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.
The following are some issues which are summarized that may result or show themselves due to vent design and or exhaust design deficiencies.
Black or burn marks on the plastic and or tool surface (Diesel effect);
These are caused by the lack of venting, and or that the vents are inadequate to accommodate the volume of air trying to escape versus the speed of the volume of plastic material being injected into the tool. The results which are usually a black mark, charring, both on the part and in the tool (on the metal of the mold), and one may even smell a burning on the plastic. If charring occurs in the tool the solution to the problem must be resolve quickly as continuous production can lead to pitting of the tool steel, and or a permanent mark in the tool which is then transferred to the plastic even after the issue has be fixed.
If one were to slow down the injection rate (speed of fill) and this issue goes away, than it may be that the vent is there but something within the vent system is inadequate, or if no vent the air has time to escape, dissipate to other locations, and or the fill pattern within the tool changes completely. Though slowing down injection rate, is a common solution to the issue it should only be considered a temporary fix, as time has been added to the overall cycle time.
If one slows down the rate of injection and the problem remains than vents, vent size need to be explored, along with possible clamp tonnage in that it is possible to collapse vents due to too high of clamp tonnage and design of shut off.
Buildup, plate out
On certain materials the issue of buildup and plate out of particulate is noticed. This may also be a result of not enough of a vent and or exhaust system. If the issue is first noticed within the cavity than venting itself should be explored, while if first noticed on the surface of the mold but not to the edge of the tool than exhaust should be looked at.
Sometimes this build up can be the direct result of too cold of a mold surface.
Sometimes this issue is one that takes time to show, and could be one hour or longer, and sometimes it is resolved by cleaning the mold face once every shift or more frequently.
Other defects that have direct/ indirect links to venting:
Porosity in the parts
Blisters
Sink marks
Dull spots
Heightened or prominent weld and or meld lines
In many cases the troubleshooting of issues with suspected venting issues follows certain steps;
1- Slow down the rate of injection
2-loosen up the clamp force
3-add/place some sort of shim to the surface of the mold
When any and all of these work production continues and everyone forgets about resolving the issue since it appears and is fixed for a limited time. Long term one should note what was done, and that a tooling issue (venting) need be looked at when the mold is pulled, or some convenient time during its production run.
In summary venting is a simple: The air inside the cavity space / runner must be allowed to escape so that the plastic being injected can replace it and fill this space. The difficulties lie in the rate of fill-exhaust of the plastic to air and that adequate pressure must be kept on the mold to keep it closed, and vent areas must be adequate to allow only the air to escape and not the plastic.
Note*** with some materials that are loaded with flame retardants, it has been found to reduce the vent size to prevent what is called juicing. Always check with the material suppliers and their technical people as to their recommendations especially if there is no local knowledge of said material and venting specifications.
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21012013.01
