Sidewalls & Draft
The plastic parison stops and begins to solidify as soon as it hits the mold. The material will stretch to fill the cavity as blowing progresses. There is no flow of material along the mold walls. The thinning caused by stretching along the mold wall will result in weakness and is susceptible to further thinning. The apparent rigidity (or strength) of any area on the part varies proportionately with the square of the wall thickness. Because of this, any cavity design which utilizes sharp 90° corners will result in parts with extremely thin, weak corners. Plastic contouring of heavy sections into these critical areas can improve the condition but not eliminate it. There are a variety of corner configurations that improve or alleviate this problem. The most common approaches are angling the sidewall and putting a radius or an angle at the corner.

This thinning along sidewalls and in corners is the reason that parts should have outside draft angles. Exterior draft is not critical to part removal from a cavity since the parts shrink away from the outer mold walls as the plastic cools. Draft is recommended when exterior walls are to be textured.

Part removal may be a problem with back-draft sections. Back-drafted areas can lock the part in the mold. If possible, a part with back draft on one side should have an equal positive draft on the opposite side. Thus, a part with a 15° back draft on one side and a 15° positive draft on the other side can be removed like a part with no draft. Otherwise, molds will need moving sections to remove the back drafted feature.