Polyethylene (PE) resin contracts for blown-film, cast-film, pipe, and injection molding facilities are often negotiated around one structural variable: whether the pricing basis is fixed or floating. Fixed or Floating: Choosing the Right Basis for a Polyethylene Supply Deal is a pricing-clause decision that determines which party carries the financial effect of ethylene feedstock and published resin index movement. The mainstream PE family covers low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE), each with defined density, melt flow rate, and mechanical performance bands under ISO 1872-1 and ASTM D4976-23. The basis decision operates independently from the technical grade specification but directly affects landed cost volatility, working capital, inventory valuation, and downstream margin protection. A fixed quarterly price embeds the supplier’s view of feedstock direction; a floating formula passes the index movement to the converter. For the basis clause to function, the contract must define the grade, publication, location, timing, and adder logic with the same precision as the resin specification.

What Drives Contract Price Basis Selection in PE Resin Procurement?

In a contract negotiation, the selection between fixed and floating basis is usually resolved after grade approval, volume band, delivery incoterm, and technical acceptance limits have been agreed. The underlying cost structure is ethylene-intensive. Published cost-curve analyses commonly place monomer, utilities, and depreciation at 50% to 70% of integrated PE cash cost, with ethane-fed North American crackers carrying lower feedstock cost per tonne than naphtha-fed European or Asian units. Monthly ethylene contract settlements and spot PE assessments published by ICIS, S&P Global Commodity Insights, and Argus Media therefore move resin formula prices by amounts that can exceed 100 USD/t within a calendar quarter. A converter supplying downstream customers with fixed quarterly selling prices faces margin compression when the purchase basis floats; a processor that sells on a monthly resin escalator may prefer a floating purchase basis to avoid carrying the opposite mismatch. The decision is not about whether one basis is cheaper, but whether the cash-flow exposure of the commercial portfolio aligns with the purchasing formula.

Contract negotiations frequently separate the deliverable resin price from the indexation mechanism. A floating formula may be expressed as the arithmetic average of the ICIS LDPE film spot CFR Far East assessment for the month prior to shipment plus a fixed adder covering silo logistics, packaging, credit, and technical service. A fixed-price clause may operate as a quarterly or calendar-year price with reopeners tied to ethylene movement. The basis selection affects inventory valuation: a converter holding 45 days of resin inventory purchased on a floating index records stock losses when the index declines before finished goods are sold. Under a fixed basis, the same inventory position does not reprice, but the buyer may have paid above the falling spot market and may face competition from converters receiving floating import offers.

Clause Structures That Reallocate Feedstock Risk

Fixed-price clauses isolate the converter from monthly resin index volatility for a defined period, but that protection is embedded in the supplier’s offer as a risk premium. When the fixed price is held for 12 months, the seller must absorb or hedge ethylene and PE index changes; the cost appears either explicitly as a basis premium or implicitly through reduced volume flexibility and tighter take-or-pay provisions. Floating clauses transfer index risk to the buyer but require disciplined contract administration. Both forms should specify the publication, grade, location, incoterm, assessment frequency, and timing lag. A clause tied to “LDPE film spot CFR China” does not automatically protect a European HDPE pipe processor whose input is HDPE pipe grade delivered by rail from a local plant.

Formula elements that require contract definition include the averaging window, the number of decimal places, the handling of non-publication dates, and the treatment of retrospective index revisions. Suppliers and converters also negotiate price collars, raw material reopeners, and minimum/maximum movement bands. A commonly used re-opener provision permits price adjustment when the contract ethylene benchmark moves beyond ±10% against an agreed baseline. The resulting invoice price is then a hybrid basis rather than a pure fixed or floating position. This hybrid form appears often in multi-year automotive or pipe-sector supply agreements where both parties accept a band of risk and share the tail risk outside the band.

The fixed quarterly price may reduce treasury workload but can leave the converter uncompetitive if spot PE falls by 15% during the quarter while regional competitors buy on a monthly index. The floating monthly index may match current market clearing prices but can complicate customer quotations and ERP purchase-price condition records.

Melt Flow Rate, Density, and Mechanical Property Windows

Melt flow rate determined to ISO 1133-1:2022 at 190 °C under a 2.16 kg load is the primary contractual rheological parameter for PE. High-load melt flow index values may be added for film and pipe grades using a 21.6 kg load to expose shear sensitivity. Density is measured by ISO 1183-1 or ASTM D792-20 and defines the PE sub-class: LDPE spans 0.910 g/cm³ to 0.925 g/cm³, LLDPE 0.915 g/cm³ to 0.930 g/cm³, and HDPE exceeds 0.941 g/cm³. These boundaries correspond to crystalline fraction, stiffness, barrier, and environmental stress-cracking resistance changes that alter processing behavior. The same nominal grade can shift within the specification window across lots, and that shift is frequently enough to change melt pressure, die swell, drawdown, and part dimensions.

Table 2 summarizes common commercial windows. Values are typical commercial sheet ranges, not specification limits; the contract must fix both the nominal target and the permitted tolerance because lot-to-lot variation within the same grade can shift melt pressure and dimensional stability.

From an extrusion line perspective, the commercial basis decision interacts with grade changeover losses and melt-pressure stability. Blown-film LLDPE with MFR below 1.0 g/10 min at 190 °C/2.16 kg typically runs on single-screw extruders with L/D ratios from 24:1 to 30:1, die gaps from 1.8 mm to 3.0 mm, and blow-up ratios from 2:1 to 4:1. The melt at low MFR has higher shear viscosity, increasing motor torque and melt temperature; a switch from LDPE to LLDPE can require a die-gap increase or a lower output set point to avoid sharkskin melt fracture. Injection molding of HDPE closures and crates uses MFR values from 20 g/10 min to 60 g/10 min, melt temperatures from 200 °C to 250 °C, and clamp force requirements near 3 kN/cm² to 5 kN/cm² of projected area. Pre-drying of virgin PE is not usually required. If surface moisture exceeds 0.05% by weight, drying at 80 °C for 2 h to 4 h in a desiccant dryer removes splay-causing surface water without altering the base resin. A fixed-price specialty HDPE used on a line requiring long purge cycles may create more financial exposure than a floating commodity LLDPE that can be re-sold or diverted to another machine.

When an Indexed Price Formula Outperforms a Fixed Quarterly Price

When a converter’s downstream sales contract contains a monthly resin escalator tied to the same published PE index, an indexed purchase formula minimizes margin compression. The purchase and sales formulas must reference the same grade, region, publication, and average period; otherwise basis risk remains. A common mismatch occurs when a buyer indexes purchases to CFR Northeast Asia LDPE film while selling to European customers on a local LDPE film index. Over 6 months, regional supply-demand balances can diverge by more than freight cost differences, eroding the expected margin protection. The selection of an index therefore requires reconciliation between the physical material actually consumed and the market assessment used in the formula.

An indexed formula also requires agreement on which weeks are averaged. If a supplier proposes the arithmetic average of the ICIS LDPE film spot CFR Far East weekly assessments for the month prior to delivery, the buyer must confirm whether that includes all published Fridays or only the last four assessments. The same precision applies to S&P Global Commodity Insights and Argus PE assessments. Contract data fields include publication name, quotation code, freight basis, polymer grade, processing method, and incoterm. The index is not a physical price; it is a market assessment. The adder therefore represents all costs not captured by the index, including packaging, silo inventory, technical service, and credit. A floating formula without a clearly identified incoterm creates disputes because CFR and DDP include different freight and customs cost layers.

An indexed formula may outperform a fixed quarterly price when the converter has strong raw-material pass-through clauses and can adjust customer invoices monthly. In that position, the buyer is not exposed to the absolute direction of PE prices but only to the spread between the purchase and sales formulas. The spread is contractually fixed as the adder, so margin is preserved when both formulas move together. The same structure fails if the customer’s escalator uses a different index or a lagged average. A lag of one month between purchase and sales repricing leaves the converter carrying one month of unresolved price movement, which is a significant working-capital item for a plant consuming 800 t/month to 1,200 t/month of film-grade PE.

Quality verification under either basis should reference a mutually agreed specification sheet containing density, MFR, ash content, tensile yield stress, and contamination limits. Lot release typically follows ASTM D4976-23 for polyethylene molding and extrusion materials; food-contact grades are further evaluated under FDA 21 CFR 177.1520 for olefin polymers and EU Regulation (EU) No 10/2011 with its migration testing framework. Tensile properties are determined by ISO 527-2 or ASTM D638-14, with specimens conditioned to ISO 291 at 23 °C and 50% relative humidity. A contract that fixes price but leaves grade quality as “standard film grade” is incomplete because the basis premium may be undermined by incoming lots that fail to meet the converter’s film gauge stability limits. Polyethylene reclaim streams also require careful compatibility: chlorinated polymer contamination can generate acid gases during processing and reduce extruder barrel life.

Logistics and inventory clauses interact with basis choice. Fixed-price contracts often impose monthly take-or-pay volumes, whereas floating contracts can be coupled with consignment stock and repricing at the time of withdrawal. A supplier-managed inventory program that reprices PE on the day of consumption reduces buyer inventory write-down risk but increases administrative complexity. For film conversion lines with gravimetric blending of reclaim, the basis determination should be synchronized with additive masterbatch cost and downstream film gauge variation.