注塑模的温度调节系统外文文献翻译/注塑射模具外文翻译/中英文翻译

附录  外文资料  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 systems 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.     英文翻译                    注塑模的温度调节系统      商用塑料是最常用的，但它是热敏感性材料。如果说因热引起的问题是可以避免的，那么象注塑模中熔化过程中精确的温度控制就是有必要的。    从温度控制的观点和一些控制方法和技术的角度来考虑（这些方法和技术因不应忘记而被叙述），好的温度控制能节约和热能。                       一、介绍      注射模过程曾引起一次会议的讨论，这次会议为模制产品的塑料原材料制定了可行性标准。一些可行性参数是重量，尺寸，颜色 和强度。所有这些参数都受材料制造环境的影响。为了决定其可行性，为这些参数已经建立了相应的公差。对注射机的正确操作起作用的众多参数中，最重要的一个参数是温度，所有的塑料产品的制造都只有在特定的温度范围内。这个特定的温度范围因材料而异。一些材料的这个温度范围相当宽，而另一些材料的这个范围却相当窄。      为使产品在允许温度限制范围内，需要某些形式的温度控制。如果应用正确，这里有大量的类型能导致正确控制形式的操作。通过正确的应用控制设备。能节省贵重的塑料和能量。系统的现实性和操作者监管要求的程度，也很大程度上依 赖于最新消耗，运输消耗，工作费用三者之间的平衡。      这章的目的是从温度控制的角度来检查注射模具和列举一些常用的温控方法以及其优点。                      二、  过程  2 1  模具的分类      从控制的角度来说，一个注射模具由许多分区和一个控制部分组成（每一个分区有一种测量温度的方法），控制器比较两者之间的不同测量价值和控制两者之间的不同，而用某种方法输入到这个分区的热移走。通过划分模具的分区，能使这些分区更容易认识，不同的分区，要求有不同的温度和不同的热输入（如图 1）为了达到这个 目的，一个典型的小模具就可以有 3 4 个桶型区和喷管区。这些离主流道衬套最近的区域是塑料要求熔化的地方。因此要求有相当大的热量进给。然而，在离主流道衬套最远的浇口处，通过增加注射压力，使塑料和浇口之间产生摩擦热。这意味着，当模具在工作时只需要相当小的热量输入。如果机器的循环周期非常短。某些材料在制造过程中比被要求的热量产生更多的热量，为了保持温度，就需要采用某些形式的冷却方式应用。  2 2  热电偶的安装      再考虑这些桶型区：一个型腔应具有足够的壁厚。用以承受足够的压力。最平常的加工方法是电加热和使用一 个带状的加热片贴在型腔周围（如图 2），在任何类型的一个控制器都只能控制一个点的测量温度的测试，而且尽可能贴近型腔。因为我们需要的是塑料的温度，而不是型腔的温度，塑料是热的不良导体。依靠纯热进去塑料，如果热电偶安放在型腔的表面或非常浅，那么测量值和实际值之间将会有非常大的差异。      任何给出的操作环境都或多或少的存在实际值和测量值之间的差异。然而如果环境变化，如模具的运动速度和周围的环境温度变化，这都可以影响到工件的熔化温度。因此，热电偶的安装位置要尽可能的靠近型腔的内壁。  2 3 温度过调量      一个 具有一个热电偶的加热片贴在一个深孔型腔的壁上。它的合模系统是最典型的塑料加工机械，而且存在着大量的控制问题，不仅在正常的模具工作期间必须完成稳定的控制，而且可行的合理的初始操作也必须完成机械可以在不用调节时尽可能完美而迅速地使它达到正常的操作温度（如果温度上升或下降，以某一频率。就是说它经过那点，但不停留在那点，而是在它返回那点时继续通过一定数量的点。在这种情况下，过量调节就出现了。如图4）      在系统中引起过量调节的基本原因是，多个热传导滞后等产生的残余热量。首先，引起受热物体的温度上升，然后，传递给 第二个受热物体，同时使第二个物体温度上升，然后从第二个受热物体传递给第三个受热物体。以次类推直到热在传递过程中达到控制温度的点附近。      举一个最简单的多个热传递的例子，如果两个受热体，如果每个受热体都是一样的，那将是过调量中最糟的。一种情况，冲加热到装入的差的热传递使环境变糟，因为加热温度（如在开始时的温度）。将使最终装入温度远高于其本身。当加热电源切断时，最终温度就达到了。（忽略温度损失和假设加热热量和吸收热量相等）。这将意味着最终电源切断时，最终各方面的温度。因此，过调量作为过调量作为热传递在装入 温度上升时变地更糟。      在特别糟的过调量（可控制）的情况出现在热传递通过热导体材料的深处，这是实际的环境。这个环境是一个具有深的安装电热偶的注射模具环境。这套热传递系统抽绘一个无限次续的多热传递系统的影响。在打开电源和在热电偶中的一次重要转变之间需要几分钟的时间。实际上，这反映的是一种延时的表现（如传导滞后），虽然热传导滞后和真正的延时之间存在着差异，在热传导滞后和真正的延时之间存在着差异，在热传导滞后的时间中，热进给到型腔，以至于热源被切断的瞬时深的热电偶开始反应，当热能已经进给通过整个型腔壁后来完 全地分配本身。      总的滞后的大部分，可以是由于发生在热阻传导体的热传导滞后引起，热阻传导体从热的基本发热体，经过电隔离在型腔外表，因为滞后通过型腔壁（