Venting

The displaced air in the mold cavity must be able to escape during mould filling process. If there are insufficient vents compression of air may take place. The pressure and local temperature rise quickly, potentially causing incomplete filling or even burning of the thermoplastic.

The number of vents is often limited by mold construction economics but ideally should be taken into account during the design stage. In general the higher the viscosity of a material the larger the vent dimensions. As with gates vents should be cut “steel safe” i.e. start at the minimum dimension and open the vent up gradually until the optimum molding is achieved. Too small and the vent will clog up and reduce or eliminate the ability to expel air from the mold cavity; too large and flash may be seen on the moldings.

Dimensions of venting channels can be read from the figure below.  The dimensions are chosen in such a way that air can escape without flash.

Construction of a venting channel.

Vents can be placed anywhere along the parting line in particular they should be located in areas that are the last to fill in particular section of the mold.  A reasonable spacing is every 25mm. If the air is trapped with no way out to the mold parting line, it is advisable to place a venting pin/ejector pin to permit the air escape through the clearance between pin and hole. Another option is to use sintered metal inserts, these inserts allow gas to pass into them without clogging up with the polymer. These inserts should only be used as a last resort and only on non-visual surfaces.

Venting locations.

To summarize, inadequate venting may result in various molding failures:

• burnt spot
• weak and visible weld lines
• poor surface finish
• poor mechanical properties
• incomplete filling, especially in thin sections
• irregular dimensions
• local corrosion of the mold cavity surface