Cold Runner and Gate system

 

Overview:

 

Function:

Cold-Runner Systems:

1	Sprue (canonical Shape for easy removal, 3-5 ° angle) ; connection between Nozzle and Runners; highly polished
2	Runners
3	Gates

 

• The gate channel serves to convey the melt coming from the nozzle of the injection molding machine to the cavity with:

1	the lowest possible pressure and heat loss,
2	in shortest possible time and
3	without thermal degradation.
4	In multi-cavity molds, the melt must be supplied to all gates uniformly

 Gate types:

 

 

Risk:

 

• location of the gate is the weakest spot (apart from weld lines)
• not balanced filling
• Spiral effect: 11, 14, 19, 22 will be filled First and mayshow splays and Mold Deposits

 

• wrong position of the gate on the molded part causes the formation of weld lines (meeting of melt fronts) and air traps

 

 

Design:

 

Key rules:

1. runners should stay open until all cavities are properly filled and packed;
2. runners should be large enough for adequate flow, minimum pressure loss and no overheating;
3. runner size and length should be kept to the minimum consistent with previous guidelines.

4. gate should be located in the thickest wall of the part in order to make it possible to pack and compensate for shrinkage by using hold pressure

5. position the gate so that the air will be swept toward a parting line or ejector pin (where conventional vents can be located)

 

Runner balancing:

1. the pressure loss in all gate paths is kept equally by “natural balancing” (equal cavities), but leads to longer gate paths, greater material losses and to larger molds
2. Alternative different cross-sections , but fine-tuning needed.

 

Design rules for runner:

 

	Thickness cavity = T
B	= T + 1mm
C	= T + 2mm
D	= T + 3mm
E	= T + 5mm
	Nozzle diameter = 1 mm less than the diameter of the sprue

•  The minimum recommended runner diameter for most materials is 1.5 mm
• For most materials, the runner surface and sprue should be polished in the line of material flow.
• Cavities and cores should be polished in the line of draw unless an alternative finish has been specified.
•  It is desirable to have multiple sprue pullers and ejection locations in extended runner systems. 
•  When designing hot runner systems, it is advisable to consult the suppliers for availability and recommendations for the correct manifold and drop

 

Gate design Rules:

• gate cross-section is typically smaller than that of the runner and the part, so that the part can be easily de-gated

• start with a fairly small gate since the gate can be increased with EDM using a slightly larger electrode if necessary

• The gate length should be as short as possible to avoid an excessive pressure drop across the gate. A suitable gate length ranges from 1 mm to 1.5 mm (0.04–0.06 in)

• Resin suppliers provide usually info about gate design