Let’s take a closer look. It is common practice that when you switch from a full round to a parabolic runner shape that you simply use the diameter of the full round runner as the depth and diameter of the parabolic shape and then add draft (10° per side is common). This is illustrated in the figure below.
This looks harmless enough, but when doing the math and calculating the additional runner volume (indicated in the red areas) you will find that you just added about 20 percent more material into your runner volume. But it is only a runner, so the overall volume increase may be small in comparison to the total shot. As such, the increase is probably only fractions of a penny in terms of material costs. So again, what’s the big deal?
Consider the following example:
For a given eight-cavity cold runner design running 640k shots per year using a material that costs $2.10/lb, the parabolic runner will end up costing an additional $1,868 per year. Some may still think that it is not a big deal. But now consider you are running this mold for five years, so your additional out of pocket expense is nearly $10,000. Now consider that people are creatures of habit and typically do the same thing over and over again. If the same thought process is done on 50 mold builds per year, your additional cost is now running close to $500,000.
Pennies can become dollars very quickly, and those dollars are coming directly out of your profits all because someone didn’t consider the details. Additionally, if you switched to a trapezoid runner the increase in material jumps to 27 percent, thus costing you even more money.