The pace of vehicle model updating accelerates with the growing competition in the automotive industry, resulting in shorter development cycles. The mold development department needs to summarize and analyze each aspect of mold development to shorten the development cycle and enhance the efficiency of mold development.
The development of automotive body panel molds is divided into two stages: design and manufacturing. Design includes stamping SE, stamping process DL diagram design, CAE analysis and mold surface compensation design, CAM mold surface design, mold structure design, FMC, structure, and mold surface programming.
Manufacturing includes FMC production, cast and forged blank preparation, primary processing, initial assembly, secondary processing, assembly, fitting, debugging, quality improvement, and delivery.
In the mold development cycle, the entire vehicle’s mold design cycle is typically about 4-5 months, while the mold manufacturing cycle can reach 12 months, with debugging and quality improvement taking up 5-6.5 months.
Therefore, improving the manufacturing efficiency of molds is significant for shortening the development cycle.
I. Status of Body Panel Mold Design and Manufacturing
In recent years, the development of body panel molds has made significant technological advances, with most achieving three-dimensional DL diagram design and full-process CAE analysis.
Mold surface compensation design has begun, and uneven gap design is implemented for mold surfaces. Mold structures are designed as three-dimensional entities, with simulations for dynamic interference, waste sliding out, and stamping automatic line.
In terms of mold manufacturing, three-dimensional processing with FMC has been implemented, primary processing is fully programmed, secondary processing is automated, and mold hardening techniques are being promoted (including medium frequency hardening and flame hardening).
Some companies have initiated laser hardening and post-hardening mold surface precision processing. A “one-flow” manufacturing model for mold manufacturing has been introduced.
The long mold manufacturing cycle is mainly due to difficulties in clamping certain parts, inappropriate gaps between the concave and convex molds, large volumes of fitting work, repeated debugging and rectification due to defects such as part cracking, wrinkling, and rebound, improper plan management, and design errors.
II. Problems in Process and Mold Surface Design
- The product’s processability is unreasonable, which can’t be fully solved from a stamping process perspective, making it difficult to meet quality requirements and complicating debugging, leading to mold modifications or repeated debugging.
- The parameters set for CAE analysis are unreasonable, or the material properties used for the analysis surpass those actually used in production. The failure to consider safety margins or special product requirements leads to significant deviations between process design data and manufacturing debugging results, causing repeated debugging.
- There are defects in process data or part product data, leading to resolutions during later debugging.
- The design of mold surface gaps is unreasonable, leading to a large amount of fitting work in the later stages. The design does not consider material thickness changes during part forming, machine tool sag compensation, or part expansion treatment. Non-colored parts of the roof cover drawing mold are designed according to material thickness gaps, causing the surface allowance of colored parts to be completely fitted, wasting over 30 hours.
- The angle of the trimming edge is unreasonable, or burrs are easily generated at the junction of the positive and side trimming knives, often leading to repeated debugging. The accuracy of the trimming edge expansion or test material verification is insufficient, leading to edge adjustment and welding processing. In particular, sometimes the modified edge is not vertical, not sharp, and uneven, leading to burrs and repeated debugging.
- The mold surface design does not consider root cleaning or large pressure areas, requiring fitters to clean roots or a lot of fitting work.
- The transfer of process information is not in place. For example, fitters do not understand the fitting requirements of various parts of the mold, and surface treatment personnel do not understand the quenching area, leading to rework or long information confirmation time.
III. Problems in Mold Design and Manufacturing Process
- Exhaust holes and screw holes in the mold are not designed, requiring fitters to make them. Fitters find it time-consuming and laborious to find exhaust hole positions for back-opening ribs, and screw hole matching requires serial processing, resulting in long cycles. The efficiency of lateral hole punching by fitters is low.
- The clamping of oblique wedges, sliders, or some small splicing blocks is difficult. The design does not consider the clamping process chuck, which is often added on-site during reviews. Without a chuck, multiple clamping processes are needed during processing, resulting in low processing precision. Some real models are added on-site, and the programming does not understand the chuck position, which can cause safety hazards of tool collision and often leads to repeated programming.
- There is no standard for mold marking, and there are no markings on the design entity. Especially in non-graphic production, when the cast parts and splicing blocks just enter the factory, it is difficult to distinguish and find parts, wasting time.
- The parameters for mold surface milling need to be optimized. The traditional setting for precision machining allowance is 0.15mm, the accuracy of the mold surface after precision milling is ±0.05mm, and the surface roughness cannot meet the requirements. The dimensional accuracy is poor, the fitting work volume is large, and the cycle is long.
- In process design, the trimming, flipping, splicing block mold surfaces, and edges are processed after the mold base and splicing block screws are tightened. The serial processing of the mold base and splicing blocks affects the manufacturing cycle of the mold.
IV. Considerations for Stamping Process and Mold Surface Design
- Upon receiving the product numerical model, the stamping SE should be initiated. Combined with the database, CAE analysis results and review form content, FEMA technology is used for analysis. Communicate product issues to the product design department in the form of an ECR report, optimizing the product’s processability to the greatest extent. When designing the process scheme, take into account the quality assurance of the part.
- CAE precision calculation. Set up standards and evaluation criteria for CAE parameters for inner and outer panels and typical parts, and create a database for numerical model reconstruction and compensation plans for typical parts. For example, the stipulation of CAE safety margin, material selection requirements, thinning rate requirements for various materials, material slippage requirements, and the qualification rate requirements for the whole process analysis, only those meeting the requirements can proceed. Mark shrink lines on the mold, set a grid on the blank, and compare CAE with debugging during part debugging, integrating the results into the data.
- Add a confirmation step for processing mold surface data and product data to the design process, evaluate the outer panel parts during CAE analysis, add mold surface compensation to the indented areas, reducing the large amount of research and matching man-hours of the fitters.
- Establish mold surface clearance design standards. When designing a drawing mold surface, consider using the die heart of the stamping machine tool (inspect and debug the die heart of the punch and the workbench and slider of the user’s machine tool, and establish a database), compensate for the drawing of the mold surface according to the size and type of the part; expand the drawing mold surface design; handle the clearance of the drawing rib management surface pressing and blanking area; consider the clearance handling of the part functional surface and the pressing and blanking area for solving springback; consider the mold surface clearance compensation for material thinning; subsequent mold surface design of the mold, the negative clearance between the pressing area of the presser and the shaping patch; compensate for the mold surface clearance at the material thinning area.
- Pay attention to the problem of trimming burrs in process design, prioritize sequential trimming under permissible conditions, summarize data verification of blade line development to improve CAE analysis accuracy, ensure accurate blank development, and cancel blade line verification content.
- When establishing standard mold surface design, root cleaning design should be carried out for non-important fillets.
- Timely and accurate transfer of design information is an important part of reducing repetition. To ensure smooth information flow, establish standards and information forms, such as data handover forms, research and matching color cards, indications of quenching areas for flipping molds, etc. Standards and information forms are all stored in PDM and ERP.
V. Optimization of Mold Structure Design and Manufacturing Process
- Exhaust holes and screw holes: The design of mold exhaust holes and screw holes is accomplished via CNC milling, or with vertical hole spotting and lateral milling. The vertical holes are drilled by a fitter, which shortens the drilling time and improves accuracy.
- Establishing standard clamping support systems for parts such as irregular convex molds, wedges, and sliders, and designing reserved process chucks can unify programming and machining, improving efficiency and precision. The design of slider chucks and clamping usage is illustrated in Figure 9.
- Mold identification: By combining the characteristics of various users and mold factories, we establish mold identification standards. Identifiers or print positions are designed on casting bases and splicing blocks, and preferably cast on the parts. This allows operators to find and install according to the identifiers, which also facilitates mold maintenance. Figure 10 shows an example of repairing punch mold sliders and installation surface identifiers.
- Research on machining process parameters of numerical milling surfaces affecting precision and efficiency, in combination with the features of various parts. Optimization of parameters such as tool speed, feed, step distance, cutting mode, and allowance setting is carried out. For example, adjusting the allowance of mold surface finishing to 0.05mm and adjusting tool speed and feed can increase the efficiency of semi-fine and fine numerical milling of mold surfaces by over 40%, significantly improving surface roughness and precision.
- Adjustment of the machining process route for trimming and flipping splicing blocks: The surfaces and edges of trimming and flipping splicing blocks are processed in steps, assembled after heat treatment, and only the allowance for fine machining after quenching is left for the trimming mold edge. This saves the cycle of processing and heat treating the splicing block surfaces and edges.
With the perfection of the cover mold database and the in-depth application of analysis software, as well as the improvement of machine tool machining precision, as long as the design work in each technical stage is detailed, and the aforementioned issues are considered during the design stage, starting from the product design, process design, and mold design sources, conducting sufficient virtual verification, preventing previously occurred issues or defects found in the analysis, taking measures early, and ensuring information is delivered in place can reduce assembly, machining clamping, and auxiliary time, improve manufacturing precision, greatly reduce research and development time and debugging frequency, meet the quality requirements of stamping parts, and ultimately achieve the goal of shortening the mold manufacturing cycle.