In the food & beverage, daily chemical packaging and other industries, the quality of plastic caps directly affects product sealing safety and user experience. During production and quality inspection, multiple indicators need to be accurately measured, covering dimensions, weight, mechanical properties, sealing pressure, etc. Different scenarios often use different units, and standardized conversion is key to ensuring consistent quality. Guangzhou Jeepine Intelligent Compression Molding Machine Co., Ltd. hereby systematically summarizes the core measurement items, standard units, and practical conversion rules for plastic caps, providing a directly applicable reference for the cap industry.

I.Cap Weight: Gram (g) as the Sole Standard Unit

Plastic caps are lightweight products; weight directly affects production cost and assembly fit.

Standard unit: gram (g) – no other weight units are commonly used in the industry.

Typical range: Common cap weights range from 1.7 to 7.2 g. For example, a drinking water cap weighs about 2 g, while a thick-walled daily chemical cap can reach 7 g.

Measurement accuracy: At least 0.01 g; for high-precision quality inspection, 0.001 g is recommended.

Key feature: No unit conversion needed – read directly from the scale.

II.Cap Dimensions: Millimeter (mm) as the Uniform Standard

Cap dimensions determine the fit precision with the bottle finish, forming the basis for sealing qualification. Key parameters include outer diameter, height, thread specifications, etc.

Standard unit: millimeter (mm) – universally used in the global packaging industry.

Measurement accuracy: All dimensional tolerances are controlled within ±0.01 mm. For example, the diameter of a standard 28 mm cap is allowed between 27.9 and 28.1 mm.

Common sizes: Mainstream neck finishes are 28 mm, 30 mm, and 38 mm, matching corresponding standard bottle finishes.

Key feature: No unit mixing – calipers and laser measuring instruments all output in mm.

III.Cap Puncture Force: Quick Conversion between kgf and N

Puncture force refers specifically to the force required to pierce the thin membrane of a "smart cap" (used on water cooler jugs). It measures opening convenience and structural strength.

Common units: kilogram-force (kgf), newton (N)

Core conversion formula: 1 kgf = 9.8 N (for engineering estimates, 1 kgf ≈ 10 N can be used)

Important note: kg is a unit of mass, while kgf is a unit of force. Their numerical values are equal, but the physical meanings are different.

Measurement accuracy: 0.1 kgf (or 1 N)

Example: A puncture force of 5 kgf ≈ 49 N (estimated as 50 N)

IV.Cap Strip Pull Force: Conversion between kgf, lbs, and N

Strip pull force is the force required to vertically break off the tamper-evident band. It directly affects opening safety and anti-tamper performance.

Common units: kilogram-force (kgf), pound-force (lbs), newton (N)

Core conversion formula: 1 kgf = 2.2 lbs = 10 N (industry standard simplified conversion, balancing accuracy and practicality)

Example: A strip pull force of 3 kgf = 6.6 lbs = 30 N

V.Opening / Breakaway Torque: Multiunit Precision Conversion

Torque is the rotational moment (force × distance) needed to turn the cap. Opening torque affects user experience; breakaway torque relates to the fracture performance of the tamper-evident band. This unit system is the most complex.

5.1 Common Torque Units

SI units: newton-meter (N·m), newton-centimeter (N·cm)

Metric unit: kilogram-force-centimeter (kgf·cm)

Imperial unit: pound-force-inch (lbs·inch)

5.2 Core Conversion Formulas

Internal metric conversion: 1 N·m = 100 N·cm = 10 kgf·cm

Metricimperial conversion: 1 N·m ≈ 0.22 lbs × 39.37 inch ≈ 8.66 lbs·inch (the value 0.0058 in the original was a typo; the industry standard is 8.66)

5.3 Practical Examples

Opening torque 2 N·m = 200 N·cm = 20 kgf·cm ≈ 17.32 lbs·inch

Breakaway torque 5 kgf·cm = 0.5 N·m ≈ 4.33 lbs·inch

VI.Application Torque: Units are Exactly the Same as Opening Torque

The application torque (tightening torque) applied by filling equipment and the opening/breakaway torque measured during quality inspection share the same physical nature. The measurement units and conversion rules are fully interchangeable; no separate conversion system is needed.

Typical range: 1–8 N·m (corresponding to 10–80 kgf·cm), suitable for different bottle finish sealing requirements.

VII.Sealing Pressure: Oneclick Conversion among 5 Unit Types

Seal testing actually measures pressure (force per unit area), commonly referred to as "sealing pressure" in the industry. It directly determines the cap's leakproof performance.

Common units: bar, kilogram-force per square centimeter (kg/cm²), pound-force per square inch (PSI), kilopascal (kPa), megapascal (MPa)

Core conversion formula: 1 bar = 1.02 kg/cm² = 14.5 PSI = 100 kPa = 0.1 MPa

Test standard: Typical seal test pressure is 0.3 MPa (3 bar); hold for 1 minute with no leakage to pass.

Example: 50 kPa = 0.5 bar = 0.05 MPa ≈ 0.51 kg/cm² ≈ 7.25 PSI

VIII.Quick Unit Conversion Table (for Highfrequency Scenarios)

Measurement Item	Common Units	Conversion Relationship
Weight	g	No conversion needed
Dimensions	mm	No conversion needed
Puncture Force	kgf, N	1 kgf = 9.8 N ≈ 10 N
Strip Pull Force	kgf, lbs, N	1 kgf = 2.2 lbs = 10 N
Torque	N·m, N·cm, kgf·cm, lbs·inch	1 N·m = 100 N·cm = 10 kgf·cm ≈ 8.66 lbs·inch
Sealing Pressure	bar, kg/cm², PSI, kPa, MPa	1 bar = 1.02 kg/cm² = 14.5 PSI = 100 kPa = 0.1 MPa

IX.Industry Application Notes

Unit consistency principle: Production, quality inspection, and customer acceptance should agree on a common unit to avoid disputes caused by conversion errors.

Accuracy for different scenarios: Use the exact conversion (e.g., 1 kgf = 9.8 N) for laboratory highprecision testing; use the simplified conversion (1 kgf ≈ 10 N) for fast production line checks.

Use of imperial units: For products exported to European or US markets, imperial units such as lbs and PSI should be indicated simultaneously. In the domestic Chinese market, metric units are preferred.

X.Summary

Although there are many measurement units for plastic caps, they mainly fall into five categories: weight (g), dimensions (mm), force (kgf/N/lbs), torque (N·m/kgf·cm), and pressure (MPa/bar). Mastering the conversion rules in this document allows rapid and accurate conversion between different units, ensuring stable cap production quality and smooth industry collaboration.