The most common defects we encounter are flash and short shots. I would think that these two defects would be complete opposites, so why do we always see both?

I agree that these two defects sound mutually exclusive… the part has too much material with flash, and too little material with short shots. Surprisingly, when these two defects are seen together… it often indicates a poorly established fill, and I’ll explain why.

When establishing a molding process… If the part is completely full during first stage, then the process cannot compensate for variability. A drop in material viscosity will cause too much material to enter the mold during fill, causing flash. A rise in material viscosity will cause too little material to enter the mold cavity, causing shorts. 

When the process is established with a short, then 2nd stage pack is used to fill out the part. any typical variation in viscosity during fill will then be compensated by pack.

The most solid and reliable processes are short during first stage fill… and then packed out during second stage.

-Andy