial price difference of 155 RMB is caught up by the ongoing cost of plastic boxes by the end of the first year.

1. Where does the tens-of-RMB price difference actually lie?

The price you pay mainly covers raw materials (Polypropylene PP about 8-10 RMB/kg), molds, marketing, and channel profits. Manufacturers do not, and cannot, reserve costs for potential chemical migration risks after six months of use. In contrast, the cost of stainless steel and glass lunch boxes is “back-loaded”; a significant portion of the price is for acquiring highly stable raw materials like 304 stainless steel (~20 RMB/kg) or borosilicate glass tubing, as well as precise welding, edge grinding processes, and strict quality control. You are prepaying for the “safety margin” and “performance guarantee” for the next 5-10 years.

2. Health risks are the most expensive “hidden bill”

According to some studies, long-term exposure to certain environmental endocrine disruptors may be associated with a few percentage points increase in the risk of certain chronic diseases. Although individual risk may seem small, once it occurs, the corresponding medical costs, time loss, and decreased quality of life cannot be measured by the lunch box price difference. Choosing stainless steel/glass is like purchasing health insurance with a “zero deductible,” minimizing this uncertain major expense risk to the lowest level.

3. Environmental costs will ultimately be paid by everyone’s wallet

A plastic lunch box weighs about 100 grams. If 13 discarded boxes are generated over 5 years, the total weight is about 1.3 kg. The recycling value of these mixed plastics is very low, sorting and recycling costs are high, most end up incinerated or landfilled, and the disposal cost (about 0.5-1 RMB/kg) is ultimately borne by society through taxes or waste fees. A stainless steel lunch box weighing 500 grams, at the end of its life, still has a scrap value of 3-5 RMB (calculated at 6000 RMB/ton), with a recycling rate exceeding 90%. Choosing products with long service life and high recyclability essentially reduces the hidden environmental tax we all share.

4. The overlooked cost of “convenience loss”

After 3 months of use, a plastic lunch box will likely start to have issues with odor transfer and staining. After holding curry once, it may require repeated washing and airing for several days to remove the smell. This “convenience loss” means your time cost and emotional cost are continuously increasing. Not to mention the huge loss caused by broth leakage due to seal aging (usually around 1 year) ruining a laptop keyboard or important documents. The design of a stainless steel lunch box with a glass lid or latch without a seal fundamentally prevents such problems. Its stable performance provides a long-term convenience of “no need to worry,” which itself has high value.

5. Recalculate from the “Cost of Ownership” perspective

In economics, Cost of Ownership = Acquisition Cost + Maintenance Cost – Residual Value. Applying this formula:

• Plastic Lunchbox (5 years): Cost of Ownership ≈ 333 RMB (Acquisition) + 0 (Maintenance, but includes hidden costs like odor transfer, leakage) – 0 (Residual Value, no recycling value) = ≥333 RMB.
• Stainless Steel Lunchbox (5 years): Cost of Ownership = 180 RMB (Acquisition) + 0 (Maintenance) – 5 RMB (Residual Value) = 175 RMB.

This calculation clearly shows that from a 5-year perspective, choosing the more “expensive” stainless steel lunchbox actually results in a lower cost of ownership, and you obtain a higher level of safety and user experience. This account is cost-effective no matter how you calculate it.

Durability and Service Life

A plastic lunch box priced around 20-30 RMB often has an average effective service life of only 6-12 months. This means you need to repurchase annually or even semi-annually. Over 5 years, the actual cost may exceed 100 RMB, resulting in a pile of plastic waste destined for disposal. A high-quality hard lunch box, while requiring an initial investment ranging from 100 RMB to 300 RMB, is designed for a service life measured in “years” or even “decades.”

Material Strength

The flexural modulus of ordinary PP or PC plastic is about 1500-2500 MPa, while that of stainless steel is as high as nearly 200,000 MPa—a difference of two orders of magnitude. An empty stainless steel lunch box weighing only 450 grams can withstand vertical pressure exceeding 100 kg, whereas an ordinary plastic box of the same size may suffer permanent deformation under just 10 kg of pressure.

Stainless Steel

Food-grade stainless steel, primarily 304 (high-end uses 316), has core advantages in strength, stability, and lifespan.

1. The numbers behind hardness and toughness

Stainless steel hardness is measured by Vickers hardness (HV). 304 stainless steel has an HV value of about 150-200. This means an ordinary pocket knife (hardness ~HV 150) can hardly scratch its surface. More crucially, its yield strength (the point where permanent deformation begins) is above 205 Megapascals (MPa), and its tensile strength (force needed to break it) is above 520 MPa. For comparison, car tire pressure is typically around 2.5 atmospheres (approx. 0.25 MPa). The pressure the lunch box material needs to withstand is thousands of times greater. So daily drops and squeezes are barely a “tickle” for it.

2. How is 316 stronger than 304?

The key is the element “Molybdenum (Mo)”. 316 stainless steel contains 2%-3% Molybdenum. This addition qualitatively improves its corrosion resistance, especially against chlorides (like salt). 304 may suffer “pitting corrosion” in salty environments, while 316 has a higher Pitting Resistance Equivalent Number (PREN). If your lunches often contain high-salt dishes, or you are in a coastal humid environment, 316 material can extend the lunchbox’s expected lifespan by over 30%.

3. How long does it really last?

With correct use and cleaning (avoiding strong chlorine environments), the corrosion rate of a 304 stainless steel lunch box is less than 0.01 mm/year. A box with a wall thickness of 0.5 mm would theoretically take 50 years to corrode through. In reality, its end of life is often not because it breaks, but because you get tired of it.

Tempered Glass

Tempered glass lunch boxes feel heavy; their strength comes from a precise physical strengthening process.

1. Surface compression is its “armor”

Ordinary glass shatters upon impact because surface micro-cracks propagate rapidly under stress. Tempered glass is heated to over 600°C and then rapidly and uniformly cooled with cold air in seconds (quenching), causing the surface to harden and contract first, creating immense compressive stress on the interior. This surface compressive stress can be as high as over 100 MPa. This means any impact or scratch must first overcome this strong pressure layer before reaching the inherent brittleness of the glass. It’s like putting an invisible suit of armor on the glass.

2. How much temperature shock can it handle?

Ordinary glass might withstand a temperature difference of only 50°C. A well-tempered glass lunch box can safely withstand temperature differences of 150°C to 200°C. For example, taking it from the refrigerator (4°C) directly into a microwave on high for 3 minutes (internal temperature can exceed 90°C), a difference of nearly 90°C, is safe for it. Its thermal expansion coefficient is very low, about 3.3 x 10⁻⁶ /K, meaning minimal deformation with temperature change, reducing the risk of thermal shock breakage.

3. Its weakness is clear

The “Achilles’ heel” of tempered glass is the sides and edges. These areas are stress relief zones from the tempering process and have lower strength. Also, point impacts from sharp objects (e.g., dropping onto a small stone on the ground) concentrate all force on one point, easily breaching the surface compression layer causing breakage. But barring such extremes, a tempered glass lunch box’s service life easily exceeds 3-5 years.

Tritan Plastic

Don’t confuse Tritan with the cheap plastic in your mind; its performance parameters are on another level.

1. Drop resistance is 10 times that of ordinary plastic, at a minimum

A key indicator for material toughness is the “Izod Notched Impact Strength.” For ordinary lunchbox plastic (PP), this value is about 20-50 J/m, while Tritan can reach 600-900 J/m, or even higher. This means its ability to absorb impact energy is tens of times greater. Dropped from a 1.5-meter high desk edge onto a tile floor, an ordinary plastic box might crack or show white marks, while a Tritan box will likely bounce a few times, intact.

2. Withstands the repeated “torture” of the dishwasher

Ordinary plastic, under the repeated冲刷 of 60-75°C hot water and highly alkaline detergents in a dishwasher, tends to age, become brittle, and lose its luster. Tritan’s material structure is more stable. According to tests by manufacturer Eastman, it can withstand over 1000 cycles of standard dishwasher cleaning with minimal degradation in mechanical properties and transparency. Assuming 5 dishwasher cycles per week, it ensures stable performance for nearly 4 years.

3. Absolutely non-stick to oil, saves 50% cleaning effort

Tritan material itself is oleophobic. Experiments show that after applying the same amount of cooking oil, using the same amount of water and cleaning action, the oil residue on a Tritan lunch box is reduced by over 50% compared to an ordinary PP plastic lunch box.

Structure and Connection Points

Data shows that over 80% of premature lunch box failures are not due to body breakage, but cracked lids, broken latches, or seal failure. The latch of an ordinary plastic lunch box may start to fatigue and loosen after 300-500 open/close cycles, reducing sealing performance by over 50%. A high-quality hard lunch box invests 30%-40% of its total cost in structural design, aiming to turn weak points into core strengths. The table below visually compares key parts between ordinary products and quality designs:

Latches

A lunchbox latch doesn’t rely on brute force; it’s actually a precision lever mechanism.

1. Engagement depth determines the safety margin

The hook engagement depth of a poor-quality latch might be only 0.3-0.5 mm, potentially disengaging with a slight squeeze. A quality latch’s engagement depth is designed between 1.2-1.8 mm. This micron-level difference provides over 300% safety margin. When you press the latch, you’re overcoming a carefully calculated elastic deformation, not bending a fragile plastic piece.

2. What does 10,000 open/close cycles mean?

Quality lunchbox latches undergo fatigue testing: opening/closing at a specific force, 10-15 times per minute, continuously for 10,000 cycles. After testing, the latch’s locking force decay must be less than 15%. This means even if you open/close the box 5 times daily (load meal, open at lunch, clean at night), about 1825 times a year, this latch can ensure stable use for over 5 years. The 0.8-1.2 mm thick reinforcing ribs added at the latch root are key to preventing breakage from the base.

二、Sealing Ring

The sealing ring is dynamic; it needs to deform under pressure to fill gaps.

1. The Compression Set of Silicone

This is the gold standard for measuring seal durability. It refers to the degree to which a seal loses its ability to rebound after prolonged compression. High-quality solid silicone (not cheap sponge rubber), after being compressed at 70°C for 24 hours, can have a permanent set rate controlled below 10%-15%. This means even after being tightly compressed all day, it can recover over 85% of its original shape upon release, without developing a compression set. Cheap materials might permanently deform over 30% after one severe compression, leading to leaks.

2. Removable design triples the lifespan

Making the seal ring removable is one of the smartest designs for extending the overall lunchbox life. As a silicone component, the seal will inevitably age (albeit slowly) after long-term contact with oil. An integrated seal aging means the entire lid is scrapped. A removable design allows you to replace just the seal for 10-20 RMB after 2-3 years of use, giving the main body a new life, easily extending the product’s effective life to 8-10 years.

Hinges and Lid Flatness

These are hidden engineering feats, but once they fail, the lunchbox is directly scrapped.

1. Hinge bending life

An ordinary integrated hinge may develop white stress marks and break after 3000-5000 bends. Quality hinges use a combination of a stainless steel core pin embedded in high-temperature resistant nylon (like PA66). The steel pin handles rotation, the nylon part handles bending. This design’s bending fatigue life easily reaches over 20,000 cycles, ensuring the hinge is worry-free for the box’s entire lifespan.

2. Lid flatness tolerance

Lid warping is a major cause of leaks. Upon leaving the factory, the flatness tolerance of a quality lunchbox lid is strictly controlled within 0.1 mm. This ensures the seal receives even pressure when latched. Lids of cheap boxes can develop internal stress during injection molding cooling, warping over time, with tolerances even exceeding 0.5 mm.

Superior Leak-Proof Performance

Statistics show that over 70% of people who bring their own lunch are most concerned about container leakage. Ordinary plastic lunch box lids are thin and prone to deformation; their sealing performance drops sharply under food hotter than 60°C or transportation颠簸. A quality hard lunch box, through 3 core design features, reduces the leakage probability to below 1%, truly achieving “no soup leakage.”

Sealing Ring

The leak-proof capability of a lunch box rests over 90% on the sealing ring, this “Achilles’ heel.” It’s like car tires; even the best car body, paired with aged or劣质 tires, can’t withstand the journey. The root cause of over 70% of leakage issues on the market lies with the seal, not the container body. An excellent seal must maintain elasticity within an extreme temperature range of -20°C (freezer) to 120°C (steam heating), with its deformation rate strictly controlled below 5% to ensure a tight fit every time.

1. The feel to the touch determines how long it will last

The seal material is directly linked to lifespan and safety. Food-grade platinum-cured silicone is the choice for high-end boxes; it feels substantially thick and smooth, not greasy. This material costs over 1.5 times more than ordinary silicone, but its probability of harmful substance leaching is below 0.01%, and it can withstand over 1000 boiling sterilization cycles, with a service life of 3-5 years. In contrast, the PVC or cheap TPE seals used in many cheap plastic boxes have a major flaw: poor heat resistance. Experimental data shows that after contact with oily food above 80°C, over 60% of PVC seals will show permanent compression set within 3 months, with hardness change rate over 15%, and may leach plasticizers in amounts between 0.05-0.1 mg/kg, posing health risks.

2. Width and thickness are the foundation of pressure

The seal isn’t better the thicker it is; it needs precise配合 with the lid structure. An effective leak-proof silicone ring typically has a cross-sectional diameter (thickness) between 2.0 mm and 3.5 mm. This size is calculated: less than 2.0 mm provides too little contact area, requiring immense latch pressure to seal, making it hard for users to open; greater than 3.5 mm may make the lid hard to close or cause insufficient compression rebound. Seals produced with high-precision molds can control cross-section diameter tolerance within ±0.1 mm, ensuring even deformation when pressed.劣质 seals may have uneven thickness, e.g., 2.5 mm in some spots, only 1.8 mm in others. This slight difference is enough to create gaps at weak points, causing broth to seep out during a 5-10 minute side-lying test.

3. The rebound when pressed is the key to sealing

This is the most critical indicator – the elastic compression rate. It refers to the seal’s ability to return to its original shape after compression. A qualified seal should rebound quickly and fully when pressed by hand, without delay or leaving a fingerprint. Professionally, its compression set (recovery ability after compression at 70°C for 22 hours) should be below 20%. This means even if the box is clamped shut overnight, when you open it in the morning, the seal can still recover to over 95% of its original height within 0.5 seconds, ready for the next perfect seal. A劣质 seal rebounds slowly after compression, even leaving a dent, with a compression set possibly over 40%. After using it for a month, you’ll find the lid feels “loose,” a sign the seal has failed.

4. Groove design determines if it will “shift”

The seal is embedded in a groove on the lid. The width and depth tolerance of this groove must be controlled within ±0.05 mm. Too loose, and the seal shifts under force, creating leak points; too tight, installation is difficult and pre-stretches the seal, reducing its elasticity. Good designs incorporate R-angles (rounded corners) at groove turns to avoid sharp angles cutting the seal. You can check manually: try gently pulling the seal; its shift幅度 should not exceed 1 mm, and it should return to position automatically when released.

Latch Design

The sealing method of ordinary plastic lids, which relies on “feeling the squeeze,” typically applies less than 2 kg of pressure. A precision latch system, using mechanical lever principles, amplifies the user’s modest pressing force into a total locking pressure of over 20 kg, reducing the leakage risk to below 1%.

1. The press and the ‘click’ sound are just the beginning

When you press the latch of a quality hard lunch box, you hear a clear “click.” This feedback isn’t decorative; it’s a definite mechanical engagement indicator. It means the latch’s internal catch has passed the dead point and self-locked. This process typically requires only 3-5 Newtons of pressing force (similar to pressing a computer key), but through lever amplification, the vertical pressure instantly acting on the seal reaches 5-8 kg.

2. The latch’s fulcrum, lever arm, and dead angle determine ease of use and reliability

A latch is essentially a first-class lever. Taking a common flip-top latch as an example, your finger’s press point is the “effort,” the pivot connecting to the lid is the “fulcrum,” and the side pressing the seal is the “load.” The fulcrum position and lever arm length are precisely calculated. The fulcrum position design in a quality latch allows a short effort arm of about 2 cm to drive a load arm movement of 5 cm, producing a pressure amplification effect of about 2.5 times. More importantly, when the latch closes, the effort, fulcrum, and load points form an over-center self-locking structure.

3. The number and layout of latches combat the ‘seesaw’ effect

A square lunch box lid bends slightly under pressure, like a board. If only one central latch is used, the ends lift, creating a “seesaw” effect, skyrocketing leakage probability to over 50%. Therefore. Four latches are placed at the center of each lid edge or one-third from the corners. This layout ensures even pressure distribution on the seal, controlling the pressure difference per unit area within 0.5 kg/cm². Even if you lift the box by one latch, the other three points remain firmly locked, preventing leakage from the other end.

4. The latch material itself determines its lifespan

Quality latches typically use engineering plastics like POM (Polyoxymethylene) or reinforced nylon, with fatigue strength over 3 times that of ordinary ABS plastic. This means a POM latch, after 10,000+ open/close tests, remains structurally intact, with precision loss less than 0.1 mm. Cheap latches made of ordinary PP material show visible wear after 1000-2000 cycles, leading to poor closure. Some high-end models even embed a small stainless steel spring plate (~1 cm²) inside the latch, specifically responsible for providing the “click” tactile feedback and self-locking force, with a lifespan exceeding 50,000 bending cycles.

5. Pressure Test: A piece of paper tells you if it’s tight enough

Here’s a simple test: take a standard A4 paper, place it over the lunch box’s seal groove, then close all latches. Afterwards, try to pull the paper out by hand. If the paper slides out easily and without resistance, the latch pressure is insufficient, and sealing performance is questionable. If the paper is firmly held, requiring some force (feeling like pulling one sheet from a stack) to move, or even tears, it indicates the latch has successfully compressed the seal against the body, reaching the effective sealing pressure threshold per unit area.