電器后蓋塑料模具設(shè)計(jì)
電器后蓋塑料模具設(shè)計(jì),電器,塑料,模具設(shè)計(jì)
附錄
外文資料
TEMPERATURE CONTROL
P. H. J. Ingham
Marketing Manager ,Eurotherm Ltd,Worthing,Sussex,UK
SUMMARY
Commercial plastic materials are organically based and are therefore heatsensitive .Accurate temperature control of melt processes such as injection moulding is therefore necessary if problems caused by thermal degradation are to be avoided.
The injection moulding process is considered form a temperature controlriewpoint and some of the control methods or techniques are described.since it should not be forgotten that good temperature control can lend to materials and energy savings.
1 INTRODUTION
The injection moulding process is concerned with the efficient conversion of plastics raw material into moulded product of acceptable standards.Some of ths parameters which determine acceptability are weight,dimensions,colour and stenght,all of which can be affected by the conditions under which the material is processed.Having established by the conditions for thwese parameters so as to deermine acceptability,limits can be set for the conditions under which the material is processed.One of the most important parameters contributing to the correct operation of an injection moulding machine is temperature.All plastics materials can be correctly processed only within a certain range of temperatures which varies from materialFor some mateials and mould types the band isvery small and for others it can be quite wide.
Any attempt to define the limits within which the product is acceptable determines the need for some form of control.There are a number of types of control which,if applied correctly,can lead to adequate performance.Significant material and energy savings can be achieved by correctly pplying the right type of control equipment.The reliability of the system and the degree of operator supervision required also depend very largely on the balance struck between initial cost and performance.
It is the purpose of this chapter to examine the injection moulding machine from a temperature control viewpoint and to outline some of the control methods can be used ,together with advantages and disadvantages.
2 THE PROCESS
2.1 Machine Zoning
From a control viewpoint,an injection moulding machine consists of a number of zones (each equipped with a means of measauring the temperature) and a controller,which compares the measured value of the set-point and controls the heat input to the zone in such a way as to remove any different between the heat input to the zone in such a way as to remove any difference between the tow. Yu dividing the machine into a number of zones the different temperature requirements of different zones and their different heat input needs can most easily be met (Fig.1).
For this purpose a typical small machine may have three or four barrel zones and a nozzle one. The zones nearest to the material feed hopper are where the plastic is melted and thus require fairly large heat inputs. However, in the zones hearest to the nozzle, the heat produced, by the rise in pressure needed to force the plastic into the mould, means that relatively little additional heat input is requied when the machine is running. Indeed, if the machine cycle very short, with some materials it may be that more heat is generated than required to maintain the temperature, which will then rise uncontrollably mless some form of additional cooling is applied.
2.2 Thermocpuple Location
Considering again the barrel zones:these consist of a metal arrel with wall thickness sufficient to withstand the high pressures produced during the mjection cycle. The most common form of heating is electrical and is ipplied using band heaters strapped around the barrel (Fig.2). A controller of any kind can only control the temperature at the point of measurement. Ideally this will be as deep into the barrel wall as possible, since it is the temperature of the plastic which is required and not that of the barrel. Plastic is a poor thermal conductor and depending on whether the net heat dow is into or out of the plastic, a thermocouple deep into the barrel wall will register a temperature above or below the actual temperature. If the measuring element is shallow or on the barrel surface, the difference between the measured and actual melt temperatures can be very large. For any given conditions of operation there will be a more or less fixed difference between the melt and measured temperatures and acceptable produce may be produced. If ,however, the conditions, e.g. machine speed or ambient temperature, change, this may give rise to a melt temperature which does not result in the production of acceptable product. It is therefore important to place the thermocouple as close to the melt as possible , i.e. deep the barrel.
2.3 Temperature Overshoot
The resultant system of an electrical band heater strapped around a thick walled barrel with a deep thermocouple is typical of most plastics processing machinery and present a number of control problems. Not only must stable control be achieved during normal running of the machine but acceptable start-up performance must also be achieved. The machine must be brought to its normal operating temperature as quickly as possible and preferably with no overshoot. (Overshoot is said to occur if the temperature is rising or falling at such a rate as it reaches set-point that it does not stop there but continues past by some amount before returning towards set-point again; see Fig.4.)
The basic cause of temperature overshoot in the system is multiple heattransfer lags, i.e. where the heat generated electrically first raises the temperature of the heater thermal mass and is then conducted from the second thermal mass to a third and so on, until the heat reaches the point of measurement which, as stated already, is as near as possible to the point in the process to be controlled.
In the simplest cast of multiple heat transfer only two thermal masses would be significantly involved, namely those of the heater and the load. If the thermal mass of each is about the same, this tends to represent about the worst case for overshoots (and hence controllability). Poor heat transfer from heater to load worsens the situation, since the heater temperature (during start-up, for example)can then become very much higher than the load temperature; when the power to the heater is cut off the final temperature reached (ignoring heat losses and assuming equal thermal masses for heater and load) will be the mean of their respective temperatures at the instant when the power is cut off. Thus ,the overshoot in load temperature increases as the heat transfer becomes worse.
A particularly bad case of overshoot (and controllability) occurs where heat is transferred through a considerable thickness of heat-conducting material. This is exactly the situation which is presented by an injection machine barrel with deep set thermocouple. This sort of heat transfer represents in effect an infinite order multiple heat transfer: several minutes can elapse between switch-on of power and a significant change in thermocouple temperature. In fact the response has almost the appearance of a delay (i.e. transport lag ) although there is really a considerable difference between this heart-transfer lag and a true delay. During the time of the heart-transfer lag, heat is being fed into the barrel, so that even if the source of heat were switched off at the instant the deep thermocouple began to respond, the thermocouple temperature would continue to rise as the heat energy already fed in distributed itself evenly throughout the thickness of the barrel wall.
A large part of the total lag can in practice be caused by the heart-transfer lag which occurs with a resistance heater. From the heater element thermal mass, via electrical insulation, to the outer surface of the barrel. For the lag through the barrel wall(or for any similar from the heat transfer) doubling the heart-transfer distance results in four times the lag. Iron, from which most injection machines are made, is a rather poor material for heat transfer: for example similar lag are obtained in aluminium and iron when the distance in aluminium is five times greater.
3. METHODS OF CONTROLLING TEMPERATURE
3.1 Measuring the Temperature
The first item in the control system to consider is the measuring element, of which there are tow basic electrical types: active and passive.
The active type are thermocouples. There are formed by the junction of tow dissimilar metals and give an output voltage proportional to the difference in temperature between the thermocouple and the point of measurement (Fig.3). The fact that the millivolt output of the thermocouple in relation to temperatures is non-linear and that it depends on a stable reference temperature for comparison purposes are factors ， Which must be taken into account in the controller. Thermocouples are very robust mechanically. (This is an obvious advantage in the environment of the moulding shop.) They also exhibit good repeatability from example to example of the same type. The two most common types used in plastic processing are both base metal thermocouples and these are nickel chrome/nickel aluminium (Type K) and iron/jconstantan (Type J).
The passive types rely on having a resistance which varies with temperature in a known manner and thus, when fed from a constant current upon temperature. Such elements do not require a reference temperature to be generated by the controller. The commonest are the platinum resistance thermometer (which occupies a larer volume than a thermocouple and is more fragile)and the thermistor(which operates on the same principle and has the same disadvantages).
The thermocouple is by far the most common measuring elcment used in practice. The siting of the thermocouple will depend upon the degree of control required, as will the choice of controller.
3.2 ON/OFF Control
The simplest form of controller provides ON/OFF control of load power. The measured temperature is compared with the set-point and if it is too low, power is applied to the load; if it is too high the power is switched off. In practice there will be a small amount of hysteresis in the controller (mainly so that spurious noise signals on the thermocouple and effects due to mains regulation should not result in rapid ON/OFF chattering of the load power control relay). If the thermocouple and heater are in very close proximity, i.e. there is no appreciable lag, the temperature will cycle with an amplitude somewhat in excess of the controller hysteresis and with the natural period of the system. There will inevitably be some overshoot on start-up because full power will be applied to the load until the set and actual temperatures become equal and any stored energy in the heater will continue to be transferred to the load even after switch-off. It can be seen that if the thermocouple is deep in the barrel (thus measuring the melt temperature more closely) the system lags will be considerably increased and the temperature cycling will be of a longer period and will become much larger. Similar comments apply to the start-up overshoot.
Thus ,in the least demanding circumstances, an ON/OFF controller with a shallow thermocouple may give acceptable results. However, with the large heaters required to give short start-up overshoot will probably be unacceptable for all but the least demanding situations and will be worse if account is taken of correct siting of the thermocouple.
The natural period of the system results from a combination of heater power and location, sensor location, and the thermal mass of the system.
3.3 Proportional Control (P only)
If we take an ON/OFF controller and force the switching of the output within the controller itself (with variable mark: space ratio)at a rate which is higher than the natural period, then we have proportional control. As the measured temperature approaches the set temperature, the relay will switch off(for a short time) the power supplied to the load. This point, at which just less than full power is applied to the load, is the lower edge of the ‘proportional band’. As the actual temperature approaches the set temperature more closely, less and les power is applied to the load until, when the two become equal, the power input is zero. It is general for the proportional band to be downscale of the set-point, i.e. at set-point the power fed to the load is zer..
The proportional band is usually defined as a percentage of the controller set-point scale span. Since the power applied to the load is proportional to the error or difference between actual and measured temperature (a so-called error-actuated system),it follows that if any power is required to maintain the temperature there must be some error in the system. This error is known as offset or droop (Fig.5). Since, on start-up, the load power will first be switched off at a temperature below the set-point, the resultant overshoot will be reduced. With a sufficiently large proportional band and sufficiently rapid cycling of the output power (compared to the system’s natural frequency) the oscillations in temperature will cease eventually. However, this does not necessarily mean that there will be no sart-up overshoot in temperature, but only that the subsequent oscillation will decay to zero amplitude.
英文翻譯
注塑模的溫度調(diào)節(jié)系統(tǒng)
商用塑料是最常用的，但它是熱敏感性材料。如果說(shuō)因熱引起的問(wèn)題是可以避免的，那么象注塑模中熔化過(guò)程中精確的溫度控制就是有必要的。]
從溫度控制的觀點(diǎn)和一些控制方法和技術(shù)的角度來(lái)考慮（這些方法和技術(shù)因不應(yīng)忘記而被敘述），好的溫度控制能節(jié)約和熱能。
一、介紹
注射模過(guò)程曾引起一次會(huì)議的討論，這次會(huì)議為模制產(chǎn)品的塑料原材料制定了可行性標(biāo)準(zhǔn)。一些可行性參數(shù)是重量，尺寸，顏色和強(qiáng)度。所有這些參數(shù)都受材料制造環(huán)境的影響。為了決定其可行性，為這些參數(shù)已經(jīng)建立了相應(yīng)的公差。對(duì)注射機(jī)的正確操作起作用的眾多參數(shù)中，最重要的一個(gè)參數(shù)是溫度，所有的塑料產(chǎn)品的制造都只有在特定的溫度范圍內(nèi)。這個(gè)特定的溫度范圍因材料而異。一些材料的這個(gè)溫度范圍相當(dāng)寬，而另一些材料的這個(gè)范圍卻相當(dāng)窄。
為使產(chǎn)品在允許溫度限制范圍內(nèi)，需要某些形式的溫度控制。如果應(yīng)用正確，這里有大量的類型能導(dǎo)致正確控制形式的操作。通過(guò)正確的應(yīng)用控制設(shè)備。能節(jié)省貴重的塑料和能量。系統(tǒng)的現(xiàn)實(shí)性和操作者監(jiān)管要求的程度，也很大程度上依賴于最新消耗，運(yùn)輸消耗，工作費(fèi)用三者之間的平衡。
這章的目的是從溫度控制的角度來(lái)檢查注射模具和列舉一些常用的溫控方法以及其優(yōu)點(diǎn)。
二、 過(guò)程
2·1 模具的分類
從控制的角度來(lái)說(shuō)，一個(gè)注射模具由許多分區(qū)和一個(gè)控制部分組成（每一個(gè)分區(qū)有一種測(cè)量溫度的方法），控制器比較兩者之間的不同測(cè)量?jī)r(jià)值和控制兩者之間的不同，而用某種方法輸入到這個(gè)分區(qū)的熱移走。通過(guò)劃分模具的分區(qū)，能使這些分區(qū)更容易認(rèn)識(shí)，不同的分區(qū)，要求有不同的溫度和不同的熱輸入（如圖1）為了達(dá)到這個(gè)目的，一個(gè)典型的小模具就可以有3～4個(gè)桶型區(qū)和噴管區(qū)。這些離主流道襯套最近的區(qū)域是塑料要求熔化的地方。因此要求有相當(dāng)大的熱量進(jìn)給。然而，在離主流道襯套最遠(yuǎn)的澆口處，通過(guò)增加注射壓力，使塑料和澆口之間產(chǎn)生摩擦熱。這意味著，當(dāng)模具在工作時(shí)只需要相當(dāng)小的熱量輸入。如果機(jī)器的循環(huán)周期非常短。某些材料在制造過(guò)程中比被要求的熱量產(chǎn)生更多的熱量，為了保持溫度，就需要采用某些形式的冷卻方式應(yīng)用。
2·2 熱電偶的安裝
再考慮這些桶型區(qū)：一個(gè)型腔應(yīng)具有足夠的壁厚。用以承受足夠的壓力。最平常的加工方法是電加熱和使用一個(gè)帶狀的加熱片貼在型腔周圍（如圖2），在任何類型的一個(gè)控制器都只能控制一個(gè)點(diǎn)的測(cè)量溫度的測(cè)試，而且盡可能貼近型腔。因?yàn)槲覀冃枰氖撬芰系臏囟?，而不是型腔的溫度，塑料是熱的不良?dǎo)體。依靠純熱進(jìn)去塑料，如果熱電偶安放在型腔的表面或非常淺，那么測(cè)量值和實(shí)際值之間將會(huì)有非常大的差異。
任何給出的操作環(huán)境都或多或少的存在實(shí)際值和測(cè)量值之間的差異。然而如果環(huán)境變化，如模具的運(yùn)動(dòng)速度和周圍的環(huán)境溫度變化，這都可以影響到工件的熔化溫度。因此，熱電偶的安裝位置要盡可能的靠近型腔的內(nèi)壁。
2·3溫度過(guò)調(diào)量
一個(gè)具有一個(gè)熱電偶的加熱片貼在一個(gè)深孔型腔的壁上。它的合模系統(tǒng)是最典型的塑料加工機(jī)械，而且存在著大量的控制問(wèn)題，不僅在正常的模具工作期間必須完成穩(wěn)定的控制，而且可行的合理的初始操作也必須完成機(jī)械可以在不用調(diào)節(jié)時(shí)盡可能完美而迅速地使它達(dá)到正常的操作溫度（如果溫度上升或下降，以某一頻率。就是說(shuō)它經(jīng)過(guò)那點(diǎn)，但不停留在那點(diǎn)，而是在它返回那點(diǎn)時(shí)繼續(xù)通過(guò)一定數(shù)量的點(diǎn)。在這種情況下，過(guò)量調(diào)節(jié)就出現(xiàn)了。如圖4）
在系統(tǒng)中引起過(guò)量調(diào)節(jié)的基本原因是，多個(gè)熱傳導(dǎo)滯后等產(chǎn)生的殘余熱量。首先，引起受熱物體的溫度上升，然后，傳遞給第二個(gè)受熱物體，同時(shí)使第二個(gè)物體溫度上升，然后從第二個(gè)受熱物體傳遞給第三個(gè)受熱物體。以次類推直到熱在傳遞過(guò)程中達(dá)到控制溫度的點(diǎn)附近。
舉一個(gè)最簡(jiǎn)單的多個(gè)熱傳遞的例子，如果兩個(gè)受熱體，如果每個(gè)受熱體都是一樣的，那將是過(guò)調(diào)量中最糟的。一種情況，沖加熱到裝入的差的熱傳遞使環(huán)境變?cè)悖驗(yàn)榧訜釡囟龋ㄈ缭陂_(kāi)始時(shí)的溫度）。將使最終裝入溫度遠(yuǎn)高于其本身。當(dāng)加熱電源切斷時(shí)，最終溫度就達(dá)到了。（忽略溫度損失和假設(shè)加熱熱量和吸收熱量相等）。這將意味著最終電源切斷時(shí)，最終各方面的溫度。因此，過(guò)調(diào)量作為過(guò)調(diào)量作為熱傳遞在裝入溫度上升時(shí)變地更糟。
在特別糟的過(guò)調(diào)量（可控制）的情況出現(xiàn)在熱傳遞通過(guò)熱導(dǎo)體材料的深處，這是實(shí)際的環(huán)境。這個(gè)環(huán)境是一個(gè)具有深的安裝電熱偶的注射模具環(huán)境。這套熱傳遞系統(tǒng)抽繪一個(gè)無(wú)限次續(xù)的多熱傳遞系統(tǒng)的影響。在打開(kāi)電源和在熱電偶中的一次重要轉(zhuǎn)變之間需要幾分鐘的時(shí)間。實(shí)際上，這反映的是一種延時(shí)的表現(xiàn)（如傳導(dǎo)滯后），雖然熱傳導(dǎo)滯后和真正的延時(shí)之間存在著差異，在熱傳導(dǎo)滯后和真正的延時(shí)之間存在著差異，在熱傳導(dǎo)滯后的時(shí)間中，熱進(jìn)給到型腔，以至于熱源被切斷的瞬時(shí)深的熱電偶開(kāi)始反應(yīng)，當(dāng)熱能已經(jīng)進(jìn)給通過(guò)整個(gè)型腔壁后來(lái)完全地分配本身。
總的滯后的大部分，可以是由于發(fā)生在熱阻傳導(dǎo)體的熱傳導(dǎo)滯后引起，熱阻傳導(dǎo)體從熱的基本發(fā)熱體，經(jīng)過(guò)電隔離在型腔外表，因?yàn)闇笸ㄟ^(guò)型腔壁（或任何一個(gè)類似的熱傳導(dǎo)）兩倍的熱傳導(dǎo)距離而產(chǎn)生了四倍的滯后。大多數(shù)注射模具制造用的鋼材對(duì)熱傳導(dǎo)是相當(dāng)差的材料。舉一個(gè)簡(jiǎn)單的例子：當(dāng)在鋁中的距離比在鐵中大五倍時(shí)。在鐵和鋁中能得到相同的熱滯后。
三、 溫度控制的方法
3·1溫度的測(cè)量
在控制系統(tǒng)中，首先要考慮的一條是測(cè)量的元素，它有兩種基本的電子測(cè)量類型：主動(dòng)的和被動(dòng)的類型。
主動(dòng)類的是熱電偶，它由兩種不同金屬片和一個(gè)外部電壓組成。這個(gè)外部電壓與熱電偶和測(cè)量點(diǎn)之間的不同溫度相稱（如圖3）；熱電偶的毫伏輸出電壓與溫度不成線性關(guān)系，它依賴一個(gè)作為比較目的的穩(wěn)定的參考溫度，這一事實(shí)都是在控制器里必須考慮的因素，熱電偶具有相當(dāng)強(qiáng)的機(jī)動(dòng)性（這在模具工廠的環(huán)境中是相當(dāng)有利的）。這些因素也表現(xiàn)好的重復(fù)性。從例子到相同的類型的例子，兩個(gè)最常用在塑料加工過(guò)程的例子都是金屬熱電偶的基本組合材料，它們是鎳鉻/鎳鋁合金（類型K）和鋼/銅合金（類型J）。
無(wú)源類熱電偶，存在一種阻力，這種阻力使溫度不同于眾所周知的那種方式。因此，當(dāng)在恒流電源的作用下，這種阻力將產(chǎn)生電壓，這個(gè)電壓依賴于所通過(guò)的材料的溫度。最常用的是鉑阻熱電偶（這種熱電偶比以前講的普通熱電偶具有更大的容量，并且更容易碎。）和熱敏電阻（它是用同樣的原理進(jìn)行工作具有同樣多的不利條件）。
熱電偶是在實(shí)踐中被大量使用的最常用的測(cè)量工具。熱電偶的定線將依賴于要求控制的度數(shù)和所選的溫度控制器。
3·2控制器的開(kāi)關(guān)
控制器的最簡(jiǎn)單的形式提供負(fù)載電源開(kāi)關(guān)的控制，測(cè)得的溫度與安裝點(diǎn)比較，假如溫度太低，負(fù)載電源將參與工作，假如溫度太高，負(fù)載電源見(jiàn)被切斷，在實(shí)際中，在控制器中有一些磁滯現(xiàn)象。如果熱電偶和加熱器非常接近，那么這就不存在滯后，溫度將以某種振動(dòng)進(jìn)行循環(huán)。這個(gè)振幅是由控制起的滯后和系統(tǒng)的自然周期引起，因?yàn)槿β实碾娫丛谝蟮臏囟群蛯?shí)際溫度相等之前一直提供負(fù)載，所以在開(kāi)始時(shí)有一定的過(guò)調(diào)量是不可避免的。很明顯，如果熱電偶在型腔壁的深層（因此測(cè)量的熔化溫度更接近）。系統(tǒng)的滯后增大，溫度的循環(huán)周期將變長(zhǎng)，振幅將變大，也同樣在開(kāi)始時(shí)有一個(gè)過(guò)調(diào)量。
因此，一個(gè)具有線的熱電偶開(kāi)/關(guān)控制器可以得出所接受到的結(jié)果，這是起碼的要求。然而具有大的熱電偶的開(kāi)/關(guān)控制器要求有一個(gè)更短的啟動(dòng)時(shí)間。如果計(jì)算考慮了這個(gè)熱電偶的正確安放位置，那么這個(gè)啟動(dòng)時(shí)間過(guò)短將可能是對(duì)于所有控制器來(lái)說(shuō)是不接受和更糟的。除這起碼的要求。
這套系統(tǒng)的自然時(shí)期來(lái)源于一個(gè)熱電偶能量與位置的聯(lián)合作用，傳感器的位置和系統(tǒng)的熱量集中區(qū)域三個(gè)因素。
3·3比例的控制（僅僅是P的控制）
如果我們使用一個(gè)開(kāi)/關(guān)控制器，并且迫使輸出量轉(zhuǎn)換。在控制器內(nèi)部本身有一個(gè)頻率，這個(gè)頻率高于自然時(shí)期的，然后我們將要進(jìn)行一個(gè)比例的控制問(wèn)題。當(dāng)測(cè)量的溫度接近安放點(diǎn)的溫度時(shí)，繼電器將在短時(shí)間內(nèi)切斷提供負(fù)載電源，在比最大電源電壓少一些的這個(gè)點(diǎn)是比例帶的最低邊緣，當(dāng)實(shí)際溫度接近安放點(diǎn)的溫度時(shí)，越來(lái)越少的電源電壓進(jìn)給量，直到兩者完全相同時(shí)，電源輸入量將變成零。總的一句話來(lái)說(shuō)，對(duì)于比例帶到安放點(diǎn)呈降低的比例趨勢(shì)。例如在安放點(diǎn)的電量進(jìn)給為零。
比例帶的定義就是一個(gè)控制器安放點(diǎn)的范圍段的一個(gè)百分率。因?yàn)殡娫簇?fù)載的誤差是成比例的，或是實(shí)際溫度與測(cè)量溫度之間存在著差異（一個(gè)所謂的誤差一個(gè)實(shí)際系統(tǒng)），這產(chǎn)生的后果將是假如任何電源要求保持溫度，這將使在系統(tǒng)中產(chǎn)生某些錯(cuò)誤，這個(gè)誤差就是眾所周知的偏差和下降（如圖5）。然而在開(kāi)始上升階段，在溫度還低于安放點(diǎn)時(shí)，負(fù)載電源將被關(guān)掉，短期內(nèi)的結(jié)果將降低，用一個(gè)足夠大的比例帶和足夠快的外部輸出電壓的循環(huán)（與系統(tǒng)本身的自然頻率相比）溫度的波動(dòng)將最終停止。然而，這并不意味著這里沒(méi)有上升的過(guò)調(diào)量，而僅僅只是意味著在此以后的波動(dòng)將減小到振幅為零。
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