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 plastic processing there are different types of venting. The tool is the primary area for venting, and there is also the vented screw. It is the first type that shall be discussed in that though much effort is placed on getting materials into a mold, in a set period of time and all, little is done to let the air escape that is in the mold in that same period of time.
An example was a mold trial many years ago, when arriving at the plant in the late afternoon I was allowed to examine the tool (still in the tool shop) for the upcoming trial the next morning. Upon examination it was noted that there were no vents in the tool. In discussions with the owner it was suggested that vents be cut prior to testing so as to allow as much chance of success as possible. The tool maker disagreed, saying to test the tool and then they would know where to place the vents. The owner asked where would I place the vents and with the magic marker I marked them out, as a starting point. Upon arrival in the morning I inspected the tool and found vents in all areas suggested, and we were able to produce good parts and optimize the process. Upon discussions with the owner he stated that he put them in, himself after coming back to the plant after our dinner out. This action by him saved numerous hours of time, not only mine but that of the machine and operators.
Vents allow air and gases to escape from the mold cavities. The mold a precision machined block of metal, when the mold closes these ground plates than create a seal around the parting line so as to contain the plastic, and if the air is not allowed (via a vent) to escape it compresses, this compression can impede the flow of resin, can ignite (diesel) and in general cause many headaches in the production of parts.
The vent is in design the same as a gate but for the air with in the cavity / runner chamber. It consists of a land area, a height and width and then flows into an exhaust (runner) that transports the air to atmosphere.
The vent height is a critical component of the design, in that it must be large enough for air to escape but not so large as to allow plastic to flow into it. There are standards per materials for these dimensions provided by material suppliers and others. (See Vol. 5 no 9)
The other aspect of the vent is the land length, which is typically 1 to 1.5mm in length. If the land is too long than pressure shall build up within this area and close off the venting action.
The exhaust to atmosphere is the real issue in too many molds. While a tool may be vented properly the exhaust may not and the vents fail, thus leading many to increase vent depths, numbers and then to have issues with flash and other defects. The exhaust must covey the air to atmosphere. The height of the exhaust much like the runner is large, say 0.5mm to 1.5mm so that the air flow is unimpeded in it quest to atmosphere. These exhaust channels may be polished and run over inserts and the mold base to the edge (exit) of the mold base. In many cases an insert set within the mold base is proud, and the tool maker explains that it exhausts out between the base and this is true for a new mold. But over time one may notice a buildup of residue on this surface and or coining to the mold inserts which than impedes the flow and clearances leading to faster build up. This is a direct result of not enough clearance.
The above gives a general view of a vent and exhaust in the mold plate. The above view shows a green line this being the exhaust to atmosphere for the vent.
The above is an exaggerated view of a vent to exhaust, as typically these are radiused down since they are generated on a surface grinder in many cases. As one looks at the vent it can be seen that it is a defined area with a land, height and width.
to be continued;
Silveys Plastic Consulting
www.silveysplasticconsulting.com
360-882-3183
SL Silvey
10122012.01
