Drag marks or drags are the most common cosmetic defects in die-casting productions.
It characterizes casting deformation with roughness or undercuts, especially on the surfaces. The cast metals/alloys may stick to the mold during ejection to initiate drag marks.
Drag marks shouldn’t be confused with solders as the two share considerable similarities in appearance. Unlike solders, several die surface finishing conditions contribute to drags. The article briefly discussed the sources, effects, and countermeasures for die-casting drags.
Sources of Drag Marks
Significant mold-metal adhesion or stickiness is the ultimate reason behind this casting defect. And multiple factors are responsible for its favorable environment –
● Excessive Temperature
–Every alloy/metal has a specific temperature limit for casting. Going beyond the threshold makes the molten liquid sticky to scratch the mold surfaces.
● Uneven Cooling
–Non-uniform cooling of different casting parts initiates random solidification. It causes various other defects to pop up across the semi-liquid or semi-solid casting.
● Limited Draft Angle
–Draft angles play a crucial role in die-casting ejection from the mold. Poor/low to zero draft angle often hinders smooth sliding during pull-outs from molds.
● Material Composition
–A higher percentage of Iron (Fe) develops a greater affinity to the mold steel. Any more than 0.6% Fe in aluminum alloys causes the defect to appear rapidly.
● Poor Mold Tooling
–Surface scratches, distorted design, and compromised gate system inherit a rough texture. This inhibits stickiness or adhesion to trigger drags and even stains.
● Improper Lubrication
–Poor lubrication directly affects the ejection process. Partial ejection from the mold develops high friction between the parts to cause sticky drag marks.
The defect interferes with aesthetics, making the final product less presentable. Die casts with drags may be discarded without proper surface finishing. Though they don’t contribute to any mechanical damages, worsening of the surface condition is also possible.
Preventing Drags in Die-casting
Die-casting drags may not affect the quality significantly, but it’s still undesirable and disappointing. The following measures can avoid its formation on casting surfaces –
1. Adequate Draft Angles
–Setting a sufficient draft angle in new dies can help reduce this problem. It shouldn’t exceed the allowable limit. Also, a smaller angle is better than a zero/no angle.
2. Improved Mold Tooling
–Gates, surfaces, and dimensions should remain perfect for the mold. It can eliminate metal flow impacts on walls and cores while reducing the mold filling speed.
3. Balanced Composition
–Ensuring the Fe content remains within the permitted range confronts drags. A balanced composition based on the desired outcome must be deployed before casting.
4. Proper Releasing Agent
–Using the suitable lubricant at the right amount saves the cast from stickiness. An even spraying is necessary to avoid random, scattered, or unequal ejection.
5. Optimized Operation
–Everything in the process, from casting to ejection, must maintain precision. Setting all the depending parameters accordingly delivers perfect cooling and ejection.
6. Changing the Die
–It’s better to use new dies instead of worn out, defective, or altered ones. In this case, fixated ones will struggle to keep up with the required accuracy after a few runs.
A plain and clean operating process is the ultimate answer to die-casting drag marks. The die should remain perfect, whereas the operation must maintain precise measurements.