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Buyer guide

Rotational molding resin: powder, grind, MI and UV level by product

We source rotomolding polyethylene as pellets for molders who grind in-house, or as ground and pre-colored powder, from one gaylord to a truckload. Send the grade you run, or the part, melt index, density and UV level, and a polymer chemist reviews it and quotes a match or a qualified equivalent.

Updated October 2026

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The short answer

Most rotomolded parts are made from linear polyethylene with a density of about 0.932–0.945 g/cm³ and a melt index of about 2–7 g/10 min (190°C / 374°F, 2.16 kg), ground to powder that nearly all passes a 35 mesh (500 µm) screen. Pick density for stiffness, melt index for flow and detail, then the UV level for outdoor life and the ESCR for anything that holds chemicals. Grade alone is not enough: grind quality, dry flow and bulk density decide whether the powder molds a clean wall.

Powder, micropellets or pellets you grind yourself

Polyethylene is sold as pellets about 3 mm (1/8 in) across. Pellets do not melt and flow into a solid, even wall in a rotomolding oven, so for polyethylene they are almost always pulverized into powder first. The question is who does the grinding, and whether color goes in before or after.

Form	What it is	When it fits	Watch for
Pellets you grind in-house	Natural or pre-colored pellets, ground on your own pulverizer	Molders with a pulverizer and steady volume of a few grades	You own grind quality: mill gap, screen, temperature and throughput. Grinding makes dust, so plan for dust control and explosion risk.
Ground natural powder	Pellets ground by a toll grinder or compounder, often to 35 mesh	Molders without a pulverizer, or with more grades than they want to grind	Ask for sieve analysis, dry flow and bulk density on each lot. Powder ships in bags, gaylords or bulk bags, not in bulk trucks.
Pre-colored (compounded) powder	Color melt-compounded into the resin, then ground	Parts where impact, color match and UV life must hold lot to lot	A compounding and grinding step, often made to order with a minimum run per color.
Dry-blended powder	Natural powder tumbled with dry pigment at the molder or grinder	Short runs and many colors	Lower impact and more color swirl than compounded color at the same pigment level. Test cold impact on molded parts.
Micropellets	Very small pellets cut from an underwater die, used without grinding	Niche, mostly small custom-color runs	Molders report pinholes unless the size is below about 400 µm, and the special die slows the extruder, which often costs more than grinding.

Particle sizes from the Association of Rotational Molders (ARM) test methods and its technical director’s published guidance. Confirm the grind on each supplier’s sheet.

Grind, dry flow and bulk density: why the powder matters as much as the grade

A good grade ground badly molds a bad part. ARM publishes test methods for particle size (based on ASTM D1921) and for dry flow and bulk density (based on ASTM D1895 and ISO R60, adapted for rotomolding powder). Poor flow can cause bridging, uneven walls, a rough inside surface, pinholes and pigment swirl. Bulk density decides how much powder fits in the mold, so a fluffy powder can make it hard to charge enough material for a thick wall.

Check	How it is measured	Typical target	Why it matters
Grind (mesh)	A 100 g sample shaken 10 minutes through a sieve stack such as 30, 35, 40, 60, 80 and 100 mesh (ARM particle size method, based on ASTM D1921)	35 mesh grind: nearly all passes 35 mesh (500 µm); average particle often about 300–350 µm	Too many coarse particles leave pinholes and bubbles. Too many fines dust, cling and can bridge.
Dry flow	Seconds for 100 g of powder at 20–25°C / 68–77°F to empty a specified funnel, with no tapping (ARM method based on ASTM D1895 and ISO R60)	Often under about 30 seconds per 100 g on supplier sheets. ARM sets no pass mark, and a powder that needs tapping to flow is a warning sign	Slow flow means the powder does not tumble evenly, which shows up as thin corners and rough inside walls.
Bulk density	Weight of powder that fills a 100 cm³ cup under the same funnel, reported in g/cm³ (same ARM method)	Often about 0.33–0.40 g/cm³ (about 21–25 lb/ft³), lower than pellets at about 31–37 lb/ft³	Sets charge volume and shot weight. Long, fibrous particles (tails) from a hot or worn mill lower both bulk density and flow.
Particle shape	Look at it under magnification	Rounded particles, few tails	Tails trap air and slow flow. A change here often explains a lot that runs differently with the same grade.

Targets are typical published values and vary by grade and grinder. Results depend on funnel finish and powder temperature, so compare lots tested on the same equipment.

Melt index and density by product

Every melt index below is at 190°C / 374°F and 2.16 kg (ASTM D1238). Density is by ASTM D1505 or D792. Higher density adds stiffness and heat resistance but often lowers impact and stress-crack resistance. Higher melt index fills detail and fuses faster but can lower impact and ESCR. Our LLDPE page has quote buttons for the common rows.

Product	Grade family	Typical window	Put on the RFQ
Potable water, rainwater and septic tanks	LLDPE or MDPE, hexene or octene, UV stabilized	MI about 3–5 g/10 min (190°C / 374°F, 2.16 kg); density about 0.935–0.940 g/cm³	UV level and how it is rated, ESCR, wall thickness, and whether your customer needs the material or the finished tank certified to NSF/ANSI/CAN 61.
Chemical, fertilizer and agricultural tanks	Higher-density LLDPE or MDPE with long ESCR, or crosslinkable PE (XLPE)	MI about 2–5 g/10 min (190°C / 374°F, 2.16 kg); density about 0.937–0.945 g/cm³	The chemicals and concentrations stored, ESCR by ASTM D1693 at a stated Igepal strength, service temperature, and whether the tank is built to ASTM D1998.
Feeders, bulk bins and stiff industrial containers	HDPE rotomolding grade	MI about 2–5 g/10 min (190°C / 374°F, 2.16 kg); density about 0.941–0.948 g/cm³	Stiffness and abrasion needs, cold impact, and ESCR. Some higher-density grades list ESCR in the tens of hours, so check it if the part sees chemicals.
Kayaks, canoes, marine floats and docks	LLDPE, MDPE or HDPE rotomolding grade, high UV level; XLPE in some boats	MI about 3–7 g/10 min (190°C / 374°F, 2.16 kg); density about 0.932–0.948 g/cm³	Low-temperature impact by the ARM drop test, UV level, abrasion, and compounded versus dry-blended color.
Playground slides, site furniture and signs	LLDPE or MDPE, high UV level, often pre-colored	MI about 3–7 g/10 min (190°C / 374°F, 2.16 kg); density about 0.932–0.940 g/cm³	UV level and color retention, cold impact, and any customer test for fading. Bright colors fade first, so approve them on molded parts.
Coolers and foam-filled parts	Higher-flow LLDPE or MDPE	MI about 5–7 g/10 min (190°C / 374°F, 2.16 kg); density about 0.935–0.939 g/cm³	Warp and dimensional control, adhesion to polyurethane foam if you foam in place, and impact at the coldest temperature the part sees.
Bins, totes and material-handling containers	LLDPE or MDPE, sometimes HDPE for stiffness	MI about 4–7 g/10 min (190°C / 374°F, 2.16 kg); density about 0.935–0.945 g/cm³	Stacking load, drop impact, UV if stored outside, and FDA letter if the container holds food.
Toys, thin walls and detailed parts	High-flow LLDPE	MI about 6–7 g/10 min (190°C / 374°F, 2.16 kg), a few grades higher; density about 0.935 g/cm³	Detail and corner fill, oven cycle, and cold impact. High flow helps detail but gives up some toughness.

Typical windows drawn from published rotomolding data sheets and ARM’s resin data guide, which tests rotomolded samples. Two grades with the same MI and density can differ several-fold in ESCR and impact, so confirm on the data sheet for the exact grade.

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LLDPE, MDPE, HDPE, crosslinkable PE and the niche resins

Most rotomolding polyethylene is linear: made with a butene, hexene or octene comonomer. Producers sell grades at about 0.935–0.940 g/cm³ as rotomolding LLDPE, while by density alone the same resin falls in the medium-density range, so you will see both names for similar material. Compare the numbers, not the label.

Resin	Typical density	Where it fits	Trade-offs
Rotomolding LLDPE / MDPE	About 0.932–0.940 g/cm³	Most tanks, kayaks, playground parts, coolers and general parts	The workhorse. Hexene and octene grades often give better ESCR and cold impact than butene grades at the same MI and density.
Rotomolding HDPE	About 0.941–0.951 g/cm³	Stiff bins, feeders, barriers and some high-abrasion parts	More stiffness and heat resistance. More shrink and warp, and some grades have much lower ESCR.
Crosslinkable PE (XLPE)	Molded like PE, then crosslinks in the oven	Chemical storage tanks, some kayaks and fuel tanks	Very high ESCR and toughness. Scrap and flash cannot be reground into the same part, the oven cycle is tied to the crosslinking package, and fewer producers make it. ASTM D1998 adds a gel test for crosslinked tanks.
Polypropylene	About 0.90 g/cm³	Parts needing more heat or stiffness than PE	Niche. PP is harder to grind than PE and some grades need cryogenic grinding, which adds cost, and it is brittle in the cold unless a copolymer is used.
Nylon (PA)	About 1.1 g/cm³	Fuel tanks and parts that need low permeation or high heat	Niche and specialized. It must be kept dry, and few grades are made for rotomolding.

PVC plastisol is also rotomolded as a liquid, but that is a different supply chain and we do not cover it here.

UV level and color

How UV levels are stated. Producers state UV protection as a number such as UV8, UV12 or UV20. It is each producer’s own scale, not a standard. Higher means more stabilizer and longer life in an accelerated weathering test, and the scale is roughly linear: a UV8 grade is meant to hold up about twice as long as a UV4 grade. Some producers tie the number to roughly thousands of hours in a xenon-arc or fluorescent UV test before a property such as tensile elongation drops to a set share of its starting value, but the test and pass rule vary: data sheets cite ASTM G155, G154 or D2565, or EN 13341 in Europe. Compare UV numbers only when the method and criterion match, and ask for both.

Lower levels suit indoor or shaded parts. UV8 is widely treated as the starting point for long-life outdoor tanks. UV12 to UV20 and higher are marketed for playground, marine and high-sun parts. Pigments change UV life too: carbon black protects the most, and some bright colors fade early even when the resin holds its strength.

Dry blend or compounded color. Dry blending is cheaper per color and quick to change, but the pigment sits on the particle surface, which can lower impact and leave swirl. Compounded (pre-colored) powder costs more and needs a minimum run, but gives better impact, color consistency and UV life. Some pigments also change shrinkage and warp, so approve each color on a molded part. See masterbatch vs. precolored resin and UV-resistant plastics.

Cold impact, ESCR, NSF 61 and ASTM D1998

ARM low-temperature impact. Specimens cut from a rotomolded part are chilled to −40°C / −40°F (at least two hours in an air-circulating freezer; stacked specimens need much longer) and hit with a 10, 15, 20 or 30 lb dart using the up-and-down (Bruceton staircase) method to find the height that breaks half the specimens. Results compare only at the same dart, and ARM warns they cannot be scaled for wall thickness, so compare parts of similar thickness. A ductile failure, a stretched hole, is what a properly cured polyethylene part should show. Brittle shattering often points to under-cure or over-cure, not bad resin.
ESCR. Rotomolding data sheets and the ARM guide often list bent-strip ESCR (ASTM D1693) as F50 hours in both 100% and 10% Igepal. The 10% figure is the harder test and spreads grades out more. Compare only at the same condition and strength. See stress cracking.
NSF/ANSI/CAN 61 for drinking water. A resin or compound can be certified and listed as a material, and a tank is certified as a finished product by the tank maker. A listed resin does not make your tank certified. Ask your customer which one the specification calls for, check the listing for the exact grade and color, and note that potable-water certification does not by itself cover storing treatment chemicals.
ASTM D1998 for chemical tanks. The current edition, D1998-21, covers flat-bottom, upright, cylindrical, one-piece rotomolded polyethylene tanks of 1,900 L (500 gal) and up, for above-ground use with chemicals at atmospheric pressure. Continuous service must stay below 66°C / 150°F for Type I (crosslinked) tanks and 60°C / 140°F for Type II (non-crosslinked) tanks; pressure, vacuum and hotter service fall outside it. It covers both crosslinked and non-crosslinked polyethylene and calls for low-temperature impact testing, plus a gel test for crosslinked tanks. If you build to it, tell us so we match a grade suited to it.
Food contact. For feed bins, food totes or coolers, ask for the producer’s FDA letter for the exact grade (21 CFR 177.1520) and statements for any pigment you add. See FDA food-contact resin.

Cycle problems that trace back to the resin

Most rotomolding defects come from the oven cycle, and the fix is in the process. A few trace back to the resin or the powder, and those are the ones to check before you change suppliers or blame a lot. Many producers publish an optimum peak internal air temperature (PIAT) for each grade, measured with an in-mold temperature probe. Cure to it, then judge the resin.

Bubbles and pinholes. Often under-cure: the melt never got hot enough for trapped air to dissolve. Resin-side causes are coarse grind, tails, low bulk density and damp powder or pigment. Check the sieve analysis and dry flow first. See voids and bubbles.
Under-cure. Bubbly inside surface, low cold impact and brittle breaks. A grade with a higher optimum PIAT or a heavier wall needs more oven time. Swapping to a lower-MI or higher-density grade without adjusting the cycle can cause it.
Over-cure. Yellowed or burnt inside surface, odor, and impact that drops again after peaking. It is degradation, and a weak heat stabilizer package or reground scrap that already saw several cycles makes it worse.
Warpage and shrink. Mostly cooling: uneven or too-fast cooling, and part design. Higher-density grades shrink more, and some pigments nucleate polyethylene and change shrink. Moving up in density for stiffness can bring warp with it. See warped parts.
Rough inside wall and thin corners. Often slow dry flow or a wide particle size spread. A new grinder or a worn mill can cause this with no change in grade.
Color swirl and specks. Dry-blended pigment, poor dispersion, or a pigment that does not wet out. Compounded color often fixes it.

Packaging, lead times and what moves the price

Package	Typical net weight	Notes
Bag	55 lb (25 kg)	Common for ground powder, pre-colored compounds and imported resin. Often about 40–55 bags per pallet.
Gaylord (box with liner)	About 1,000–1,600 lb for pellets; powder nets less	Powder packs lighter than pellets, so check the net weight on the quote.
Bulk bag	About 1,000–2,200 lb	Common from grinders. Needs a bag unloader or discharge frame.
Bulk truck or hopper railcar	About 40,000–50,000 lb by truck; about 180,000–210,000 lb by railcar	Pellets only, for molders who grind in-house and have silos.

Base resin. Rotomolding PE follows the polyethylene market. Hexene and octene grades sit above butene grades, and fewer producers make some rotomolding grades, so one outage can move them more than film grades.
UV level. A higher UV rating means more stabilizer and costs more. Buy the level the part needs, not the highest on offer.
Grinding. A toll grinding charge on top of the pellet price, plus freight to and from the grinder. Finer or tighter grinds cost more.
Color. Pre-colored compounds add pigment, compounding and a minimum run. Dry blending costs less but can cost impact.
Specialty resins. XLPE, PP and nylon rotomolding grades come from few sources and carry a premium.
Order size and freight. A bag of powder costs the most per pound and a railcar of pellets the least. See how resin is priced and the resin market notes. We do not post prices.
Lead time. Pellets already packaged at a supplier’s warehouse often ship first. Grinding and pre-color add a queue, so tell us the date you need powder on your floor.

What to put on the RFQ

The grade you run now, or MI (190°C / 374°F, 2.16 kg) and density (ASTM D1505 or D792), each with a range.
Form. Pellets to grind, natural powder, or pre-colored powder. For powder: mesh, and your limits for dry flow and bulk density.
Comonomer and resin family. Butene, hexene or octene LLDPE, MDPE, HDPE or XLPE.
UV level, with its test method, and the outdoor life the part needs.
Color. Natural, black or a custom match, and compounded or dry blend.
Performance. ARM cold impact with the dart and wall thickness, and ESCR by ASTM D1693 with the Igepal strength.
Approvals. NSF/ANSI/CAN 61 (material or finished tank), ASTM D1998, FDA letter, or a customer specification.
The part and your oven. Product, wall thickness and the PIAT you cure to now.
Quantity and ship-to city, plus your yearly usage and packaging. See send an RFQ package.

Common questions

What mesh size is rotomolding powder?

Most polyethylene rotomolding powder is a 35 mesh grind: nearly all of it passes a 35 mesh screen, which opens at 500 µm. The average particle is smaller, often about 300–350 µm, with a spread of sizes. Ask for the sieve analysis by the ARM particle size method, not just the mesh.

Can I rotomold pellets without grinding them?

Not standard polyethylene pellets. They fuse but do not melt into a solid, even wall before the resin starts to degrade. Micropellets under about 400 µm can work, but they cost more to make than grinding in most cases, so they are a niche for small custom colors.

What melt index should a rotomolded water tank use?

Many water tank grades run about 3–5 g/10 min at 190°C / 374°F and 2.16 kg, with a density of about 0.935–0.940 g/cm³. Large or tall tanks lean toward the lower MI and higher ESCR. Confirm the UV level and any NSF/ANSI/CAN 61 requirement before you choose.

What does UV8 mean on a rotomolding resin?

It is the producer’s rating for UV stabilizer level, not a standard. Higher numbers mean longer life in accelerated weathering, roughly in proportion, so UV8 is meant to last about twice as long as UV4, and some producers tie each step to about a thousand test hours. The weathering method and pass rule differ between producers, so ask for both and compare only like with like.

If the resin is NSF 61 listed, is my tank certified?

No. A listed resin is a material listing. A finished tank is certified as a product, and that is the tank maker’s certification. Check which one your customer’s specification requires, and keep the listing for the exact grade and color.

What is the minimum order for rotomolding resin?

One gaylord of pellets, about 1,000 to 1,600 lb, or bags of ground powder. Pre-colored compounds often carry a minimum run per color, which we confirm on the quote. A truckload can mix pellets and powder or several colors.

If your grade is not listed, tell us what you run. We source commodity, engineering and specialty resins.

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What to send for an accurate quote

The grade name, from any supplier, or a data sheet
Color (natural, black or a custom match)
Fill type and percentage, if any
Melt flow, if you know it
Quantity for this order and what you use in a year
Ship-to city and state or province

Rotomolding resin, matched by grade, grind and UV level

Send the grade you run, the part, and the form, color and UV level you need, and a polymer chemist matches a grade and quotes it, as pellets or powder. We aim to send your quote within one to three business days.