What matters should be paid attention to in die processing of auto parts?

The die processing of automobile parts is a high-precision and complex manufacturing process, which needs to be strictly controlled from many dimensions such as design, materials, technology, equipment, precision control and safety management. The following are key considerations:

I. Design and Process Planning Stage

1. Optimization of die structure design

Reasonable parting surface design;

Avoid complex surface parting, and give priority to plane or simple inclined plane, which is convenient for machining and demoulding.

The parting surface needs to consider the strength of the mold (such as thickening the die core edge) and the exhaust performance (opening an exhaust groove with a depth of 0.02-0.05mm and a width of 5-10mm).

Gating system design:

The taper of the main runner is 3-5, and the roughness of the inner wall Ra is less than or equal to 0.8 μ m, thus reducing the melt flow resistance; The cross section of the shunt channel is preferably circular or trapezoidal to balance the filling speed of each cavity.

For precision parts (such as gearbox dies), hot runner system is adopted to reduce waste and improve molding consistency.

Cooling system layout:

The diameter of the cooling water channel is φ6-φ12mm, and the distance from the surface of the cavity is 1-2 times of the diameter of the water channel, so as to avoid the mold temperature difference caused by "near cold and far hot".

Adopt spiral, baffle or conformal cooling (3D printing channel) to ensure the temperature uniformity of the mold ≤ 5℃.

2. Material selection and pretreatment

Matching of die steel grades:

Aluminum alloy die casting die: H13, 8407 and other hot working die steels are selected, and the quenching hardness is 48-52HRC. Vacuum quenching and tempering are needed to eliminate internal stress.

Injection mold: S136 stainless steel is used when corrosion resistance is high, and NAK80 pre-hardened steel (hardness 37-43HRC, which can be directly processed) is used for mirror polishing mold.

Material defect detection:

Before purchasing, suppliers are required to provide ultrasonic flaw detection reports to eliminate metallurgical defects such as interlayer and porosity; Important molds need to be analyzed by PMI composition to prevent inferior quality.

Second, the key control of the processing process

1. Rough machining: removing the allowance efficiently.

Tools and cutting parameters:

Carbide milling cutter: the cutting speed is 80-120m/min, the feed rate is 0.2-0.5mm/z, and the cutting depth is 3-5mm (steel parts), so as to avoid "gnawing on the cutter" or cutting edge collapse.

Rough machining of graphite electrode: special coated cutter is selected, with rotating speed of 2000-3000r/min and feed speed of 1500-2000mm/min to prevent graphite from cracking.

Clamping and positioning:

Using one side and two pins for positioning, the positioning accuracy is ≤ 0.02 mm; Large-scale molds use hydraulic clamps or vises, and the clamping force is evenly distributed to avoid deformation.

After rough machining, a finishing allowance of 2-5mm is reserved, and stress relief annealing (for example, heat preservation at 550℃ for 2 hours) is carried out to reduce subsequent machining deformation.

2. Finishing: ensure the size and surface accuracy.

Precision milling and EDM;

High-speed milling (HSM): Machining cavity fillet, narrow seam and other details, with spindle speed of 15,000-40,000 r/min, feed rate of 0.05-0.2mm/z and surface roughness Ra ≤ 1.6 μ m..

Electrical discharge machining (EDM): the electrode loss is controlled to ≤0.5%, and low current and high frequency parameters are adopted in finishing, so that the discharge gap is 0.01-0.03mm and the surface roughness Ra of the cavity is ≤ 0.8μ m..

Five-axis linkage machining:

Complex curved surfaces (such as automobile panel dies) need a five-axis machine tool. Through the linkage of swing head and turntable, multi-faceted machining can be completed at one time, and the positioning accuracy is ≤0.005mm, which reduces the repeated clamping error.

3. Special processing and surface treatment

WEDM (wire cut):

When machining precision pin holes and inclined guide rails, use φ0.1-φ0.3mm copper wire, the cutting speed is 10-20 mm/min, the dimensional accuracy is ±0.002mm, and the surface roughness Ra is less than or equal to 0.4 μ m..

Surface treatment process:

Nitriding treatment: the surface hardness of the die is increased to 1000-1200HV, and the depth is 0.3-0.5mm, which is suitable for easy-to-wear cores and sliders.

PVD coating (such as TiN and TiCN): The coating thickness is 1-3μm, and the surface hardness is 2000-3000HV, which reduces the friction coefficient and improves the demoulding performance of injection mold.

Iii. Precision detection and mold test verification

1. Coordinate measurement (CMM)

Critical dimension detection:

Cavity/core size: tolerance ±0.01mm (precision mold), ruby probe (diameter φ2-φ5mm) is used, and 3-5 points are detected for each feature.

Draft inclination: the deviation between the detected value and the designed value is ≤ 0.1, and it is calculated by angle measuring module or fitting straight line.

Form and position tolerance control:

Flatness: ≤0.005mm/100mm, detected by marble platform and dial indicator; Verticality: ≤0.01mm/100mm, measured by square and dial indicator.

2. Die test and process adjustment

First piece trial:

Injection mold: the initial injection pressure is 60-80MPa, the holding pressure is 30-50MPa, and the cooling time is 20-40s. Observe whether the mold is full and whether there is flash.

Die casting die: the injection speed is 3-5m/s (low speed) and 20-30m/s (high speed), the die temperature is 180-250℃, and the dimensional qualification rate of die casting parts is ≥95%.

Defect analysis and die repair;

Shrinkage: increase cooling channels or increase gates;

Burr: grind the parting surface, and the fit clearance is ≤ 0.03 mm;

Out-of-tolerance: correct the worn part of the core/cavity, and use laser cladding or electric spark repair welding (thermal deformation should be avoided).

Fourth, equipment maintenance and safety management

1. Machine tool precision maintenance

Spindle accuracy monitoring: use the spindle runout tester to check every week, and the radial runout is ≤0.003mm and the axial runout is ≤ 0.002 mm. If it exceeds the tolerance, it is necessary to rebalance or replace the bearing.

Maintenance of guide rail and lead screw: clean the oil stain on the guide rail every day, lubricate with lithium-based grease (NLGI Grade 2), and regularly check the preload of the ball screw with a torque wrench to ensure that the transmission accuracy is ≤ 0.005 mm/m..

2. Safety operation specifications

Tool management:

The new tool needs to be tested for dynamic balance (unbalance ≤ 5 GMM), and the mating surface between the handle and the spindle taper hole (such as BT40 and HSK63) is clean and free from bumping.

Tool life management: when machining HRC50 die steel with cemented carbide milling cutter, the service life is about 8-12 hours, and it is forced to be replaced after reaching it, so as to avoid equipment failure caused by tool collapse.

Cutting fluid management:

The concentration of water-soluble cutting fluid is maintained at 5%-8%, and the pH value is 7.5-9.0. The electrical conductivity (≤1000μS/cm) is tested every week, and the filter element is replaced regularly to prevent bacteria from breeding and corroding the mold.

V. Typical case: Key points of automobile panel die processing

Large stamping die:

Material: GGG70L ductile iron (tensile strength ≥700MPa) is selected, and after rough machining, it needs to be aged for three times (600℃×24h) to relieve stress.

Machining sequence: firstly, milling the datum plane of the die holder (flatness ≤0.01mm), then milling the profile with a five-axis gantry, and finally polishing it with a CNC grinder (roughness Ra≤0.4μm).

Inspection key points: the depth tolerance of drawbead is ±0.05mm, the straightness of cutting edge is ≤0.02mm/m, and the whole profile is inspected by blue light scanning (accuracy is ±0.01mm).

summary

The die processing of automobile parts should focus on "accuracy first, controllable process and stable equipment", avoid the processing difficulties in advance through DFM (manufacturability design), and realize the whole process quality control from blank to finished product by combining on-line inspection (such as machine tool probe) and intelligent process system (such as MES traceability). At the same time, it is necessary to pay attention to environmental protection requirements (such as cutting fluid recovery and waste sorting) and operator skills training (such as five-axis programming and EDM parameter optimization) to ensure efficient and safe delivery of qualified molds.