Design of the overflow discharge system for die casting molds

Die-casting technology is an advanced process, has a long history, mainly used in the production of aluminum alloy, zinc alloy, magnesium alloy, copper alloy, and other non-ferrous metal parts, motorbike, and automobile engine oil sump, cylinder body, box, throttle body, engine bracket, and other parts are more widely used, the main principle is through the die-casting machine will be alloyed (aluminum, zinc, magnesium, copper alloy, etc.) in high speed, high temperature, high pressure and other conditions into the mold The product is called die-casting. 

The die-casting mold is the main process equipment for die-casting production. In the economy of mass production, the casting quality qualification rate is high and low; the operation cycle is fast and slow, and all have a great relationship with die-casting die; die-casting die in the trial production often appears as flying edge burr, under-casting, cold isolation, deformation, sticky mold, strains, skin, internal gas shrinkage holes, and other quality defects. When these defects occur, the first should adjust the die-casting process, investigate the equipment, check the mold parting surface, etc., followed by a reasonable re-examining of the mold within the gate design. 

If the key influencing factors, such as the die casting process, die casting equipment, and mold gate, are excluded and still cannot be solved, then the overflow system of the die casting mold should be the main improvement direction for the follow-up. This article focuses on the design direction of the overflow system for die-casting molds to improve the process.

The role of the drainage system and structural design

Overflow system is the process of filling the cavity with molten metal, the exclusion of air, residual paint, and the initial filling of cold alloy channels and places, the quality of the casting play an extremely important role, mainly by the slag bag, slag bag mouth, exhaust tank 3 parts. The role of the slag bag mainly has the following 7.

(1) To accommodate the cold alloy liquid generated during the initial filling of the die-casting process, and at the same time with the exhaust slot to lead out the gas in the mold cavity, to strengthen the exhaust effect.

(2) Guide die-casting alloy filling flow distribution can reduce the casting local filling defects, to avoid turbulence.

(3) Can transfer the air shrinkage holes in critical parts of the casting, hard partition to non-critical parts, reducing the quality risk.

(4) Adjust the mold temperature uneven distribution, especially since the casting weight volume is small; filling the end of the mold temperature is difficult to meet the standard; setting the slag bag can also raise and balance the mold temperature.

(5) Can increase pack tightness.

(6) Can adjust the pressure of the cavity metal liquid.

(7) To prevent the metal liquid from entering the venting tank.

 The design of the slag bag should be fully considered its purpose and role, and the most important is to use a variety of purposes, conditions, shapes, and sizes of the slag bag; even 1 slag bag can play more than 2 roles. Usually set in the last impact of the alloy liquid or the last filled parts, as well as the alloy liquid convergence, easy to wrap into the gas and produce vortex parts, or castings too thick, too thin parts (Figure 1), the total volume of the slag bag accounted for 20%-50% of the volume of the cavity.

The role of the slag bag mouth and design principles

The role of slag bag mouth is mainly connected to the casting, and the slag bag is also one of the main exhaust gas channels.

(1) A single slag bag should not open more than one slag bag mouth to avoid backflow of metal liquid.

(2) In general, the cross-sectional area of the slag ladle is designed to be 30%-60% of the cross-sectional area of the internal gate, leaving a certain adjustment margin as far as possible.

(3) Slag bag mouth and die casting connection should be designed into R0.3-0.5 rounded or the chamfer C0.3-0.5 to avoid the slag bag castings dropping meat.

(4) Aluminum alloy slag bag mouth thickness is generally (0.8-2.0) mm, and zinc alloy is generally (0.6-1.0) mm.

(5) Slag bag mouth length is generally (2.0-3.0) mm, and the width is generally (8.0-12.0) mm.

Main principles of exhaust slot design

The main role of the die casting exhaust slot

The main role of the exhaust slot has 2: one is in the die-casting filling, especially when the low-speed filling discharges molten cup and mold cavity gas; two is the timely discharge of release agent, punch particles at high temperatures produce gas (Figure 2). In the die-casting molding process, if the mold exhaust is poor, the gas in the cavity by compression will produce considerable back pressure, preventing the normal high-speed filling of alloy liquid. Gas is also easy to be wrapped in the castings inside, which will cause the deep cavity parts of the filling to be insufficient and a large number of internal pores, castings thick wall parts, especially in the filling end parts of the mold exhaust slot more practical design. Generally speaking, the cross-sectional area of the exhaust slot should reach about 20%-40% of the cross-sectional area of the internal sprue; 30% is recommended.

 In the die-casting blank such as no surface quality defects, casting internal cut, no obvious gas shrinkage holes, die-casting, and no leakage phenomenon, it generally can be considered that the mold cavity exhaust slot design is reasonable. According to many years of die-casting die debugging improvement and summary, exhaust slot cross-sectional area is best in the design range gradually adjust large, not once in place. Otherwise, it will cause mold flying edge, burr, and other defects.

The specific design principles of the exhaust slot

(1) The general exhaust slot and the slag bag mouth have a certain stagger to avoid premature closure of the exhaust slot, weakening the exhaust function.

(2) The cross-sectional area of the air vent is not less than 20% of the cross-sectional area of the inner gate and cannot be larger than the cross-sectional area of the slag ladle.

(3) In principle, to increase the cross-sectional area of the exhaust slot, you can increase the width or the number and try not to increase the thickness.

(4) Exhaust slots should be easy to clean.

(5) to avoid mutual backflow through.

(6) The design of the exhaust slot should allow for a 20%-30% correction.

(7) The exhaust slot should not be opened directly to the sides of the mold but should be bent and then led out.

Exhaust slot opening method

 Depending on the mold, there are different ways to open the exhaust slots, but the ultimate goal is to exhaust the maximum amount of gas from the mold cavity. There are 8 main ways of opening the mold exhaust slots.

(1) From the slag bag up and down the left and right direction, straight open exhaust slot connected to the template, referred to as direct exhaust, exhaust slot depth is best ≤ 0.12 mm, otherwise the exhaust slot will flow into part of the alloy liquid, the exhaust slot blocked.

(2) From the shape of the casting directly into the exhaust slot, the disadvantage of this way is that it will lead to flying burrs, increasing the post-cleaning process and costs.

(3) The exhaust slot is led from the bottom of the slider; this method is more common in zinc alloy molds, aluminum alloy molds are not advocated, and the depth of the exhaust slot under the slider is recommended to be no more than 0.1 mm. Otherwise, a small burr is likely to occur, resulting in the movement of the slider stuck, size being over-poor, or even cracked.

(4) Simple exhaust block way, the general use of the occasion is the casting internal air shrinkage hole quality requirements are high, the exhaust effect of the way than ordinary straight row good, the disadvantage is that it will die-casting equipment clamping force requirements, is not conducive to reducing the cost of remelting back to the furnace material, at the same time will increase the risk of mold flying material, the way is also often blocked at the end of the exhaust channel, the design level requirements are high.

(5) Through the top bar to open the exhaust slot, but now the die-casting industry is increasingly demanding cost requirements, the top bar exhaust, although there is a certain effect, will occur in the orifice burr, the corresponding increase in post-processing processing costs, while the top bar hardness, surface treatment and with the clearance design to be very reasonable, otherwise the mold production process often occurs in the top out of the card, mold failure is frequent, the cost is higher.

(6) In some deep cavity parts, the filling process is too easy to produce local back pressure. Gas is not easy to discharge, often in the deep cavity at the bottom to open the exhaust core, core end face, and with the section design, 4 direction of the small exhaust slot, the disadvantage of the way is also will increase the cost of cleaning burrs. Still, the effect of exhaust on a small deep cavity is obvious.

(7) Through the slide with the gap to form a natural exhaust channel, manufacturing costs have increased this way on the mold manufacturing high precision.

(8) By opening the vacuum exhaust at the end of the exhaust channel, there are 2 kinds of simple vacuum and precision vacuum. Among them, simple evacuation and exhaust block simple evacuation, and cylinder core evacuation 2 kinds of ways; exhaust block evacuation is low cost, simple design, but easy to flow into the alloy liquid blocked exhaust channel; some manufacturers use high heat transfer material to make exhaust block, but the cost is too high, at this stage, there is no mature promotion, easy to block the problem is not a better solution. The precision vacuum exhaust method works well, but both mold manufacturing costs and maintenance costs are on the high side, not conducive to the use of large-scale promotion. Exhaust opening unreasonable, whether for product quality or mold flying material, flying edge, burr improvement, are a hundred harmless.


A set of reasonably designed die-casting mold discharge systems can reduce the die-casting machine clamping force, reduce the casting filling pressure, and speed, increase the scope of process adjustment, the casting quality to improve, mold flying material risk reduction, mold failure reduction, mold maintenance cost reduction, downtime reduction, production efficiency improvement, etc. have a significant effect.


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From V1Diecast, I’m a the marketing manager. I am familar with die casting parts and die casting technologies, including vacuum die casting, aluminium die casting, and related surface finishings. Ask for a quote for your ongoing or upcoming projects now!

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