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The Tab Gate
 
Definition: 1 a small removable tab of approximately the same thickness as the molded item, usually located perpendicularly to the item. The tab is used as the site for edge-gate location. 2: tab; a small appendage 3: gate; an opening allowing access or entrance
 
 
If you mold lenses and or just mold polycarbonate, acrylic or other clear material you may be acquainted with the tab gate. The basics of the tab is to allow someplace for a gate from the runner to be attached, and than an area for the material to change flow direction and finally to flow into a part or impression and hopefully avoid jetting, weld lines and or other part imperfections. In research of the tab gates in various references available it is discussed as the area of where a 3 plate mold gates into or a hot runner may gate into this. It is also discussed as an appendage to prevent jetting in the part.
 
A tab gate is composed of 3 distinct areas: the gate to the part, or part attachment, the tab itself and the gate between the runner and tab.
 
The gate between the part and tab. In many cases this area is the same height as the part and width is adjusted for flow and or volume of material into the part. The name tab should be a hint in that the area is usually large and must be clipped from the part. To aid in this clipping this area is slightly smaller than the part wall, though thought has to given to placement and detachment of this gate.
 
The gate to the tab from the runner should be located so as to be 90 degrees from the gate to the part (perpendicular). The size and volume through this gate should be calculated. This is the area where maximum shear can occur in the feed system.  The reason for the offset of 90 degrees is so as not to jet into our part. This offset allows an impingement on the side wall of the tab so as to build up a flow front.
 
The tab is the critical part as this is where we have a mass of material being injected into prior to flowing into the part impression. The height of the tab and width create a volume, which must be filled prior to flowing into the part, though the second gate area into the part / impression itself.  The idea here is to look at flow rate, if you were to conduct a shear rate calculation through the gate to the tab and compare it to the tab gate (into part /impression) the rate decreases tremendously.
 
 
Why use a tab, number one would be flow front, prevention of jetting. This is allowing the material to shear through our small gate but than due to impingement and area change create a larger mass flow into the part.  Secondary this than eliminates stress at the part as the gate to the tab is our pressure regulator it than sees the higher pressure, plus with a larger opening to the part end we may even have some suck back or shrinkage allowing undue stress in the part to escape. The first gate freezes between the runner and tab.
 
Other benefits to using a tab gate design can be color dispersion, creation of high shear point aiding in flow of material and separation of runner from a tab. Note that this does not eliminate the downside of trimming the tab from the part but the part is not attached to the runner.
 
In the use with a hot runner or 3 plate mold design some references refer to the use of a tab gate as that which is being gated into. In examples it appears to be a short runner, or as some would call them a chicken runner, i.e. not wanting to gate directly into the part. In either case it accomplishes what is necessary and that is to provide an area to reform our material into a manageable flow front to provide filling of the part with minimal defects.
 
Which ever reason you may have to using a tab gate using it correctly can prevent issues with our finished part. It is important to remember that there is one gate to our part and a second gate from our runner. What happens between them is as critical as how we design both gates themselves, and if using this method many issues can be eliminated.

SL SILVEY

360-882-3183