A brief discussion on measures to ensure the quality of stamped parts
The stamping process is the first manufacturing link of the vehicle, and its product quality directly affects the quality level of subsequent processes. Many vehicle manufacturers have listed the quality of stamping parts as a key improvement and guarantee project. How to design high-quality stamping parts during the product development stage? This article summarizes some control points in the company's model development process and shares them with you.
During the project implementation, our company adopted a series of process assurance measures to well control the quality of stamped parts. In particular, through the use of springback compensation and other design methods, the quality changes of stamped parts in the later period were reduced. 40%, and the overall quality pass rate of stamping parts has reached more than 98% (calculated based on key control points of the entire vehicle), basically reaching the level of Japanese and Korean companies.
Early SE analysis stage
The focus of SE analysis includes part formability, processability, positioning and tolerance accuracy, etc.
1. Formability analysis
Formability analysis is to analyze problems such as product cracking, wrinkling, slip lines, impact lines, and rebound deformation, and provide solutions.
The formability analysis mainly includes: whether the part has negative angles, avoiding sharp corner forming (for example: in order to ensure the formability of the part, sharp corner transitions must not occur in the transition area between the back door and the rear taillight), and choosing a symmetrical shape for the outer cover model ( To avoid the occurrence of slip lines during the forming process), the height of the inner/outer curved flanging of the part (to avoid the occurrence of flanging cracking/wrinkling), the hemming and flanging angle (usually between 90° and 105°), the hemming For changes in the hemming shape of the product (the change area is greater than 30mm) and sharp corners, pay attention to the hemming width (usually the height is not higher than 3 to 5mm), etc.
The review of the difficulty of forming mainly includes: appropriately disassembling large parts and changing the shape of the parts (for example: try to ensure that the parts are formed in a straight line, and then choose a large arc transition; try to choose one matching ridge line between the four doors; Eliminate part springback and increase spring ribs; reduce design forming depth and avoid sharp changes in shape, etc.).
2. Process determination
The stamping process requires inspection of the workpiece process arrangement and trimming angles; analysis of problems such as poor trimming, excessively large and too long burrs in the later stage, and solutions; and review of the applicability of the wire arrangement, etc.
(1) Forming condition requirements: main strain of drawing: outer plate > 0.03, inner plate > 0.02; thinning rate < 0.2; wrinkling: outer plate A-level surface 0%, inner plate < 3% of material thickness;
(2) Trimming condition requirements: The minimum angle requirements for vertical cutting are shown in Table 1. See Table 2 for bevel trimming angle requirements. See Table 3 for punching condition requirements.
3. Positioning analysis
The selection of RPS datum should meet the 3-2-1 (or N-2-1) principle and the principle of coordinate parallelism and unity. The RPS point should be selected as a part that has sufficient rigidity and no deformation; it should be as parallel to the coordinate line of the vehicle as possible, and should be selected at a position with a consistent cross-sectional shape (changes in the cross-section can easily cause deformation of the part, making it difficult to position accurately); because the datum hole and the datum The positioning direction of the surface is different, and the positioning datum hole should not coincide with the positioning datum surface as much as possible (theoretically 90°); in order to reduce positioning errors, the datum should be consistent during subsequent production and use; in order to improve the quality of the product, the positioning The datum should also be selected as far as possible where the parts to be welded have fit or functional requirements.
For the same parts, the positioning reference positions should be as unified as possible; the positioning of the parts cannot be relied on the fit of the shapes of adjacent parts, and the parts must be positioned individually; for parts with poor rigidity, additional positioning points can also be added to meet the positioning of the parts stability requirements.
4. Tolerance accuracy determined
The quality requirements of parts in different parts are different, and the tolerance accuracy requirements are also different (for example: the flanging contour tolerance of outer panels that affects the body gap is generally ±0.5mm or ±0.7mm, while the flanging contour tolerance of other parts is ± 1.0mm or larger; the flanging height tolerance with matching requirements is generally about 0.5mm, while the other flanging height tolerances are above 1.0mm). Since the outer cover affects the appearance quality of the entire vehicle, the size and appearance requirements are stricter than those of other structural parts. The car body is divided into areas A, B, C and D according to different areas. From A to D, the quality requirements decrease in sequence. The quality requirements for different positions of the same part are also different. For example, the accuracy requirements for reference holes and reference surfaces are higher, followed by assembly holes and mating surfaces, and other parts without mating requirements are lower (generally above ±1.0mm). . Tolerances should be maximized when designing while ensuring quality.
Process development and monitoring
1. Inspection tool development
It is usually necessary to determine the stamping parts of the inspection tool:
(1) Important parts (such as outer panels, parts with special characteristics, etc.);
(2) Parts with complex structures and high precision requirements that cannot be detected using general measuring tools (such as left and right front wheel covers, front apron panels, floors, side panel inner panels, etc.) that require higher quality and have many overlapping parts. pieces);
(3) Parts that are difficult to form and are prone to springback, deformation and other defects (such as B-pillars, door inner panel reinforcements, anti-collision beams and other parts that are easy to spring back and produce quality defects, and whose material yield strength exceeds 340 MPa );
(4) Parts that have had problems during the debugging process of previous models or have affected the accuracy of lap joints and bodywork (such as the oil filler, roof rear cross member, sill beam, and rear section of the sill beam, etc.).
Technical requirements for inspection tools: The positioning surface, support surface and clamping points of the inspection tools must be set according to the RPS system in the product part drawing; the accuracy requirements are the position of the reference hole ±0.05mm, the diameter of the reference hole, the outer diameter of the positioning pin, and the reference Surface position ±0.10mm, datum plane parallelism/perpendicularity 0.05mm/1000mm, scribing pin hole position ±0.10mm, scribing pin outer diameter tolerance, shape or template blade surface error ±0.10mm, bottom plate parallelism /Verticality 0.05mm/1000mm.
2. Mold development
(1) Equipment requirements
① The forming force of the part should account for less than 75% of the equipment's output capacity, and the forming force stroke should meet the curve requirements of the equipment's output force;
②The equipment parameters meet the requirements of mold installation (not exceeding the work surface and 50mm smaller than the work surface);
③The closing height is within the equipment requirements (usually the limit size is reserved at 10~20mm);
④ Offset installation size ≤75mm;
⑤The ejector stroke, air cushion pressure/slider adjustment, etc. meet the mold usage requirements;
⑥Repetitive positioning accuracy of mobile workbench <0.05mm;
⑦ The parallelism between the workbench and the slider is <0.12/1000; the perpendicularity between the slider stroke and the workbench is <0.3/150.
(2) Supplier selection
During the supplier inspection process, while ensuring that the hardware meets the development requirements, more emphasis should be placed on the confirmation of the software (development capabilities, system operation and quality assurance capabilities, etc.), and attention should be paid to collecting other customers' evaluations of the supplier. During the supplier selection process, suppliers should be selected based on the difficulty of the mold or parts, and the expertise of each supplier should be considered when subcontracting molds or parts.
For the development of external panels, we choose internationally renowned or domestic first-class suppliers, such as Germany's Bartz, Japan's Miyazu, Dongfeng Mold and Tianqi Mold, etc. For high-strength plate parts, you can choose suppliers with rich development experience and successful cases. During the development process, we focused on suppliers such as Volkswagen, Toyota, Honda, and Hyundai, and paid attention to the resource collection of suppliers.
(3) Process monitoring
After signing the contract, the supplier is required to deliver the development plan signed by the project manager, and before the actual production is put into production, the supplier is required to make regular progress reports.
After the actual prototype is invested, suppliers are required to add photos during regular progress reports to ensure the authenticity of the progress.
Conduct inspections on suppliers from time to time, evaluate the suppliers based on the inspection results, and send a copy of the evaluation results and improvement opinions to their senior leaders, informing them that the evaluation results will be used as the basis for assessment in subsequent cooperation.
When the project is abnormal, on-site supervision is required. The on-site engineer (SQE) must report the work progress every day and inform the supplier's senior management of the project progress and abnormal situations in order to receive better support.
(4) Mold technical requirements
The die fillet of the drawing die should be R> (6~10) times the material thickness; the CH hole should be set on a flat surface as much as possible (the maximum angle on the inclined surface should not exceed 5°); when the thickness of the high-strength sheet is >1.2mm, the edge ring must be It is made into an insert structure; the inserts are generally divided into pieces according to the 5° angle between the joint surface and the center of the mold; the seams are made through arcs (10~15mm); the drawing mold adopts a conforming guide form; the integration rate of the outer panels is >95% , Surface roughness Ra0.8; inserts in areas with severe material flow require special processing (TD, PVD and laser).
Repair and punching dies: The pressing force is selected according to the upper limit of the design requirements (all pressing forces for outer panels are nitrogen cylinders); a guide device must be used when trimming with a wedge; when the material thickness is >1mm, a counter-side device must be used for side trimming; The structure of the mating parts of the parts must be compact; the trimming edge of the parts must fit within 15mm.
Flanging shaping mold: The flanging top maker is required to work synchronously; the end exceeds the flanging boundary by 5mm; anti-warping measures must be taken for the outer panel (such as turning both ends first); the flanging line cannot be formed if it changes in one stamping direction. It is divided into two sequences to complete. The overlap between the two sequences must be at least 20mm, and the length of the transition zone is 40 to 50mm.
In order to ensure the stability of the product, the flanging clearance meets the requirements of Table 4.
3. development process control
(1) Control of mold development quality
The preliminary construction method and mold structure of mold development must pass multiple reviews and be put into production after approval.
Monitoring of casting quality: Materials must not have defects such as pores, shrinkage cavities, shrinkage porosity, trachoma, cracks and sand.
Monitoring of machining quality: The size and shape accuracy and surface roughness requirements of mold parts must be ensured; different processing methods should be distinguished and appropriate machining amounts should be reserved.
Monitoring of assembly quality (including the use of standard parts): Each insert is assembled in place, the bottom surface is not less than 80%, the seam gap is less than 0.03mm, the working surface is evenly colored, the positioning is accurate, and the fastening is firm (and there are relaxation measures) .
Debugging quality control: Ensure that the materials of stamping parts are the same as those of mass production; try to use mechanical presses for debugging equipment, and the debugging strokes are the same as those during production; lubricating oil is not allowed to be applied during debugging; cracking, cracking and peeling are not allowed on the parts. Wrinkles and other surface defects are not allowed; for exterior panels, no defects that affect the appearance quality are allowed to occur to meet the phased quality goals.
(2) Quality control of inspection tool development
The early stage structure of the inspection tool development needs to pass multiple reviews and approval before being put into production and processing.
Quality control of the inspection tool base: the materials meet the design requirements; the welds of the base weldments must be beautiful and full, and defects such as weak welding, missing welds and undercuts must not occur, and welding spatter must be removed; welded structural parts must be completely stress-relieved and annealed.
BASE board quality control: flatness meets design requirements; roughness Ra1.6; coordinate lines are required to be engraved on the base plate (the coordinate lines must be complete) and extended to the inspection tool body, and the coordinate line position relative to the reference error is 0.2 /1000; the scribing depth and width are both 0.1~0.2mm (a scribing machine is required for scribing).
Quality control of the body: after the processing of the lowest resin surface is completed, the thickness must be more than 60mm to meet the stability and reliability requirements of the use of the inspection tool; for parts with lower flanges, the lowest point of the measurement point on the surface of the inspection tool must be at the bottom The height of the upper surface of the platform is greater than 100mm; normal measurement of the steel plate ruler, surface difference table and gap ruler is ensured; the accuracy meets the design requirements.
The clamp fixed seat is stable and firm; it can move freely without interference; the clamp adopts JiaShou and Jiahe standards.
Debugging and production phase
During the debugging and production phase, reasonable testing points and testing standards can be formulated based on factors such as product stability, compliance with tolerance dimensions, and impact on on-site production and subsequent customers. The specific work is as follows:
1. Establishment of testing points
Based on the actual loading situation and product requirements, reasonable stamping quality inspection points are formulated.
The implementation steps are:
(1) Based on the functions of the stamping parts, identify their key and important parts as one of the basis for testing the stamping quality;
(2) Based on the product quality standard book and the overlap and positioning relationships during the welding process, determine the key and important parts of the product as one of the required quality inspection items;
(3) During the PT production process, pay attention to the changes in stamping production process parameters and the quality instability points caused by the product. Summarize and record them as one of the required quality inspection items; for detection points that are stable and have no impact on loading , which can be used as a production control point after review and verification.
2. Formulation of testing standards
Identify the parts that affect the accuracy of the body-in-white, carry out key rectifications, analyze the test data for the parts that do not affect the accuracy of the body, and revise the data based on the actual test values of the product.
Implementation steps:
(1) Collect testing data during the development process of parts (data from each batch of samples and PT1, and the average of the testing data of no less than 3 parts in each batch);
(2) Analyze the test data according to the same parts and parts to determine whether the distribution of the data tends to a stable value;
(3) Based on the impact of historical data on customers during the part development process, the welding and loading results of each batch of parts, and the distribution of part data, the actual detection values of the product are revised and adjusted.
Finally, based on the revised data of the actual product detection values and the determined final detection points, the inspection benchmark document is modified and distributed to the workshop.
