Cold Runner Design – Attention to Details, Part 2

Whenever designing a cold (solidifying) runner system, a decision must be made as to the shape of the runner channels. The most common options for runner shapes are shown in Figure 1. In terms of the preferred shapes, start on the left side and work your way to the right. However, it is not recommended to go much further than the trapezoid shape. The other three runner shapes are inefficient, have no value and will waste a great deal of plastic and energy.

The full round runner shape is the most efficient, which simply means that it provides for the lowest pressure drop given the same volume of plastic. The main disadvantage of the full round runner is that the mold maker has to machine the runner into both halves of the mold. Along with that, the two halves must closely align to avoid mismatch which could result in filling variations. Due to the additional machining and alignment concerns, designers and mold makers often prefer to use either a parabolic or trapezoid shape.

Figure 1

There are also cases when a designer used a full round runner shape in the mold design, but the mold maker took it upon himself to change it to a parabolic runner for whatever reason. The thought being that the mold will still fill, right? So what’s the big deal?

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 Figure 2.

Figure 2

This looks harmless enough, but when doing the math and calculating the additional runner volume (indicated in the red areas in Figure 2) you will find that you just added about 20% 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 8-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%, thus costing you even more money.

Many times it’s the little decisions we make that eat away at the profitability of a company, and often times we don’t give them a second thought. As with anything, the devil is in the details.