Thermal how it reacts to build a precise part.

Thermal Modelling of FDM 3-D PrintersAbstractFused deposition modelling (FDM) process is driven by a moving heat source, and thermal modelling plays an important role in mechanical and physical properties of the end-part. Fused Deposition processes are performed by melting, extrusion and solidification of thermoplastic materials. This paper focuses the thermal modelling of a FDM printer to explain how it reacts to build a precise part. It highlights the heating and cooling processes seperately.1Literature ReviewAdditive Manufacturing (AM) processes are characterized by a layer-by-layer material deposition so that no forming tool is necessary. It is possible to produce very complex parts which are impossible or very expensive to manufacture by conventional manufacturing processes. The starting point for every AM process is a 3D CAD model from which the layer information is generated in the data preparation. |Figure 1Rapid Prototyping Processes|Fused deposition modelling (FDM) is one of the additive manufacturing process for building thermoplastic parts. FDM introduced onto the market in 1991 by Stratasys. It is also the first additive manufacturing process which uses material extrusion for building a part. This clean running, single step operation uses non-toxic, thermoplastic wire-like filaments eliminating liquid photopolymers, powders or lasers from the process?1?. In FDM process the part is melted through a extrusion head on the base plate. The nozzle is controlled by a computer to melt the thermoplastic filaments. As shown in the ?Figure 2? this nozzle “squeeze” the heated filament layer by layer on the build sheet until the part is finished ?2?. |Figure 2FDM Process|s?cakl?k olay?na gir genel olarak |Figure 3Temperature Changes for Different Materials|2MethodologyDeposition materials change density with temperature when they solidifies and these density changes cause physical anomalies. Also different areas of the printed material cool at different rates, the areas’ dimensions change at different speed. Thermally solidifiable systems are subject to both “curl” or “warping”( 4 ) and “plastic deformation” distortion mechanisms. Curl is manifest by a curvilinear geometric distortion which is induced into a prototype during a cooling period 5 .There are some techniques to reduce the curl on the object. One of them is heating the build environment to prevent sudden temperature changes or at least keeping the region temperature higher than the glass transition temperature of the thermoplastics. Another technique is to choose materials that have lower thermal expansion coefficients. For example ABS has higher glass transition temperature and coeeficient of thermal expansion than PLA and PETG and that is why it is more sensitive to curl 4 . Other technique is to apply the material at the lowest possible temperature. Also increasing the adhesion between the platform and part may prevent curl. Various material, including waxes, thermoplastic resins, and metals may be used to form three-dimensional objects. The material is preferably one which will melt at a preselected temperature and rapidly solidify without shrink distortion upon bonding to the previous layer. A temperature controller responsive to temperature sensors on the dispensing head is used to closely control the temperature of the supply material to a level of about 1°C, above its solidification temperature at the point of discharge. This ensures consistent flow and that the material will solidify instantly while cooling, after discharge, with resultant efficiency in the object-forming process as multiple layers are discharged, solidify, and build-up. A supplemental heater on the nozzle tip responsive to the temperature controller provides the close control of the temperature of the material as it is discharged, to ensure that it is in a fluid state slightly above its solidification temperature?3? .Also a thermal seal is provided on the material receiving end of the dispensing head to ensure the material does not rise to a level of temperature where the material folds or bends. Additionally, there is a tubular guide member acts like a passage through the extrusion head. They are choosen from highly conductive metals such as aliminium or silver so that they dissipates the heat to keep the flexible filament smooth at a suitable temperature during its movement.Figure 4 is a diagram showing the liquefier temperature and air temperature requirements to build a good part. Each material has upper and lower liquefier and air temperature limits and if these limits are exceeded it may cause poor quality surface and low strength.Surface rippling is occured by too high air temperature and too high liquefier temperature. On the other hand, low air temperatures end up with poor bonding strength between the layers for some materials and delamination of the model in some cases. Low liquefier temperatures result in low limits for material flow rates due to the high viscosity of the material and also poor bonding. For this reason the design should complete the optimum balance between strength and surface finish?4? .|Figure 4Modelling zone temperature parameters |As a result of these experimental datas Stratasys delivered some design modifications to their customers in 1993?4? . These modifications are adding new seals, fan heater boxes and a new cabinet door design to keep the air temperature uniform, and using a longer and more powerful liquefier to improve the delivery of the material and consistency of the set point temperature. Changing the seals , door and the heat boxes eliminates the cold spots in the environment. Longer liquefier provides less variation in the temperature at high and low flow rates. In the process of the intellegent nozzle’s ( ?Figure 5? ) working, the printing raw material of the FDM 3D printer gets into the feeding pipe through the wire feeder, and then enters the heating pipe through the transition pipe, and then the thermoplastic material is heated in the heating pipe which regulates heating temperature with feedback regulation via thermistor. Squeezed by the non- melting material, the material of heating and melting parts squeezes out the thermoplastic filament of predetermined diameter from the nozzle. The influence of the nozzle structure on the FDM printing is mainly the internal flow structure of the nozzle and the control of heating temperature of the nozzle and also the different pressure difference in each flow generated by flow fields in different flows. The temperature of the nozzle affects various performance of material, such as cohesiveness, accumulation resistance, wire flow, extruded wire width and so on. For one thing, if the temperature is too slow, it will lead to partial solid material, as a result, too large viscosity of material will affect the velocity of wire and even the block nozzle; For another thing, if the temperature is too high, it will lead to partial liquid material, as a result, the wire material appears sallow and increasing mobility and also too fast extruded speed, which can’t control the wire precision accurately. The nozzle flow mainly applies Teflon material which has acid-base resistance and various organic solvent resistance characteristics and even almost insoluble in all solvents?5? .|Figure 5Intelligent Nozzle of a FDM Printer |The operation of the liquefier is controlled through two different channels: whereas flow control is facilitated through servo controllers that command the motion of the traction rollers; temperature regulators adjust heater power to keep liquefier temperature at the constant pre-set value. The decoupled nature ofthe temperature regulation from motion or flow control generates unique challenges for liquefier design, e.g. the temperature at the liquefier entrance shall not exceed a critical value even though the liquefier temperature is constant at pre-set temperature, the liquefier length should be long enough to achieve the desired temperature at its end for all flowrate conditions?6? . The process of the FDM 3D printing is mainly to heat and melt the raw material and then print in a plane by the nozzle, and finally the product is fixed and formed through layer by layer accumulating. Therefore, in the process of printing the heating temperature of the nozzle and the modeling temperature of the working platform are demanded to control. The temperature control mechanism is shown in ?Figure 6? . The heating chamber of the nozzle is heated by the heating rod and the temperature controller. The heating rod is responsible for heating, and the temperature controller controls the heating temperature and through the thermistor controls the heating circuit by timely feedback. The regulation of nozzle temperature is mainly to control the heating speed and the feedback speed. The materials of the FDM 3D printing mainly are PLA plastic and ABS plastic, and the melting point of these two materials is all about 170°. In order to shorten the printing time and improve the printing precision, the heating speed and the over-temperature feedback ability need to be improved. The temperature controlling of working platform is mainly to avoid the warp in the process and molding defects when the product is forming. So, it is needed to stabilize the working platform in a relatively stable temperature, which can be achieved by a long time control of thermistor at a definite temperature range. |Figure 6Temperature Control Mechanism|
3Results and Conclusions