Cold flakes are the chipped-off solidified metal alloy chunks under the shot sleeve. A layer of poured molten alloys in the cold chamber machine solidifies to initiate this defect. The plunger’s forward and backward movements push those chips into the casting and the runner.
These fragments are primarily covered with an oxide layer with impurities like lubricants and pores. However, aluminum and its alloys are the most susceptible constituents to encounter this defect. The formation of cold flakes often acts as the leading catalyst for more serious defects.
This article briefly looks into the causes,effects,and remedies to counteract the formation of cold flakes.
Causes of Cold Flakes in Die Casts
It’s difficult to break down this defect’s source or nature without mentioning many technical details. Still, a simple explanation for cold flakes’ occurrence concerns the shot sleeve’s improper, erroneous, or compromised operational involvement.
Low pouring temperature, inconsistent pouring rate, high dwelling time – anything related to the shot sleeve can set off its formation. However, low system temperatures (particularly the sleeve and the metal) mainly trigger the problem.
These broken-off chips have a straight surface on one side and roughness on the other. The straight side holds a lubricant coating against the rough side’s irregularities from partial bonding. And the straight or smooth side tends to cause further defects.
Effects of Cold Flakes in Die Casting
Some of the notable outcomes of having cold flakes in alloy casting include –
1.The straight surface fails to bond with the remaining alloy due to the lubricant coating.
2.Formation of cracks at points due to the poor to no bond between the alloy materials.
3.Irregular or random breakouts at the gate system due to the lodging of chipped flakes.
4.Fracture across the straight side to cause potential breakage at the casting’s gate system.
5.Adhered/stuck cold flakes to the gate alter the molten flow to cause partial blocks.
6.Additional surface defects due to the alteration of the Al alloys stream during the casting.
7.The eventual development of a leak path/location for future cracks in thinner wall castings.
Preventing Die Casting Cold Flakes
Optimum operation consistency and careful management implementation are the keys to addressing this chipping defect. Some of the crucial countermeasures to achieve the operational accuracy include –
● High-Sleeve Temperature
–Maintaining a sufficiently high temperature for the sleeve can prevent solidification. However, avoiding excessive temp is also necessary to eliminate potentially rapid erosion under the pouring hole.
● Minimized Dwelling Time
–Lowering the dwell time for the sleeve can greatly alter the chances of cold flakes. However, it may affect the intended acceleration schedule regarding minimizing trapped air/gas/impurities.
● Significant Percentage Fill
–The percentage fill on shot sleeves should remain as high as possible. It only reduces the formation of chipped flakes upon pouring without affecting the possible intensity or skin’s thickness.
● Consistent Pouring Temp
–Avoiding too low a pouring temperature is preferable in the case of die casting. For most of the metal alloys, any less than 650°C or 1200°F can initiate the formation of solid chips. So, proper temperature control can play a critical role here.
● Trimming Runner System
–Breaking the solidified metal layer or skin often directly pushes the chips into castings and runners. A suitable trimmer to shear the runner off is recommended to avoid such an issue.
● Pre-Heating of the Sleeve
–An electric shot or heater may come in handy to pre-heat the sleeve. Using scrap castings to keep up the operation until the sleeve temp reaches 350°C or 650°C can help with this handicap.
Eliminating cold flakes is nearly impossible due to its complex, uncertain, and random nature. All the controlling actions mentioned above may not apply to every casting process either. So, setting the variations accordingly is essential to ensure flawless castings.