HDPE

HDPE (high-density polyethylene) is the workhorse polyolefin you reach for when the part must live in water, dilute acid, caustic cleaner, or food. It is semi-crystalline, density 0.94 to 0.97 g/cc, with tensile strength in the 22 to 31 MPa range, elongation at break often above 500 percent, and moisture absorption under 0.01 percent. That combination buys you a tough, waxy, self-lubricating surface at roughly USD 1.40 to 2.10 per kilogram of resin.

The trade-off is modulus and heat. HDPE softens near 80 C and melts around 130 C, and nothing sticks to it without surface activation, so you cannot paint, glue, or overprint a finished HDPE part without flame, plasma, or corona treatment. You also cannot run it outdoors without carbon black or HALS stabilizers, because unprotected HDPE chalks and cracks in under a year of UV exposure.

The guide below is written for engineers sourcing injection molded bottles, blow molded tanks, rotomolded buoys, and CNC-machined HDPE sheet for chemical process fixtures. It assumes you already know polyethylene families exist and want to know exactly which grade, process, and design rules land a working part at the price and schedule you committed to.

HDPE earns its place on three axes. First, chemical breadth: the material shrugs off 10 percent sulfuric acid, 50 percent sodium hydroxide, bleach, isopropanol, and most aqueous surfactants at room temperature, which is why every dairy carrier and agrochemical tote in your supplier catalog is HDPE. Second, impact toughness at cold: Charpy notched values stay above 10 kJ per square meter down to -40 C, so HDPE jerrycans survive winter drop tests that shatter PP.

Third, piece-price. An injection molded HDPE cap costs USD 0.03 to 0.08 in volume, a blow molded one-liter bottle USD 0.12 to 0.25, and a rotomolded 200 liter tank USD 45 to 90. Compared to PP you pay a small premium for the extra density and stiffness; compared to PET or nylon the resin is roughly one third the cost per kilogram. The resin is also resin code 2, meaning it feeds the most mature polymer recycling stream in North America and the EU, relevant if your ESG report counts post-consumer recycled content.

The table below benchmarks a 500 mL squeeze-bottle geometry at 50,000 units annually, FOB Taiwan, tool amortized over the run. Use it to frame early sourcing conversations, not as a final quote.

HDPE is not one resin. Injection grades run high melt flow and low molecular weight for speed; blow and rotomold grades run low melt flow for part wall integrity; pipe grades (PE100) are bimodal for slow crack resistance. Pick a grade family before you pick a process.

Process choice locks in 60 percent of the part cost before you draw a single rib. HDPE welds to itself beautifully via hot-gas, extrusion, or butt fusion, but it fights every other joining method. Read the notes column carefully before you commit tooling.

HDPE covers a wide chemistry window, but not all of it. The table below reports 30-day immersion at 23 C for unfilled HDPE. For hot or pressurized service, derate one category (Excellent becomes Good, Good becomes Fair).

Start from the duty cycle, then pick grade and process. The matrix below is how we route incoming RFQs on the manufacturing floor.

HDPE shrinks 1.5 to 3.0 percent during injection, roughly three times what ABS or PC do. Keep wall thickness uniform within 20 percent, put gates on the thicker section, and add generous corner radii (minimum 1.0 mm, preferably wall thickness divided by two) to stop shrinkage pulling corners. Nominal wall for IM parts lives between 1.5 and 3.0 mm; above 4 mm you are paying for cycle time and will still see sink marks over ribs.

Draft needs to be at least 1 degree, 2 degrees on textured surfaces, because HDPE's waxy surface grabs mold steel on ejection. For threaded closures, design a 30 degree buttress profile rather than a standard 60 degree vee; it lowers stress concentration and survives more on/off cycles. For snap fits, size cantilever length to keep strain under 3 percent, and prefer living hinges only on PP (HDPE hinges fatigue within a few hundred cycles).

Where you need load-bearing bosses or press-fit inserts, machine in brass heat-set inserts after molding. Do not rely on self-tapping screws; HDPE creep will strip threads within a year of assembly load.

HDPE ships with a natural matte, slightly waxy surface. You can leave it as-molded, emboss a logo in-tool (the mirror pattern transfers cleanly), or add SPI B-1 sandblast texture for grip. Anything beyond that needs surface activation. Flame treatment at 900 C for 0.2 seconds raises surface energy from 31 mN/m to around 45 mN/m, enough for ink and cyanoacrylate to bond; corona and atmospheric plasma achieve similar results at lower throughput.

For paint, use a 2K polyurethane with a polyolefin primer such as 3M 94 or Morton Adcote 548. For decoration, in-mold labeling (IML) with a PE label is the most durable choice and skips secondary operations. Pad printing requires corona-treated stock and UV-cure inks. Hot-air plastic welding (300 C, PE rod) gives structural bonds up to 80 percent of parent strength for tank repairs and custom fabrication. Sanding, milling, and drilling are straightforward with sharp tools; chips are long and stringy, so use a vacuum and watch for chip welding on the cutter.

A Taiwan fab tier-2 supplier came to us in 2025 to replace stainless 316 sulfuric-acid day tanks that were pitting at the weld heat-affected zones after 14 months of service. We rotomolded 1,500 L tanks in Alathon H5112 black HDPE with a 12 mm wall, integrally molded flanges, and PTFE-lined nozzle ports. Tooling cost USD 28,000 amortized over 40 tanks, per-tank price landed at USD 780 including the PTFE nozzle assembly.

The first five tanks were in service 9 months later. Pitting was eliminated. Wall thickness at the weld-equivalent zone (the parting line) was verified by ultrasound at 10.8 mm, within spec. The customer reordered the remaining 35 tanks on a 10 week rolling schedule. Total program value USD 31,200; replacement program budget vs the stainless alternative saved roughly USD 48,000 over a five-year horizon.

A Hong Kong food-service chain needed 12,000 color-coded cutting boards (red, yellow, green, white) for HACCP compliance across 240 outlets. We specified DMDA-8907 NT7 compounded with FDA-listed pigments, CNC-machined from 20 mm pressed HDPE sheet to 450 by 300 mm with chamfered edges and a drilled hang hole. Sheet cost USD 42 per panel; finished board landed at USD 6.80 including packaging.

The project ran 7 weeks from PO to last pallet. Two design iterations addressed a cupping complaint: we moved the grain direction 90 degrees and added a shallow rib on the back. Warp dropped from 2.5 mm to under 0.6 mm across the diagonal. The chain reorders roughly 3,000 boards per quarter on replacement cycles, worth USD 20,400 per order at steady state.

An Italian yacht OEM replaced teak deck inserts with HMW PE100 black HDPE after three seasons of teak cracking and UV bleaching on their 48-foot range. We CNC-machined inserts from 30 mm extruded HDPE sheet with a diamond-pattern non-slip surface and counterbored M8 fasteners. Per-yacht set (42 inserts) landed at USD 1,180; the teak equivalent ran USD 3,400.

The key engineering decision was the carbon-black stabilizer package: we specified a grade guaranteed to UL 746C f1 for outdoor UV life over 10 years. After one full Mediterranean season on 14 yachts, no color shift or surface chalking was measured. The OEM rolled HDPE inserts into three additional model lines, adding USD 54,000 annual revenue. Lead time per yacht set ran 12 business days.

HDPE wins when the job is containment of water, dilute chemistry, or food; when impact toughness at cold matters; and when piece-price has to start with a zero. It loses when you need heat above 80 C, cosmetic paint-to-substrate adhesion, structural bolted joints without heat-set inserts, or outdoor service without UV stabilization. Every one of those failure modes has a cure if you call it out in the spec, and every one of them bites if you do not.

The cheapest HDPE program is rarely the cheapest: the second-tier resin with no food-contact certificate, no UV package, and 40 percent regrind will save you USD 0.03 per bottle and cost you USD 30,000 in rework when the line audit fails. Pay for the grade you need, verify each lot, and HDPE will run for decades.