Why one polymer is sold in many grades
"PP" or "HDPE" is not a single product but a whole family of grades. A producer offers dozens of variants of one polymer that are chemically similar but deliberately set differently in melt flow. The reason: each processing method places fundamentally different demands on the melt.
The key lever is the melt mass-flow rate (MFR) to ISO 1133, closely tied to mean molecular weight and distribution. Simply put: high MFR means a thin, free-flowing melt; low MFR means a stiff melt with high strength. A grade that shines in injection moulding therefore fails in extrusion – and vice versa. Buying granulate means buying a grade optimised for a method, not just "a polymer".
Important: MFR values are only comparable when measured at identical temperature and load and to the same standard (ISO 1133 or ASTM D1238). A number without its test condition is worthless.
MFR, melt strength and molecular weight distribution
Three linked properties decide a grade's suitability:
- MFR / molecular weight: inversely related. Higher molecular weight means more chain entanglements, higher melt viscosity and thus lower MFR. High molar mass improves strength, toughness and resistance but hinders flow.
- Melt strength: the resistance of the molten polymer to stretching and sagging. It decides whether an extruded profile or a blow-moulding parison stays in shape rather than sagging.
- Molecular weight distribution (MWD): a broad or bi-/multimodal distribution combines short chains (flow) with long chains (melt strength and long-term performance). A narrow distribution melts more evenly but gives less melt strength.
A textbook example is bimodal PE100 for pressure pipe: made in two reactors in series – a low-molecular fraction for processability, a high-molecular fraction for long-term hydrostatic strength and slow-crack-growth resistance. A broad distribution also improves die-swell control and reduces melt fracture.
Injection moulding: high flow for thin, complex walls
In injection moulding the melt must flow quickly under pressure into a closed, often thin-walled cavity. Injection grades therefore have a relatively high MFR (low viscosity). Typical ranges (always with test condition):
- PP: roughly 10–100 g/10 min (230 °C/2.16 kg). Standard parts 10–40, thin-wall packaging 30–100. Example datasheet value: SABIC PP 576P at MFR 19; high-flow impact copolymers for closures reach about 70.
- HDPE (caps & closures): roughly 8–20 g/10 min (190 °C/2.16 kg). Examples: SABIC CC862 at 8.0 and an ExxonMobil closure grade with melt index 20.
- ABS: usually given as melt volume-flow rate (MVR) at 220 °C/10 kg – a deliberately different test condition.
- PA66 (unfilled): often specified as MVR at 275 °C/5 kg; easy-flow injection grades sit well above tough grades.
The trade-off: high MFR eases filling but tends to cost strength and toughness. Extremely high MFR values (several hundred g/10 min) belong not to injection moulding but to fibre/meltblown applications.
Extrusion: low MFR, high melt strength, broad distribution
In extrusion the melt leaves the die as an open profile (pipe, profile, sheet, film) without a supporting cavity, so it must stay in shape. That calls for low MFR, high molecular weight and a broad or bimodal distribution. Typical ranges:
- PE100 pressure pipe: MFR about 0.2–0.5 g/10 min (190 °C/5 kg). Examples: Borealis BorSafe HE3490-LS-HW at 0.22 and LyondellBasell Hostalen CRP100 at 0.23 (both bimodal).
- PP-R pipe: e.g. Borealis RA130E at MFR ~0.25 g/10 min (230 °C/2.16 kg) – explicitly "high molecular weight, low melt flow rate".
- PP in general (sheet/profile/film): mostly below 3 g/10 min (230 °C/2.16 kg), while injection PP is often 30–70.
- HMS-PP (high melt strength): long-chain branching gives strain hardening – important for foam extrusion and thermoforming.
Extrusion compound is almost always supplied as a fully additivated pellet (for pipe including carbon black and UV stabiliser). One exception is PVC, processed mostly as a dry blend (powder) on twin-screw extruders.
Blow moulding: the parison must stand up
In blow moulding a preform is inflated. To keep it from sagging or tearing, the grades need high melt strength – low MFR and usually a broad/bimodal distribution.
- Extrusion blow moulding (HDPE): low MFR, typically about 0.3–1.0 g/10 min (190 °C/2.16 kg) for parison stability; very stiff grades are specified via HLMI (190 °C/21.6 kg). Bimodal HDPE combines stress-crack resistance (ESCR) with processability.
- Stretch blow moulding (PET): the leading metric is not MFR but intrinsic viscosity (IV) of about 0.72–0.85 dl/g.
Which HDPE grade suits canisters and which PET grade suits bottles, how coextrusion adds a barrier layer and what machines are used, is covered in Blow moulding of bottles and canisters.
Rotational moulding: powder, not pellets
Rotational moulding (rotomoulding) is the big special case for feedstock form: the material is used not as granulate but as fine powder. In the slowly rotating, externally heated mould the powder melts and sinters evenly onto the wall.
- Particle size: industry norm around 300–500 µm; 500 µm equals 35 US mesh. The powder is ground from roughly 3 mm pellets on high-speed mills.
- MFR: typically about 2–8 g/10 min (190 °C/2.16 kg), often optimal at 3–7. Examples: SABIC R50035E (LLDPE) at 5.0 and Borealis Borecene Compact RM8346RC (LMDPE) at 6.0.
- Distribution: fairly narrow (often metallocene PE) for even melting and short cycles – but with lower melt strength and thus a higher sagging tendency in vertical sections.
Polyethylene dominates the process at around 90 % of volume because it is cheap, easy to grind and thermally stable. Typical machines are rock-and-roll (long, large parts), carousel/turret (highest productivity), shuttle (compact, cheaper) and clamshell (single-arm, maximum flexibility). Oven temperature is typically around 300 °C, the decisive peak internal air temperature (PIAT) for PE about 200–240 °C. More in Rotational moulding in detail.
Film quality: blown film vs. cast film
Film grades also depend on the method. Two routes compete:
- Blown film: the melt leaves a ring die, is air-cooled and inflated into a biaxially stretched bubble. This gives balanced machine- and cross-direction properties and, through slower cooling, higher crystallinity and better barrier. Blown film needs higher melt strength.
- Cast film: the melt leaves a flat die onto a cooled chill roll. Fast quenching gives better optics (less haze, more gloss) and tighter thickness tolerance; grades with higher MFR and lower melt strength run well.
By material: LDPE has high melt strength from long-chain branching and stabilises the bubble. LLDPE is very tough but inherently low in melt strength and is often modified or blended. mLLDPE (metallocene) allows thinner film (downgauging) with a narrow distribution. HMW-HDPE for stiff high-stalk blown film is again specified via HLMI because it barely flows under 2.16 kg. The full comparison is in Blown film vs. cast film.
Overview: leading metric by method
The overview summarises which metric drives which method. All figures are typical guideline values, comparable only at identical test conditions; the manufacturer's datasheet is always authoritative.
| Method | Leading metric | Typical range | Feedstock |
|---|---|---|---|
| Injection moulding | MFR (ISO 1133) | PP ~10–100 g/10 min (230 °C/2.16 kg); HDPE closures ~8–20 (190 °C/2.16 kg) | Granulate |
| Extrusion (pipe/profile) | MFR / HLMI | PE100 ~0.2–0.5 g/10 min (190 °C/5 kg); PP-R ~0.25 (230 °C/2.16 kg) | Granulate (PVC: powder) |
| Blow moulding (EBM) | MFR / HLMI | HDPE ~0.3–1.0 g/10 min (190 °C/2.16 kg) | Granulate |
| Stretch blow (PET) | IV (intrinsic viscosity) | ~0.72–0.88 dl/g | Granulate (bottle chip) |
| Rotational moulding | MFR | ~2–8 g/10 min (190 °C/2.16 kg) | Powder ~300–500 µm |
| Film (blown) | MFR / HLMI | LLDPE lower MFR; HMW-HDPE via HLMI | Granulate |
Requesting granulate: method first, then grade
For purchasing this means one clear rule: the polymer name alone ("PP", "HDPE") is not enough. The processing method and the target part determine which grade of the same polymer fits – with the right MFR window, the right melt strength and the correct feedstock form (granulate or powder).
When requesting material, always state: method (injection, extrusion, blow moulding, rotation, film), part or wall thickness and special requirements such as food contact, flame retardancy, UV resistance or colour. That narrows the grade precisely instead of ordering an unsuitable standard type. An overview of all grade families is in the materials database.