What are the key points of quality control in stamping of auto parts?

The quality control of stamping process of automobile parts is centered on the four core requirements of stamping parts, namely, dimensional accuracy, surface quality, forming integrity and mechanical properties. The key nodes of the whole stamping process (raw materials entering the factory, die debugging, stamping production, finished product inspection, storage and transportation) are controlled, and the typical defects of automobile stamping parts such as cracking, wrinkling, springback, burr and surface scratch are avoided. The core points cover five dimensions: raw material control, mold and equipment control, production process control, finished product inspection control, and subsequent protection control, and meet the production requirements of mass production, high precision and high consistency in the automobile industry. The following are the practical points that can be directly implemented:

1. Control of incoming raw materials: avoid molding defects from the source and ensure the consistency of raw materials.

The material, size and surface state of stamping raw materials directly determine the forming effect of stamping parts. Cold-rolled steel plates, galvanized plates and aluminum alloy plates are the main stamping materials for automobiles. The control of raw materials needs to be tested in all items and in the same batch. The key points are as follows:

Material and performance testing: check the material certificates of raw materials (such as the material lists of Baosteel and Shougang), and test the tensile strength, yield strength, elongation and yield ratio of the sheet (the elongation required for automobile panels is ≥30%, and the yield ratio is ≤0.8) to ensure that the plasticity of the sheet meets the forming requirements such as deep drawing and bending; The hardness and stamping formability of aluminum alloy plate should be tested to avoid cracking caused by hard material.

Dimension and accuracy detection: detect the thickness tolerance (within ±0.05mm required for automobile parts), flatness (≤1mm/m) and width tolerance after uncoiling (±0.1mm). Uneven thickness will lead to the difference of local forming force of stamping parts and cause wrinkling/cracking; Out-of-tolerance flatness will lead to feeding deviation and affect dimensional accuracy.

Surface quality inspection: check that there are no scratches, pits, oxide scale and galvanized layer falling off on the surface of the plate (galvanized plate), and the surface defects will be directly copied to the surface of the stamping parts, which will become the appearance defects of the finished automobile parts; At the same time, ensure that the plate is free of oil pollution and corrosion, which will lead to feeding slippage, and corrosion will damage the mold and affect the surface of stamping parts.

Batch control: the same batch of stamping parts needs to use the same steel mill, the same batch and the same specification of raw materials, so as to avoid the performance difference of different batches of raw materials, which leads to poor adaptability of stamping parameters and inconsistent quality between parts.

Second, mold and equipment control: the core guarantee of stamping accuracy, reducing human and equipment errors.

Mould is the "template" for the size and shape of stamping parts, and equipment is the output carrier of forming force. Their accuracy and stability directly determine the quality of stamping parts, which is also the key to quality consistency in batch production.

(1) Mold control: fine adjustment, maintenance and wear prevention, ensuring stable mold precision.

Core of die debugging: after the new die is put on line or die is changed, focus on debugging four key parameters: die gap, blank holder force, drawbead and convex-concave die fillet:

Die clearance: the blanking/punching die clearance is 8%~12% of the plate thickness, and uneven clearance will lead to excessive burr and deformation of parts; The gap between the convex and concave die of the drawing die is 1.05~1.1 times of the thickness of the sheet. If the gap is too small, it will scratch the surface and cause cracking, and if it is too large, it will wrinkle easily.

BHF: The BHF should be accurately controlled in the drawing process (hydraulic BHF is preferred). If the BHF is too small, the sheet will wrinkle; if it is too large, it will reduce the sheet fluidity and cause local cracking;

Drawbead: By adjusting the height and spacing of drawbead, control the feeding speed of all parts of the plate to avoid wrinkling/cracking caused by local feeding too fast/too slow (for example, the feeding speed around the door outer panel should be uniform);

Convex and concave die fillet: the fillet radius of the punch of the drawing die is ≥3~5 times the thickness of the plate, and the fillet radius of the die is ≥5~8 times the thickness of the plate. If the fillet is too small, it will lead to stress concentration of the plate and crack, so it is necessary to regularly check whether the fillet is worn or broken.

Daily maintenance of the mold: check the surface smoothness of the mold before delivery for each shift, and make sure that it is free from abrasion and adhesion. The working surface of the mold needs to be coated with special stamping grease to avoid the scratch caused by the adhesion between the plate and the mold; Regularly grind the cutting edge of punch die (blanking/punching die), and the wear of the cutting edge will lead to burr out of tolerance; Check whether the mold locating pin and the guide sleeve of the guide post are loose and the gap is too large, so as to avoid mold clamping deviation and affect the part size.

Mould error prevention: locating pins, blocking blocks and feeding detection sensors are arranged on the mould to prevent plate feeding deviation, insufficient/excessive feeding, and avoid stamping parts with out-of-tolerance size or incomplete forming.

(2) Equipment control: ensure the stability of pressure, speed and stroke, and there is no operation deviation.

Mechanical presses (mass production) and hydraulic presses (large panels) are commonly used in automobile stamping. The core of equipment control is stable parameters, smooth operation and up-to-standard accuracy:

Equipment accuracy detection: regularly detect the parallelism of the press slider, the flatness of the table and the stroke accuracy of the slider (deviation ≤0.03mm). If the parallelism of the slider is out of tolerance, it will lead to uneven clamping of the mold, resulting in deformation of the parts and wear of the mold;

Fixed stamping parameters: during mass production, the stamping pressure, slider stroke, stamping speed and feeding speed of the press are locked, and it is forbidden to adjust them at will; The drawing process adopts "slow drawing and fast return" to ensure the uniform forming of the plate and avoid cracking caused by too fast speed;

Equipment operation monitoring: install equipment operation sensors to monitor the real-time data of stamping pressure and feeding position, and automatically stop when parameter deviation occurs to avoid batch failure; Regularly check the positioning accuracy of the feeder (coil straightening machine, manipulator), and the feeding deviation shall be ≤0.1mm to prevent the size of the parts from exceeding the tolerance.

Third, production process control: prevent defects in the whole process and ensure the consistency of batch production.

The production process is the main link of stamping defects, and the core of control is standardized operation, key process control and real-time defect investigation, focusing on avoiding five typical defects such as cracking, wrinkling, springback, burr and surface scratch. The core points are as follows:

Standardized operation control

Operators shall operate strictly according to the operation instruction, and unify the feeding mode and the part taking mode (mechanical hand/vacuum sucker is used to take the part, and manual direct contact with the forming surface of stamping parts is prohibited);

Before stamping, the sheet is leveled and cut to length, and the leveled sheet has no warping, and the deviation of blanking size is less than or equal to ≤0.2mm, so as to avoid forming defects caused by uneven blank size;

Rational use of stamping lubricant: Apply special stamping oil (oily lubricant for cold-rolled plate and water-based lubricant for galvanized plate/aluminum alloy plate to avoid corrosion) on the surface of plate in deep drawing and complicated bending process, and apply the lubricant evenly and with moderate dosage (too much is easy to slip, too little is easy to scratch and crack).

Key control of key processes

Drawing process: monitor the forming state of the sheet metal in the whole process, without local wrinkling, cracking and necking (necking is a precursor of cracking, so it is necessary to stop immediately to adjust the blank holder force or drawbead);

Bending process: control the bending angle and springback, and offset the springback of the plate (for example, bending 90, and designing the die to be 85 ~ 88) through die compensation (pre-bending reverse angle) to ensure the tolerance of bending angle ≤ 0.5; At the same time, ensure that there is no crease or crack at the bend, and the bending radius is greater than or equal to the thickness of the plate (the hard steel needs to be increased);

Blanking/punching process: strictly control the burr height (burr of automobile exterior parts ≤0.05mm, structural parts ≤0.1mm), and immediately repair and grind the die edge if the burr exceeds the tolerance;

Trimming/shaping process: the edge of the trimmed part is flat and free from jagged, and the shaping process corrects the springback and deformation of the stamping parts to ensure the shape accuracy and assembly fit of the parts (for example, the fitting gap between the inner door panel and the outer door panel is ≤0.1mm after shaping).

Real-time defect inspection and treatment

Implement the three-inspection system for first articles: before production, after mold changing and after equipment adjustment in each shift, process 3-5 first articles, and carry out full-size inspection+surface quality inspection before mass production;

Carry out inspection system: every 30~50 pieces, sample and detect the size and surface quality of parts, and find out the batch quality problems caused by mold wear and equipment deviation in time;

Isolation of defective parts in time: If cracks, wrinkles, serious scratches and other defective parts are found, stop the machine immediately to investigate the causes, and isolate and mark the defective parts at the same time, and it is forbidden to flow into the next process.

Iv. finished product inspection and control: all items are inspected, which meets the precision and quality requirements of the automobile industry.

Automobile stamping parts need to meet the four inspection requirements of dimensional accuracy, form and position tolerance, surface quality and forming integrity. The inspection is divided into appearance inspection, dimensional and form tolerance inspection and forming performance inspection, and some key parts need to be rechecked for mechanical properties. The core points are as follows:

Visual inspection (100% full inspection)

Surface quality: no scratches, scratches, pits, stains, galvanized layer falling off (galvanized parts), no visible defects on the surface of exterior parts (such as door outer panel and hood), and the surface defects of structural parts should be within the allowable range (no scratches with depth > ＞0.1mm and length > 5 mm);

Forming integrity: no cracking, necking and wrinkling, no edge warping and cracking at flanging, and no unclear rib pattern and collapse at pressing rib;

Edge quality: no burr, flash, serrated edge, smooth trimming, chamfering/fillet meeting the drawing requirements.

Dimensional and geometric tolerance inspection (full inspection of first article+batch sampling inspection)

The fixture, three-coordinate measuring instrument, dial indicator and caliper are used for inspection. The fixture is a special tool for batch inspection of automobile stamping parts, and the key dimensions (such as hole position, outline and assembly surface) of the parts are detected, and the deviation is ≤±0.1mm；;

The three-coordinate measuring instrument is used for full-size detection of the first piece and the sampling piece, focusing on the detection of coaxiality, parallelism, verticality and flatness (such as coaxiality of the mounting hole ≤0.05mm and flatness of the assembly surface ≤ 0.1 mm/m);

The key hole position and installation surface are the key inspection items, and the hole position deviation is ≤±0.08mm to avoid affecting the subsequent welding and assembly accuracy.

Formability inspection (sampling inspection)

For deep-drawn parts and complex bending parts, the hardness is tested by sampling (the hardness deviation of the local hardened area after forming is ≤20HV) to avoid subsequent welding cracking caused by uneven hardness;

Sampling aluminum alloy stamping parts for tensile test, rechecking the elongation to ensure that the mechanical properties of the formed parts meet the design requirements;

Check the springback of the parts to ensure that the springback is within the design compensation range, and it fits closely with the matching parts during assembly.

Retention of inspection records: All inspection data (first article inspection, inspection and sampling inspection) shall be recorded and kept for at least 3 months to facilitate quality traceability (the automobile industry requires quality records to be traceable).

V. Follow-up protection and control: avoid secondary damage to finished products and ensure the quality of storage and transportation.

The surface and dimensional accuracy of finished stamping parts are easily damaged by bumping, scratching and deformation, especially for automobile exterior parts. The core of subsequent protection management and control is to prevent scratching, bumping and deformation, covering all links of warehousing, transportation and circulation. The key points are as follows:

Pick-up and circulation: Take-up and circulation by manipulator, vacuum sucker and special material rack, and it is forbidden to drag the forming surface of stamping parts directly by hand; In the process of circulation, parts are separated by pearl cotton and kraft paper to avoid bumping and scratching each other.

Warehousing and storage: Stamping parts are classified and stored according to specifications and batches, and special workstation appliances (racks and boxes) are adopted, and the workstation appliances need to be treated with anti-skid and anti-scratch (lined with rubber and pearl cotton); Large stamping parts (such as doors and hoods) are stored vertically to avoid deformation due to their own weight; Keep the storage environment dry and ventilated, and the relative humidity is ≤60%, so as to prevent galvanized parts and aluminum alloy parts from rusting and oxidation.

Transportation protection: during batch transportation, stamping parts are fixed on special material racks and fastened with straps to avoid shaking and bumping during transportation; Transport vehicles shall take rainproof and dustproof measures to prevent rain and dust from polluting the surface of stamping parts.

Clear marking: each piece/batch of stamping parts shall be marked with the name, specification, batch number, production date and inspection status (qualified/pending inspection/unqualified), so as to prevent mixed materials and unqualified parts from flowing into the next process (such as welding and painting).

VI. Prevention and rectification control of typical defects: targeted solutions to avoid batch recurrence.

Typical defects of automobile stamping parts have fixed incentives, so it is necessary to establish a closed-loop management of defect analysis, rectification and verification. The incentives and rectification measures for core defects are as follows, so as to achieve "finding one place, rectifying one place, and eliminating batch":

Targeted rectification measures for core incentives of typical defects

The fillet of the cracking die is too small, the blank holder force is too large, the plasticity of the plate is poor, and the lubricant is insufficient. Increase the fillet of the convex-concave die, reduce the blank holder force, replace qualified raw materials, and evenly apply lubricant.

The blank holder force of wrinkling is too small, the die gap is too large, the feeding speed of sheet metal is uneven, and the drawing bead design is unreasonable. Improve the blank holder force, adjust the die gap, optimize the drawing bead and control the feeding speed of sheet metal.

The rebound plate material is soft, the bending angle is not compensated, and the punching force is insufficient. The die makes reverse rebound compensation, improves the punching force, and replaces the raw materials with matching materials.

Excessive burr, worn die edge, uneven die gap and insufficient stamping pressure. Grind die edge, adjust die gap and improve stamping pressure.

Surface scratch, rough mold surface/sticky material, manual picking, polishing the mold surface without isolation between parts, cleaning the sticky material, picking the parts by manipulator, and adding isolation pads between parts.

Seven, personnel and system control: establish a quality responsibility system to meet the requirements of the automobile industry system.

Personnel training: Operators and inspectors need to be trained and tested, familiar with stamping process, quality requirements and defect identification methods, and master standardized operation procedures;

Quality responsibility system: define the quality responsibility of each post, and the quality problems can be traced back to individuals to avoid quality defects caused by human operation errors;

System landing: Strictly follow the requirements of IATF16949 automobile industry quality system, and establish quality exception handling process, mold maintenance process and raw material inspection process to standardize and standardize the whole process;

Continuous improvement: for the quality problems in the production process, the root causes are analyzed by using the 8D report, and corrective measures are formulated and the effect is verified, thus forming a closed loop of continuous quality improvement.

Core summary

The core of quality control in stamping process of automobile parts is * * "source control, process strict control, strict inspection of finished products, follow-up protection and defect closed loop". Focusing on the core requirements of high precision, high consistency, no appearance defects and assembly fit of automobile parts, quality control is integrated into every node of the whole stamping process, and at the same time, relying on standardized operation, special inspection tools/molds and automatic equipment to reduce human error and ensure the quality stability of mass production.