Definition: Vent: 1. to exhaust unwanted air or fluid from an area. 2. A passageway between the mold cavity and outside edge of the mold face which allows air to escape as molten plastic is injected into the cavity.
In the previous part the designs shown were for vents on the parting line of the mold, to release the compressed air in the cavity that is displaced by the filling of the cavity with plastic. This air can compress and impede material flow into the cavity, may compress and ignite (dieseling) and if trap at flow fronts decrease the strength of weld and meld lines in the part.
Other types of venting include the use of ejector pins, inserts, dummy pins, porous materials, and vacuum venting.
In venting around ejector pins the clearance may have to be tighter than what one uses for the parting line, and the exhaust clearance needs to be cut into the mold, and or one side of the pin. In cutting into one side of the pin it is important to not cut too far as to cause the pin to wobble when extended, and to be held in place via the remaining radius on the pin. The land length is critical and in the ejector pin acts as the bearing surface for the ejector point to ride in and can commonly be referred to as self-cleaning as pin moves up and down any excess residue produce is moved aside. This can also be an issue if this residue is deposited onto the cavity side and then is incorporated into the next shot.
Dummy Pins are that in that a hole is drilled and a pin inserted with a vent incorporated into this pin. Since the pin does not move one issue tends to be build up which after a time can clough the vent rendering it ineffective and causing problems. It is important in cleaning of the pin to notice where the buildup is, and how many cycles. It should never be that it is not cleaned between runs because no issues were observed, but taken out and inspected as to residue build up, cleaned and put back.
Between inserts, a vent can be cut as one would cut it on the surface but possibly starting thinner and with a larger exhaust. This is a great way to aid in getting out of gas and volatiles in the mold. Sometimes in blind corners or details that are complex that are made with inserts the fit to the mating piece allows for this. The exhaust is especially critical as somehow there has to be an open path to atmosphere which is not impeded as to allow for an easy flow of air.
The use of a porous material such as Porcerax*** is needed to allow details to be molded as there is no way to place a vent in the area. This material is a sinter material thus it is porous, allows air to pass through it. The main issue with these materials can be maintenance, as they well clough. Since the flow paths are very small pressure build up can occur and gaseous material with much residue can build up within the material blocking the air flow. Cleaning is typically done in a bath with chemicals and or solvents for the materials, and special design features for the piece which allows it to accept an airline for cleaning purposes.
In all the above ways to vent the exhaust has to be to atmosphere in each one. While the vent is critical as to allow only air to escape and no plastic, that air must be able to get to atmosphere. So a path must exist from the vent area to outside the mold.
Vacuum venting is a process in which a vacuum is pulled on the cavity. There are a few suppliers of these systems, and CAE has supplied the MoldVac® for many years. The issue is the sealing of the cavity, which can be done with an O-ring and or using a larger volume pump / tank dependent on the system used. Some mold suppliers such as Cumsa has a mold mounted system to pull a vacuum on the cavities, which uses a venturi method to pull this.
In any of these system it becomes an issue as to where to pull the vacuum from, meaning through the ejector pins, though a venting area on the parting line or some other point. The seal is also important, but must be weighed against the volume and efficiency of the amount /volume of vacuum pulled.
To be continued...
MoldVac® is a registered trade mark of CAE Services Corporation Batavia, IL 60510
*** International Mold Steel, Inc. is a licensed distributor of Porcerax II, a SINTOKOGIO,LTD. product.
Contact information
International Mold Steel Inc.
(859) 342-6000 | www.imsteel.com
www.silveysplasticconsulting.com
19122012.01
