In order to improve quality of die casting, during process of fill cavity with metal liquid, gas in cavity should be excluded as much as possible, forward-flow cold-stained metal liquid mixed with gas and contaminated by coating residue should be excluded, which requires overflow and exhausting system.
Overflow and exhaust system can also make up for some casting defects caused by irrational design of gating system.
Design of die casting mold usually considers overflow, exhaust system and pouring system as a whole.

1) Exhaust gas in cavity, store forward cold metal liquid mixed with gas and paint residue.
2) Control flow state of molten metal to prevent local eddy currents.
3) Adjust temperature field distribution of mold to improve thermal balance of mold.
4) It is used as push-out position when die casting part is demoulded, to prevent deformation of die casting part, and to avoid trace of push-rod on the surface of die casting part.
5) Overflow groove provided on movable mold can increase tightness of die casting to movable mold, so that die casting is taken out with movable mold when mold is opened.
6) As an additional part of support, hanging, clamping or positioning during storage, transportation and processing of die castings.

 

Figures a and b show overflow grooves in fixed mold section

 

Figure c shows that trapezoidal cross section overflow groove is opened in movable mold part, which can increase clamping force of die casting to movable mold part.
Figure d shows that when a large capacity of overflow tank is required, overflow tank is opened on both sides of parting surface, and push rods are provided.
Overflow groove provided on parting surface has a simple structure, convenient processing, and is most widely used.
Cross-sectional shape of overflow groove on parting surface is generally semi-circular or trapezoidal, which is convenient for processing with a ball end mill or an end mill with a taper, and can be opened in fixed or moving mold part.

 

Figure a shows an annular overflow groove formed by a stepped core. In order to facilitate demolding, small end of core should have a larger demolding slope.
Figure b shows a cylindrical overflow groove opened at the end of core or movable mold part. A push rod is set at the bottom of overflow groove, which is conducive to pushing out die casting part and also to exhaust.

 

Figure c shows a cone-shaped overflow groove, which is similar in structure to a ring-shaped overflow groove and is easier to pull out of fixed mold.
Figure d shows a conical overflow groove opened at the end of core to exclude gas and cold metal liquid deep in cavity. At the same time, an exhaust block is added to exhaust ejection of overflow groove by push rod.

 

1) Overflow tank should be set to facilitate removal of gas in cavity, eliminate cold-stained metal liquid in front stream mixed with gas and contaminated by paint residues, and improve thermal balance of mold.
2) It shall be easy to remove overflow groove from die casting and try not to damage appearance of die casting.
3) Pay attention to avoid heat knots between overflow groove and die casting.
4) There should not be multiple overflow ports or a wide overflow port on an overflow tank to prevent metal liquid entering overflow tank from flowing back into cavity.
5) Thickness h of overflow should be smaller than thickness of gate.
6) Sum of cross-sectional areas of all overflow gates should be less than cross-sectional area of gate, and it should be maintained at about 60% -70% of cross-sectional area of in-gate to discharge the first metal.
7) Set of position of overflow tank as shown below.

 

 

 

According to flow direction and path of molten metal when filling cavity, cavity is divided into several zones, one end of each zone is entrance of molten metal, and the other end is provided with an overflow groove.
In an ideal state, molten metal flowing into a zone stays only in this zone, or enters into overflow groove provided at the other end through this zone, there is no obvious molten metal flow between zones.
Relationship between volume of overflow groove and volume of adjacent cavity area is shown in table below.

 

Relationship between volume of overflow groove and volume of adjacent cavity area

Recommended trapezoidal overflow tank dimensions are shown in table below.

 

 

During die casting production, filling speed of metal liquid is very fast, and filling time of cavity is very short. Removal of air in cavity and gas generated by volatilization of coating is an extremely important issue.
Exhaust groove is used to exhaust air and gas generated by volatilization of coating from cavity, position of exhaust groove is related to position of inner runner and flow state of metal liquid.
In order to discharge gas in cavity as much as possible by metal liquid during injection, exhaust groove should be set at the place where metal liquid is finally filled.
Exhaust groove is generally matched with overflow groove, it is set at rear end of overflow groove to enhance effect of overflow and exhaust.
Exhaust groove can also be provided separately in necessary part of cavity.

Exhaust groove provided on parting surface has a simple structure, and cross-sectional shape is generally a narrow rectangle, which is convenient for processing.
Vent groove on parting surface is flexible in setting and can be changed according to actual situation during mold test, so it is most widely used.
See following table for structure of exhaust groove on parting surface.

 

Structure of exhaust groove on parting surface
Exhaust tank dimensions are shown in table below.

 

 

 

 

Cross-sectional area of exhaust groove is generally 20% -50% of cross-sectional area of runner. It can also be calculated by following formula:
Aq = 0.00224V / tk
Aq is cross-sectional area of exhaust groove (mm2);
V is volume of cavity and overflow groove (cm3);
t is gas discharge time (s), which can be selected approximately according to filling time;
K is opening coefficient of exhaust groove during filling process, and K = 0.1-1.
When selecting K value, following factors should be considered: when die casting is small, metal liquid flow rate is low, and exhaust tank is located at the last filling place of metal liquid, K value is larger; otherwise, K value is smaller.