Plastics processing
Blow molding
air pressure pushes hot plastic against a cold mould to make hollow parts
Bottles, jerry cans, chemical drums and fuel tanks all come from one idea: form a soft tube or preform, close a mould around it and inflate it until it touches the wall. This article covers EBM, IBM and ISBM, the parameters that matter and the defects you meet most often at the machine.
Marketing Department, Advanced Chem Solutions · 21 October 2026 · about 8 min read
The principle in one minute
There are three main variants. All of them use compressed air to inflate softened plastic against a cooled mould and hold it there until it sets. What differs is the starting shape before blowing.
- EBM (extrusion blow molding) An extruder pushes melt through a die head to form a hanging hollow tube, the parison. The two mould halves close, pinch the bottom shut (the pinch-off) and air is blown in. Material outside the cavity becomes flash that has to be trimmed. Integrated handles are possible. Used for HDPE bottles, jerry cans and large drums.
- IBM (injection blow molding) A preform is injection moulded onto a core rod, then the rod carries it to a blow mould and air is blown through the rod. No flash and a very accurate neck, which suits small bottles for pharmaceuticals and cosmetics.
- ISBM (injection stretch blow molding) A PET preform is injection moulded and brought just above Tg. A stretch rod draws it lengthwise, then blow air expands it around the circumference. The PET chains orient in two directions (biaxial orientation), giving a bottle that is clear, strong and a better gas barrier than an unstretched one. This is how water and carbonated soft drink bottles are made.
The heart of EBM is that the parison has to hold its own shape while it waits for the mould. If the melt is too fluid, the parison stretches under its own weight and the upper wall thins. That is why EBM needs high melt strength and low-MFI grades. ISBM poses a different problem: getting the preform temperature right so it stretches evenly.
The production cycle
One EBM cycle has six steps:
- Extrude the parison Melt is pushed through the die head. Parison programming changes the die gap during extrusion so the wall is thicker where it will be stretched most, such as shoulders and corners.
- Close the mould and pinch off The mould pinches the bottom shut and a knife cuts the parison from the die. Shuttle machines then move the mould to the blow station. Large drums use an accumulator head that stores melt and pushes the whole parison out in one stroke.
- Blow A blow pin or needle injects air and the parison inflates against the mould wall.
- Cool under pressure Air pressure is held so the skin stays in contact with the cold wall. This step usually takes the largest share of the cycle, because HDPE walls are thick and conduct heat poorly.
- Exhaust, open, eject
- Trim and leak-test Flash is removed from the neck, base and handle opening, ground and fed back as regrind. Many bottle lines run an in-line leak tester.
Two-stage ISBM separates moulding from blowing: dry the PET → inject preforms → store them → reheat in an infrared oven to a set temperature profile along the length → stretch rod plus pre-blow → high-pressure final blow → cool and eject. Single-stage machines do everything in one machine using the heat left over from injection.
Parameters to control
| Parameter | What it controls | What goes wrong if it is off |
|---|---|---|
| Melt temperature (EBM) | Viscosity, parison sag, pinch-off weld quality | Too high: sag, thin upper wall, longer cooling · Too low: weak pinch-off weld, rough surface, high pressure |
| Parison program / die gap | Wall thickness along the part | Thin corners and shoulders, heavy base, extra weight with no added strength |
| Die centring and die-head temperature around the circumference | Wall thickness around the part | Thick on one side and thin on the other, parison curling |
| Parison drop time | Sag and die swell | Parison length drifts, part weight varies |
| Blow pressure and pre-blow | Contact with the mould, surface detail, cooling rate | Too little air: soft detail, slow cooling, more shrinkage · Pre-blow mistimed: parison walls stick together or burst |
| Mould cooling water temperature | Cooling rate, shrinkage | Rocker bottom, dimensions drift after ejection |
| Mould closing speed and pinch-off geometry | Strength of the base weld | Thin weld that fails in the drop test |
| Preform temperature profile (ISBM) | Material distribution and PET orientation | Too hot: haze from thermal crystallisation · Too cold: pearlescence (stress whitening), poor stretch |
| Stretch rod and pre-blow timing (ISBM) | Material along the length and in the base | Heavy base, thin shoulder, off-centre gate |
| PET moisture and melt temperature | IV and acetaldehyde (AA) | Wet: hydrolysis, IV drop, brittle bottles · Too hot or long residence: high AA, which affects the taste of water |
Reference temperatures: HDPE melt for EBM about 180–220 °C · PET preform injection about 270–290 °C · preform temperature before stretch blowing about 90–120 °C, above the Tg of PET (about 70–80 °C). Typical range — follow the TDS of your grade.
Typical parts and resins
| Variant | Typical parts | Main resins |
|---|---|---|
| EBM | Milk bottles, detergent bottles, motor-oil bottles, jerry cans, chemical drums, fuel tanks (multilayer with a barrier layer), automotive air ducts | Mainly HDPE, also PP, LDPE and EBM-grade PET |
| IBM | Pharmaceutical and cosmetic bottles, roll-on bottles, small bottles with tight neck tolerances | HDPE, PP, PET, PS |
| ISBM | Water, carbonated soft drink and edible-oil bottles, clear cosmetic bottles | Mainly PET, some PP |
Which way MFI goes: blow molding uses clearly lower MFI than injection moulding. HDPE blow grades often have an MFI below 1 g/10 min at 190 °C/2.16 kg (fractional MFI), so many datasheets also report a value at 21.6 kg (HLMI). Large drums use even lower-MFI grades. Higher molecular weight gives the melt strength the parison needs and the ESCR that chemical containers require.
Melt strength is not set by MFI alone. Molecular weight distribution and long-chain branching matter too, so two grades with similar MFI can sag differently. PET is not specified by MFI at all but by IV (intrinsic viscosity); carbonated drink bottles use a higher IV than water bottles because they must hold CO₂ pressure.
Common defects
Most blow molding problems are solved at the machine, the mould or by changing the resin grade. Only a few are genuinely helped by additives. The table separates the two.
| Symptom | Likely cause | How to check | Can an additive help? |
|---|---|---|---|
| Parison sag, thin upper wall | Melt too hot, low melt strength grade, long hang time, regrind or PCR with higher MFI than the base resin | Cut bottles and weigh top against bottom, measure wall at several heights, see whether parison length changes with hang time | Mostly a process and grade issue. If you raise the PCR share, control MFI lot by lot — see inconsistent recycled resin |
| Uneven wall around the circumference | Die off-centre, uneven die-head temperature, program not suited to the shape | Section bottles at several heights and measure around; if the same side is thin on every bottle, look at the die | Not an additive problem — fix the die and the parison program |
| Weak pinch-off, base weld cracks | Melt too cold, pinch-off land too sharp or narrow, mould closes so fast that material is not pushed into the weld | Cut through the base and look at the inside of the weld; a thin, sharp groove (notch) means too little material. Confirm with a drop test | Mould and temperature issue, not an additive one |
| Rocker bottom | Thick base still hot at ejection keeps shrinking, too little base cooling · PET: base too hot or material collecting in the base | Stand bottles on a flat plate right after ejection and again after storage, measure base thickness | Mainly cooling and material distribution |
| Die lines along the length | Degraded deposits or dirt on the die lip, damaged die lip, torn screen | A line at the same circumferential position on every bottle points to the die lip | A processing aid (PPA) can reduce die-lip build-up. With PCR, filtration matters — see recycled resin |
| Containers crack when filled (ESCR failure) | HDPE grade with too little ESCR for the contents (surfactants, oils), stress concentrated at handles, corners and the pinch-off, local thin walls | Find the crack origin, test resin ESCR to ASTM D1693, and test real bottles with the real contents | Additives do not raise resin ESCR directly. Bottles stored in sunlight crack sooner once the surface degrades — see plastics turning brittle in sunlight · PCR with lot-to-lot ESCR swings — see recycled resin |
| Bottles attract dust on the shelf | PE is an insulator and holds static charge from friction during production and transport | Dust in streaks or only where the surface was rubbed; measure surface resistivity | Yes — see dust-attracting parts, and test labelling and printing at the same time |
Additives in blow molding
In EBM, masterbatch is added at the extruder throat through a gravimetric or volumetric doser. In ISBM it goes in at the preform injection machine, because the blowing step does not remelt the material.
- Include regrind in your let-down maths Ground flash already contains additive. If you calculate the MB percentage on total throughput including regrind, the level in the bottle ends up above target. Calculate MB on the virgin portion and keep the regrind ratio constant.
- UV for jerry cans, chemical drums and containers stored outdoors — see UV masterbatch
- Antistatic for bottles that collect dust — see antistatic masterbatch
- PCR such as rHDPE in detergent bottles, where MFI, ESCR, odour and colour must be controlled — see PCR resin
- Colour and opacity the MB carrier should have an MFI close to the base resin, otherwise it disperses poorly and leaves colour streaks
- Processing aids and stabilisers to reduce die-lip build-up and protect regrind that goes round the loop many times
- PET uses dedicated additives such as UV blockers to protect the contents and AA scavengers for water bottles; they must survive PET injection temperatures
ACS does not blow-mould parts. We design masterbatch formulations and produce them with a network of partner compounders. What we can do is separate the problems a formulation can solve from the ones that need fixing at the machine.
FAQ
Blow molding questions we hear often
How do I choose between EBM and ISBM?
EBM suits HDPE or PP bottles and containers with handles, from small bottles to large drums. Moulds cost less and shapes are easier to change, but there is flash to trim. ISBM suits clear, strong PET bottles with accurate necks and no flash, but it needs a preform mould and higher blow pressure, and integrated handles are difficult. The real deciders are the material, the volume and the shape.
Why do HDPE blow grades have such a low MFI?
The parison has to carry its own weight without sagging while it waits for the mould to close. Low-MFI grades have higher molecular weight, so they have more melt strength and usually better ESCR. With an injection-type high-MFI grade, the upper wall thins, bottle weight varies and containers crack more easily with chemicals.
How much flash regrind can I add back?
Recycling flash is normal practice in EBM and there is no single figure that fits every job. What matters is keeping the ratio constant across shifts, because regrind shifts MFI, colour and melt strength slightly each time it goes round, and it already contains additive that must be counted in the let-down. Test bottles at the highest regrind ratio you plan to run before making it the standard.
Bottles attract dust straight off the machine. How do I fix it?
PE is an insulator and picks up charge easily from friction. A migrating antistatic masterbatch moves to the surface over time and draws in moisture from the air to bleed off the charge, so full effect takes some time after moulding and depends on ambient humidity. Test labelling and printing at the same time, because additive at the surface can affect adhesion.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
Send us a problem bottle
Send a part or photos together with your resin grade and current formulation. We will help you tell whether the problem sits in the formulation or in the process. Free of charge, no obligation to buy.
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Injection molding
Preforms and everyday parts: the injection cycle, parameters and machine-side defects
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Extrusion
Three screw zones, the die and the downstream line for pipe, sheet and film
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Rotational molding
Another route to large hollow parts such as water tanks, with no pressure involved
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