A part that can be made with a straight pull mold has all its features designed so that when the two halves of the mold pull straight away from each other. Undercuts on the part require mold pieces to pull out sideways, perpendicular to the direction of pull. These are called side actions.

Rib Design

Avoid deep & thin ribs if possible

Deep ribs require draft and clearance in order to allow the milling tool access. The max tool lengths average between 8-12 times the tool diameters.

Also worth noting is that deep, thin ribs tend to increase the mold milling time and make hand polishing difficult and time consuming.
 
Draft
  
Include sufficient draft
  
Draft facilitates the removal of the part from the mold. The guidelines associated with the number of degree of draft required will vary with geometry and other part characteristics (e.g. surface texture requirements), but in general the more the better.
Here are some rough guidelines to follow:
• We strongly advise using at least 0.5 degrees on all "vertical" faces.
• 2 degrees works very well in most designs.
• 3 degrees is recommended for a shutoff (metal sliding on metal).
• 3 degrees is preferred for light texture.
• 5 or more degrees is suggested for heavy texture.