Attention to Details

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 the Figure above. 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 moldmaker 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 moldmakers often prefer to use either a parabolic or trapezoid shape.

There are also cases when a designer used a full round runner shape in the mold design, but the moldmaker 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? Read more here.