Die castings with complex structures and uneven wall thickness are prone to heat nodes, especially in thick and large parts, which lead to final solidification. Failure to compensate for shrinkage in time here leads to shrinkage holes, shrinkage and other defects in castings. In products with high air tightness requirements, scrap rate is high. Local pressurization technology is one of effective means to solve this defect.

Local pressurization structure is mainly composed of extrusion cylinder, extrusion pin sleeve, extrusion pin, etc. (As shown in Figure 1 below).

 

1. Extrusion pin 2. Extrusion pin sleeve 3. Pressing cylinder 4. Extrusion pin fixing sleeve 5. Connecting head 6. Extrusion cylinder

The more representative statements at present are as follows:
(1) 20-30 times system pressure.
If system pressure is 14Mpa, and 20 times is selected, local pressurization pressure is 280Mpa;
(2) About 3 times casting pressure.
If casting pressure is 90Mpa, local boost pressure is 270Mpa;

Generally, it is selected according to situation of boosting part. If diameter is too small, it is easy to break, and if diameter is too large, it is easy to get stuck. Generally, it is between φ5-φ25.

2.3.1 Cylinder diameter
P local boost * A extrusion pin = P system pressure * A cylinder piston. In this formula, P local boost is local boost pressure determined in 2.1; A extrusion pin is cross-sectional area calculated by extrusion pin diameter determined in 2.2; P system pressure is system pressure of die-casting machine, and this value can be found in die-casting machine manual; A cylinder piston is piston area of extrusion cylinder. In formula, P local boost, A extrusion pin, and P system pressure are known items, so cylinder diameter of piston can be calculated.
2.3.2 Stroke
Measure volume W of hot joint shrinkage part of casting that needs to be extruded, solidification volume shrinkage rate β of aluminum alloy, which generally does not exceed 6.5% (β=6.5%), and radius R of extrusion round pin. Calculation formula for reserved movement stroke L of extrusion pin is: L=nβW/(πR2), where n is safety factor, which is generally selected as n=3.

(1) Special attention: When mold design is completed, it is necessary to confirm that when extrusion pin is in retracted state, matching section of extrusion pin should be 1-2mm higher than bushing. Otherwise, if it is designed according to Figure 2, it is very easy to get stuck during die casting production. It is more reasonable to design according to Figure 3.

 

(2) Head shape of extrusion pin also has an important influence on local pressurization effect, which can be simulated and verified using CAE software.
(3) In actual production, since mold steel will expand after being heated, if clearance between extrusion pin and extrusion pin sleeve is designed too small, die casting process will cause jamming and wear. If clearance is designed too large, water will leak and jam.
Generally, if extrusion pin is not cooled by water, clearance between it and extrusion pin sleeve can be reduced to 0.04-0.06mm; if cooling water is passed, clearance between it and extrusion pin sleeve can be reduced to 0.02-0.04mm.
(4) In addition to nitriding treatment of extrusion pin sleeve and extrusion pin, if conditions permit, PVD nano-titanium plating and other measures can be used to increase surface hardness of mold to increase its life.

Generally, extrusion starts 0.1-2 seconds after high-speed injection is completed, and extrusion pin is retracted after 1.5-2 seconds of extrusion.
Start-up time of extrusion device plays a decisive role in whether extrusion is successful. Alloy liquid filled in cavity must be extruded during crystallization and solidification process to achieve extrusion shrinkage compensation effect.
If extrusion is started too early, alloy liquid is still liquid, and extrusion ends before it begins to solidify. When alloy liquid solidifies, extrusion action is lost. If extrusion is started too late, alloy liquid in hot spot of casting or shrinkage feeding channel has solidified into a solid before extrusion begins. Solid alloy material has lost its fluidity, and extrusion pressure has lost ability to transmit in an opposite direction in alloy liquid. It requires a much greater pressure than extrusion to push alloy material to plastic deformation. At this time, pressure can only make alloy material move a very small distance at extrusion head, and it cannot play a shrinkage compensation role for shrinkage cavity inside casting.

When extrusion depth is unstable due to internal leakage in cylinder or other reasons, large die-casting machine will suffer a large loss due to downtime for maintenance of lower die. We have made a small improvement, as shown in Figure 4 below. An oil pressure gauge is installed between system pressure and oil inlet pipe of extrusion cylinder, as shown in Figure 5 below, which can easily determine cause of fault.

 

Local pressurization technology is closely related to mold design, die-casting process, etc. To achieve a stable quality of castings after local pressurization, it is also necessary to control mold temperature, pouring temperature, cycle time and coating spraying amount, otherwise local extrusion effect will change.