A luggage compartment mold compensation processing method considering thermal coupling error

Abstract

The application discloses a luggage compartment mold compensation processing method considering thermal coupling error. In the application, a global compensation vector is divided into a plurality of sub-regions according to a surface curvature, the deformation main direction in each sub-region is relatively consistent, and a nonlinear compensation constitutive relation is established in the direction, so that the compensation amount is quadratically increased with the increase of equivalent stress, and the insufficient representation of linear compensation when facing the thermal softening and hardening coupling effect is overcome. The addition of the boundary continuous constraint enables the displacement on the public boundary to be automatically kept in coordination after independent compensation of each sub-region, and surface ripples or step-like defects caused by sudden changes of tool paths are avoided. After the partition nonlinear processing, the spatial distribution of the compensation field and the geometric shape of the real deformation field are more consistent, the surface at each position can uniformly rebound to the theoretical design position after demolding and cooling, and therefore the accuracy in the whole processing process is improved.

Description

Technical Field

[0001] This invention belongs to the field of automotive parts manufacturing technology, specifically a method for compensating luggage compartment molds by considering thermal coupling errors. Background Technology

[0002] As a crucial forming tool for key exterior body panels of automobiles, the surface accuracy of the luggage compartment mold directly affects the assembly clearance and surface difference quality between the luggage compartment cover and surrounding components such as the side panels and rear bumper. With OEMs continuously increasing their requirements for the dimensional accuracy of the body-in-white, the contour tolerance of the mold surface has been compressed from the traditional 0.3 mm to within 0.15 mm, placing extremely high demands on the mold compensation machining process after heat treatment. Currently, the most commonly used surface compensation method in the mold precision machining field is the overall offset method. This method involves acquiring discrete point clouds of the surface using a coordinate measuring machine after the mold has been quenched and rough-machined. All measured points are compared with the normal deviation of the theoretical digital model to calculate the average deformation of the entire surface. Then, a machining path is generated by uniformly offsetting the toolpath along the normal direction. During multiple trial cutting cycles, the operator repeatedly adjusts the global compensation coefficient based on each measurement result until the surface clearance meets the assembly requirements. For some small molds with relatively simple structures and gentle curvature changes, this method can still meet basic accuracy requirements.

[0003] However, in existing technologies, due to the different curvatures of different surface regions, different material flow directions, and extremely uneven distribution of residual stress, the main deformation direction and deformation amount exhibit strong nonlinear differences in local areas, and global uniform compensation cannot capture these local differences. Summary of the Invention

[0004] The purpose of this invention is to provide a baggage compartment mold compensation processing method that takes into account thermal coupling errors in order to solve the problems mentioned above.

[0005] The technical solution adopted in this invention is as follows: A method for compensating for baggage compartment mold errors considering thermal coupling, comprising the following steps:

[0006] S1: Collect the three-dimensional geometric model and injection molding process parameters of the car luggage compartment mold, and discretize the model into a finite element mesh. At the same time, record the thermophysical parameters, elastic modulus and thermal expansion coefficient of the mold material. Input the above data into the thermo-mechanical coupling analysis platform as the initial boundary conditions for subsequent error calculation.

[0007] S2: Based on the mesh model and process parameters established in S1, temperature fluctuations and internal pressure of the mold during the injection molding process are applied. Through thermo-coupling finite element simulation, the nodal displacement field of the mold surface at each time step is extracted. The displacement field is time-series averaged to obtain the comprehensive deformation distribution of the surface in the stable production stage. The deformation is then reverse-symbolized as a preliminary global compensation vector.

[0008] S3: The global compensation vector obtained in S2 is divided into multiple sub-regions according to the surface curvature change. Within each sub-region, the main compensation direction is determined based on the main deformation direction, and a nonlinear relationship is established between the compensation amount and the residual stress of the surface. This constructs an iterative compensation function under the condition that the boundary displacement of each sub-region remains continuous. After each iteration updates the compensation amount, the new compensation amount is immediately sent back to the simulation model in S2 to recalculate the surface displacement under temperature fluctuations and internal pressure. This process is repeated until the overall change of the compensation amount between two adjacent iterations tends to stabilize. At this point, the non-uniform compensation mapping table for each sub-region is output. The partition structure of this mapping table is directly used for toolpath generation in S4. The simulation model in S2 is called multiple times in this step to update the displacement data.

[0009] S4: Read the compensation mapping table generated by S3, convert the spatial compensation amount of each grid node in the table into the offset vector of the tool center point, and according to the inverse kinematics of the machine tool, superimpose the offset vector onto each cutting point in the original machining tool position file; during superposition, adjust the tool axis vector segment by segment according to the sub-region sequence of S3 to ensure a smooth transition of feed rate between adjacent tool positions. The corrected tool position file is the CNC program for the initial compensation machining.

[0010] S5: The CNC program output by S4 is used to process the test piece. After processing, the actual dimensions of the surface are measured by an online 3D scanner. The measured point cloud is compared with the theoretical model in S1 to obtain the residual error distribution. If the maximum value of the residual error exceeds the tolerance zone, the residual error is used as a new initial deformation amount and fed back to the simulation input of S2. S2 to S4 are executed once. Otherwise, the current compensation program is determined as the final batch processing scheme.

[0011] In a preferred embodiment, in step S1, the mold surface for data preparation and model processing includes complex features such as rear sidewalls, bottom plates, side walls, multiple reinforcing ribs, and mounting bosses.

[0012] The entire CAD solid model of the mold was imported into the finite element preprocessing software in a common format. Second-order tetrahedral elements were used to discretize the entire geometry. A denser mesh was applied to areas with severe curvature, such as the roots of stiffeners, the edges of bosses, and corners, to capture temperature and stress gradients. A sparser mesh was used on large, gently sloping surfaces to balance computational efficiency, while ensuring that all mesh quality parameters, such as aspect ratio and warpage, met the solver requirements. The thermophysical and mechanical properties of the mold material were entered, including thermal conductivity, specific heat capacity, density, linear expansion coefficient curves as a function of temperature, as well as elastic modulus and Poisson's ratio. These parameters were selected from standard manuals based on the material grade and covered the entire temperature analysis range.

[0013] In a preferred embodiment, in step S2, a time-varying thermal convection load is applied to the outer surface of the mold to simulate heat exchange with air and cooling channels. The heat transfer coefficient is selected according to different parts of the mold surface, with higher values ​​near the cooling channels and lower values ​​on the exposed surface. A pressure distribution is applied to the entire inner cavity surface, decreasing exponentially or linearly from the peak pressure to zero. This pressure reaches its maximum at the completion of filling and is then gradually unloaded during the holding and cooling stages. An implicit time integration scheme is used to perform transient solutions on the entire model, with the time step automatically adjusted according to the temperature change rate to ensure convergence. The total computation time fully covers the entire process from melt filling, holding and shrinkage compensation to complete cooling and demolding.

[0014] In a preferred embodiment, in step S3, the global nodal displacement field obtained in S2 is transformed onto the surface mesh, and the deformation gradient of each mesh element is calculated to obtain the Green strain tensor. Based on the Gaussian curvature and mean curvature at each node, region growing clustering is performed, grouping nodes with gentle curvature changes and consistent principal strain directions into the same sub-region. The entire surface is thus divided into dozens of surface patches. For each sub-region, the principal strain direction of the mean strain tensor in that region is extracted using the principal direction determination formula, and this direction is used as the principal feed direction during compensation machining of that sub-region. This direction not only provides an anisotropic coordinate reference for subsequent nonlinear compensation but also indicates the preferred adjustment axis for tool axis adjustment in S4.

[0015] In determining the principal direction of the compensating cut within a sub-region, this direction is defined as the eigenvector corresponding to the largest eigenvalue of the green strain in the local deformation gradient tensor:

[0016]

[0017] in Let be the unit vector of the principal compensation direction for the p-th sub-region. Search for the direction vector for any unit. Let T be the Green-Lagrange strain tensor constructed for the average deformation gradient of nodes in the p-th subregion, with the superscript T indicating vector transpose. This formula extracts the direction of the maximum principal strain in the region through extremum search, serving as the preferred reference axis for subsequent tool axis adjustment.

[0018] In a preferred embodiment, in step S3, within each sub-region, the second invariant of the nodal stress deviator tensor output from simulation S2 is taken as the equivalent driving force. Considering the hardening-softening coupling effect of the die steel under repeated thermal shock, the compensation depth exhibits a quadratic growth relationship with the equivalent driving force. This relationship is calibrated using material tensile data at the corresponding temperature of the sub-region, forming a nonlinear compensation constitutive formula, which includes a quadratic term and a linear term adjusted with the average temperature of the region, thus appropriately increasing the compensation amount in the high-temperature region. The formula coefficients for each sub-region are determined independently, thus reflecting the differences in local thermodynamic states.

[0019] The linear compensation constitutive equation is used to transform the local equivalent stress into the normal compensation depth at that node:

[0020]

[0021] in Let be the normal compensation amount for the node with planar coordinates (x, y) in the p-th sub-region. This is the Mises equivalent stress at this node, derived from the second invariant of the deviatoric stress tensor. This represents the secondary strengthening coefficient of the sub-region. To vary with the average temperature of this sub-region The softening coefficient changes linearly. This is the elastic rebound reference constant for this sub-region. The quadratic term in this formula dominates the compensation increment in the large deformation region, while the linear term controls the compensation principal quantity in the medium stress range.

[0022] In a preferred embodiment, in step S3, after combining the compensation amounts independently calculated for each sub-region into the initial field of the entire surface, a compensation step will inevitably occur at the common boundary of adjacent sub-regions, which will disrupt the toolpath continuity. Therefore, a sum of squared displacement differences is constructed at the common boundary nodes and added as a penalty term to the global objective function. The penalized nonlinear equations are solved iteratively using fixed points. After obtaining a new compensation amount in each iteration, this compensation amount is superimposed onto the original surface node coordinates to form a corrected geometric model. The corrected model is then sent back to S2 to re-execute the thermodynamic simulation to obtain a new displacement field. This displacement field is used to update the residual error, which is then substituted into the constitutive formula to generate the next round of compensation. The iteration continues until the relative rate of change of the root mean square of the compensation vectors in the two rounds of the iteration convergence criterion formula is lower than the preset tolerance. At this point, the compensation values ​​of each node are locked and organized into a mapping table according to the sub-region number. The table simultaneously records the region number to which the node belongs and the main direction angle of that region, directly providing a partition index for the tool position offset in S4.

[0023] The iterative convergence criterion under continuous boundary constraints is used to terminate the iterative solution process of the entire compensation field:

[0024]

[0025] in Let N be the relative rate of change at the k-th iteration, and N be the total number of mesh nodes on the surface. Let be the normal compensation amount of the i-th node after the k-th iteration. The numerator is the compensation amount for the corresponding node in the previous iteration, the denominator is the root mean square value of the difference between the full surface compensation in the two iterations, and the denominator is the root mean square value of the compensation value in the previous iteration. This is a convergence tolerance limit pre-defined in engineering. This criterion also implies that the boundary continuity penalty term has converged, because the compensation value of each node has been corrected in the iteration through the displacement difference constraint of the common nodes of adjacent subdomains.

[0026] In a preferred embodiment, in step S4, the three vertices of the triangular facet where the cutting point is projected and their respective stored normal compensation depth values ​​are retrieved from the compensation mapping table output in S3. The actual normal compensation amount at the point is obtained by linear interpolation of the three vertex compensation values ​​using an area coordinate algorithm. The unit outward normal direction of the surface at the point is calculated and obtained by normalizing the cross product of the two edge vectors of the triangular facet where the point is located. The interpolated normal compensation amount is superimposed along this normal direction onto the nominal tool contact point coordinates to obtain the compensated actual tool contact point spatial position. Based on the radius of the selected ball end mill, the compensated tool contact point is pushed outward along the same normal direction by a tool radius to the center of the ball; this position is the theoretical tool center point without considering the rotation axis oscillation.

[0027] In a preferred embodiment, in step S4, during the inverse kinematics solution of the machine tool, the offset tool axis attitude angle is decomposed into command values ​​for each rotary axis based on the topology of the five-axis machining center. This solution process includes solving the transcendental equation containing the arctangent function and selecting the set with the smallest change in rotary axis angle from multiple feasible angles to ensure smooth movement between adjacent tool positions. The additional translation of the tool center point caused by the rotary axis angle is subtracted from the sphere center coordinates using the position coupling relationship in the machine tool kinematic chain. This additional translation is determined by the tool length and spindle swing length, thus separating the actual command coordinates of the three linear axes. The above five coordinate values ​​are subtracted from the corresponding values ​​in the original tool position file to obtain the five-axis offset vector of the cutting point. This vector is superimposed on the corresponding point in the original tool position file to complete the point correction. After sequentially traversing all cutting points of the entire luggage compartment mold, the CNC program for the initial compensation machining is output.

[0028] In a preferred embodiment, in step S5, a large feed rate is used to quickly remove excess material during the roughing stage, while the finishing stage switches to a smaller feed rate and a high spindle speed to ensure surface quality. After machining, the workpiece is not removed; instead, a line structured light 3D scanning device is used to perform a full-field scan of all surfaces to obtain measured point cloud data.

[0029] In a preferred embodiment, in step S5, if the maximum absolute value of the residual error on the entire profile exceeds the tolerance zone specified in the drawing, the residual error distribution is fed back to the simulation input of S2 as a new initial deformation, replacing the original nodal displacement conditions and re-triggering the transient thermo-mechanical coupling simulation of S2 to update the displacement field. Subsequently, the compensation processes of S3 and S4 are executed again to generate the corrected CNC program. If the scanning results after the second round of machining show that the residual error in all areas of the entire profile falls within the tolerance zone, the current CNC program is locked as the final solution for formal mass production. If there are still local deviations in the second round, the above feedback loop is repeated until all profiles pass inspection.

[0030] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0031] 1. In this invention, the global compensation vector is divided into multiple sub-regions based on the surface curvature, ensuring that the principal deformation direction within each sub-region is relatively consistent. A nonlinear compensation constitutive relation is established in this direction, causing the compensation amount to increase quadratically with the increase of equivalent stress, overcoming the shortcomings of linear compensation in representing the coupled effects of thermal softening and hardening. The addition of continuous boundary constraints ensures that each sub-region automatically maintains displacement coordination on the common boundary after independent compensation, avoiding surface ripples or step-like defects caused by abrupt toolpath changes. After this partitioned nonlinear processing, the spatial distribution of the compensation field better matches the geometry of the actual deformation field, and all parts of the surface can uniformly spring back to the theoretically designed position after demolding and cooling.

[0032] 2. In this invention, during the iterative solution process, after each update of the compensation amount, it is immediately sent back to the simulation model in step S2 to recalculate the displacement field. This is equivalent to completing multiple rounds of pre-verification within the software, ensuring that the output initial compensation program has high reliability. The residual error of the actual processed scan is fed back to the input boundary. If an out-of-tolerance error is detected, a new round of compensation calculation is automatically started, forming a closed loop from simulation correction to cutting verification and then to error backfeedback. This significantly narrows the residual error after the first trial cut, and the number of subsequent iterations is effectively controlled within a limited range. This saves time and material costs associated with repeated trial cuts and manual retesting, and also significantly reduces the number of trial cuts and the process debugging cycle. Attached Figure Description

[0033] Figure 1 This is a schematic diagram illustrating the process principle of the present invention;

[0034] Figure 2 This is a schematic diagram comparing data from an embodiment of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0036] Reference Figure 1-2 A method for compensating for baggage compartment mold errors considering thermal coupling errors includes the following steps:

[0037] S1: Collect the three-dimensional geometric model and injection molding process parameters of the car luggage compartment mold, and discretize the model into a finite element mesh. At the same time, record the thermophysical parameters, elastic modulus and thermal expansion coefficient of the mold material. Input the above data into the thermo-mechanical coupling analysis platform as the initial boundary conditions for subsequent error calculation.

[0038] S2: Based on the mesh model and process parameters established in S1, temperature fluctuations and internal pressure of the mold during the injection molding process are applied. Through thermo-coupling finite element simulation, the nodal displacement field of the mold surface at each time step is extracted. The displacement field is time-series averaged to obtain the comprehensive deformation distribution of the surface in the stable production stage. The deformation is then reverse-symbolized as a preliminary global compensation vector.

[0039] S3: The global compensation vector obtained in S2 is divided into multiple sub-regions according to the surface curvature change. Within each sub-region, the main compensation direction is determined based on the main deformation direction, and a nonlinear relationship is established between the compensation amount and the residual stress of the surface. This constructs an iterative compensation function under the condition that the boundary displacement of each sub-region remains continuous. After each iteration updates the compensation amount, the new compensation amount is immediately sent back to the simulation model in S2 to recalculate the surface displacement under temperature fluctuations and internal pressure. This process is repeated until the overall change of the compensation amount between two adjacent iterations tends to stabilize. At this point, the non-uniform compensation mapping table for each sub-region is output. The partition structure of this mapping table is directly used for toolpath generation in S4. The simulation model in S2 is called multiple times in this step to update the displacement data.

[0040] S4: Read the compensation mapping table generated by S3, convert the spatial compensation amount of each grid node in the table into the offset vector of the tool center point, and according to the inverse kinematics of the machine tool, superimpose the offset vector onto each cutting point in the original machining tool position file; during superposition, adjust the tool axis vector segment by segment according to the sub-region sequence of S3 to ensure a smooth transition of feed rate between adjacent tool positions. The corrected tool position file is the CNC program for the initial compensation machining.

[0041] S5: The CNC program output by S4 is used to process the test piece. After processing, the actual dimensions of the surface are measured by an online 3D scanner. The measured point cloud is compared with the theoretical model in S1 to obtain the residual error distribution. If the maximum value of the residual error exceeds the tolerance zone, the residual error is used as a new initial deformation amount and fed back to the simulation input of S2. S2 to S4 are executed once. Otherwise, the current compensation program is determined as the final batch processing scheme.

[0042] In step S1, the mold surface for data preparation and model processing includes complex features such as the rear sidewall, bottom plate, side wall, multiple reinforcing ribs, and mounting bosses.

[0043] The entire CAD solid model of the mold was imported into the finite element preprocessing software in a common format. Second-order tetrahedral elements were used to discretize the entire geometry. A denser mesh was applied to areas with severe curvature, such as the roots of stiffeners, the edges of bosses, and corners, to capture temperature and stress gradients. A sparser mesh was used on large, gently sloping surfaces to balance computational efficiency, while ensuring that all mesh quality parameters, such as aspect ratio and warpage, met the solver requirements. The thermophysical and mechanical properties of the mold material were entered, including thermal conductivity, specific heat capacity, density, linear expansion coefficient curves as a function of temperature, as well as elastic modulus and Poisson's ratio. These parameters were selected from standard manuals based on the material grade and covered the entire temperature analysis range.

[0044] In step S2, a time-varying thermal convection load is applied to the outer surface of the mold to simulate heat exchange with air and cooling channels. The heat transfer coefficient is selected for different parts of the mold surface, with higher values ​​near the cooling channels and lower values ​​on the exposed surface. A pressure distribution is applied to the entire inner cavity surface, decreasing exponentially or linearly from the peak pressure to zero. This pressure reaches its maximum at the completion of filling and is then gradually unloaded during the holding and cooling stages. An implicit time integration scheme is used to perform transient solutions on the entire model. The time step is automatically adjusted according to the temperature change rate to ensure convergence. The total calculation time fully covers the entire process from melt filling, holding and shrinkage compensation to complete cooling and demolding.

[0045] In step S3, the global nodal displacement field obtained in S2 is transformed onto the surface mesh, and the deformation gradient of each mesh element is calculated to obtain the Green strain tensor. Based on the Gaussian curvature and mean curvature at each node, region growing clustering is performed, grouping nodes with gentle curvature changes and consistent principal strain directions into the same sub-region. The entire surface is thus divided into dozens of surface patches. For each sub-region, the principal strain direction of the mean strain tensor in that region is extracted using the principal direction determination formula, and this direction is used as the principal feed direction during compensation machining in that sub-region. This direction not only provides an anisotropic coordinate reference for subsequent nonlinear compensation but also indicates the priority adjustment axis for tool axis adjustment in S4.

[0046] In determining the principal direction of the compensating cut within a sub-region, this direction is defined as the eigenvector corresponding to the largest eigenvalue of the green strain in the local deformation gradient tensor:

[0047]

[0048] in Let be the unit vector of the principal compensation direction for the p-th sub-region. Search for the direction vector for any unit. Let T be the Green-Lagrange strain tensor constructed for the average deformation gradient of nodes in the p-th subregion, with the superscript T indicating vector transpose. This formula extracts the direction of the maximum principal strain in the region through extremum search, serving as the preferred reference axis for subsequent tool axis adjustment.

[0049] In step S3, within each sub-region, the second invariant of the nodal stress deviatoric tensor output from simulation S2 is taken as the equivalent driving force. Considering the hardening-softening coupling effect of the die steel under repeated thermal shock, the compensation depth exhibits a quadratic growth relationship with the equivalent driving force. This relationship is calibrated using material tensile data at the corresponding temperature of that sub-region, forming a nonlinear compensation constitutive formula. This formula includes a quadratic term and a linear term adjusted with the average temperature of the region, thus appropriately increasing the compensation amount in the high-temperature region. The formula coefficients for each sub-region are determined independently, thereby reflecting the differences in local thermodynamic states.

[0050] The linear compensation constitutive equation is used to transform the local equivalent stress into the normal compensation depth at that node:

[0051]

[0052] in Let be the normal compensation amount for the node with planar coordinates (x, y) in the p-th sub-region. This is the Mises equivalent stress at this node, derived from the second invariant of the deviatoric stress tensor. This represents the secondary strengthening coefficient of the sub-region. To vary with the average temperature of this sub-region The softening coefficient changes linearly. This is the elastic rebound reference constant for this sub-region. The quadratic term in this formula dominates the compensation increment in the large deformation region, while the linear term controls the compensation principal quantity in the medium stress range.

[0053] In step S3, after combining the compensation values ​​calculated independently for each sub-region into the initial field of the entire surface, a compensation step will inevitably occur at the common boundary of adjacent sub-regions, which will disrupt the toolpath continuity. Therefore, a sum of squared displacement differences is constructed at the common boundary nodes and added as a penalty term to the global objective function. The nonlinear equations with penalty are solved iteratively using fixed points. After obtaining a new compensation value in each iteration, this compensation value is superimposed onto the original surface node coordinates to form a corrected geometric model. The corrected model is then sent back to S2 to re-execute the thermodynamic simulation to obtain a new displacement field. This displacement field is used to update the residual error, which is then substituted into the constitutive formula to generate the next round of compensation value. The iteration continues until the relative rate of change of the root mean square of the compensation vectors in the two rounds of iteration convergence criterion is lower than the preset tolerance. At this point, the compensation values ​​of each node are locked and organized into a mapping table according to the sub-region number. The table simultaneously records the region number to which the node belongs and the principal direction angle of that region, directly providing a partition index for the tool position offset in S4.

[0054] The iterative convergence criterion under continuous boundary constraints is used to terminate the iterative solution process of the entire compensation field:

[0055]

[0056] in Let N be the relative rate of change at the k-th iteration, and N be the total number of mesh nodes on the surface. Let be the normal compensation amount of the i-th node after the k-th iteration. The numerator is the compensation amount for the corresponding node in the previous iteration, the denominator is the root mean square value of the difference between the full surface compensation in the two iterations, and the denominator is the root mean square value of the compensation value in the previous iteration. This is a convergence tolerance limit pre-defined in engineering. This criterion also implicitly assumes that the boundary continuity penalty term has converged, because the compensation value of each node has already been corrected during iterations through the displacement difference constraints of common nodes in adjacent subdomains.

[0057] In step S4, the three vertices of the triangular facet where the cutting point is projected and their respective stored normal compensation depth values ​​are retrieved from the compensation mapping table output in S3. The actual normal compensation amount at this point is obtained by linear interpolation of the three vertex compensation values ​​using an area coordinate algorithm. The unit outward normal direction of the surface at this point is calculated and obtained by normalizing the cross product of the two edge vectors of the triangular facet where the point is located. The interpolated normal compensation amount is superimposed along this normal direction onto the nominal tool contact point coordinates to obtain the compensated actual tool contact point spatial position. Based on the radius of the selected ball end mill, the compensated tool contact point is extrapolated outward along the same normal direction by a tool radius to the center of the ball. This position is the theoretical tool center point without considering the rotation axis oscillation.

[0058] In step S4, during the inverse kinematics solution of the machine tool, the offset tool axis attitude angle is decomposed into command values ​​for each rotary axis based on the topology of the five-axis machining center. This solution process includes solving the transcendental equation containing the arctangent function and selecting the set with the smallest change in rotary axis angle from multiple feasible angles to ensure smooth movement between adjacent tool positions. The additional translation of the tool center point caused by the rotary axis angle is subtracted from the sphere center coordinates using the position coupling relationship in the machine tool kinematic chain. This additional translation is determined by the tool length and spindle swing length, thus separating the actual command coordinates of the three linear axes. The above five coordinate values ​​are subtracted from the corresponding values ​​in the original tool position file to obtain the five-axis offset vector of the cutting point. This vector is then superimposed onto the corresponding point in the original tool position file to complete the correction for that point. After traversing all cutting points of the entire luggage compartment mold, the CNC program for the initial compensation machining is output.

[0059] In step S5, a larger feed rate is used to quickly remove excess material during the roughing stage, while the finishing stage switches to a smaller feed rate and a higher spindle speed to ensure surface quality. After machining, the workpiece is not removed; instead, a line structured light 3D scanning device is used to perform a full-field scan of all surfaces to obtain measured point cloud data.

[0060] In step S5, if the maximum absolute value of the residual error on the entire profile exceeds the tolerance zone specified in the drawing, the residual error distribution is fed back to the simulation input of S2 as a new initial deformation, replacing the original nodal displacement conditions and re-triggering the transient thermo-mechanical coupling simulation of S2 to update the displacement field. Subsequently, the compensation processes of S3 and S4 are executed again to generate the corrected CNC program. If the scanning results after the second round of machining show that the residual error in all areas of the entire profile falls within the tolerance zone, the current CNC program is locked as the final solution for formal mass production. If there are still local deviations in the second round, the above feedback loop is repeated until all profiles pass inspection.

[0061] Comparative example:

[0062] To visually verify the effectiveness of the baggage compartment mold compensation processing method that considers thermal coupling errors, a comparative example is designed as a reference. This comparative example adopts the traditional global uniform offset compensation scheme in the mold processing field, and is accompanied by a data table to compare the results from three dimensions: accuracy, efficiency, and consistency.

[0063] Specific implementation methods of proportionality

[0064] The comparative model adopts a traditional overall offset compensation scheme for the mold surface. After the initial trial machining, this scheme uses a coordinate measuring machine to acquire discrete point cloud data of the luggage compartment mold surface. All measured points are compared with the theoretical CAD model to calculate the arithmetic mean of the normal deviation of the entire surface. Based on this, a set of global compensation vectors of equal size covering the entire surface are generated. During the toolpath planning stage, the theoretical model is directly offset by an equal amount along the global normal, without dividing any sub-regions based on changes in surface curvature, nor distinguishing the differences in the direction of residual stress release after quenching in different areas such as luggage compartment corners, deep cavity sidewalls, and large flat surfaces. Trial assembly inspection is performed immediately after a single compensation machining. If local gaps exceed tolerances or fit is insufficient, a second equal offset is applied to all surfaces by enlarging or reducing the overall compensation coefficient, the toolpath is regenerated, and trial cutting is performed again. This process is repeated until manual inspection deems it acceptable. Throughout the process, the compensation amount is not sent back to the thermo-coupled simulation model to recalculate displacement updates under the action of the temperature and stress fields, nor are boundary steps or tool marks that may be caused by abrupt changes in compensation amount in adjacent areas considered.

[0065] Performance comparison data table and parameter calculation method

[0066] To quantify the differences between the two schemes, the same batch of quenched luggage compartment inner panel molds were selected as the test objects. The mold has a surface size of 1200 mm by 800 mm and a maximum depth of 150 mm. It includes three corner areas with a radius of less than 30 mm and a large side wall plane with a length of more than 400 mm. Figure 2 The core indicators of the two schemes in terms of post-processing surface accuracy, boundary quality, and efficiency are listed.

[0067] Table 1 Comparison of Baggage Compartment Mold Compensation Processing Effects

[0068] The specific calculation methods for each parameter in the table above are as follows:

[0069] The maximum residual profile error and the average profile error of the profile are both taken from the best fitting alignment results between the measured point cloud after processing and the theoretical model. The absolute value of the normal deviation is used for statistics. The maximum value is the peak value among all measuring points, and the average value is the arithmetic mean of the deviations of all measuring points.

[0070] The maximum step height of the partition boundary is only clearly defined in the present invention. However, although the comparative scheme does not partition, 10 measuring points are also selected at the same spatial coordinate position, that is, near the intersection of the corner and the plane. The actual surface height difference between two adjacent processing areas is compared, and the maximum absolute value is taken to measure the degree of surface step caused by the sudden change in compensation amount.

[0071] The number of trial cuts that meet the accuracy requirements refers to the total number of processing and inspection cycles from the start of initial processing until the quality inspector confirms that the surface gap is less than the 0.2 mm tolerance zone.

[0072] The deviation between the predicted and measured values ​​of springback after heat treatment is determined by the amount of surface deformation after quenching. The absolute value of the difference between the springback displacement calculated by the simulation software under given process parameters and the actual displacement measured by the three-coordinate measuring machine is taken, and then the average is calculated for all measuring points.

[0073] As can be seen from the data in the table, the present invention, by means of non-uniform partitioning compensation and iterative convergence mechanism, compresses the maximum contour error to one-quarter of the comparative example, reduces the boundary step height by more than an order of magnitude, and meets the tolerance requirements on the first trial cut without the need for repeated machine stops for adjustment. This fully verifies the practical beneficial effect of the thermo-coupling error compensation method in improving the processing accuracy of large deep cavity molds and shortening the trial production cycle.

[0074] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the term "comprising" or any other variations thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0075] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for compensating for baggage compartment mold errors considering thermal coupling, characterized in that: The method includes the following steps: S1: Collect the three-dimensional geometric model and injection molding process parameters of the car luggage compartment mold, discretize the model into a finite element mesh, and record the thermal properties, elastic modulus and thermal expansion coefficient of the mold material. S2: Based on the mesh model and process parameters established in S1, apply the temperature fluctuations and internal pressure of the mold during the injection molding process, and extract the nodal displacement field of the mold surface at each time step through thermo-mechanical coupled finite element simulation. S3: Divide the global compensation vector obtained in S2 into multiple sub-regions according to the curvature change of the surface. In each sub-region, determine the main compensation direction of the region according to the main direction of deformation, and establish the nonlinear relationship between the compensation amount and the residual stress of the surface. Construct an iterative compensation function under the condition that the boundary displacement of each sub-region remains continuous. After each iteration, update the compensation amount and send the new compensation amount back to the simulation model of S2 to recalculate the surface displacement under the action of temperature fluctuation and internal pressure. Repeat this process until the overall change of the compensation amount between two adjacent times tends to be stable. At this time, output the non-uniform compensation mapping table of each sub-region. S4: Read the compensation mapping table generated by S3, convert the spatial compensation amount of each grid node in the table into the offset vector of the tool center point, and according to the inverse kinematics solution of the machine tool, superimpose the offset vector onto each cutting point in the original machining tool position file; S5: The CNC program output by S4 is used to process the test piece. After processing, the actual dimensions of the surface are measured by an online 3D scanner. The measured point cloud is compared with the theoretical model in S1 to obtain the residual error distribution. If the maximum value of the residual error exceeds the tolerance zone, the residual error is used as a new initial deformation amount and fed back to the simulation input of S2. S2 to S4 are executed once. Otherwise, the current compensation program is determined as the final batch processing scheme.

2. The baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S1, the mold surface for data preparation and model processing includes the rear sidewall, bottom plate, side wall, and multiple reinforcing ribs and mounting bosses. Enter the thermal and mechanical properties of the mold material, including thermal conductivity, specific heat capacity, density, linear expansion coefficient curve as a function of temperature, elastic modulus and Poisson's ratio. These parameters are selected from the standard manual according to the material grade and cover the entire temperature analysis range.

3. The baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S2, a time-varying thermal convection load is applied to the outer surface of the mold to simulate heat exchange with air and cooling channels. The heat transfer coefficient is selected according to different parts of the mold surface, with a higher value in the area near the cooling channels and a lower value on the exposed surface. A pressure distribution is applied to the entire inner cavity surface, which decreases exponentially or linearly from the peak pressure to zero. The pressure reaches its maximum at the moment of filling and is then gradually unloaded during the pressure holding and cooling stages.

4. The baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S3, the global nodal displacement field obtained in S2 is transformed onto the surface mesh, the deformation gradient of each mesh element is calculated, and then the Green strain tensor is obtained; based on the Gaussian curvature and average curvature at each node, region growth clustering is performed, and nodes with gentle curvature changes and consistent strain principal directions are grouped into the same sub-region, and the entire surface is divided into dozens of surface patches accordingly. For each sub-region, the principal strain direction of the mean strain tensor of that region is extracted using the principal direction determination formula, and this direction is used as the principal feed direction during the compensation machining of that sub-region. This direction not only provides anisotropic coordinate reference for subsequent nonlinear compensation, but also indicates the priority adjustment axis for tool axis adjustment in S4. In determining the principal direction of the compensating cut within a sub-region, this direction is defined as the eigenvector corresponding to the largest eigenvalue of the green strain in the local deformation gradient tensor: in Let be the unit vector of the principal compensation direction for the p-th sub-region. Search for the direction vector for any unit. The Green-Lagrange strain tensor is constructed for the average deformation gradient of nodes in the p-th subregion, with the superscript T indicating vector transpose. This formula extracts the direction of the maximum principal strain in the region through extremum search, which serves as the preferred reference axis for subsequent tool axis adjustment.

5. A baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S3, within each sub-region, the second invariant of the nodal stress deviator tensor output from S2 simulation is taken as the equivalent driving force. Considering the hardening and softening coupling effect of the die steel under repeated thermal shock, the compensation depth and the equivalent driving force have a quadratic growth relationship. The relationship is calibrated using the material tensile data at the corresponding temperature of the sub-region, forming a nonlinear compensation constitutive formula, which includes a quadratic term and a linear term that adjusts with the average temperature of the region, so that the compensation amount in the high-temperature region is appropriately increased. The formula coefficients of each sub-region are determined independently, thus reflecting the differences in local thermodynamic states. The linear compensation constitutive equation is used to transform the local equivalent stress into the normal compensation depth at that node: in Let be the normal compensation amount for the node with planar coordinates (x, y) in the p-th sub-region. This is the Mises equivalent stress at this node, derived from the second invariant of the deviatoric stress tensor. This represents the secondary strengthening coefficient of the sub-region. To vary with the average temperature of this sub-region The softening coefficient changes linearly. This is the elastic rebound reference constant for this sub-region; the quadratic term in this formula dominates the compensation increment in the large deformation region, while the linear term controls the compensation principal quantity in the medium stress range.

6. A baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S3, after combining the compensation amounts calculated independently by each sub-region into the initial field of the entire surface, a compensation step will inevitably appear on the common boundary of adjacent sub-regions, which will destroy the continuity of the toolpath; therefore, a sum of squared displacement differences is constructed on the common boundary nodes and added to the global objective function as a penalty term. The nonlinear equations with penalties are solved by fixed-point iteration. After each iteration, a new compensation amount is obtained and superimposed on the original surface node coordinates to form a corrected geometric model. The corrected model is then sent back to S2 to re-execute the thermodynamic simulation to obtain a new displacement field. The residual error is updated using this displacement field and then substituted into the constitutive formula to generate the next round of compensation amount. The iteration continues until the relative rate of change of the root mean square of the compensation vectors in the two rounds of iteration convergence criterion formula is lower than the preset tolerance. At this point, the compensation values ​​of each node are locked and organized into a mapping table according to the sub-region number. The table records the region number to which the node belongs and the main direction angle of that region, directly providing a partition index for the tool offset in S4. The iterative convergence criterion under continuous boundary constraints is used to terminate the iterative solution process of the entire compensation field: in Let N be the relative rate of change at the k-th iteration, and N be the total number of mesh nodes on the surface. Let be the normal compensation amount of the i-th node after the k-th iteration. The numerator is the compensation amount for the corresponding node in the previous iteration, the denominator is the root mean square value of the difference between the full surface compensation in the two iterations, and the denominator is the root mean square value of the compensation value in the previous iteration. This is the convergence tolerance limit predetermined in the engineering process.

7. A baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S4, the three vertices of the triangular facet where the cutting point is projected and their respective stored normal compensation depth values ​​are retrieved from the compensation mapping table output in S3. The actual normal compensation amount at the point is obtained by linear interpolation of the compensation values ​​of the three vertices using the area coordinate algorithm.

8. A baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S4, during the inverse kinematics solution of the machine tool, the offset tool axis attitude angle is decomposed into command values ​​for each rotary axis based on the topology of the five-axis machining center. The solution process calculates the transcendental equation containing the arctangent function and selects the set with the smallest change in rotary axis angle from multiple feasible angles to ensure smooth movement between adjacent tool positions. The position coupling relationship in the machine tool kinematic chain is used to deduct the additional translation of the tool center point caused by the rotary axis angle from the sphere center coordinates. This additional translation is determined by the tool length and spindle swing length, thereby separating the actual command coordinates of the three linear axes. Subtract the five coordinate values ​​mentioned above from the corresponding values ​​in the original toolpath file to obtain the five-axis offset vector of the cutting point. Superimpose this vector onto the corresponding point in the original toolpath file to complete the correction of that point.

9. A baggage compartment mold compensation processing method considering thermo-coupling errors as described in claim 1, characterized in that: In step S5, a larger feed rate is used to quickly remove the excess material during the roughing stage, and a smaller feed rate and a higher spindle speed are used during the finishing stage to ensure the surface quality. After machining, the workpiece is not removed, and the entire surface is scanned using a line structured light 3D scanning device to obtain measured point cloud data.

10. A method for compensating baggage compartment mold errors considering thermo-coupling errors as described in claim 1, characterized in that: In step S5, if the maximum absolute value of the residual error on the entire surface exceeds the tolerance zone specified in the drawing, the residual error distribution is fed back to the simulation input of S2 as a new initial deformation, replacing the original nodal displacement conditions to re-trigger the transient thermo-mechanical coupling simulation of S2 to update the displacement field. Then, the compensation process of S3 and S4 is executed again to generate the corrected CNC program.

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