Blow molds for PET bottles: how they are built and what to consider

How a PET bottle is blown

First, a preform is injection molded — a blank with a finished neck and thread. The preform is then heated, placed in the blow mold, stretched with a rod and blown with air. The plastic presses against the cooled walls of the mold and takes on its contour.

The neck does not change during blowing: it comes from the preform, not from the blow mold. That is why the mold is designed for a specific preform.

What a blow mold consists of

A typical mold for a PET bottle has several parts:

• two body halves — they shape the side of the bottle with its relief, panels or label area;
• a base insert — a separate part that can be replaced without changing the whole mold;
• cooling channels — they determine the cycle time and how stable the dimensions are;
• vents — they let the air out so that the relief and edges come out sharp.

Which base to choose

Carbonated drinks usually get a petaloid base with several feet: it withstands the internal pressure. For still water, oil and household liquids a simpler base works; it is easier to blow and needs less material. Hot filling requires a special design of both the bottle and the mold.

What is needed to quote the mold

To quote the mold without extra questions, prepare:

• a drawing or 3D model of the bottle, or a sample if there is none;
• the bottle volume and the product it will hold: carbonated, still, hot-filled;
• preform data: neck type, weight and a drawing if you have one;
• the blow molding machine model and the number of cavities;
• the planned volume.

New bottle or mold modification

If only the relief, the label area or the base changes, it is sometimes enough to rework separate parts of the existing mold, for example to replace the base insert. A new mold is made when the volume, height or overall contour of the bottle changes. The decision is made after inspecting the mold and the drawings.