In venting air out of closed cavity molds, one must understand the way plastic flows into such cavities and how air becomes trapped. Air entrapment, viscosity, and pot life are all related and must be considered prior to pouring. The distance a plastic will travel horizontally before falling vertically will determine what size detail pockets must be vented.

A rule of thumb is to vent all pockets with vertical walls with wall angles of 45 degrees or more from horizontal in the intersection of vertical and horizontal, vents should be 3/4" to 1", apart at all inside and outside corners. Also, all pockets with wall angles of 45 degrees or less at the highest point of the cavity on horizontal must be vented.

It also must be understood that plastic will flow in all directions simultaneously while choosing the path of least resistance. This can cause venting problems around bosses, so any free standing boss should be vented completely around where the boss meets the horizontal. This happens when the crest of the plastic flow is divided to form two crests, and because this material meets tangically, air is often entrapped at the meeting.

Another common venting problem is when air becomes trapped within the plastic and rises and gathers in areas that are horizontal and unvented. This problem can be eliminated in three ways; (1) Vacuum or de-gas the plastic prior to pouring, (2) Vent broad flat areas (however, unless these vents are extremely close together, it may not solve the problem), and (3) Most widely accepted is to tilt the mold so all entrapped air may rise to a common point that is vented. High spots of the mold must be vented, but these areas may change as the mold is tilted.

A physical law is that lower density materials will float on higher density materials. Air, being of extremely low density will always rise above liquid plastic. So for best surface quality, always pour closed cavity molds so air can rise to backing or parting line.
Let us turn our attention to venting changes and additions. Whenever possible, this type of mold should be poured from the back of the pattern or corebox and vented the same way. This solves two problems; (1) It gives a mechanical lock for the repair and (2) Any air entrapped will not be at the face, but underneath at the backing.

However, in some cases, the pattern is too thick to be drilled through, so other means must be used. The mold must be vented through the top which will necessitate removing and finishing the vents. This type of mold still should be poured from the lowest point.. These types of pours may also utilize the "tilt" method as described above so that venting can be kept to a minimum. The tilt should eliminate as many 180 degree surfaces as possible.

In summarizing, the point to remember when venting is that air rises, so by eliminating horizontal impasses by changing the angle or creating an escape route for the rising air, venting should become less of a task.