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Injection molding runner system

For a good mold flow analysis you must manually select and calculate as follow:
Runner geometry, the ideal runner cross section is circular as this ensures favorable melt flow and cooling.
Runner dimensions, the diameter of a runner highly depends on its length in addition to the part volume, part flow length, machine capacity, and gate size. Generally they must never be smaller than the largest wall thickness of the plastic parts and usually lie within the range 3 mm to 15 mm.

Initially runner diameters can be calculated with the this formula
D= w^1/2 x L^1/4/3.7
Where
D= runner Diameter (mm),
W= part weight (g),
L= runner length (mm).
If the material has glass fiber and carbon fiber reinforced LNP, composites. The cold runner sizes have to be 30% to 60 % larger than a runner for an unfilled material. An alternative way for determining the runner dimensions is Reverse Engineering.

Rule of thumb
  1. 1.5 x by the thickest section of the part in this case it is 3 mm.
  2. Add 20% to each leg or turn only if the material is unfiled add 30 to 60% only if the material has filling in this case we have big steel inserts + Glass+ crystalline for this reason a big diameter runners system is better.
  3. For each intersection use a big radius for filled material .25," for unfiled material use small radius 1 to 2mm it will minimize shear.
  4. For sizing the orifice use .5" foot for taper or 1 ° to 2 ° each side use the primary runner diameter as follow
If the primary runner diameter is .5" and 6 long and if we use a .5 in/ft taper. The orifice size will be = .25" Max standard drill. If the calculated value is not a standard size drill, the mold maker has to pick a standard drill size. For example if the value is .2475" the standard drill will be .25".

The level of glass fiber and carbon fiber reinforced LNP* composites will cool faster than unfilled compounds due to the higher thermal diffusivity of these materials. The sprue or runner system must never control the cooling phase and/or overall molding cycle time. Also the dimensions of the sprue depend primarily on the dimensions of the injection molded part in particular the wall thickness. As a guideline:
The sprue must not freeze before any other cross section in order to permit sufficient transmission of holding pressure and must be as short as possible.

At all runner intersections there should be a cold slug well. The cold slug well helps the flow of material through the injection molding runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity. The length of the well is usually equal to or greater than the runner diameter and this is achieved by extending the length of the primary runner at the intersection with the secondary runner.
Performance Sprue Bushings feature a high-conductivity, corrosion-resistant, copper-based alloy bushing body with a hardened 420 stainless steel nozzle seat insert. Performance Sprue Bushings reduce sprue cooling time and cycle time. The product's stainless steel nozzle seat provides wear resistance and insulates to reduce transference of heat from machine nozzle to the sprue area.

If you are using a cold runner system with and edge gate The dimensions of the gates are very critical land length .5mm to 1mm, cold will extension 1 x runner diameter (min). Gate with 2 to 3 times gate thickness, gate depth 60% to 100% of wall thickness. A good venting system is a big part of the plastic injection molding process.

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