Blow Moulded Bottle Weight Trimmed until the Drop Test Fails
Trimming blow moulded bottle weight reduces unit resin costs but narrows drop test safety margins along bottom pinch seams and heel radii.
Melt Index
Meaning
Standardized rheological measurement quantified under ASTM D1238 or ISO 1133 determines the mass of molten thermoplastic polymer extruded through a standard capillary die under a specified load and temperature within ten minutes. Evaluation of melt index provides an inverse indication of average polymer molecular weight, where lower values signify longer polymer chains, higher melt viscosity and increased melt strength. The metric governs raw material classification and quality control for polyolefins and engineering thermoplastics, stopping where non-thermoplastic thermosets or liquid resins require rotational rheometry.
Measurement Mechanics
Laboratory testing loads dried polymer pellets into a vertically heated barrel maintained at a constant test temperature. A standardized piston loaded with specified weights applies downward gravitational pressure, pushing molten resin through an orifice measuring 2.095 millimetres in diameter. Operators slice the extruded polymer strand at precise time intervals, weighing the cooled extrudate on analytical balances to calculate the mass flow rate in grams per ten minutes.
Standard test conditions vary by polymer type, utilizing 190 degrees Celsius and 2.16 kilograms for polyethylene, and 230 degrees Celsius and 2.16 kilograms for polypropylene.
Processing Correlation
Extrusion blow moulding operations depend on low flow rate resins to maintain parison structural integrity. High density polyethylene grades with a fractional melt index below 0.5 grams per ten minutes exhibit high melt strength, preventing parison sag during vertical extrusion. Conversely, high flow materials flow rapidly under gravity, causing severe wall thinning and uncontrolled parison curtaining.
Blending high-flow regrind into virgin resin elevates the bulk flow rate, requiring immediate process parameter corrections such as barrel temperature reductions or faster cycle times. Process stability requires tight incoming material tolerances to avoid wall thickness drift.
Procurement Standards
Resin purchase contracts specify acceptable flow index tolerance bands to ensure consistent processing performance. Incoming lot inspection tests raw material samples to detect supplier polymerization drift or batch cross-contamination before silos are filled. Sourcing teams compare technical datasheets to match virgin polymer flow characteristics with specific machine capabilities and container sizes.
Uncontrolled flow variations lead to flashing, incomplete mould filling or compromised drop impact strength in finished articles. Tight material specifications protect converting plants from unnecessary downtime and elevated scrap rates.