Experimental Study and Numerical Simulation of Injection Molding Process for Special-Shaped Plastic Part

Taking the special-shaped plastic part as the research object, numeric simulation and experimental study of the injection molding process are completed based on the synthetical application of numeric simulation technology, orthogonal experiment method, Moldflow, injection machine and CMM. Optimal feeding system and molding process parameters are obtained and qualified products are produced. The research results show that the efficiency of the simulation guidance could be significantly improved by means of combining the CAE technology and production experience in actual production.


INTRODUCTION
Widely application of CAE technology has significantly improved the quality of injection parts.However, the technology of numerical simulation software is often based on a large number of simplification and assumptions, so varying degrees of deviation between the simulation results and the actual production situation will appear.Undoubtedly, this will greatly influence the use effect of the software.For this reason, it is necessary to compare and analyze the simulation results with the actual production situation in order to improve the accuracy and applicability of the simulation results.
In this paper, taking the special-shaped plastic part as the research object, numeric simulation and experimental study of the injection molding process are completed based on the synthetical application of numeric simulation technology, orthogonal experiment method, Moldflow, injection machine and CMM.Based on the analysis and comparison of the numeric simulation results with actual experiment results, application characteristics and rules of the Moldflow in optimized analysis of injection molding process are discussed.Application examples and experiences are provided for production enterprises in order to perfectly optimize the efficiency of Moldflow.

STRUCTURE ANALYSIS OF THE SPECIAL-SHAPED PLASTIC PART
The special-shaped plastic part, as shown in Fig. (1), is used to install the machine.It is connected to the machine positioning hole through the cone at the end of small cylindrical surface in order to ensure the machine in the horizontal position.The product is consisted of a large square, some small cylinder and a cone.The cone is composed of two symmetrical parts with 0.3 mm crack between them.When assembling, the crack is compressed and elasticity deformation of the cone will be produced, so that the plastic parts and matched part will be connected reliably.
During injection molding process, the cone is at the farthest place of the plastic part, so large warpage will be produced.In order to prevent the disappearance of the crack due to the too large warpage, the maximum warpage of the crack should be smaller than 0.15 mm.Therefore, the size of the crack is an important dimension.
Taking the lower surface quality requirements and small size of the plastic part into account, the mold structure form of two cavities with rectangle-shape side gate is adopted.In order to optimize the mold structure and improve the quality of injection molded parts effectively, numerical simulation of the injection molding processes based on CAE Technology is used.

NUMERICAL SIMULATION OF INJECTION MOLDING
Firstly, complete the 3D modeling of plastic parts and convert it into STL format in the Pro/E, then import the STL file to the Moldflow and generate the mesh.After the gate location analysis is completed, preliminary filling analysis should be executed in order to accurately determine the filling situation of the plastic part.

Preliminary Design of Feeding System
According to the previous analysis results of the best gate location and the structure characteristics of the plastic, two preliminary design schemes of the feeding system are determined as shown in Figs.(3, 4).Side gates of the two schemes are rectangular gates with the size of 3 mm×2 mm×0.8 mm.The side gate of the first scheme is arranged on the large front plane of the plastic part.Length of the main runner is 45 mm, small end diameter of it is 3 mm and angle of the cone is 2 degrees.The second branch runner is located between the two side gate and the first branch runner.Section of the first runner is cylindrical with 20 mm long and 5 mm in diameter, while section of the second runner is cylindrical with 10 mm long and 4 mm in diameter.
The side gate of the second scheme is arranged on the side of the plastic part.The Length of the main runner is 45 mm, small end diameter of it is 3 mm and the angle of its cone is 2 degrees.The first branch runner is located between the two side gate and the main runner.Section of the first runner are cylindrical with 8 mm long and 4 mm in diameter.

Numerical Simulation of Feeding System
To accurately judge the advantages and disadvantages of the two schemes respectively, filling analyses for two schemes of the feeding system based on Moldflow are carried out.The same data are taken for the simulation of the two schemes in order to avoid the influence of other factors.Parameters are set as follows: mold temperature is 60 degrees, melt temperature is 210 degrees, filling control method is automatic, velocity/pressure switching mode is 100% filling volume, holding pressure control mode is filling pressure and time control mode, pack pressure is 80% of the maximum injection pressure, maintenance time is 10 seconds.
Analysis result of the first scheme is shown as Fig. (5).The filling is completed in 1.819 seconds.The last filled area is cylindrical ends of the plastic parts and edges of the four top square hole.Filled time of the cylindrical ends is 1.662 seconds while filled time of the top is 1.819 seconds.Difference between the two filling time is 0.157 seconds, so the unbalance rate is 8.63%.It can meet the need of injection molding.The pressure at the end of filling is 29.30MPa, as shown in Fig. (6).
Filling time result of the second scheme is shown as Fig. (7).The filling is completed in 1.911 seconds.The last filled area is the same as that of the first scheme.Filled time of the cylindrical ends is 1.677 seconds while filled time of the top is 1.911 seconds.Difference between the two filling time is 0.334 seconds and unbalance rate is 17.47%.Generally, filling unbalance rate should be less than 5%, so the unbalance rate at this time is unable to meet the need of Injection molding.Too much unbalanced flow will cause unbalanced pressure at both ends of the plastic part, create over packing pressure and has a greater impact on the quality of products.The pressure at the end of filling is 30.36MPa, as shown in Fig. (8).Comprehensive comparison of the simulation results of the two feeding system scheme shows that the first scheme is the better plan.The filling unbalance rate of the first scheme is only 8.63%.It can be reduced to 5% by means of appropriate adjustment measures so as to meet the need of injection molding.However, the filling unbalance rate of the second scheme is 17.47%.It will cause large warpage deformation and can't meet the requirements of molding plastic parts.Pressure at the end of the filling of the first scheme is 29.30MPa while that of the second scheme is 30.36MPa, so clamping force of the first scheme is relatively small.It means that it is easy to suit the requirements of the injection molding machine.In addition, there are not much more differences between the remaining parameter values of two schemes such as weld lines, air pockets etc.Therefore, the first scheme should be selected as the mold feeding system.

EXPERIMENTAL STUDY OF INJECTION MOLDING
Based on the result of the optimization design of feeding system before, Moldflow and orthogonal experimental method are used to optimize the parameters of the injection molding process.The results show that the optimal injection molding process parameters of the special-shaped plastic part are composed of 210 degrees melt temperature, 60 degrees mold temperature, 2.2 seconds injection time, 31 MPa packing pressure and 20 seconds packing time [1].Among them, mold temperature is the most important parameter of the five process parameters which influence on the warpage, followed by melt temperature and packing time, the minimum is injection time [1].In order to verify the accuracy of the software simulations and determine the best molding process parameters, verification experiment is carried out using the manufactured injection mold for the special-shaped plastic part.

Experimental Variables and their Ranges
Unlike the numerical simulation, dimension of the plastic part can be measured by the CMM in the actual experimental process, therefore, quality standard of the special-shaped plastic part is determined as that the crack value at the cone endpoints is not more than 0.3 mm.

Orthogonal Experiment Design
Orthogonal experiment method is used in order to verify the analysis results of Moldflow.Considering the control difficulty of mold temperature in the actual experiment, five main factors which influence the experiment are determined as follows: melt temperature, cooling time, injection time, packing pressure and packing time.
For the convenience of narration, these factors are referred to as A, B, C, D and E (the same below).In the range of each factor, four levels are selected scientificly as shown in Table 1.Corresponding to the L16 (4 5 ) orthogonal experiment design scheme, sixteen experiments are arranged.In order to ensure the accuracy of the experimental results, ten products (five groups ) are measured in each experiment and the average is used as the measurement result.
Table 1.Processing parameters and their levels.

Results and Analysis
Experiments are completed in the school laboratory and qualified products are produced.Produced plastic parts are measured by CMM and all data are recorded.Crack values at the cone endpoints obtained from the experiments are shown in Table 2 [2].
In this case some experimental equations such as formula (1) and formula (2) are provided for range analysis.where X is the data of the experiment, K is the summation of the data for relevant factor, P is the average value of the relevant K, R is the area of variation between upper and lower limits on a particular scale.Range analysis results of the data are shown in Table 3.According to the data in the Table 3, effect curve of each factor is acquired as shown in Fig. (12) [3].Range analysis results in the table3 lead to the conclusion that optimal process parameters are A2, B3, C3, D3 and E2.Optimal process parameters are composed of 210 degrees melt temperature, 60 degrees mold temperature, 2.2 seconds injection time, 31 MPa packing pressure and 20 seconds packing time.Comparing the simulation results with the actual production situation, it is found that the results of melt temperature, packing pressure and packing time are exactly the same.Only the injection time becomes 1.9 seconds, while the cooling time is 15 seconds.Therefore, analysis results of the software is basically correct and can be used in actual production process.Reasonable adjustments should be taken in the application according to the actual production conditions.
It can be seen from the Table 3 and Fig. ( 9) that effect order of the process parameters is CBADE and injection time is the most significant parameter which influences on the crack value, followed by cooling time and melt temperature, the minimum is packing time.

CONCLUSION
In this study, numerical simulation and experimental study of the injection molding process are completed and qualified products are produced.The conclusions of the study are as follows: (1) Comprehensive comparison of numerical simulation and experimental study indicates that analysis results of the software is basically correct and can be used in actual production process.Reasonable adjustments should be taken in the application according to the actual production conditions.
(2) From the optimization design process of feeding system for the special-shaped plastic part, it can be seen that unbalance rate is a key factor that influence on the feeding system, so the first scheme is the plan.
(3) Based on the range analysis results in the table3, optimal process parameters are composed of 210 degrees melt temperature, 60 degrees mold temperature, 2.2 seconds injection time, 31 MPa packing pressure and 20 seconds packing time.
Injection time is the most significant parameter which influences on the crack value.

3. 1 .
Analysis of the Best Gate Location Select the Cycolac G364 ABS plastic which produced by USA GE company first, and then analyze the best gate location of the plastic part.It can be seen from the result as shown in Fig. (2) that the best gate location is in the middle part of the side of the plastic part.
Equipment and materials are used as follows: Haifeng HF800 injection molding machine manufactured by Ningbo Haifeng Plastics Machinery Company, as shown in Fig. (9).CMM manufactured by Qingdao Fodi Company, as shown in Fig. (10).Mold for the special-shaped plastic part, as shown in Fig. (11).And PA-750 ABS plastic which is produced by Zhenjiang Qimei Company.