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遼寧科技大學本科生畢業(yè)設(shè)計 第11頁
Injection Molding
Many different processes are used to transform plastic granules，powders，and liquids into final product．The plastic material is in moldable form，and is adaptable to various forming methods．In most cases thermoplastic materials are suitable for certain processes while thermosetting materials require other methods of forming．This is recognized by the fact that thermoplastics are usually heated to a soft state and then reshaped before cooling．Theromosets，on the other hand have not yet been polymerized before processing，and the chemical reaction takes place during the process，usually through heat，a catalyst，or pressure．It is important to remember this concept while studying the plastics manufacturing processes and the polymers used．
Injection molding is by far the most widely used process of forming thermoplastic materials．It is also one of the oldest．Currently injection molding accounts for 30％of all plastics resin consumption．Since raw material can be converted by a single procedure，injection molding is suitable for mass production of plastics articles and automated one-step production of complex geometries．In most cases，finishing is not necessary．Typical products include toys，automotive parts，household articles，and consumer electronics goods．
Since injection molding has a number of interdependent variables，it is a process of considerable complexity．The success of the injection molding operation is dependent not only in the proper setup of the machine variables，but also on eliminating shot—to—shot variations that are caused by the machine hydraulics，barrel temperature variations，and changes in material viscosity．Increasing shot-to-shot repeatability of machine variables helps produce parts with tighter tolerance，lowers the level of rejects，and increases product quality ( i．e．，appearance and serviceability)．
The principal objective of any molding operation is the manufacture of products：to a specific quality level，in the shortest time，and using a repeatable and fully automatic cycle,molders strive to reduce or eliminate rejected parts in molding production．②For injection molding of high precision optical parts，or parts with a high added value such as appliance cases，the payoff of reduced rejects is high (Fig．4．2．1)．
A typical injection molding cycle or sequence consists of five phases：
①Injection or mold filling
②Packing or compression
③Holding
④Cooling
⑤Part ejection
Fig.4.2.1 Injection molding process
Plastic granules are fed into the hopper and through an opening in the injection cylinder where they are carried forward by the rotating screw．The rotation of the screw forces the granules under high pressure against the heated walls of the cylinder causing them to melt．As the pressure builds up，the rotating screw is forced backward until enough plastic has accumulated to make the shot．he injection ram(or screw)forces molten plastic from the barrel，through the nozzle，sprue and runner system，and finally into the mold cavities．During injection the mold cavity is filled volumetrically．When the plastic contacts the colc mold surfaces，it solidifies(freezes)rapidly to produce the skin layer．Since the core remains in the molten state，plastic flows through the core to complete mold filling．Typically，the cavity，is filled to 9 5％～9 8％during injection．Then the molding process is switched over to the packing phase
Even as the cavity is filled，the molten plastic begins to cool．Since the cooling plastic contracts or shrinks，it gives rise to defects such as sink marks，voids，and dimensional instabilities．③To compensate for shrinkage，addition plastic is forced into the cavity．Once the cavity is packed，pressure applied to the melt prevents molten plastic inside the cavity from back flowing out through the gate．The pressure must be applied until the gate solidifies．The process can be divided into two steps (packing and holding) or may be encompassed in one step (holding or second stage)．During packing，melt forced into the cavity by the packing pressure compensates for shrinkage．With holding the pressure merely prevents back flow of the polymer melt．
After the holding stage is completed，the cooling phase starts．During cooling，the part is held in the mold for specified period．The duration of the cooling phase depends primarily on the material properties and the part thickness．Typically，the part temperature must cool below the material’s ejection temperature．While cooling the part，the machine plasticates melt for the next cycle．The polymer is subjected to shearing action as well as the condition of the energy from the heater bands．Once the shot is made，plastication ceases．This should occur immediately before the end of the cooling phase．Then the mold opens and the part is ejected．
Blow Molding
The rapid growth in the use of advanced materials in a large number of? highly demanding automotive,electronic and cunsumer goods applications has promoted the development of? new and more complex material forming processes. In the last twenty years injection molding and blow molding have seen? a rapid growth due to the development of? new application and packaging industries,. this success can be traced to the optimization of existing processes and to the development of? new processing techniques employing novel concepts, injection molding? has seen the introduction of? techniques such as co-injection ,gas assisted injection? molding ,lost core molding and injection/compression.
(a)Parison extrusion stage (b)Clamping and blowing stage
Fig.4.2.2 Extrusion blow molding
Blow molding has been able to deal with much more complex? parts through? the development of 3D and sequential? blow? molding , complex molds? for? deepdrawn parts and cryogenic mold cooling . The introduction of new materials? has also made? possible the production of parts? having? multilayer structure
The complexity of these? new molding techniques calls for? a much better understanding? of the material behavior? during the basic stages of the process and its relation to the properies and performance of the final part, which are directly dependent? upon? die and mold designs and on the? operating conditions during extrusion ,?injection ,inflation and cooling in? the mold.?It?is? in these? areas? that? the computer? simulation? fot the? coupled??phenoment of??fluid? flow and heat transfer has proven to be a very valuable tool for the equipment manufacturer,mold designer and process engineer!
Blow molding process
Blow molding can be carried reciprocating screw injection machine．About either on an extruder or asection of molten polymer tubing (parsion) is extruded into an open mold．By means of compressed air or steam the plastic is then blown into the configuration of the mold．This technique is widely used for the manufacture of bottles and similar articles．In the case of large articles，such as liter beverage bottles，the parison may previously have been injection molded and oriented to provide additional strength to the final blown piece．
In the extrusion blow molding process(Fig．4．2．2)，the raw material is fed to a plasticating extruder in granular or pellet form．The plastic is melted by heat which is transferred through the barrel by the shearing motion of the extruder screw．The helical flights of the screw change configuration along its length from input to output(solids conveying,melting and metering sections)to assure a uniformly homogeneous melt at the screw tip．
In continuous extrusion blow molding，the screw feeds the melt directly into the head-die assembly．The meit flows around the mandrel and into an annular die of the convergent or divergent type．A hollow tube or“parison”is extruded continuouslv and cut at preset time intervals for transfer into the blow mold．
In the case of intermittent extrusion blow molding，the extruder feeds the material to an accumulator／head device．Once the desired volume has accumulated a ram or plunger pushes the material rapidly through the head-die assembly．The mold clamp mechanism does not need to transfer to a blowing station．The next parison is only extruded after the part is blown，cooled and removed from the mold．
Once a parison of the desired length has been formed，the mold is closed and the parison is inflated by internal air introduced through the die-head assembly．The mold walls are vented，and a vacuum may be applied．The molten polymer is thus forced to conform to the shape of the mold cavity．The article iS then cooled，solidified and ej ected from the mold．
In both methods the annular die may be designed to incorporate a hydraulic mechanism to vary or program the annular gap size．In this way，the extrusion process can be programmed to impart a specific wall thickness distribution or controlled weight to the parison．
Injection／stretch blow molding(Fig．4．2．3)is a two—stage process．In the first stage，the material is injection molded around a core rod to form a preform．In the second stage，the preform is then stretched through the action of a stretch rod，inflated and cooled in much the same manner as in the extrusion blow molding process．The result is a lighter product biaxially oriented in the axial and radial directions．Biaxial orientation provides increased tensile strength(top load)，less gas，liquid and odour permeation due to an increased molecular alignment and improved drop impact，clarity and light weighting of the container．Injection／stretch blow molding also produces scrap—free，close-tolerance，completely finished bottles or containers that require no secondary operations．Preform design and its relationship to the final container properties remain one of the most critical aspects of the process．The part thickness distribution has to be mapped onto the preform and through the knowledge of the material properties (degree of crystallinity and shrinkage after molding；stretching characteristics and their temperature dependence among others) the preform dimensions(form and thickness distribution)can be established．
(a)P reform injection stage (b)Stretching and blowing stages
Fig.4.2.3 Injection/stretch blow molding
塑料注射成型
許多不同的加工過程習慣于把塑料顆粒、粉末和液體轉(zhuǎn)化成最終產(chǎn)品。塑料材料用模具成型，并且適合用多種方式成型。在大多數(shù)情況下，熱塑性材料可以用許多方法成型，但熱固性塑料需要用其他方法成型。對于熱塑性材料有這種事實的認識，它常常被加熱成為另一種柔軟狀態(tài)，然后在冷卻以前成型。對于熱固性塑料，換句話說，在它加工以前還沒有形成聚合物，在化學反應加工過程中發(fā)生變化，如通過加熱、催化劑或壓力處理。記住這個概念在學習塑料加工過程和聚合物的形成是很重要的。
塑料注射成型越來越廣泛地運用于熱塑性材料的成型工藝。它也是最古老的一種方式。突然間，塑料注射成型材料占所有成型材料消費的30%。塑料注射成型適合于大批量生產(chǎn)，當原材料被成單一的步驟轉(zhuǎn)換成為塑料物品和單步自動化的復雜幾何形狀制品。在大多數(shù)情況下，對于這樣的制品，精加工是不需要的。所生產(chǎn)的各種各樣的產(chǎn)品包括：玩具、汽車配件、家用物品和電子消費物品。
因為塑料注射模具有很多易變的相互影響，那是一種復雜的虛慎重考慮的加工過程。塑料注射模具設(shè)備的成功是不依賴于機器變化到恰當?shù)牟襟E，只有淘汰了需要注射變化的機器，才會導致適應液壓變化、料筒溫度變化和材料黏度變化的機器的產(chǎn)生。增加機器重復注射的能力的變化可以幫助減少公差，降低次品等級和增加產(chǎn)品質(zhì)量。（即外觀和適用性）。
任何成型作業(yè)的產(chǎn)品制造其主要目的是：在滿足一定的質(zhì)量下,在最短的時間內(nèi)，可以重復制造，并且是全自動循環(huán)。模具設(shè)計者努力減少或避免出現(xiàn)不合格的部件。For injection molding of high precision optical parts, or parts with a high added value such as appliance cases, the payoff of reduced rejects is high .對于注塑生產(chǎn)高精密注塑模具的光學零件、配件和高附加值的器械，減少不合格品所產(chǎn)生的利潤高的現(xiàn)象(圖4.2.1)。
一個典型的注塑成型周期或序列分為五個階段。
①注射或充填
?、诎b或壓縮
③保壓
?、芾鋮s
⑤部件彈出
圖 4.2.1 注塑成型周期
塑料顆粒輸入料斗，并通過注射缸的開口和螺絲的旋轉(zhuǎn)力作用使部分顆粒使顆粒進入注射缸，同時在加熱器的作用下使顆粒融化。隨著壓力增大，造成氣缸壓力高使塑料分子激烈的碰撞側(cè)壁。當壓力達到一定的值時，旋轉(zhuǎn)螺絲受力的作用而移動，開始注射。注射腔的熔融塑料從噴嘴噴出，通過澆口和流道系統(tǒng)，并最終進入型腔。在注射時，型腔填充塑料。 When the plastic contacts the cold mold surfaces, it solidifies(freezes) rapidly to produce the skin layer.Since the core remains in the molten state , plastic flows through the core to complete mold filling .Typically ,the cavity is filled to 95 ﹪ -98 ﹪ during injection . 當熔融塑料接觸模具表面時，冷卻、凝固（凍結(jié)）迅速產(chǎn)生凝固層。由于塑件的內(nèi)部仍然是在熔融狀態(tài)，模具型腔通過填充熔融塑料完成注塑。通常情況下，模具型腔一般注射至9 5%～9 8%。然后成型工藝是轉(zhuǎn)向了包裝的階段。
當型腔充滿，熔化塑膠開始冷卻。由于塑料冷卻會收縮,它會產(chǎn)生缺陷,如凹痕、孔洞、尺寸的不穩(wěn)定等。為了補償收縮,除將模具的型腔充滿融化塑外, 一旦模具的型腔被充滿，為了防止熔從注射口處流出，壓力必須加到注射口，知道熔塑凝固為止。這個過程可分為兩個步驟(包裝、保壓)或可能包含在一個步驟(保壓或第二階段)，包裝時，保持型腔口的壓力，然后進行補償收縮包裝。保持的壓力和流量的僅僅是防止后面的聚合物熔體流動。
保壓階段完成后,冷卻開始。在冷卻過程中,一部分是在指定時間模具內(nèi)冷卻，主要取決于材料的性能和厚度。一般來說,這部分必須冷卻到材料溫度的噴發(fā)溫度。當冷卻完成后，注射機使聚合物熔化進入下一循環(huán)。聚合物受到剪切作用以及加熱圈的作用。在冷卻階段結(jié)束前，注射機停止注射，塑化作用也停止。然后模具打開,廢料被清除。
吹塑
先進材料的應用在快速增長，汽車、電子和消費者的產(chǎn)品要求大量的提高，因而需要發(fā)展新的和更復雜的材料的成型工藝。在過去的二十多年里，注塑、吹塑有了較快發(fā)展,開發(fā)了新的應用和包裝工業(yè), 該成果可以追溯到優(yōu)化現(xiàn)有的工藝和發(fā)展新工藝采用新穎的概念、注塑模引進技術(shù),如共注塑、氣體輔助注射成型,消失模成型和注射或熱壓成型機。
吹塑已經(jīng)能夠通過三維造型、發(fā)展時序吹塑、復雜的模具深腔制品件和低溫模具冷卻來應付更復雜零件的。同時引進新材料也可能使生產(chǎn)零件有多層結(jié)構(gòu)。
一些復雜的新的成型工藝要求在初級階段和其性能的相互關(guān)系及最后部分的性能對材料性能有更好的理解,它直接決定模具設(shè)計和操作條件,包括擠壓,膨脹與冷卻等。它是在這些地方, 對于制造商和模具設(shè)計工程師而言，用計算機仿真來處理來流體的流動和傳熱已被證實是一個非常有價值的工具和設(shè)備!
吹塑工藝
吹塑可以用一種往復式螺桿塑化注塑機。而擠出機或型坯熔化的聚合物導管則是擠壓成的一個敞模。采用壓縮空氣或蒸汽塑料則是吹塑模具的配置。該技術(shù)已廣泛用于瓶子的制造和類似的物品。對于大型的日常用品,如公升瓶裝飲料, 型坯可能事先已經(jīng)注塑，并以增加強度最終被吹塑成一體。
在擠壓成形吹塑過程(圖片.4.2.2)、原材料以顆粒狀或球團狀送入擠出塑煉機，塑料是通過加熱融化而通過桶的雙螺桿擠出機螺桿的剪切動作轉(zhuǎn)移。 為確保塑料顆粒融化均勻，對螺旋轉(zhuǎn)螺桿的長度改變是沿著螺桿的尖端從輸入到輸出(固體物料搬運技術(shù)、熔煉、計量部分)。
（a）管坯擠壓階段 （b）夾緊和充氣階段
圖 4.2..2 擠出吹塑成型機
連續(xù)的擠出吹塑成型，螺旋喂送器將融化物直接送入機頭模集中，熔態(tài)流體環(huán)繞心軸進入收斂或發(fā)散類型的環(huán)形口模。一個空心管或管坯受到連續(xù)的擠壓，按照預先設(shè)定的時間間隔連續(xù)地擠出管坯。
對于間歇擠壓吹塑成型，擠出機喂料累加器/頭裝置。一旦所需的物料已積累了一定數(shù)量或柱塞推動物料快速通過機頭模。模夾鉗裝置不需要轉(zhuǎn)到吹塑站，接下來管坯經(jīng)過吹塑、冷卻從模具中擠出。
一旦管坯預定長度形成,模具閉合, 管坯通過機頭模裝置向其內(nèi)部吹進空氣。鑄態(tài)聚合物因此被擠壓成符合形狀的模具。模件逐漸冷卻、固化后從模具中取出。
在這兩種方法都可以設(shè)計出環(huán)形口模成以液壓機制來改變或編制的環(huán)形間距大小，以這種方式,擠出工藝可編程的某一特定的壁厚分布的管坯或控制重量
注塑或拉伸吹塑成型(圖片4.2.3)是一種兩級工藝。第一階段,材料被注塑成一個圍繞環(huán)形預制芯棒。在第二階段,預制的芯棒通過一個彈性桿、膨脹與冷卻被拉伸，這就是間歇式擠出吹塑成型。結(jié)果是軸向和徑向的雙軸取向產(chǎn)品，為提高板料取向的拉伸強度(上部荷載)，減少氣體、液體、氣味滲透，主要取決于對輕壓容器分子排列的改進和提高分子沖擊,。無論是注塑，還是拉伸吹塑成型，制成完整的瓶或容器,無需二次加工。預設(shè)方案,最終容器性能保持其中最關(guān)鍵的部分，壁厚的部分被設(shè)計到預成型坯中,通過材料性能的知識（結(jié)晶度和收縮成型后,伸展特性及其對溫度等等）可以將制件的尺寸確定（組織和厚度的分布）下來。
（a）預成型注塑階段 （b）拉伸而和吹塑階段
圖 4.2.3 注塑和拉伸吹塑成型
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