Blow Molding
Blow molding inflates a heated plastic tube or preform against the walls of a mold with compressed air, producing hollow one-piece containers and tanks.
- Part
- Forming
- Variants
- 3
- Revised
- 2026-08-11
At a glance
- Family
- Plastics and Rubber
- Typical tolerances
- ±0.020 in (±0.5 mm), roughly ±1%, on blown body dimensions; injection-molded neck finishes hold about ±0.005 in (±0.13 mm). Wall thickness is not directly controllable and commonly varies 30% or more across a part.
- Surface finish
- Smooth gloss to matte or grained, set by the mold surface; the pinch-off seam is always visible on extrusion blow molded parts
- Typical volumes
- 1,000 to many millions; rotary lines run tens of thousands of containers per hour
- Lead time
- 3–8 weeks for an aluminum EBM tool; 6–12 weeks for an ISBM preform tool plus blow mold; hours per production run thereafter
- Materials
- Plastic
What it is
Blow molding inflates a hot thermoplastic tube or a reheated preform against the walls of a two-part mold with compressed air, producing hollow, seamless containers in one piece. It makes essentially every plastic bottle, jerry can, fuel tank, duct, and air intake in production today, overwhelmingly in HDPE, PP, PET, and PVC.
Three families cover almost all of it. Extrusion blow molding (EBM) clamps a continuously extruded parison and inflates it, and is the route for handleware, industrial containers, and automotive ducting. Injection blow molding (IBM) molds a preform onto a core pin and blows it at a second station, giving precise neck finishes on small pharmaceutical bottles. Injection stretch blow molding (ISBM) reheats an injection-molded preform and stretches it axially while blowing, biaxially orienting PET into clear, pressure-capable bottles.
Inflation pressure runs roughly 25–150 psi (1.7–10 bar) for extrusion blow molding and 500–600 psi (35–40 bar) for PET stretch blow molding.
How it works
Extrusion blow molding
- Extrude the parison. A single-screw extruder pushes melt through an annular die to form a hanging tube. HDPE melt runs about 350–420 °F (175–215 °C). Parison wall is programmed point by point along its length so the regions that will draw thinnest start out thicker.
- Clamp. The two mold halves close on the parison. Pinch-off edges at top and bottom weld the tube shut and shear away the tail and the moil.
- Blow. A blow pin or needle admits air at roughly 25–150 psi (1.7–10 bar). The parison expands to the cavity in well under a second. Blow-up ratio — cavity diameter divided by parison diameter — is normally kept at or below 3:1.
- Cool. The part solidifies against the mold wall. Cooling is the cycle bottleneck exactly as in injection molding; a 1 L HDPE bottle typically cycles in 10–20 seconds, a large tank in minutes.
- Deflash and trim. Pinch-off flash at top and bottom is removed, in line or in a separate operation.
Injection stretch blow molding
- Injection mold the preform. PET is dried hard — typically 4–6 hours at 300–350 °F (150–175 °C) — and injection molded into a test-tube-shaped preform with the finished neck thread already on it.
- Condition. The preform is reheated through infrared ovens into PET's orientation window, roughly 210–250 °F (100–120 °C): above the glass transition but well below the melting point.
- Stretch and blow. A stretch rod drives down the axis while pre-blow air, then final blow air at 500–600 psi (35–40 bar), expands the preform against the cavity. Typical ratios are 2–3× axial and 3.5–4.5× hoop, for a planar orientation around 10–16×.
- Set and eject. Biaxial orientation is what gives PET bottles their clarity, stiffness, and gas barrier. An unoriented PET bottle of the same weight is hazy and creeps under carbonation pressure.
Which variant fits which part?
EBM handles handles, integral spouts, and thick industrial walls but leaves a pinch seam and holds looser tolerances. IBM produces no pinch scrap and a precise neck, but is limited to small parts. ISBM is the only route to biaxially oriented PET and dominates beverage packaging.
Design guidelines
Wall thickness is an output, not an input
You do not specify blow-molded wall thickness directly. You specify a shape and a shot weight, and the material distributes itself according to how far each region has to stretch. Corners and the deepest draw thin the most. Container walls typically land at 0.012–0.040 in (0.3–1.0 mm); industrial tanks and ducts at 0.080–0.250 in (2–6 mm). Design so the thinnest predicted section still meets the requirement, and let parison programming even out the rest.
Blow-up ratio
Keep the ratio of maximum cavity diameter to parison diameter at or below 3:1, and aim for 2:1–2.5:1. Above 3:1 the material thins unpredictably at the extremes and corners go translucent and brittle. If one region demands a much larger blow-up than the rest, split the part or reprogram the parison for that zone.
Radii
Generous radii everywhere. An internal radius of at least one wall thickness is the floor; 2–3× wall is far safer. Sharp corners are where material is stretched hardest and therefore ends up thinnest — the opposite of the injection-molded case, where sharp corners run thick and hot.
Draft
0.5–1° per side works on smooth surfaces because the part shrinks away from the cavity as it cools — less than injection molding needs. Textured surfaces want 1–1.5°. The draft-versus-texture-depth relationship tabulated on /charts/injection-molding-design-guidelines applies to blow molds as well, and that chart's wall thickness table governs the injection-molded preform in any IBM or ISBM job.
Ribs and features
Blow molding cannot fill a deep, narrow rib — available pressure is one to two orders of magnitude below injection pressure. Stiffen with broad, shallow features instead: swage rings, domed panels, and recessed label areas. Depth beyond about 1× the local wall rarely forms cleanly.
Parting line and pinch-off
Put the parting line where flash removal is easy and cosmetically acceptable. The pinch-off weld at the base of an EBM part is the weakest section in the container — never route a load path or place a mounting feature across it.
Neck finishes
In IBM and ISBM the neck is injection molded rather than blown, so it holds injection-molding tolerances of about ±0.005 in (±0.13 mm) while the blown body does not. Put every closure, seal, and thread requirement in the neck and keep tight dimensions off the body. Standard finishes (the SP-400 and SP-410 series) let you buy stock closures.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Blow-up ratio | 2:1 to 2.5:1 | 3:1 | Beyond this, corners thin out and go brittle |
| Internal radius | 2–3 × wall | 1 × wall | Corners stretch most and end up thinnest |
| Draft, smooth | 0.5–1° per side | 0.25° | Part shrinks away from the cavity as it cools |
| Draft, textured | 1–1.5° per side | — | Texture depth adds required draft |
| Rib or emboss depth | ≤ 0.5 × wall | about 1 × wall | Low blow pressure cannot force material into deep detail |
| Container body wall | 0.012–0.040 in (0.3–1.0 mm) | — | Typical bottle range |
| Industrial EBM wall | 0.080–0.250 in (2–6 mm) | — | Tanks, ducts, cases |
| Tight-tolerance features | Neck finish only | — | Necks are injection molded; bodies are blown |
Cost drivers
Tooling is cheap relative to injection molding, because a blow mold is a cooled shell that sees clamp force but never 20,000 psi of melt pressure — aluminum is the normal mold material for EBM. The expensive items in a blow molding program are machine time and, in ISBM, the preform tool, which is a full injection mold with all the cost that implies.
Volume breakpoints:
- Under about 1,000 parts: single-cavity EBM in a machined aluminum tool is possible, but setup and purge scrap dominate the bill.
- 1,000–100,000: single or twin cavity EBM, or two-stage ISBM running purchased stock preforms.
- Over 1,000,000: multi-cavity rotary EBM wheels or rotary stretch-blow lines, which run at tens of thousands of containers per hour.
Cost reduction, in order of leverage:
- Lightweight the part. Resin is the dominant per-part cost in high-volume containers. One gram off a bottle at high annual volume is the entire conversation, and parison programming is how you take it off without thinning the corners past their limit.
- Use a stock neck finish. Standard finishes let you buy off-the-shelf closures and, in ISBM, off-the-shelf preforms instead of paying for a preform tool.
- Design for a single parting line. Every moving core or insert for a handle, spout, or undercut adds cycle time and maintenance.
- Reduce the blow-up ratio. A squatter part distributes material more evenly, which lets you take shot weight out without dropping below minimum wall.
- Keep tight tolerances in the neck. Body dimensions are cheap to loosen and expensive to tighten.
Variants
3 named
Extrusion Blow Molding (EBM)
Injection Blow Molding (IBM)
Injection Stretch Blow Molding (ISBM)
Questions
7 questionsWhat is the difference between extrusion blow molding and stretch blow molding?
Extrusion blow molding clamps a hot extruded tube (a parison) and inflates it immediately, which suits HDPE and PP handleware, tanks, and ducts. Injection stretch blow molding reheats a previously injection-molded PET preform and stretches it axially while blowing, biaxially orienting the polymer. That orientation is what gives PET bottles their clarity, stiffness, and carbonation resistance.
What blow-up ratio can blow molding handle?
Keep the ratio of maximum cavity diameter to parison diameter at or below 3:1, and design toward 2:1–2.5:1 where you can. Higher ratios stretch the material unevenly and leave corners thin, translucent, and prone to cracking.
What wall thickness does blow molding produce?
Consumer container walls typically land at 0.012–0.040 in (0.3–1.0 mm) and industrial tanks and ducts at 0.080–0.250 in (2–6 mm). Wall thickness is an outcome of shot weight and local stretch rather than something you specify directly, so design to the thinnest predicted section.
Can blow molded parts have ribs?
Only shallow ones. Blow pressure is roughly 25–150 psi in extrusion blow molding, one to two orders of magnitude below injection pressure, so deep narrow ribs will not fill. Stiffen instead with broad swage rings, domed panels, and recessed label areas no deeper than about half the wall thickness.
What tolerance can blow molding hold?
Expect about ±0.020 in (±0.5 mm), or roughly ±1% of the dimension, on blown body features. Necks made by injection blow or injection stretch blow molding are the exception — they are injection molded and hold about ±0.005 in (±0.13 mm), so put sealing and thread requirements there.
Why is the pinch-off seam the weak point on a blow molded part?
At the pinch-off, the mold shears the parison closed and the two inside surfaces weld together while already partly cooled. That weld never reaches the strength of the surrounding wall, so mounting features, load paths, and drop-impact zones should be kept away from it.
Which blow molding variant should I use for a small pharmaceutical bottle?
Injection blow molding. It molds the preform onto a core pin and blows it at a second station, so there is no pinch-off scrap and the neck finish holds injection-molding tolerances of about ±0.005 in (±0.13 mm). The trade is that it is limited to small parts and cannot produce integral handles.