Sidewalls & Draft
Half of the parison is draped over the mold core to form the interior of the part when the mold closes. As the plastic cools, it shrinks onto the metal mass of the mold core. Positive draft is needed on all sides of the mold core in order to remove the plastic part after it has shrunk. The more generous the draft, the easier the part can be removed from the mold. If a core design requires a no-draft or back-draft section, a positive draft should be provided on the opposite side of the core, if possible.

A part with 5°-positive draft on all sides of the core can be easily removed with the assistance of ejector pins. Parts with lesser draft can also be removed with ejector pin assistance but as draft on the core decreases, the risk of damaging the part during ejection increases. Snap-fits and small undercuts can be fine-tuned to allow ejection.

With core-cavity molds, the parison becomes fixed at two levels, the top of the core and the pinch-off. When the part is blown, this trapped plastic wall stretches (no flow) to meet the sidewall of the core. A deep core with little draft and a sharp corner will produce a thin, weak-walled part. This characteristic is similar to the corner thinning discussed in cavity sidewall & draft design and can be resolved in much the same way.