Chapter 2 Literature Review
2.2 Investigation previous research about warpage and shrinkage
Because warpage and shrinkage are two major reasons affect to larger quality of product, many papers had been researched itself. The following is several articles research about warpage and shrinkage:
Mirigul A., [6] researched reducing shrinkage in injection moldings via the Taguchi, ANOVA and neural network methods. The study had designed rectangular specimen (length x width x thickness: 110 x 10 x 3,2mm) for experimental and used two kinds of material which are polypropylene (PP), and polystyrene (PS). These materials
12
were injected mold cavity of specimen under various parameters process change included: melt temperature, injection pressure, packing pressure, packing time.
According to the analysis results, the research found out the optimal set of parameters, which make minimum shrinkage of specimen. The optimal set of parameters are melt temperature at 260oC, injection pressure at 60MPa, packing pressure at 50MPa, packing time at 15s, which gave minimum shrinkage of 0.937% for PP and 1.224% for PS.
Statically, the most significant parameters were found to be as packing pressure and melt temperature for the PP and PS moldings, respectively. Injection pressure had the least effect on the shrinkage of either material. Daniele A et al. [7] had studied a methodology for shrinkage measurement in micro-injection molding.
Tuncay E., and Babur O., [8] presented minimization of warpage and sink index in injection-mold thermoplastic parts using the Taguchi optimization method. The objective of this study consist of minimization of the warpage and sink index in terms of
process parameters of the plastic parts have different rib cross-section types, and rib
layout angle using the Taguchi optimization method. Based on, considering the process parameters such as mold temperature, melt temperature, packing pressure, in addition to rib cross-section types, and rib layout angle, a series of mold analyses are performed to exploite the warpage and sink index data. The polymeric materials were selected
13
PC/ABS, POM, and PA66. Taguchi optimization method was used by exploiting mold analyses based on three level factorial design. Orthogonal Arrays of Taguchi, the signal-to-noise (S/N) ratio, the analysis of variance (ANOVA) are utilized to found the optimal
levels and the effect of process parameters on warpage and sink index. Confirmation
analysis test with the optimal levels of process parameters are carried out in order to demonstrate the goodness of Taguchi method. From this, it can be concluded that Taguchi method is very suitable to solve the quality problem occurring the injection-molded thermoplastic parts. Subramanian N.R., et al. [9] using the same method presented optimizing warpage analysis for an optical housing. Babur O., and Ibrahim S., [10] had studied warpage and structure analysis of thin shell plastic in the plastic injection molding.
Hasan O., Tuncay E., [11] researched application of Taguchi optimization technique in determining plastic injection molding process parameters for a thin-shell part. The results show that warpage and shrinkage are improved by about 2.17% and
0.7%. A verification test is also performed to prove the effectiveness of Taguchi
14
technique after the optimum levels of process parameters are determined. It can be
clearly inferred from this conclusion that Taguchi optimization is sufficient to solve the
warpage problem with shrinkage for thin-shell plastic components of orthose part.
Eghbal H., and Abu B.S., [12] using the same method shown analysis of warpage and shrinkage properties of injection-molded micro gears polymer composites using numerical simulation assisted by the Taguchi method.
Azaman M.D., [13] with partners have been researched shrinkages and warpage in the processability of wood-filled polypropylene composite thin-walled parts formed by injection molding. In this study, the injection molding of shallow, thin-walled parts (thickness 0.7 mm), composed of lignocellulosic polymer composites (polypropylene (PP) + 50 wt% wood), was simulated. The volumetric shrinkages and warpage in the thin-walled parts were evaluated under different process conditions, with varying post- filling parameters, such as mold temperature, cooling time, packing pressure and packing time. The analysis showed that the cooling time and packing time had less of an effect on the shrinkage and warpage; nevertheless the optimal levels for both parameters are required in the molding process for the thin-walled part to achieve the best results.
The volumetric shrinkage was lower near the gate than at the end-of-fill location along the flow path. The results also showed that the volumetric shrinkage correlates with the
15
warpage measured on the molded part. The optimum parameters ranges is 40–45oC for the mold temperature, 20–30s for cooling time, 0.85 from injection pressure (Pinject) for packing pressure, and 15–20s for the packing time to achieve the best results with the least amount of volumetric shrinkage and warpage.
In addition, Sánchez R., Aisa J., and Martinez A., [14] researched about the relationship between cooling setup and warpage in injection molding. In this study some measurement results are shown, focused on cooling parameters effect on warpage, using non-contact techniques on several part areas. Results allow visualize warpage changes when melt temperature, cooling time or cooling conditions vary. Babur O., [15]
researched optimization of injection parameters for mechanical properties of specimen with weld line of polypropylene using Taguchi method. Mustafa K., et al. [16]
presented influence of cavity pressure and mold temperature on the quality of the final products. In this research, cavity pressure and mold surface temperature have been measured and recorded by pressure and temperature–pressure sensors using a Kistler CoMo 2869A injection-type apparatus. The influences of the measured factors on the quality of the final parts have been investigated experimentally. The results of this experimental study indicate that cavity pressure and mold temperature are the dominant factors determining the quality of the final product in plastic injection molding. Kuo M.T., et al. [17] researched a study of the effects of process parameters for injection molding on surface quality of optical lenses, and the same method, Hamdy H., [18]
researched modeling the effect of cooling system on the shrinkage and temperature of the polymer by injection molding.
16 2.3 Structure of the thesis
The thesis has six chapters included introduction, literature review, research methodology, analysis result, optimization minimum warpage and shrinkage, and conclusion. Chapter 1 introduces general background of the injection molding process, and objective of the thesis. Then, chapter 2 presents the influence of warpage and shrinkage to quality of product, investigation prior research about warpage and shrinkage in the injection molding process, and structure of the thesis. The next chapter shows research methodology includcing material for simulation, FEM method, and Taguchi method. The chapter 4 shows influence results of hot runner system to quality of the product, process parameters to warpage and shrinkage, process parameters to deformation of the product dimension. Analysis and computing mean S/N, and results of dental floss box quality using the optimal set of parameters are shown in the chapter 5. Finally, conclusion and future works are presented in chapter 6. The Fig.2.1 shows the structure of the thesis.
Chapter 1
1.1General introduction of injection molding plastic process
2.2 Investigation prior research about warpage and shrinkage
2.3 Structure of the thesis
17
Fig.2. 1 The structure of thesis
18
Chapter 3 Research Methodology
This chapter clearly describes the research methodology used in this study including material for simulation, Finite Element Method (FEM), and Taguchi method.
The purpose of the chapter is to build 3D model for simulation process and presents Taguchi method to apply in the thesis.
3.1 Material for simulation
Polymer material is selected to make the dental floss box, that base on characteristic requirements of its (as presented in section 1.2 goal and objective of the thesis). Because the dental floss box is a kind of product serve to human daily life, so polymer using for product have to no chemical affect human health, and dental floss box requirement can be many times opening/closing. Polycarbonate (PC) polymer has small toxic, elasticity, and PC usually uses to create products served to human in daily life. Therefore, polycarbonate polymer (PC) is chosen to create the dental floss box, that is suitable for requirement of product in simulation injection molding procecss.
Table 3. 1 Summary of polycarbonate (PC) properties [19]
1 Elastic modulus
19
manufacturing injection molding of the PC such as melt temperature range of 280 - 330oC, mold temperature range of 60 - 120oC, injected temperature range of 133-153oC, freezer temperature of 153oC.
The Fig.3.1 shows the characters of the PC, which based on them to choose suitable parameters for injection molding process. The Fig.3.1 (a) shows relationship between viscosity and shear rate at three level temperature (280oC, 305oC, 330oC), and the Fig.3.1 (b) shows relationship between specific volume and temperature range from 10 oCto 300oC.
(a) (b)
Fig.3. 1 The characters of PC [19]
3.2 Finite Element Method
3.2.1 Introduction Finite Element Method
The Finite Element Method (FEM) is a numerical technique for finding approximate solutions to boundary value problems using differential equations. It uses variational methods (the calculus of variations) to minimize an error function and produce a stable solution. Analogous to the idea that connecting many tiny straight lines can approximate a larger circle, FEM encompasses all the methods for connecting many
20
simple element equations over many small subdomains, named Finite Elements, to approximate a more complex equation over a larger domain [20].
Modex3D R.12 software is a kind of FEM to analyze defects of products using in the injection molding process field. The software provides a complete solution to help users simulate find out suitable for design mold and process parameters before application to real production environment. Covering a wide spectrum of injection molding processes, Modex3D R.12 help part designers, mold designers, and mold makers detect potential molding problems in advance at early stage of product design process such as weld lines, air traps, short shots, sink mark, burning, warpage, shrinkage, etc. With Moldex3D R.12, design revisions and optimizations can be make much more quickly and more easily. Addition, the software also allows an accurate analysis of quality of polymer flow into complicated mold shapes to reduce defects, create quality of product. Therefore, Moldex3D R.12 is selected to analyse quality of product and specially uses to optimize minimum warpage and shrinkage of dental floss box in this study.
3.2.2 Design 3D model for simulation process
Moldex3D software has two modules include Moldex3D Design and Mold3D eDesign. Moldex3D Design project is used to prepare for simulation process such as mold base, runner system, cooling system, meshing, etc. After completing design conditions in Moldex3D Design, process parameters (temperature, pressure, time) are set up to run analysis in Moldex3D eDesign,
3.2.2.1 Mold for dental floss box
21
Dental floss box is a small plastic product. The length, width, high of dental floss box dimensions are 105.5mm, 42mm, 35.5mm, respectively, and mass of the dental floss box is 9.549g. The Fig.3.2 shows 2D CAD of the dental floss box. Based on mass and dimension of the dental floss box above, mold dimensions are selected with length of 450mm and width of 350mm suitable for four cavities in the mold (The standards design amount of cavity in the mold depend on dimension of product and amount of product demand production). The Fig.3.3 shows the cavity plate and core plate dimension. The Fig.3.3 (a) shows the cavity plate dimension, and Fig.3.3 (b) shows the core plate dimension. Cavity plate and core plate are similar dimension, and value of length, width, high they are 450mm, 350mm, 60mm, respectively.
C
1) Dafts not specified shall be 1°30"
2) Material : PC 3) Mass: 9.549(g) open mold
Fig.3. 2 Dimension of the dental floss box
22
(a) (b)
Fig.3. 3 The cavity plate and core plate dimension
The Fig.3.4 presents some main plates of the dental floss box mold which are prepared to simulate by moldex3D eDesign. These plates are clamp plate, manifold plate, front cavity plate, and core plate. The dimension of each plate is clearly shown in Table 3.2.
Fig.3. 4 Main plates of the dental floss box mold
23
Table 3. 2 The dimension of the plates of the dental floss box mold Plate name Length (mm) Width (mm) High (mm)
Core plate 450 350 60
Cavity plate 450 350 60
Front cavity plate 450 350 30
Manifold plate 450 350 60
Clamp plate 450 350 45
After the dental floss box and the dental floss box mold are completely designed by SolidWorks software, they are installed in Modex3D Design as shown in Fig.3.5. A fixed haft of mold involves clamp force plate, manifold plate, and front cavity plate. So total dimensions of the fixed half of mold is 195mm. A moved half of mold is the core plate dimension whose dimension is 60mm.
Fig.3. 5 Dental floss box and mold base in the Moldex3D Design 3.2.2.2 Runner system
24
Normally, there are two main kinds of runner system to use in mold design, they are cold runner system and hot runner system. Cold runner system is usually used in a conventional mold to make normal products, that products are not necessarily high quality. Cooling system is easily design, manufacturing, and low manufacturing mold cost. Oppositely, hot runner system is complexity designed for high quality products, special products, and large products size. Several advantages using hot runner system ensure stability of uniform temperature of melted plastic flow from nozzle of injection machine to mold cavity, make cycle time faster, do not use robot to remove runner, save plastic material. Some disadvantages are high investment for equipments, difficult to change of color plastic material, hard design structure mold, and hot runner system can make cavity plate crack. Normally, hot runner system includes four main parts such as controller, piton-cylinder, manifold, and nozzle. The controller is used to control temperature of plastic to ensure temperature requirement in injection molding process.
Piton-cylinder controls to open or close nozzle for melted plastic flowing mold cavity.
Manifold provides melted plastic for each gate. The nozzle is directly provides melted plastic into cavity to make product shape. The following are standards of design runner system which is applied to design the hot runner system for dental floss box mold [21].
1.Runner must be designed to be the shortest path from the nozzle of injection machine to the cavity part.
2. Runner should be designed symmetrically when mold has multiple cavities.
3. Runner dimension is chosen based on the plastic mass of products as shown in Table 3.3. The plastic mass of dental floss box product is 9.549g as presented in Fig. 3.1.
25
Therefore, type valve and runner diameter of 4mm are secleted for hot runner system of dental floss box mold.
Table 3. 3 Relation between the plastic mass of product and runner dimension [21]
Runner dimension (mm) process. Hot runner system of case (a) is used two gates to inject one cavity, but the hot runner system of case (b) is used one gate to inject one cavity as illustrate in Fig.3.6.
Dimensions of the two cases of hot runner are uniform. For example, the diameter of each gate is 0.7mm, length of the nozzle is 195mm, and diameter of the nozzle is 4 mm.
The Fig.3.8 shows hot runner system, which is installed in Moldex3D Design to prepare for simulation process. In the chapter 4, section 4.1 analyze influence result of each hot runner system to quality of product. Based on that results, the optimal method of design hot runner is found out to obtain hight product quality.
26
Fig.3. 6 Two cases of design hot runner for dental floss box mold (unit: mm)
Fig.3. 7 Model of two case hot runners prepare to simulation process 3.2.2.3 Cooling system
Cooling system is an important part in the structure mold, that purpose is to cool product from high temperature and high pressure to freeze temperature, then products are injected outside by inject system. For the structure dental floss box mold, cooling system is designed with four cooling lines for each plate (cavity plate, and core plate),
27
and two inputs, outputs. Diameter of line each cooling is 8mm. The Fig.3.8 shows the cooling system of cavity plate. The core plate has cooling system same to the cavity plate.
Fig.3. 8 Cooling system of cavity plate (dimension unit: mm)
Fig.3. 9 All systems the dental floss box mold
The Fig.3.9 shows all systems of the dental floss box mold, they are setup by Moldex3D Design to prepare simulation process. These systems include mold base, hot runner system, cooling system, input and output coolant, connection part of the cooling system.
3.2.2.4 Meshing
28
Moldex3D software is provided several meshing methods to analyze model such as Auto Tetrahedral method, Hybrid Mesh method, and Boundary Layer. The following present characters of each meshing method, based on that the suitable method is chosen to mesh product.
Auto Tetrahedral meshing method is the simplest method for three-dimensional solid mesh creation. Users can easily create tetrahedral mesh from a closed surface. The disadvantage of this method is that it requires more elements per unit volume to achieve the same mesh quality as other types of solid mesh. The mesh quality described here is defined by the quality tables in Moldex3D Mesh and the element layer count across thickness direction. For Auto Tetrahedral meshing, users do not have full control of the element layer count of parts. Thus, sometimes the CAE analysis cannot provide correct temperature distribution in poor quality regions.
On the other hand, the Hybrid meshing is very different from the tetrahedral meshing. People can easily control the mesh quality to meet the solver’s requirement.
The disadvantage of this method is that inexperienced users have to spend more time constructing the mesh. The constructing time of hybrid mesh is three times or more than that of the auto tetrahedral mesh. For most users, it is a big drawback despite the possibility of achieving higher mesh quality.
Moldex3D Mesh provides the Boundary Layer Mesh (BLM) method. For BLM, users do not have to spend much time in solid mesh generation. In addition, the quality of solid mesh provided by BLM is good enough for analysis to obtain accurate results.
Generally, it provides at least five-element layer count across thickness direction for the
29
entire part. In that way, the increase in temperature caused by shear heating phenomena at the cavity boundary can be simulated more accurately. Furthermore, the analysis results of the filling pattern, pressure profile, and so on, can be predicted more accurately as well [19]. The Fig.3.11 shows the comparison among different meshing methods in the Modex3D software.
For the dental floss box product to ensure good quality mesh for analysis to obtain accurate results, Boundary Layer Mesh method with level 3 is chosen for analysis simulation. The Fig.3.12, Table 3.6 show product be meshed by Moldex3D 2012 software, and mesh summary, respectively.
Fig.3. 10 The comparison difference among meshing methods [19]
30
Fig.3.11 The product is meshed by Moldex3D 2012 Table 3.4 Summary of mesh properties
Mesh Type Boundary Layer Mesh (BLM)
Level mesh 3
Cavity(Part) volume 31.5324 (cc)
Hot runner volume 17.3585 (cc)
Element number 985542
Part elements 985542
Node number 748117
In addition, the injection machine is chosen in the catalogue of Moldex3D software to simulation analysis, when mold base, runner system, cooling system, and mesh all of systems are completed design. The main parameters of injection machine to simulation process are clamping force at 490kN, injection pressure maximum at 240MPa, injection rate maximum at 128cm3/sec.
31 3.3 Taguchi method
3.3.1 Introduction
The Taguchi method developed by Taguchi consists of three stages which are system, parameters, and tolerance designs, respectively. The system design phase is the
The Taguchi method developed by Taguchi consists of three stages which are system, parameters, and tolerance designs, respectively. The system design phase is the