Dr. Joseph Greene Copyright 2000 all rights reserved Venting Chapter 11

Dr. Joseph Greene Copyright 2000 all rights reserved Venting Background and Theory Definitions and Rules Parting Line Venting Vent Pins and Insert Venting Miscellaneous Venting

Dr. Joseph Greene Copyright 2000 all rights reserved Background and Theory Theory –Air inside cavity must be allowed to escape to that the in rushing plastic can fill the whole space. –Air or any gas follows simple law Pressure x Volume = Constant x Temperature –PV=nRTwhere n is the number of moles and R is the gas constant –As pressure increases, the temperature increases –As pressure decreases, the temperature decreases –Standard pressure (atmospheric pressure) is 1 atmosphere or psi. For example, –Volume of the cavity is 100 cm 3, and the pressure is 1 atm and the temp is room temperature. –If the cavity is not vented then the 100 cm3 of air is compressed to less than 1cm 3 and heats up because of PV=nRT. –The result is burning of the edges

Dr. Joseph Greene Copyright 2000 all rights reserved Definitions and Rules Definitions –Vent: The clearance (depth) between two surfaces through which air escapes. –Land: Distance air travels through vent. –Vent Groove; Collector close to the cavity space into which the vents lead. –Vent Channel: Channel connecting the vent groove to the outside. –Natural Vents: A vent created by the clearance between the mold parts (e.g., between an ejector pin and the core) Rules –Provide as large a vent as is practical for the plastic injected and for the injection pressure anticipated to create minimum resistance for the air to escape, but not large enough for the plastic to enter the gap and flashing. –Place a vent at the end of all plastic flow paths anticipated and at weld lines –Cleaning of vents is important Parting lines vents can be wiped clean as mold opens Moving vents by ejector pins are self cleaning

Dr. Joseph Greene Copyright 2000 all rights reserved Venting of Bull Buddies Part

Dr. Joseph Greene Copyright 2000 all rights reserved Venting of Plastic Part

Dr. Joseph Greene Copyright 2000 all rights reserved Parting Line Venting Continuous vent (Fig 11.2B) is best vent –Guarantees the maximum vent area through which air can escape. –Disadvantage is a certain amount of loss of support at closed mold. Spot venting (Fig 11.2A) may be used to vent in areas where continuous vent doesn’t work or difficult to make. –Spot vents can be directly connected to a vent channel leading to the outside (Fig 11.4A) –Shape of the vent channels and grooves should be trapezoidal or rounded to prevent plastic from catching within. –Trapezoidal shape will be affected less from repeated grinding of channel during repairs than a rounded shape.

Dr. Joseph Greene Copyright 2000 all rights reserved Vent Channels and Grooves Size of vent channels and grooves depends upon –The volume of plastic entering the cavity and the speed of injection –Vent groove must not be too deep –Vent grove must not be too close to surface or could weaken steel Channel cross section –should be at least as much as the total cross section of the vent leading into that portion of the vent from which the channel leads to the outside. Dimensions –Depth: mm depending on plastic –Land: 1.0, 1.5, or 2.0 mm depending on size of cavity –Width (spot vents): 3mm minimum, 5-6 mm preferably

Dr. Joseph Greene Copyright 2000 all rights reserved Vent Channels and Grooves Dimensions Depth: mm depending on plastic Land: 1.0, 1.5, or 2.0 mm depending on size of cavity Width (spot vents): 3mm minimum, 5-6 mm preferably –Example, Select vent for a round cavity with a 200mm Diameter with fast injection –Vent depth: 0.01, Total vent area: 200x(pi)x0.01mm= 6.28mm 2 –Groove: W = 5mm, DG=0.8mm, area = 3.64 mm 2 –Channel: W= 5mm, DC=1.0mm, area = 3.40mm 2 –Number of channels required: minimum 2 (2x3.4=6.80 >6.28); 4 or 8 are better –Table 11.1

Dr. Joseph Greene Copyright 2000 all rights reserved Vent Pins and Insert Venting Vent Pins –Standard design for vent pins has a light push fit (clearance from to mm on Diameter) in the portion of the bore below the vent groove to ensure that vent is concentric. Fig 11.9 Insert Venting –Necessary to ensure filling of any deep rib created with insert Fig –If rib’s depth is more than 1.5 times its width, vents are needed. –Vent groove and vent channel dimensions should be as large as possible and consistent with size of insert. –Vent channel can be one or more drilled holes or created chamfer –Vent dimensions of of Fig are suitable where vent is in line with approaching plastic flow.

Dr. Joseph Greene Copyright 2000 all rights reserved Vent Pins and Insert Venting Runner Venting –Added at end of cold slug and allows some of the air pushed by advancing plastic to escape before it reaches the gates. Fig –Vents should be same size as for parting line vents. Venting the Bottom of a Cavity –Vents some of the air ahead of in-rushing plastic stream, similar to venting of runners. –Vent also breaks vacuum from filling and helps open the mold –Fig (left): vent is in line with the plastic flow –Fig (right): vent is at right angles and could be larger The hole must be large enough not to restrict rapid venting. Vent Runner Gate Vent groove Vent channel