Why Do Disposable Plastic Portion Cups with Lids Leak Easily?

1. Introduction

2. The Impact of Plastic Material Characteristics on Sealing Performance

2.1 Analysis of the Basic Properties of Commonly Used Plastic Materials

• PET (Polyethylene terephthalate): Good transparency and strong water resistance (oxygen permeability 100-150 cm³/(m²·day), water vapor permeability 20-30 g/(m²·day)), but poor heat resistance (<70℃). High temperatures or prolonged contact with grease can easily release antimony, a heavy metal, making it unsuitable for long-term storage of acidic/oily sauces.
• PP (Polypropylene): The only reusable food-grade plastic, with excellent heat resistance (up to 130℃), resistance to acids, alkalis, and oils, low density (0.9 g/cm³), high strength, and strong chemical stability (acid resistance test mass change rate ≤5%, tensile strength retention rate ≥80%; alkali resistance test mass change rate ≤4%, elongation at break retention rate ≥75%). It is the optimal choice for various sauces.
• PS (Polystyrene): High brittleness and narrow temperature range (-20℃ to 70℃). High temperatures or contact with strong acids, alkalis, or oils can easily release styrene monomers (suspected carcinogen). It is only suitable for low-temperature, non-irritating sauces, and has poor sealing reliability.

2.2 Relationship between Material Microstructure and Sealing Performance

• Molecular chain and crystalline morphology: Isotactic molecular chains are regularly arranged, resulting in high crystallinity and good strength; the smaller the spherulite size (1-10 micrometers), the higher the impact strength and heat distortion temperature. If the PE molecular chains are tightly packed, it exhibits excellent airtightness and is suitable for sealing components; LDPE has loosely packed molecular chains, resulting in high permeability and poor sealing performance.
• Intermolecular gaps and microscopic defects: Smaller intermolecular gaps lead to stronger barrier properties, while larger gaps result in easier leakage; improper processing during injection molding can easily cause defects such as cracks, voids, and shrinkage, which become stress concentration points and accelerate seal failure.

2.3 Adaptability of Different Materials to Various Sauces

• Acidic sauces (ketchup, vinegar): Accelerate the dissolution of PET components; PP/HDPE are preferred;
• Oily sauces (chili oil, peanut butter): PET is prone to leakage at high temperatures, PS easily releases styrene, and PP has the best oil resistance;
• Hot sauces (>70℃): Only PP can withstand these temperatures; PET/PS are prone to deformation and release harmful substances.

2.4 Impact of Material Aging on Sealing Performance

• Types of aging: Thermal aging (temperature changes causing molecular chain breakage), photoaging (UV degradation), chemical aging (reaction with sauces), all of which can make plastics brittle and crack, especially causing loss of elasticity in the cup lid sealing area, leading to leakage;
• Storage impact: High temperature, high humidity, and light exposure accelerate aging; even unused clear plastic portion cups with lids may have reduced sealing performance due to improper storage.

 

3. Impact of Manufacturing Process on Sealing Performance

3.1 Impact of Injection Molding Process on Dimensional Accuracy

• Mold temperature: Uneven temperature or improper cooling design can easily lead to deformation and shrinkage of thin-walled products (cups/lids);
• Injection/holding pressure: Insufficient pressure leads to incomplete filling, and insufficient holding pressure leads to poor density, both of which can cause dimensional deviations;
• Material shrinkage rate: A high shrinkage rate (such as PET) can easily lead to uncontrolled dimensional accuracy, requiring precise mold design matching.
• Mold accuracy requirements: Hole diameter tolerance (±0.1mm for Ø10mm and below), outer diameter tolerance (±0.05mm for Ø10mm and below).  Excessive clearance between guide pins and guide bushings will directly reduce dimensional consistency.

3.2 Influence of Cup Body and Lid Connection Method

• Snap-fit connection (most common): Achieved through the cooperation of the cup body's locking groove and the lid's locking tab. It needs to meet the standard of "no leakage for 30 seconds under 0.02MPa pressure and 5 Newton torque".
• Disadvantages include: thin snap-fits leading to insufficient locking force, uneven distribution causing pressure imbalance, and poor material elasticity leading to failure after repeated use.
• Threaded connection: Provides uniform sealing pressure and a secure connection, but has high processing costs and complex assembly. Excessive thread size tolerance can easily lead to stripping.
• Heat sealing connection: Offers the best sealing performance, but requires breaking the seal, making it unsuitable for multiple-use scenarios.

3.3 Analysis of Advantages and Disadvantages of Sealing Technologies

• Ultrasonic welding: Uses 10-70KHz high-frequency vibration to locally melt the plastic, removing residues in the sealing area, achieving 100% sealing without compromising barrier properties, suitable for high-end products;
• Heat sealing technology: Relies on precise control of temperature (to prevent degradation), pressure (to prevent damage), and time (to prevent insecure sealing). The equipment is simple, but parameter adjustment is difficult;
• Adhesive bonding: Low cost but poses food safety risks, poor temperature/chemical resistance, only suitable for products with special structures.

3.4 Influence of Common Molding Defects on Sealing Performance

• Flash: Overflow of material at the mold parting line, resulting in uneven sealing surfaces of the cup mouth/lid, creating leakage channels;
• Shrinkage/deformation: Uneven cooling leads to surface depressions or shape changes, compromising the fit accuracy between the cup body and lid;
• Bubbles/cracks: Bubbles reduce strength, and cracks directly form leakage channels, both requiring scrapping. 

 

4. Influence of Design Structure on Sealing Performance

4.1 Influence of Cup Mouth Shape Design

• Cylindrical shape: Simple to process and low cost, but the contact area is small, requiring the addition of annular protrusions/grooves to improve sealing;
• Conical shape: Good guidance, generates radial pressure during fastening, excellent sealing, but the taper needs to be precisely controlled;
• Rolled edge shape: High-end design, smooth edges (improves user experience), high strength (reduces deformation), optimal sealing fit, but requires special equipment and is more expensive.
• Edge and thickness: Serrated/wavy edges increase friction; the cup mouth thickness is 20-30% thicker than the cup body to prevent deformation.

4.2 Influence of Cup Lid Shape and Size Design

• Annular protrusion: Height/width needs to be balanced (too high makes it difficult to fasten, too low results in poor sealing);
• Silicone sealing ring: Good elasticity, adapts to size deviations, but is expensive and prone to aging at high temperatures;
• Multi-layer sealing: Base layer + reinforced lock, improves reliability, suitable for high-end applications;
• Thickness design: Variable wall thickness treatment, thickening the sealing area to ensure strength and sealing performance.

4.3 Influence of Cup Mouth and Lid Fit Design

• Tolerance fit: Using H8/f7 grade, too loose leads to leakage, too tight is prone to damage, requiring consideration of material shrinkage rate, process fluctuations, and environmental deformation;
• Sealing surface: Sufficiently large contact area (improves reliability), uniform pressure (achieved through buckle elasticity), low surface roughness (reduces leakage channels);
• Fastening force: Number of buckles ≥4 (evenly distributed), geometric shape (hook-shaped/trapezoidal) adapted to the material's elastic modulus, avoiding loosening and falling off, or being too tight and difficult to open. 

4.4 Necessity of Exhaust Structure Design

• Core function: Hot beverages cool down and create negative pressure (difficult to open), and liquid obstruction during pouring (easy to splash). Exhaust can solve these problems;
• Common structures:
• Vent hole: The hole diameter needs to be moderate (to prevent liquid leakage and ensure proper venting), and a one-way valve can be added;
• Vent groove: Located at the mating part, without affecting the seal;
• Special structures: Such as "PP anti-backflow plug," which combines anti-spill and pressure balancing functions.

 

5. Improvement Suggestions and Solutions

5.1 Material Selection Suggestions

• Prioritize PP material: Suitable for acidic/oily/hot sauces, ensure 100% new food-grade PP is used, avoiding recycled materials;
• Adapt to sauce characteristics: Avoid PET for acidic/oily sauces, and only use PP for hot sauces;
• Surface treatment optimization: Plasma/corona treatment to improve surface polarity and improve compatibility with silicone sealing rings.

5.2 Process Improvement Directions

• Optimize injection molding parameters: Precise temperature control (uniform cooling), optimized injection/holding pressure (stable dimensions), progressive cooling (preventing internal stress);
• Improve mold accuracy: Use five-axis machining (accuracy ±0.01mm), regular maintenance (prevent wear), select SKD61/H13 mold steel (high temperature and wear resistance);
• Improve sealing technology: Use ultrasonic welding for high-end products, optimize buckle design using finite element analysis (prevent stress concentration);
• Strengthen process monitoring: Real-time monitoring of key parameters to ensure batch consistency.

5.3 Structural Design Optimization Suggestions

• Cup mouth improvement: Use a rolled edge design for mid-to-high-end products, add a ring-shaped sealing structure, and increase the cup mouth thickness by 20-30%;
• Lid improvement: Multi-layer sealing (basic layer + auxiliary layer), add a silicone sealing ring for high-end products, variable wall thickness design (thickening at the sealing part);
• Mating optimization: Tolerance ≤ ±0.1mm, add guiding bevels, ≥4 buckles (evenly distributed);
• Functional additions: Add controllable vent holes to the lid, add anti-slip patterns to the cup body, and design easy-to-open structures (tear strips/easy-open buckles).

5.4 Quality Control and Testing Standards

• Raw materials: Material verification + performance testing + chemical compatibility;
• Process: First-piece inspection (beginning of batch), in-process inspection (middle of batch), batch inspection (end of batch);
• Finished products: Pressure testing (200-350kPa underwater bubble observation), vacuum testing (leakage rate measurement), drop testing (1.2-meter drop), torque testing (5 Newtons to ensure sealing);
• Execution standards: Follow GB/T 17876, QB/T 4049, T/CNFIA 177, and establish internal control standards (e.g., 0.02MPa pressure maintained for 30 seconds without leakage);
• Traceability system: Batch number management, recording of production parameters and inspection data, and timely handling of customer feedback.

 

6. Conclusion