Cold runner sizes

It is always interesting to see simulations and other pieces of silliness used to determine optimum fill.  Simulations are great for gate location, but they aren’t very good about runner sizing.  Usually we make gates to small, runners too large, and sprues are picked off the shelf.  This lack of attention comes back to bite us in hard to fill, slow cycle molds.  But there are some useful rules of thumb:

The runner immediately next to the gate should be 150% in diameter of the part’s nominal wall.  Each branch beyond this primary runner should increase in diameter by .020 - .050 inches.  When the base of the sprue finally mates with the runner system, its diameter should be 120% larger than the runner it feeds.  The sprue then needs to taper to the machine’s nozzle but not smaller than .040 inches.  The machine nozzle’s orifice should be 20% smaller than the sprue orifice. Amorphous materials need larger runners than semicrystalline materials; filled materials need larger diameter runners than amorphous materials.  Consult your material supplier for the best gate dimensions and initial runner sizes.  Remember, you want to make fully filled out parts not over packed runners.

The longer the plastic has to travel in the runner system the more it will strangle itself by cooling.  While there are those who say they have some magic gizmo that will increase shear heat and therefore slow down the self-strangling behavior of a cooling runner; shear heating is a function of speed and distance and there really isn’t too much generated in most runner systems.  Tiny runners are usually effectively frozen off before they can fill or pack the parts.  While large runners fill easily, they end up controlling the cycle.

Consult your experts, use common sense and rules of thumb and you’ll find the balance to make a profit.  Think about it.

Contact me with questions put in the subject line QUESTIONS.
Bill Tobin, WJT Associates, E-mail: bill4012@hotmail.com