Modeling Injection Molding Witness Line 2

[uwbsme]

I am trying to 3D model a witness line in an injection molded plastic part that is shaped like half of a sphere. Average thickness is 1/8" (Acrylic DS or Polycarbonate). We want to have a mold with interchangeable inserts, but are worried about the witness line affecting our part photometrically. What is an average dimension (cut or boss) that I can rotate around my circular part where the insert and base mold meet? Will the witness line be on the cavity side --> ) or the outer surface -->( or both? Not too familiar with injection molding, sorry.

Let me know if you need more information.

 

[HDS]

Interchangeable inserts can be a very good idea and have worked well for me on several parts.

The geometry of the witness line will vary based on construction of the mold the texture of the mold and characteristics of the material. I doubt you can accurately model it. Your best bet is to look at the witness lines of ejectors or other mold features in similar parts and make a judgment based on real parts.

The best plan is to design in a feature to hide the witness line. For example if the insert is for a logo put a ring around it so the witness line is in the ring or on the edge of it.

 

[dgallup]

Best thing is to decide how much of a surface defect your design can tolerate, then specify that in a note on the drawing. If the tool maker knows what he has to accomplish up front he can take that into account. It is possible to have the inserts almost invisible but it will cost more.