During injection process, mold is subjected to combined effects of mechanical force and heat transfer, such as extrusion and collision of parting surface during mold opening and closing process, pressure of melt in cavity on cavity wall and inner wall of core, thermal stress caused by temperature gradient, and clamping force of injection molding machine [1]. Under simultaneous action of many factors, mold deforms elastically in a short period of time. Large-scale molds with high rigidity requirements are more likely to deform. While dimensional accuracy of mold decreases, flashing or even spraying may occur, product demolding is also affected. Current main countermeasures for enterprises are to replace high-quality mold materials or increase thickness of mold plate based on experience to ensure mold rigidity and strength, but correspondingly, unnecessary production costs will be increased and competitiveness of enterprises will be reduced. In order to solve above problems, it is necessary to optimize based on analysis of mold structure.
In mold structure analysis, cavity load and other data manually extracted from injection CAE software will have large errors when loaded in structural CAE software. Using interface between injection CAE software and structural CAE software, inputting mold flow analysis results into structural analysis as boundary conditions, more realistic and effective simulation results can be obtained. Co-simulation is carried out through software interface of injection CAE software Moldex3D and structural CAE software ABAQUS, mold structure can be analyzed more accurately.

 

(A) Mould

 

(B) Injection parts
Figure 1 3D model of mold and injection parts
Take a stent injection mold as research object, product material is PC, shape size is 25mm * 32mm * 8mm, wall thickness is 1.5mm, mold size is 350mm*280mm*260mm, layout is 8 cavities, and side gate feeds, as shown in Figure 1.

 

Figure 2 Simplified model and size
Effects of large-size features in injection mold on deformation of mold are studied, order is as follows: cooling system, guiding parts, and gating system. Based on research content, guide components and push-out system are simplified, two supporting cylinders with a diameter of ϕ34mm are retained, detailed features of non-formed parts are deleted, as shown in Figure 2.

 

(A) Mould base (b) Cavity core
Figure 3 Mold base and cavity core
Divide mold into two parts: cavity core and rest of mold base, as shown in Figure 3. According to actual process parameters, with cavity core as mold base, mold flow analysis of product is performed in Moldex3D. Mlodex3D is an injection CAE software that can perform analysis of filling, packing, cooling, warpage, stress and optics. It is easy to operate and highly accurate, has powerful calculation speed of 3D grids.

 

Figure 4 Moldex 3D analysis model
First, import product in Moldex3D and establish cooling water circuit, gating system and mold base, divide solid mesh, import performance parameters of injection material, and select mold material as P20. Set injection pressure of 85MPa, injection time of 2s, holding pressure of 80MPa, and holding time of 5s for mold flow analysis. Moldex3D analysis model is shown in Figure 4.

 

(A) FEA interface

 

(B) ABAQUS analysis model
Figure 5 FEA interface and ABAQUS analysis model
Based on results of mold flow analysis, in FEA interface of Moldex3D, surface mesh node pressure of product at a certain moment is mapped to cavity core surface mesh node by interpolation mapping method, cavity core with node load is derived INP file, import it into ABAQUS to assemble with mold base and perform structural analysis. FEA interface and ABAQUS analysis model are shown in Figure 5.

 

Figure 6 Interaction of contact surfaces between parts of mold
Sufficient clamping force is applied to mold, corresponding pressure is applied to main runner of mold base, parting surface, core cavity and contact surface of mold base are selected for hard contact. Interaction and boundary conditions are defined as shown in Figure 6.
Constraints: Interface 1 (contact surface of fixed mold base plate and injection molding machine mold plate): fixed in x and y directions; interface 2 (contact surface of fixed mold base plate and fixed mold plate): fixed in x and z directions; interface 3 (contact surface of core cavity insert and mold base) and interface 4 (parting surface): surface hard contact; interface 5 (contact surface of pad and column with movable mold plate and movable mold seat plate): binding; interface 6 (contact surface between movable mold base plate and injection molding machine mold plate): completely fixed.

Calculation of maximum allowable deformation of mold mainly considers three issues: ① no flash can be generated during injection process; ② product can be demolded smoothly; ③ to ensure dimensional accuracy of product. Injection material here is PC, and its overflow value is 0.04mm as maximum allowable deformation value.
During injection process, melt pressure gradient is the largest, gate is 85MPa, pressure at flow front is close to 0, and cavity pressure field is not established completely. After injection is completed and pressure is maintained, internal pressure field of cavity is established. At this time, force of cavity and core is more uniform and deformation range is larger. Therefore, maximum cavity pressure will appear during period of time when pressure is just started. As holding pressure continues, product begins to cool and solidify, cavity pressure also decreases.

 

Figure 7 Stress distribution of contact surface between cavity core and mold base when pressure is held for 0.17s
In order to obtain real contact stress between core cavity and mold base, when exporting from FEA interface, select INP files at the time before and after packing start, import them into ABAQUS to assemble with mold base and analyze contact surface stress, as shown in Figure 7. Results show that maximum contact stress between core cavity and mold base is 17.9 MPa when pressure is maintained for 0.17 s.
Based on results of contact stress analysis, assuming that mold base is subjected to a uniform load of 20 MPa, mechanical analysis and calculation of mold base are carried out. Objects of analysis and calculation are fixed mold plate and movable mold plate, force models of side wall and bottom plate are established respectively. Generally, stiffness or strength calculation is selected according to size of mold, and criticality is L Lin. The longest dimension is greater than L Lin, and stiffness calculation is appropriate. Otherwise, strength calculation is appropriate.

 

Where: E——material elastic modulus, MPa; P——uniform load, MPa; [δ]——allowable maximum deformation; [σ]——allowable stress, MPa.
Material of mold base is No. 45 steel, so E=210000MPa, [σ]=222MPa, [δ]=0.04mm, P=20MPa, calculated Ln=128.5mm, the longest side wall of mold base is 250mm, so stiffness calculation is selected.

 

Figure 8 Simplified model of side wall

 

Figure 9 Force model of rectangular plate with three sides fixed and one side free
For side wall can be regarded as a rectangular plate with one side free and three sides fixed, it is assumed that maximum deformation δmax and maximum stress σmax occur at midpoint of free side. Simplified model of side wall is shown in Figure 8, and force model is shown in Figure 9.
Calculated by stiffness: δmax=C1Ph4ES3≤[δ] Where: h——total height of side wall, mm; L——long side of side wall, mm; S——thickness of plate, mm; C1——calculation coefficient of a rectangular plate with three sides fixed and one side free.
Table 1 Coefficient of rectangular plate with three sides fixed and one side free

 

C1 can be through look-up table 1 or approximate formula

 

By calculation, take E=210000MPa and [δ]=0.04mm. Long side of fixed mold plate L=250mm, total height of side wall h=30mm, uniform load P=20MPa, take C1=1.485,

 

Long side of movable formwork L=250mm, total height of side wall h is 40mm, uniform load P=20MPa, take C1=1.45,

 

 

Figure 10 Simplified model of movable mold base plate

 

Figure 11 Force model of rectangular plate fixed on four sides
For movable mold base plate, it can be regarded as a rectangular plate with four sides fixed. It is assumed that maximum deformation δmax and the maximum stress σmax occur in the center of plate. Simplified model of movable mold base plate is shown in Figure 10, and force model is shown in Figure 11. Calculated by stiffness,

 

In formula: l——length of short side of bottom plate, mm; C2——calculation coefficient of rectangular plate fixed on four sides.

 

Table 2 Coefficient of fixed rectangular plate on four sides
C2 can be obtained by looking up Table 2 or an approximate formula.

 

Fixed mold plate and movable mold plate have same L/l, long side L=250mm, short side l=135mm, uniform load P=20MPa, and C2=0.0269 through interpolation look-up table method,

 

Structure of mold base is optimized to minimize thickness of mold base while ensuring rigidity. According to calculation result of stiffness, reduce thickness of side wall to 25mm and thickness of movable mold seat plate to 30mm.

 

(A) Filling end time

 

(B) Holding pressure 0.17s

 

(C) Holding pressure 0.57s
Figure 12 Surface pressure distribution of products at different times
Based on software interface of Moldex3D and ABAQUS, surface pressure of product is regarded as pressure on the core of cavity, so surface pressure of product before and after packing is selected as cavity load. Surface pressure distribution of product at different times is shown in Figure 12.

 

(A) Filling end time#

 

(B Holding pressure 0.17s

 

(C) Holding pressure 0.57s

Figure 13 Displacement analysis results at different moments
In ABAQUS, INP models at three moments of filling end time, 0.17s and 0.57s were respectively assembled with optimized mold base and analyzed for structure. Displacement analysis results at different times are shown in Figure 13.
Maximum deformation at three moments all occurred inside cavity, and maximum deformation values were very close, respectively 0.00735, 0.00743, and 0.00734mm. Main reason was that maximum pressure at three moments was not much different. At the end of filling, cavity pressure distribution is uneven and gradient is large. Filling end pressure is close to 0, runner and gate pressure are close to maximum injection pressure. Therefore, internal deformation of cavity is mainly concentrated near runner and gate. After pressure is maintained, internal pressure of cavity is gradually uniform, deformation range becomes larger, and maximum deformation is mainly in stress concentration part of cavity. True deformation of maximum cavity pressure is less than maximum allowable deformation of 0.04mm, so optimized mold meets needs of use.
In the study of mold deformation, cavity load input method of structural analysis has always been research focus. Map product surface pressure load in mold flow analysis to core cavity grid node through FEA interface of Moldex3D, import it into ABAQUS for structural analysis to improve accuracy of simulation calculation. To optimize structure of mold base, through calculation of mold stiffness and finite element analysis, under premise of ensuring requirements of mold use, thickness of mold plate is reduced and cost of mold development is reduced.