What Is Blow Moulding? A Simple Guide for Businesses

You do not need to understand manufacturing to buy packaging. But knowing roughly how bottles are made explains several things that otherwise look arbitrary — why custom shapes carry high minimums, why some bottles cost more than others, and why lead times vary so much.

Here is the short version.

The basic idea

Blow moulding forms a hollow plastic container by inflating softened plastic inside a mould. Air pressure pushes the plastic against the mould walls; it cools, hardens, and takes that shape. The mould opens, the bottle comes out.

Everything else is variation on that theme. Three processes account for nearly all rigid plastic bottles.

Extrusion blow moulding

Molten plastic is extruded as a hollow tube — called a parison — which hangs down between the two halves of an open mould. The mould closes around it, pinching the bottom shut. Air is blown in, inflating the parison against the mould walls. It cools, the mould opens, and the bottle is trimmed.

This is how most HDPE bottles and jerry cans are made. It handles handles — literally: an integrated moulded handle, as on a jerry can, is straightforward with this process and difficult with the alternatives.

What it means for you: tooling is relatively less expensive than injection processes, and the process suits opaque, tough containers in a wide range of sizes.

Injection blow moulding

Plastic is first injection moulded around a core rod to form a preform with a finished neck. The preform is transferred, still on the rod, into a blow mould where air inflates it into the final shape.

Because the neck is injection moulded rather than blown, it is dimensionally very precise. That matters for small containers with tight closure requirements — pharmaceutical bottles and small cosmetic containers in particular.

What it means for you: excellent neck accuracy and good finish, but tooling costs more and the process suits smaller containers.

Stretch blow moulding

Used for PET bottles. A preform — which looks like a thick-walled test tube with a finished neck — is heated, then stretched lengthwise by a rod while air inflates it outward.

That biaxial stretching is what gives PET its characteristic properties: high clarity, good strength at low weight, and gas barrier performance good enough for carbonated drinks.

Stretch blow moulding is often a two-stage process. Preforms are made in one place, and blown into bottles elsewhere — sometimes by the filler themselves. If you run your own blowing equipment, you would buy preforms rather than finished bottles.

What it means for you: it is why PET bottles are clear, light and strong, and why preform weight is such an important cost lever.

Why this affects what you pay

Tooling is a fixed cost. A mould is made before any bottle exists, and that cost has to be recovered. Spread over a large order it is negligible per bottle; over a small one it dominates. This is the entire explanation for high minimums on custom shapes.

Material is the largest variable cost. Bottle weight drives resin consumption, which drives cost. A few grams per bottle compounds quickly at volume — which is why “how much does it weigh?” is often a more useful question than “what material is it?”

Cycle time affects capacity. How many bottles a machine produces per hour affects both cost and lead time. Larger and more complex containers run slower.

Colour changes cost time. Switching a machine from one colour to another involves purging. This is part of why custom colours carry higher minimums than stock colours.

Stock shapes versus custom moulds

Manufacturers hold a substantial library of proven stock shapes. Using one means no tooling cost, lower minimums, faster delivery, and a shape whose production behaviour is already known.

You can still make it yours: colour, closure, decoration and label design differentiate a pack considerably without touching the mould.

A custom mould makes sense when volumes are high enough to absorb the tooling cost, when no stock shape fits the fill volume or the shelf position, or when the pack shape is genuinely part of the brand.

We discuss custom packaging development where the volumes justify it, and arrange it through our supplier network. We do not operate our own moulding facility today — establishing one is a longer-term goal, and we will say so plainly on this site when it happens.

The practical takeaway

You do not need to specify a process when you buy packaging. But if a supplier tells you a shape carries a high minimum, or that a custom bottle will take months, this is why — and it usually means the honest answer is to start with a stock shape and revisit a custom mould once the product has proven its volume.