Blow Molding Part Design

Except for injection molding, there are not extremely specific design guidelines to follow for any of the plastic manufacturing processes. Blow molding is not an exception. This discussion will present a few general best practice rules.

Blow Molded Parts are Double Walled

First, the designer should think of a blow molded part as double-walled (as opposed to injection molding, where single walls are used). There are many structural advantages to double walled designs, since a much higher moment of inertia is achievable without increasing wall thickness.

Use Draft

As with any process where the part has to be removed from a mold, all surfaces perpendicular to the mold opening direction must be drafted. Click here for the draft section for injection molding (it is not all relevant to blow molding, but the concept is explained). For blow molding, 1° draft is minimum, 2° is recommended.

Layout Part as Hollow

Remember that blow molding makes hollow parts. Layout the part a hollow, where the outside surface is the part.

Watch for Irregular Geometry

Look for geometry that will cause excessively high blow ratios, resulting in thin sections. Blow ratio is a measure of the amount of stretch the plastic will see as it is blown into the part’s shape. Click here for information on calculating blow ratio.

Use Radii on all Sharps

Consistent with good part design, use radii on all sharp edges and angles. Click here for more information on radii (it is not all relevant to blow molding, but the concept is explained). In blow molding, sharp corners cause the parison to stretch excessively, causing a thin section. And since stress concentrates in corners, a thin corner will be very weak. For blow molding, recommended radii is 2 times the wall thickness.

Use Corrugated Surfaces on Flat Surfaces

As with rotomolding and thermoforming, is recommended to use a corrugated (ribbed) design on large, flat surfaces. It will increase stiffness and reduce warpage.

Symmetrical Parts are Better

Try to design the part as symmetric as possible. If the part is not symmetric, uneven stretching will occur during the blowing phase, resulting in a large variation in wall thickness.