# Cold Runner Design: Attention to Details, Part 3

### Sizing Up Your Runner System

The topic of runner sizing always comes up during our plastics education courses. Everyone wants a magic formula for sizing every runner for every mold regardless of machine capabilities, pressure to fill the part, cycle time requirements and packing. So let’s take a look at various methods people use for sizing runner systems.

Consider the mold layout shown in Figure 1. The first question we ask is “should each diameter of each leg of the runner system be the same, or should the diameters change at each leg of the runner?” The typical response is that they should change diameter at every branch. Then the question is asked “why?” The response is typically either a pondering look or “that’s the way we always do it”. NOTE: for discussion purposes we will call a geometrically balanced runner that uses different sizes at each progressive branch a “graduated” runner.

Figure 1

The discussion then dives into the various methods people use for sizing the progressive runner branches of a graduated runner system. Numerous methods emerge that include use of various formulas, rules of thumb, mold filling simulation, and whatever the next common cutter size is. Each method has some theory behind it and they may all work for a given mold. But is the science behind each theory true? If so, is it important to the part and/or process? Which method is best? To throw a curve into the discussion, have you ever considered making the runner branches all the same size? What would the benefits and downsides be of using a constant diameter runner system?

Figure 2 below is a table of two different runner sizing approaches for the same mold as shown. In these examples the minimum diameter of 0.150” was selected to help assure good packing. This requires that the graduated runner branches become larger as they progress back to the sprue. The result is that the graduated runner has significantly more volume than the constant diameter runner, and the graduated runner may in fact increase the overall cycle time due to its size.

Figure 2

Refer back to Tech Tip #6 on runner shapes and consider the potential loss (or gain) in profits when you couple both the runner shape and runner sizing methods used in your molds. The table in Figure 3 compares an engineered graduated runner system where the total runner volume is nearly equal to that of the constant diameter runner system. Yet again it is possible that the graduated runner system would increase the required cooling time along with introducing potential packing problems.

Figure 3

The question now becomes “which of the numerous runner sizing techniques works on every mold, material, and part?”. The answer: NONE. When sizing a runner system you need to ask yourself “what’s important to you?”. Below are five common replies to that question, and most people list several of them together for a given mold.

1. Material volume
2. Cycle time
3. Maximum injection pressure
4. Packing