The methods to solve casting parts sticking molds trouble--(4)
2 Methods to prevent die casting parts sticking on Cavity
2.1 Methods to prevent die casting parts sticking on cavity from die casting mold
In the newly produced mold test, or die-casting production beginning stage, it has often the phenomenon of sticking mold. If it happens when the casting mold is operated in proper condition, the main problem is not die-casting process, but casting structure design,mold design or mold manufacturing problems. The problem can be improved a little bit by die casting artwork, surface finishing treatment, but the effect of remediation not good, not very stable,
If the phenomenon of castings sticking in the mold is easy to occur , before die casting production, the mold should be preheated well, and before the start of low-speed injection, to paint anti-stick mold paste in cavity , and blow it evenly by compressed air, Apply once per shot, try to produce about 20 shots. If it still sticks mold, indicating a mold problem, need to repair mold.
For the casting parts which design is completed already, is indeed the tightening force against the cavity is bigger than t, it is necessary to allow to set ejector bar to eject parts out on the side of cavity, allowing ejecting marks leaving on the casting parts surface. Or to make the marks easy to remove. it should design the ejecting structure on design state.
Pay attention to the calculation of tightening force against cavity & core. for the casting parts those tightening force against cavity bigger than core; tightening force against cavity & core similar, those parts are possible to stick in both cavity and core, It is necessary to change the casting or mold structure, draft angle, surface roughness, etc on design stage., try to make the tightening force of casting part against core bigger than cavity.
For the casting parts with bigger tightening force in cavity, we should select the parting line location deviating from core side as far as possible. Let’s more part of casting component in core side, to enlarge the tightening force against core. To decreasing tightening force against cavity, the designer should re-consider the draft angle of cavity, and try to enlarge the draft angle slope, while to decrease the draft angle slop on core side.
To prevent undercut and rough surface occurring during manufacturing & polish affecting casting parts release after the test or during die casting process, should correct the mold cavity crush, bumps deformation; use of polishing or chemical cleaning agent adhesion traces to eliminate alloy on the mold surface, If alloy adhesives in the mold not promptly removed, after a long time sticky mold phenomenon will become more serious; The better is to polish the rough surface on cavity.
But the cavity is not conducive to paint adhesion if make mirror polishing, also between the casting and die will produce a tight vacuum gap when mold is opened, resulting increase the mold release resistance, so the bottom of the fixed cavity can not be polished into a mirror surface. For the mold with nitriding, should be discreet to polish them, to prevent the destruction of the surface of the nitride layer, to prevent the more polished the more sticky mold situation.
Modify the mold runner, the appropriate change to the location of the runner, size and filling flow, to eliminate or reduce the impact of the runner on the emergence of erosion, viscous defect. For example: ① change the flow of aluminum liquid filling, try to reduce the metal liquid on the mold cavity of the impact, you can change the direct impact of metal liquid diagonal toward the core or wall; ② appropriate to increase the cross-sectional area of the runner, (3) to change the position of the inner runner so that the inner runner is in the thick and thick position of the casting and to avoid the impact on the mold side wall; (4) try to take the bottom of the casting deep ⑤ for the internal runner impact parts or cores, you can use the tungsten carbide bar coater, the mold surface to the electric surface of the mold Spark metallurgical method of spraying tungsten carbide micro-particle layer, metal tungsten particles combined with the base metal will not fall off, can improve the mold surface of the anti-viscous mold, such as die-casting mold surface layer deposited 2 to 4 micron thick coating, the hardness Up to HV4 000 ~ 4 500, the use of temperature up to 800 ℃.
In order to pull the casting to the core side, you can make the top of ejector bar the wedge-shaped inverted pull hook (hook length 5 ~ 8 mm, casting parts thickness 1 ~ 2 mm, see Figure 1), so the inverted hook pull the casting part to movable mold, and remove the inverted hook on casting parts. In order to increase the casting tightening force against movable mold, you can increase the surface roughness on corresponding mold area which not affect the parts surface requirement, the tightening force against movable will be much better.