HDPE Injection Molding: The Easy Resin That Is Hard to Hold to Size

Why HDPE is not just a softer PP

Most moulders treat high-density polyethylene as polypropylene with a different label. The two share a chemistry — both are semi-crystalline polyolefins — and they share a family of problems, but the numbers behind those problems are not the same, and the difference decides your tool.

HDPE has a simpler, more linear chain. It crystallises faster and to a higher degree, typically 60 to 80 percent against 40 to 60 for PP. Faster crystallisation means the part stiffens earlier in the cavity, which is good for cycle time and bad for packing: once the gate freezes, nothing you do at the screw reaches the far end of the part.

The practical consequences stack up in one direction. HDPE needs more packing pressure applied earlier, it shrinks more and more unevenly, and it warps in a way that follows flow direction rather than wall thickness. A tool that was cut for PP and re-run in HDPE will produce parts that are dimensionally correct at the gate and 2 percent small at the far end.

Crystallinity is the whole story

Every property that makes HDPE useful or annoying comes from the same source. The chains fold into lamellae as the melt cools, and the fraction that folds determines stiffness, barrier, chemical resistance and shrinkage simultaneously.

The degree of crystallinity is not fixed by the grade alone. It depends on how fast the part cools. A 1 mm wall quenched against a 15 °C mould surface will be less crystalline near the skin than in the core, because the skin freezes before the chains can organise. That skin-core difference is a built-in stress gradient, and it is why a thin HDPE part can bow after it leaves the mould even when the dimensions were right on the gauge.

For a moulder, the lever is mould temperature. Cold moulds give faster cycles and lower crystallinity, which means more shrinkage later and more post-mould movement. Warm moulds give higher crystallinity, better dimensional stability, and a longer cycle. There is no free option; you are choosing which problem to own.

Table 1 — Mould temperature against the four outcomes it controls. Values are for a 2 mm wall in a homopolymer grade.

The four settings that actually matter

HDPE is forgiving on most of the barrel. It has a wide processing band and will not degrade if you overshoot by 20 °C, which is why inexperienced operators set it hot and leave it. The cost of that habit is a longer cooling time and more shrinkage, not a scrap part — so nobody notices.

Melt temperature should sit between 200 and 260 °C. Below 200 the melt is too stiff to fill a thin section without excessive pressure; above 260 you gain nothing and lose cycle time, because the part must shed more heat before it is rigid enough to eject.

Injection speed is where HDPE differs most from amorphous resins. Because it is semi-crystalline and shear-thinning to a moderate degree, it fills well at high speed, but high speed across a restricted gate creates molecular orientation that later becomes differential shrinkage. Fill fast to the gate, then slow down through the cavity.

Packing is the setting that separates a good HDPE part from a variable one. Because the gate freezes quickly, packing pressure must be applied within the first fraction of a second after fill, and it must be high — 60 to 90 percent of injection pressure is normal, against 40 to 60 for ABS. Holding pressure after the gate has sealed does nothing except build stress in the sprue.

Table 2 — Starting settings for HDPE homopolymer. Adjust melt temperature down as wall increases; the part has more heat to shed and less need for fluidity.

Shrinkage is directional, and it follows the flow

The headline figure for HDPE is 1.5 to 3.0 percent, but that range hides the part that matters: shrinkage along the flow direction is different from shrinkage across it. Chains orient as they fill, and oriented chains shrink more when they relax. A rectangular part gated at one end will be shorter along its length than across its width by a measurable margin.

The ratio is typically 1.2 to 1.6 to 1, flow against cross-flow. On a 150 mm long part with 2.5 percent flow shrinkage, the width shrinks about 1.7 percent. If the tool was cut with a single uniform shrinkage factor, the part is correct in one direction and wrong in the other, and no amount of tuning fixes both.

Wall thickness changes the number too. Thick sections cool slowly, reach higher crystallinity and shrink more — a 4 mm wall can shrink 0.4 to 0.6 points more than a 1.5 mm wall in the same part. Where a part has both, the thick region pulls the thin region and the result is warp rather than a size error.

Table 3 — Shrinkage by direction for HDPE homopolymer, 30 °C mould. These are the numbers to give a toolmaker, not the single figure on the datasheet.

Warp, sink and the packing that cannot reach

Three defects account for most HDPE scrap, and all three come back to the same cause: the part freezes before the pressure equalises.

Warp

Warp in HDPE is almost always differential shrinkage between regions, not a cooling problem on its own. The classic case is a part with a thick boss on a thin base. The boss shrinks 2.8 percent, the base 1.8 percent, and the base bows toward the boss. Adding cooling to the boss helps; adding a rib around it helps more, because it equalises the section.

Sink

Sink over a rib or boss appears when the thick section keeps shrinking after the surface has frozen. HDPE is worse than ABS here because its shrinkage is larger, so the same rib-to-wall ratio that is invisible in ABS shows as a shadow in HDPE. Keep rib thickness at 50 to 60 percent of the adjoining wall, and boss wall at 50 percent.

Short shots that are not short shots

A part that fills 97 percent and stops is often reported as a short shot and fixed with more pressure. In HDPE it is usually a venting problem: the cavity cannot expel air fast enough, the gas compresses, and the last corner never fills no matter what the pressure. Vent depth for HDPE is 0.02 to 0.03 mm, wider than for ABS because the melt is more viscous at the flow front.

The parts HDPE is genuinely the right answer for

HDPE earns its place in four kinds of part, and in each the property that decides it is different.

• Living hinges. HDPE and PP are the only commodity resins that survive being bent a million times. The hinge needs a 0.25 to 0.4 mm web, oriented across the hinge line by fast fill, and it must be ejected hot enough to close immediately. A cold hinge cracks on the first closure.
• Chemical containers. HDPE resists acids, bases and most solvents. A 5 litre jerry can in HDPE will hold 98 percent sulphuric acid for years; the same part in PP will stress-crack in months.
• Food contact closures and caps. HDPE is dimensionally stable enough for a thread and compliant with food contact rules without additives. The limitation is heat: a cap that sees 85 °C filling must be PP or a blend.
• Wear parts. HDPE has a low coefficient of friction and good abrasion resistance, which is why it appears in guide rails, chain guides and conveyor wear strips.

Where HDPE is the wrong answer

Anything above 80 °C in service. Anything that needs to be clear — HDPE is translucent at best in thin sections and milky above 1.5 mm. Anything that needs to hold a tight tolerance across a large dimension, because the shrinkage variation will exceed it. And anything that must take a structural load at stiffness, where ABS or glass-filled PP will do the job at half the wall.