How to Select an Extrusion Blow Molding Machine
Work from the finished part to the machine, mold and production line. This checklist helps turn a product idea into a clear equipment brief.
ExploreTechnical guide / Part design
Designing Parts for Extrusion Blow Molding
Practical design rules for bottles, containers and technical hollow parts, so the part can be molded with an even wall and a reliable mold.
What makes a part easy to blow mold?
A part blow molds well when the parison can reach every area without over-stretching: moderate blow ratios, generous radii instead of sharp corners, enough draft at parting-line steps and cores, enough space between opposing walls, and features such as handles and necks placed where the parison can catch them.
| Feature | Guideline |
|---|---|
| Cavity–cavity sections | Depth no greater than width |
| Cavity–core sections | Width at least twice the depth |
| Parting-line steps | Draft of 10° or more |
| Faces over a core | About 5° draft for ejection |
| Shrinkage | About 1.65% at 1.5 mm wall, 1.85% at 3.2 mm wall |
| Space between walls | Parts up to 0.09 m²: about 9.5 mm at sidewalls, 4.8 mm top and bottom; larger parts: about 16 mm and 8 mm |
| Tack-offs (welded walls) | Mold gap 60–80% of the combined wall thickness |
Start from the parison, not the wall
You do not specify wall thickness directly in blow molding: you specify the shape and the part weight, and the parison wall distributes itself according to how far each area must stretch. A parison controller can thicken the parison where the part stretches most, but the design still decides how even the wall can be. Define the target weight and the minimum wall at critical points together with the supplier.
Blow ratios and depth
For a section formed between two cavity halves, keep its depth no greater than its width. Where one half forms over a core, half of the parison makes each side, so keep the width at least twice the depth. Grooves between core sections follow the same rule: groove depth no greater than rib width.
Corners, radii and ribs
Corners stretch the most and end up thinnest. Use generous radii on outside corners and avoid deep, narrow ribs. Where a rib or groove is needed for stiffness or stacking, keep it shallow relative to its width.
Draft and parting line
Smooth container walls release with little draft because the part shrinks away from the cavity. Steps at the parting line need 10° or more, and features formed over a core need about 5° on all sides. Place the parting line where flash can be trimmed cleanly and away from sealing surfaces.
Double walls and tack-offs
Double-wall parts such as panels and cases need space between the opposing walls for the parison to form. Tack-offs, where the two walls are welded together for stiffness, hold best when the mold gap is 60–80% of the combined wall thickness.
Necks, handles and closures
The neck finish is formed by the neck insert and blow pin, then sized by trimming; design it to the closure drawing. A hollow handle must sit where the parison will span it when the mold closes, so its position and width affect the parison size. Recessed label panels and a slightly pushed-up base help keep containers stable and flat.
Shrinkage and tolerances
Blow-molded polyolefins shrink after molding, and more with thicker walls. Build the shrinkage into the mold, specify only the dimensions that matter as critical, and confirm values with a trial. For troubleshooting after the first trial, see extrusion blow molding defects and fixes.
What to send with your design
Send the 3D model or drawing, nominal volume, target weight, resin grade, neck and closure drawings, handle and label areas, critical dimensions, and the drop, stacking or leak tests the part must pass. This lets the supplier check moldability and propose the machine and mold layout.
Part design FAQ
What is the blow ratio in blow molding?
The relation between how deep a section of the cavity is and how wide it is. For parts formed between two cavity halves, keep the depth no greater than the width; for parts formed over a core, keep the width at least twice the depth. Deeper sections stretch the material too far and leave thin walls.
What draft angle does a blow-molded part need?
Smooth containers release easily with little draft, but steps at the mold parting line are generally given 10° or more, and parts formed over a core need about 5° on all core faces so they can be ejected.
How much does a blow-molded HDPE part shrink?
Shrinkage depends on resin and wall thickness. One design guide gives about 1.65% for a 1.5 mm (0.060 in) wall and about 1.85% for a 3.2 mm (0.125 in) wall. Confirm the value with the resin supplier and the mold maker.
Why are sharp corners a problem in blow molding?
Corners are where the parison has to stretch furthest, so they end up thinnest and weakest. Generous radii keep the wall more even and improve top-load and drop performance.
What should I send a blow molding supplier with my design?
The 3D model or drawing, nominal volume, target weight, resin, neck and closure details, handle, critical dimensions, and the drop, stacking or leak tests the part must pass.
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