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Design and Simulation of Plastic Injection Moulding Process
Wong, C. T., Shamsuddin Sulaiman, Napsiah Ismail & A.M.S. Hamouda
Department of Mechanical and Manufacturing Engineering
Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia
ABSTRACf
This paper presents the design of plastic injection mould for producing a plastic product.The plastic part was designed into two different types of product, but in the same usage(n.使用) function. One part is using clip(vi.修剪，引申為凹模) function and another part is using stick function. In the computer-aided design (CAD), two plastic parts were drawn in 3 dimension (3D) view by using Pro-Engineer (Pro-E) parametric（adj.參數(shù)的） software(n.軟件). In the computer-aided manufacturing (CAM), Pro-Manufacturing from Pro-E parametric software was used to develop the machining program. For mould design, the product was designed into two changeable（adj.可變的） inserts （n.嵌入物）to produce two different types of plastic product in one mould base（模架）. Before proceeding （n. 進(jìn)行，進(jìn)程）to injection(n. 注射) machine and mould design, this part was analysed and simulated(v.模擬) by using Mold Flow or Part Advisor software. From the analysis and simulation（n. 模擬） we can define（vi. （給詞、短語(yǔ)等）下定義，構(gòu)成釋義） the most suitable injection location（n. 位置）, material temperature and pressure for injection. The predicted weld lines and air trap were also found and analysed.
Keywords: Changeable insert mould, injection pressure, air traps, injection location,mould design
INTRODUCTION
Nowadays, the technology of the tool and die fabrication （模具制造）in plastic injection is one of the world's fastest growing industries. Plastic is now used in almost every application,ranging from household articles to space travel, from transportation to packing, from medicine to toys, from bridge building to sports. Generally, injection moulding is a process that forms the plastic into a desired（adj. 渴望的，想得到的） shape by melting（adj. 熔化的；融化的） the plastic material and forcing the plastic material under pressure into the mould cavity（鑄型腔）. The shape of the plastic that is desired is achieved by cooling in thermoplastic or by chemical reaction for thermosetting（adj. 熱硬化性的；熱固）.
Mould design and fabrication is a costly and high technology process because it uses science-based computer-aided engineering (CAE) software to analyse and simulate the plastic parts, computer-aided design (CAD) software to design the complicated （adj. 結(jié)構(gòu)復(fù)雜的）plastic product and computer-aided （計(jì)算機(jī)輔助的）manufacturing (CAM) to do the programming fabrication
to run the computer numeric control (CNC) for milling or latch. Advances in computer technology have led to an increasingly （adv. 越來(lái)越多地）favorable （adj. 有利的）power to cost ratio (n. 比，比率)for computers. So this advantageous and costly technology will improve productivity and process consistency(n.前后一致).
Thus, for this fast growing industry, new technologies are vital(adj. 至關(guān)重要的) to ensure that this technology reaches perfection. S.o computer-aided engineering (CAE) is the assistant (adj.輔助的)to process and calculate(vt. 預(yù)測(cè)，推測(cè)) the plastic material flow inside the injection moulding.
Theory
How Plastic Fills a Mould
The injection moulding process can be broken into three phases:
1. Filling phase(n.階段)
2. Pressurization（n. 增壓） phase
3. Compensating(n. 補(bǔ)償) phase
Filling Phase
When designing plastic parts for the injection moulding process, the important element (n. 要素)to understand is how the plastic is filling in the mould. In the mould injection filling phase, molten plastic is injected into the cavity until the cavity is just filled. As plastic flows into the cavity, the plastic in contact with the mould wall quickly freezes (v.凍結(jié)，凝固)and this will create a frozen layer （n. 層）of plastic between the mould and the molten plastic.
Plate 1 shows how the flow front expands as material from behind is pushed forward. The edge of the flow front comes into contact with the mould and freezes. The molecules(n. 分子) in the frozen layer are therefore not highly orientated(高度取向), and once frozen, the orientation（n. 方向） will not change.
The red arrows(n. 箭頭) show the flow direction of the molten plastic. The dark blue layers against the mould walls show the layers of frozen plastic. The green arrows indicate(vt. 表明) the direction of heat flow from the polymer (n. 多聚物；[高分子] 聚合物)melt into the mould walls.
The frozen layer gains(v. 獲得) heat as more molten plastic flows through the cavity, and loses heat to the mould. When the frozen layer reaches a certain thickness, equilibrium is reached. This normally happens early in the injection moulding process, after a few tenths of a second.
Pressurization Phase
The pressurization phase will happen after the filling phase completely fills the cavity area. At this stage, all the flow paths will be filled up by molten plastic, the edges and corners of the filling area in the cavity may not contain plastic. To completely fill out
the geometry(n. 幾何形狀), extra (adj. 額外的)plastic is pushed into the cavity during the pressurization phase.
Plate 2 shows the difference between the end of the filling phase (left image) and the end of the pressurization phase (right image).Notice the unfilled corners (inside the blue circles) that are left at the end of the filling phase. The yellow cone indicates the polymer injection location and the plastic is red.
Compensating Phase
Plastic material has a high volumetric shrinkage(體積收縮量), form the melting plastic to solid, it will shrinkage average of 25%. Therefore after the pressurization phase, more material must be injected into the cavity to compensate for the plastic shrinking after it cools. This is the compensating phase(相位補(bǔ)償) (MoldFlow Help Files).
Mould Basic Construction
Mould design and mould making can be considered the most critical processes of the injection(n. 注射) moulding system. This is because the mould design and mould making have tremendous (adj. 巨大的)impact (n. 影響)on productivity(n. 生產(chǎn)率) and product quality. Basically, the function of the mould is two fold. The first is imparting(v. 通知) the desired shape to the plasticized polymer, the second is cooling the injection moulded parts. The mould, which contains one or more cavities, consists of two basic parts:
? A stationary(adj. 固定的) mould half on the side where the plastic is injected
? A moving half on the closing or ejector side. The separation (n. 分離)of the two moulds is called the parting line.
The mould will determine the final product properties(n. 特性) such as size, shape, dimension (n. 尺寸)and finishing surface(加工表面). Usually, the melted plastic material is filled through the central feed channel (n. 渠道)called sprue (n. 鑄道)to the cavity. The mould will coincide with the injection machine cylinder nozzle.( 缸噴嘴) Table 1 is the summary list for mould component and the function respectively(adv. 各自地).
Mould Design
To start up a new mould design, the designer should know some important points to avoid some mistakes before going further Le. product outlook design, material usage(材料的使用),correct shrinkage of the material, number of cavities and selection of the mould base. Making a mistake can cause an ill-fitting(adj. 不合的) mould at the final assembly（n. 裝配） and also make the plastic product out of tolerance （n.公差）and be rejected by the customer. Fig. 1 shows the mould designer's char
Moldflow software. Plate 5 shows the solid model （實(shí)體模型）with gate location placing（澆口位置） on top of the model. The yellow colour cone shape shows the i~ection location in this analysis.
In injection and transfer moulding（自動(dòng)成型）, the gate is considered the portion of the moulded piece （鑄造零件）that allows the molten resin to flow the runner or from the sprue into the cavity. The gate may be the same size and shape as the runner, but generally it is much smaller. There is an optimum（adj. 最適宜的） gate size. It should be large enough for suitable fill rate and small enough to seal off and prevent backflow or overpacking.
Product Simulation Result
Before starting the simulation process, some information has to be set up, such as material supplier （材料供應(yīng)商）and material grade（材料規(guī)格）. This is normally already set in the Moldflow database and the user just selects the specific material specification（n. 規(guī)格）. Table 2 shows the element used.for this analysis:
Material Grade：材料牌號(hào)
Mould Temperature：模具溫度
Melt Temperature：熔體溫度
Max Injection Pressure：最大注射壓力
Fill Time
Fill time is the time taken to fill up the part inside the cavity, it is also to show how the plastic material flows to fill the mould. From that we know that the short shot (part of the model which did not fill) part will be displayed. From that result one can also understand how the weld line and air trap will form. Plate 6 shows the material filling
into the mould.
Confidence of Fill
The confidence of fill result displays the probability of the region（n. 地區(qū)） within the cavity with plastic. This result is derived from the pressure and temperature results. The confidence of fill will display in three colours: green, yellow, red and translucent（adj.半透明的）. Showing in green will definitely fill, in yellow may be difficult to fill or may have quality problem, in red will be difficult to fill or will have quality problem and in translucent will not fill (short shot).
If the cavity does not fill, some changes must be made to either design, gate location, choice of plastic or processing conditions. However, to ensure the finished part is of good quality, the cavity must also be adequately(adv. 足夠地) packed with plastic. Plate 7 shows confidence of fill in this analysis.
Injection Pressure
The colour at each place on the model represents the pressure at the place on the model. Two colours show the highest pressure (red) and lowest pressure (blue). The injection pressure can be used in conjunction(n. 結(jié)合物) with pressure drop result. For example,even if a section of a part has an acceptable pressure drop, the actual injection pressure in the same area may be too high. High injection can cause overpacking（超緊密堆積作用）.
To reduce the chance of this happening, follow these steps:
1. Increase the maximum injection pressure
2. Alter the polymer injection location
3. Alter part geometry
4. Select a different material
Plate 8 shows the result of injection pressure from 0 MPa to 47.02 Mpa.
Air Traps
Plate 10 shows the air traps result after analysis. The small blue bubbles are showing the air traps in the parts. Air traps result shows the regions where the melt stops at a convergence of at least 2 flow fronts or at the last point of fill, where a bubble of air becomes trapped（adj. 陷入困境的）. To prevent air traps occurring when converging (adj. 會(huì)聚的)flow fronts surround and trap a bubble of air, balance flow paths by either:
1. Using flow leaders / deflectors(導(dǎo)流板)
2. Changing part wall thickness
3. Changing polymer injection locations
Mould Development
Mter doing all the Moldflow simulation for the product, mould design will be preceded (v. 在…之前發(fā)生)based on the simulation result. Only one mould base is used to produce two different plastic parts; this is because almost 70% of the plastic product shapes are similar, so changeable inserts are used in this case. Core insert will be as permanent insert and change the slider and cavity insert for T G Clip version and T G Stick version. Plate 11 shows the cavity and core insert for T G Clip and Stick version.
Mould Base Size
For this project, three-plate type system mould base is used with pinpoint gating(點(diǎn)澆口). The sizes for the mould base are 250 mm x 250 mIll with FCI type, A-plate size 100 mm, B-plate size 70 mm, Spacer block size 100 mm and with striper plate (referred （vi. 涉及）from LKM(Lung Kee Metal) mould base handbook). The function of the stripper plate is to remove the runner from the plastic part and the plastic part will be considered a good product with no need to touch up.
For this mould base just one cavity will be injecting each time, the molten material of plastic will flow through sprue to the cavity. After cooling, the plastic parts will harden and stick to the core and then be ejected by using the shoulder-type ejector pin.
Cooling System (Water Channel)
A water channel is to control the temperature of the mould surface and to chill up the molten plastic material to become rigid states that eject from the core side. For this project the water channels are designed to be drilled along（鉆） the length of cavities, core and slider. The cooling system is vertically to the mould. Plate 12 shows the flow of the water channel in the mould insert.
Gating System
The gate is the connection between the runner system（澆注） and the moulded part. It must permit （vt. 允許）enough material to flow into the mould to fill out the part plus additional (adj. 額外的)material as is required to overcome the normal thermal shrinkage(熱縮) of the part. In this project, pin point gates are used, when the mould is open the part will automatically be removed from the runner and there is no need to touch up for the plastic product. Plate13 shows the pinpoint gate system.
Complete Mould Assembly
Top Clamp Plate：釘夾板 Stripper plate：卸料版
Slider：滑塊 Spacer Block：間隔塊
Bottom Clamp plate：底部夾緊塊 Ring：鎖緊閥
Sprue：澆口 Support pin：支撐桿
Cavity Insert：型腔嵌 Return Pin：復(fù)位桿
Spring：彈簧 Ejector Return Plate：引射器返回板
CAD/CAM for Mould Design
In the plastic industry, CAD/CAM has emerged(v. 出現(xiàn)) to the point where it now shows the promise of being one of the most significant(adj. 重要的) technological advances of the century. CAD/CAM is enabling the creative energies of plastic part and mould designers to be spent in producing better designs in a shorter time period rather than in doing repetitive(adj. 重復(fù)的) mould design tasks.
The Advantages of CAD/CAM
The advantages of the CAD/CAM in mould design are:
a) CAD/CAM technologies are normally used for the numerically controlled (NC) machining technology to fabrication for the moulds and also its ability to create three-dfmensional product models in the data base and to generate (vt. 形成)automatically(adv.自動(dòng)地).
b) CAD/CAM can help designers to speed up design for the plastic part and mould design process and reduce the long lead-time
c) CAD/CAM used in the repetitive and routine tasks of mould making will enhance(vt. 提高) the quality of work life for those skilled mould makers by providing mOl"'e challenge and job satisfaction.
d) Easy for users to redefine for part geometry(n. 幾何形狀); this means it can enhance quality in the mould design by reducing in the number of errors.
Application of CAD/CAM and the Methods for Mould Design
In this project, the type of CAD/CAM software employed is Pro/Engineer 2000i. ProfMoldesign is an extension of Pro/Engineer to the mould manufacturing(n. 制造業(yè)) environment,covering all aspects (n. 方面)of production mould design such as mould bases, runner system creation, waterlines, ejector-pin holes, shrinkage and hundreds of other features. The Plate 14 for mould assembly is using Pro/Moldesign to create and design the mould.Prof NC and Pro/MFG are the manufacturing extensions of Pro/Engineer into C NC production arena, .and have a wide breadth(n. 廣泛) of capabilities (n. 能力)covering from 3 to 5 axis milling, 2 and 4 axis wire EDM, turning(n.車削), laser（n.激光）, flame, etc. After designing all the moulds, Pro/NC and Pro/MFG for the manufacturing will play an important role again in the process of creating the detail （vt. 詳述）drawing needed and generating NC toolpaths(刀具路徑) to machine the actual mould. The NC toolpaths are derived(v. 得到) from the product model database. Product paths completed in 3-D model will be directly(adv. 直接地) extracted(v. 提取) from the mould design database (n. 數(shù)據(jù)庫(kù))for manufacturing, for example, core inserts from the mould database that want to do some manufacturing. Before starting CAD/CAM some of the important features have to be considered like tool feed rate(刀具進(jìn)給率), spindle speed(主軸轉(zhuǎn)速), step depth(步深度), material to mill, Plunger speed（活塞速度）, coordinate system（座標(biāo)系）, and what kind of end mill used. To start CAD/CAM, dimension of work piece has to be decided first and then setting the coordinate system to the work piece（被加工件）. The work piece coordinate must be of the same setting with the coordinate in the CNC machine. The wrong setting will cause the machine to crash with the work piece.
DISCUSSION AND CONCLUSION
From the analysis simulation, Moldflow provides sufficient （adj. 足夠的）information results such as fill time, injection pressure and pressure drop. With this result, users can avoid the defect of the plastic in actual injection such as sink mark, hesitation, air traps, and overpacking. The analysis will also help the mould designer to design a perfect mould with minimum modifications and it will also reduce the mould setup time. With this analysis and simulation it will help to reduce time and cost.