Semi-automated Gating System Design with Optimum Gate and Overflow Positions for Aluminum HPDC

Abstract

Die casters usually carry out a die casting test before producing new castings. At the die-casting test stage, the runner and gate parts are always repeatedly corrected, which lead to a lengthened processing time and increased processing cost. In this study, a computer software was developed to calculate the gating system design with optimum gate and overflow positions for cold chamber High Pressure Die Casting (HPDC) die design by getting the part dimensions and choosing the suitable machine from a pre-defined database in the software. A design of experiment is used to formulate the objective function of the gate and overflow optimum position. At the end of this paper, a case study was developed as part of the software validation. The results show that the developed software can calculate the gating system design and eliminate the correction in the test stage.

1 Introduction

Die-casting is similar to permanent mold casting except that the metal is injected into the mold under a high pressure of 10–210 MPa (equiv. to 1450–30,500 psi). This results in a more uniform part with a generally good surface finish and good dimensional accuracy, as well as 0.2% of the casting dimension. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size.

However, because of the high injection speed, die-casters usually carry out a die-casting test before producing new castings to eliminate part defects. In this stage the runner and gate parts are always repeatedly correctly, which leads to a lengthened processing time and increased processing cost.

In this area, some researches study the gating system only, while others only study the runner system and its effects of product defects. In this research developing a software, using visual basic that give a recommended gate and runner design (Gating system) related to the product’s dimensions and machine used. This will minimize the part defects/porosity. Moreover, the software lets the user choose from the different machines and its plunger diameter from a pre-defined database to get a desired number of cavities for the user’s case.

Mentioned, some researches only study the runner system, such as Tai and Lin [15] which use a simulated annealing optimization algorithm with a performance index to get the optimum runner network. Weishan et al. [17] developed a CAD/CAE system for die casting. By using the CAD package, die designers can determine the location, the shape and the dimension of the runner and gating systems of die castings according to the characteristics of the die-casting machine, the geometry of the casting, and the properties of the alloy. Through the interactive process of CAD/CAE, and an ideal design can be provided with the best filling pattern and the optimal thermal conditions for both the die and the die casting. Lee and Lu [11] proposed a new mathematical model for the calculation of the back pressure in a die cavity. Fuh et al. [3] described a prototype system structured by several functional modules as specific add-on applications on a commercial CAD system for die casting die design. These modules include data initialization, cavity layout, gating system design, die-base, parting, standard components design, among others. The focus of the development was placed on gating, runner and die-based design. Yue et al. [19] established a CAD/CAE/CAM integrated system dies for application in the primary stage. The use of this integrated system can shorten the cycle of die design and manufacturing, as the lead-time of die castings is shortened greatly. Shehata and Abd-Elhamid [14] presented main die designing procedures and related equations in logical way. A computer program is developed to estimate the main die elements based on the geometry input of casting shape. After the initial inputs given, the system does full calculations, optimizes selections, and lists main die element sizes. The program can present die characteristics and casting machine characteristics. Cleary et al. [2] explored the effects of die temperature, metal super-heating and volume filling on the short shots for the casting of a simple coaster. The research used simulation methods that proved particularly suited to modeling HPDC is Smoothed Particle Hydrodynamics (SPH). SPH was used to study three industrial case studies. Hangai and Kitahara [5] proposed a fractal analysis by comparing the porosity in two types of aluminum alloy die casts manufactured by different die-casting processes, and to confirm that the fractal analysis of the spatial distribution of pores can quantitatively characterize the porosity. The same researchers [6] proposed another two types of fractional analysis to characterize the porosity in terms of the shape of individual pores and the spatial distribution of multiple pores. Their analysis shown that these are indicators of whether the predominant cause of the porosity is shrinkage or gas. these parameters are expected to indicate the action that should be taken to manufacture pore-free die castings. Park and Yang [12] proposed a linear programming (LP) model that maximizes the average efficiency of processing time for casting in real foundries. Lee et al. [10] presented a methodology for obtaining optimal designs of 4-cavity thin electronic component housings. Their study analyzed the fluid behavior and amount of air entrainment caused by the overflows and air vent using a computer fluid dynamics (CFD) simulation. Furthermore, the effect of vacuum systems on the porosity and mechanical properties of the castings was studied in their work. Queudeville et al. [13] modularized a proven instrument in reducing costs in manufacturing processes. Their methodology tries to assist design engineers throughout the development of new dies, helping to choose an appropriate set of modules based on technical as well as economic criteria. Gulagoudar et al. [4] developed a robust design approach to analyze casting defects in foundry. Thus process parameters are set in order to reduce the rejection of castings. Kittur et al. [9] determined the optimal process parameters by using the desirability function. The process parameters, namely, fast shot velocity, injection pressure, phase changeover point, and holding time, have been considered as the input to the model. Porosity, surface roughness, and hardness are measured and represented as the responses. Wu et al. [18] developed a parametric design software for die-casting die gating system, so the designing and modification will be more easy to maintain by using parametric design.

2 Framework of the Gating System

Gating system dimensions directly affect part quality. This requires designer to have detailed information for the part and the machine, so that accurate dimensions for gating system elements as runner, gate overflow and air vent can be achieved, see Fig. 1.

Fig. 1

Gating system design flow chart

2.1 Part Details/Dimensions

The part details as width, length and height also its thickness and angles, are important to take into account in the designing stage. In this study a standard parallelogram is taken to study the part details with gating system dimensions/details.

2.2 Material of Part/Mold

The material of the part and the mold also affect the design parameters. The study uses the popular part material (Al 306) as seen in Table 1, and also mold material (H13), see Table 2. So designers can cover large variant of part application.

Table 1 Al 306 (AlSi12) chemical composition

Table 2 H13 1.2344 (X40CrMoV5-1) chemical composition

2.3 Machine Specifications

For the material selected, the designer has to follow the specification of the cold chamber HPDC machines. Machine tonnage and casting volume identifies the number of cavities used, also plunger diameter and position affect the runner dimensions and mold design. In this study a pre-defined machine database is created to cover parts details as much as possible. These machines database collected from those in market and compared with many others to be sure that the deviation between machines in the same type almost none see Table 3.

Table 3 Machines database

2.4 Gate Detailed Dimensions/Location

“The gate is a narrow opening into the cavity from the gate runner” [18]. Its area is related to casting volume and thickness. The detailed dimensions of the gate can be calculated using functions and rules from handbooks and textbooks ([1], [7], [8], [16]), but the study add experts’ reviews to smoothing the gate details for better efficiency. Table 4 show functions used in gate details.

Table 4 Gating system equations

The gate detailed dimensions are given by 8 sections so it can be drawn along the gate path on any CAD modeling software i.e., SolidWorks. Figure 2 shows how sections are drawn with the required smoothness.

Fig. 2

Gate detailed sections

The gate location is identified using an experimented model developed using the design obtained by the experimental methods so that the gate location can be related to part dimensions and details.

2.5 Runner Detailed Dimensions

Runner is responsible to feed the gates from the machine sleeve. It is composed of the main runner and branch runners. Branch runners are connected to the main runner and the part, its cross sectional area is equal to that of the gate at Sect. 8. The main runner has a different cross sectional area, depending on the branch runners connected. For every two branches runners connected the main runner area must be more than 1.1 times the sum of their areas.

2.6 Overflow Detailed Dimensions/Location

The overflow is a volume of aluminum that is responsible for reducing non-metallic inclusions (porosity) and air entrapment, localized after the part. The size of the overflow is depending on the part volume and thickness. The rule of overflow volume selection was implemented by the IF/THEN rule. Overflow location is identified via the same experimental model used for gate location.

2.7 Air Vent Dimensions

Air vent is just a narrow area that enables air to escape without letting aluminum out. It helps the air entrapment of the gating system. Its area can be calculated from the gate area Table 4.

3 Experimental Model

The study developed an experimental model to study the effect of the part factors on the its porosity. Also creates an objective function for porosity percentage so it can minimize. These factors are related to part dimensions i.e., Width, length, height, angle and thickness. Furthermore, there are two factors that have been created to study gate and overflow positions effect. GSS (Gate Shift Scale) and OSS (Overflow Shift Scale) are the factors responsible for gate and overflow positions, see Fig. 3.

Fig. 3

GSS and OSS factors

The study used Box-Behnken experimental design to study the effect of the factors Table 5 on the porosity percentage. A simulation model was developed for this experiment using ProCAST¹ software. To study the effect of the 7 factors only the other parameters must be fixed, Table 4 shows simulation parameters used (Table 6).

Table 5 Box-Behnken design factors

Table 6 Simulation model parameters

The experiment shows how the porosity affected from the part shape specially the thickness and also that GSS and OSS affect it, see Fig. 4. The experiment also built a model that described the effect of all the seven factors on the porosity percentage, see Fig. 5. Once the all 5 factors are known from equations from handbooks and experts’ reviews it will be easy to get the values of GSS and OSS that gets the minimum porosity for the chosen part, see Eq. (1).

Fig. 4

Main effects plot for porosity %

Fig. 5

Normal probability plot

Equation (1)—Describes the relation between Width, Length, Thickness, Angle, GSS and OSS factors and the part Porosity % as an output from the experimental design

$$ \text{Porosity}\,\% =5.75-0.00009 \, {\cdot} \, Width-0.00953 \, {\cdot} \, Length-0.0195 \, {\cdot} \, Height+0.208 \, {\cdot} \, Thickness-0.1313 \, {\cdot} \, Angle-0.327 \, {\cdot} \, GSS-0.456 \, {\cdot} \, OSS-0.000012 \, {\cdot} \, Width^2+0.000017 \, {\cdot} \, Length^2+0.000106 \, {\cdot} \, Height^2+0.2283 \, {\cdot} \, Thickness^2+0.000946 \, {\cdot} \, Angle^2-0.0934 \, {\cdot} \, GSS^2+0.1598 \, {\cdot} \, OSS^2+0.000028 \, {\cdot} \, Width \, {\cdot} \, Length-0.000068 \, {\cdot} \, Width \, {\cdot} \, Height+0.0017Width \, {\cdot} \, Thickness+0.000031 \, {\cdot} \, Width \, {\cdot} \, Angle-0.00003 \, {\cdot} \, Width \, {\cdot} \, GSS+0.00008 \, {\cdot} \, Width \, {\cdot} \, OSS+0.000058 \, {\cdot} \, Length \, {\cdot} \, Height-0.002092 \, {\cdot} \, Length \, {\cdot} \, Thickness-0.000031 \, {\cdot} \, Length \, {\cdot} \, Angle+0.00077 \, {\cdot} \, Length \, {\cdot} \, GSS+0.00072 \, {\cdot} \, Length \, {\cdot} \, OSS-0.00148 \, {\cdot} \, Height \, {\cdot} \, Thickness+0.000064 \, {\cdot} \, Height \, {\cdot} \, Angle+0.00179 \, {\cdot} \, Height \, {\cdot} \, GSS+0.00015 \, {\cdot} \, Height \, {\cdot} \, OSS+0.00053 \, {\cdot} \, Thickess \, {\cdot} \, Angle+0.0020Thicknes \, {\cdot} \, GSS+0.0191 \, {\cdot} \, Thickness \, {\cdot} \, OSS+0.00136 \, {\cdot} \, Angle \, {\cdot} \, GSS $$

(1)

4 Validation

The study validating of the developed software and the model output was performed by creating two HPDC aluminum dies designs. The part dimensions in the two models are the same but the first model has one cavity and the other has two cavities. The validation uses two methods, first by ProCAST simulation software and the other by built the mold itself with all its stages, beginning from full design using SolidWorks CAD software through manufacturing and injection on cold chamber HPDC machine.

The software is developed to be as simple as possible to meet user needs. The following steps are the steps used in the software:

• 1. The user inputs the dimensions and the thickness of the shape as shown in Fig. 6.

  Fig. 6

  

  User input

• 2. The user also selects the type of machine and temperature used for his application.

• 3. The software calculates from the data input and machine database the maximum number of cavities can be used in this die design. It is calculated from the minimum number of cavities gets from the maximum number of cavities calculated from the machine volume, machine plunger speed and machine locking force.

• 4. User again chooses the plunger and selects the number of cavities needed or presses back to select another machine if this machine not suitable for his application as in Fig. 7.

  Fig. 7

  

  Number of cavities

• 5. The software calculates the plunger speed of the machine as an output for the user.

• 6. It also calculates the gate area and dimensions with all its sections.

• 7. Moreover, the software calculates the overflow and vent area with detailed dimensions.

• 8. Finally, it calculates the optimum gate and overflow positions.

• 9. After these calculations, the software gives the output calculations with a simple form as viewed in Fig. 8.

  Fig. 8

  

  Design output

In Fig. 8 the developed software gives that the maximum porosity of 0.8% with this design. And that was the same results obtained from ProCAST simulation model in Fig. 9.

Fig. 9

The two ProCAST validation models, one cavity model on the left and two cavities model on the right

The other validation for the developed software is done by manufacture two molds. Figure 10 shows the design of the two molds. Figure 11 shows the 2 molds after manufacturing.

Fig. 10

The two validation molds design, one cavity model on the left and two cavities model on the right

Fig. 11

The two validation molds after manufacturing, one cavity model on the right and two cavities model on the left

After manufacturing the molds, it has been tested on a HPDC cold chamber machine 250TON. Figure 12 presents validation parts after testing the molds on the HPDC machine.

Fig. 12

The two validation parts, one cavity validation model on the top and two cavities validation model on bottom

The components have been sectioned in random sections to show if there is any porosity in the component or not. Figure 13 shows that there is no porosity in the components even after polishing the sectioned surface.

Fig. 13

Part sections with no porosity, polished section on bottom and without polishing on top

5 Discussions and Conclusions

In this work, a computer software has been developed to help cold chamber HPDC die designers by generating a detailed design for the gating system with optimum gate and overflow positions, which traditionally require a long time to design. The software also is validated, which will guarantee the user that the software output eliminates further corrections and adjustments in the gating system testing stage.

The experimental design done in this study gives an equation to relate all the 7 factors to the porosity percentage, it also concludes the following:

• 1. Part thickness is the most effective parameter for part porosity %.

• 2. The best gate location is to be shifted to the right side of the part (GSS = 1), regardless to all parallelogram parameters including its angle.

• 3. Also the best location for the overflow is around the center depending on the angle of the part.

This study recommends the following:

• 1. Extend the study to cover the injection process with its factors.

• 2. Study more complex shapes.

• 3. Make the cooling rate and its parameters a part of the model.