1.The burn marks ,underfilled areas(short shot) or other cosmetics defects,which resulted from the compressed air between the flow front in the knit-lines or at the end of flow(the last filled area), is visible on the part surface when the mold has been poor vented or the vents have not been designed properly. Two major types vents used:Individual Vents: Individual part vents are placed around the cavity.
Perimeter Venting: Perimeter vents run around the entire parting line, which then vents to open air.

2. Vent depth

Polymer	EasyFlow	Stiff Flow orGlass Filled
	Depth of vent (inches)
ABS SAN HIPS	0.002	0.003
Acetyl	0.001	0.0015
Acrylic	0.002	0.003
Cellulose/acetate/butyrate	0.001	0.0015
Nylon 6/6	0.0005	–
Nylon 6/6, 13% glass		0.0015
Polyallomers	0.0005	0.001
Polycarbonate	0.0015	0.003
Polyethylene	0.001	0.002
Polypropylene	0.001	0.002
Polyphenylene oxide	0.001	0.003
Polyphenylene sulfide	0.0005	–
Polysulfone	0.001	–
Crystalline Polystyrene	0.001	0.002
Polyvinylchloride (rigid)	0.001	0.003
Polyvinylchloride (plasticized)	0.001	0.002

3.Runner venting

4.Vent the core pins and ejector sleeve

5. Vent the ejector pins

6.Minimizing the fitting length/area between A and B block

7.Porous Mold Steel