Definition: Post: an upright shaft which is adjoined to the surface of the part, an inside the part tab gate for use with tunnel or submarine gate design.

Tunnel/submarine gate: a type of edge gate located below parting line of mold, which also is self degating from the part.

 

The gate as it is known comes in many variations and types, and its primary focus or function is to allow plastic material to flow into the part, and then to act as a dam to freeze first, thus preventing out flow of material from the cavity. In the design of part, one of the issues with a finished part is what type of gate was used, and is there a vestige and or protrusion which now has to be trimmed. An example is a tab gate, which typically is trimmed from the edge of the part.

 

With tunnel gates, and or submarine gates one can have a vestige and self-trimming gate all in one, in that the gate location is typically below the parting line surface. If one wanted no vestige, or possibly to eliminate a blush issue one could use this type of gating but now with the use of a shorten ejector pin / or creating a D pin (post), now gating into the post which shall now act as a tab gate but to the inside surface of the part. Multiple issues may now become apparent, the gating is now deeper in the mold base, though still self degating, but now we have a longer flow of material up the post to the part, and finally we have to trim this since it protrudes the height of the part wall surface.

 

The last part trimming is the issue, by using a post we now have to decide how to trim, or not, and multiple issues arise. Does the post need to trimmed flush to part surface? Can there be a variance in post height if not cut flush? Is there an allowance for remnants of the cut? Does anything fit into the inside of the part which the post interferes with?

 

While these are all considerations the fact of the matter with the post is the location, and then how are to actually trim this. The first question to be answered is does the post have to be flush cut? If yes than location and how to cut become a major concern. If no than how much height to the post can be left, and what is the tolerance are the questions to ask, and location is now not as critical.

 

In most cases the post dependent of material may be a D shaped design which might allow for bending and breakage. It may also be round or of some oval shape, but the fact remains that in most cases it has to be cut off. If cut than a pair of cutters may work, but now comes the issue of how to get them inside the part to cut that gate, and further how do to make it consistent, is there room?

 

Given the complexities of the tunnel gate into a post, the post is usually located close to the wall of the part; this is just because of angles and distance. The problem with that is that in some cases the pin now touches or adjoins to the side wall, and this makes it hard to trim correctly, (flush cut) as now the wall and post are joined, so two planes are involved. This than creates two cuts and should be avoided. Also if a non-flush cut is allowed is the height enough to clear the side wall contact?

 

The other issues with these types of pins are that the complexities inside the part may not allow clear access to the post and inside plane of the part to cut flush or to a height. There may be a rib, or side wall, or other detail in the general area, thus making it more difficult to trim or cut.

 

The solutions could be to allow for a height on the pin, or tolerance such that it has to be below the side wall height. The other would be that placement was reconsidered and moved to an area that was non critical, easier to trim, or even changed to a hot runner which now eliminated all trimming. Another route that can be taken is to use a mechanical means such as a router, or end mill like device to trim off the post. The downside of this type of trimming is the dust and debris that are generated, and must be considered.

 

The critical eliminate of this was the gate location, and in this case of the use of a post to accept the tunnel gate, one than also has to consider how the trimming/ removal of the post is to be done at the design stage.

 

Illustrations:

 

 

 

As the above shows the post is close to the walls though not touching, but the rib area in front of the post now impedes the easy removal of the post to a flush cut. Small delicate cutters would need to be used if a flush cut was necessary.

 

 

And if one were to draft inside wall, we now have contact to the post, and how is one to trim flush?