1? 、? Influence of material properties: the change of material properties directly affects the forming process and the accuracy of the formed parts, and the material passes through the solid in the process. Melting? During the second phase transformation of solid, the stress and deformation caused by material shrinkage will affect the accuracy of formed parts. Such as ABS resin, its shrinkage factors mainly have two points:

(1) thermal shrinkage. That is to say, the volume change caused by the inherent thermal expansion rate of materials is the main reason for shrinkage. Shrinkage caused by thermal shrinkage:

(2)? Molecular orientation contraction. During the molding process, the molten ABS molecules are elongated in the filling direction, and then contract in the subsequent cooling process. The orientation will make the shrinkage rate of the accumulated filaments in the filling direction greater than that perpendicular to this direction.

Therefore, in order to improve the accuracy, we should reduce the shrinkage of materials. This can be achieved by improving the formula of the material, and the most basic method is to consider shrinkage in the design for size compensation. In the current data processing software, it can only be applied in X, Y and Z directions? Contraction compensation factor? That is to say, according to the different shapes and structural characteristics of parts, different factor sizes are adopted according to experience, so that the size of parts when they are molded is actually slightly larger than that of CAD models. When it cools and solidifies, it is assumed that the size of the part will eventually shrink to the size of the CAD model according to a predetermined shrinkage rate.

2、? Influence of nozzle temperature and molding chamber temperature: the nozzle temperature determines the adhesion, stacking performance, wire flow rate and extrusion wire width of materials. If the nozzle temperature is too low, the viscosity of the material will increase and the extrusion speed will slow down, which will not only increase the burden of the extrusion system, but also cause the nozzle to be blocked in extreme cases, reduce the bonding strength between materials and cause delamination; However, when the temperature is too high, the material tends to be liquid, the viscosity coefficient becomes smaller, the fluidity is strong, and the extrusion is too fast, so it is impossible to form precisely controlled filaments. When making, the previous layer of material will be pressed on it before cooling and forming, which will lead to the collapse of the previous layer of material. Therefore, the nozzle temperature should be selected within a certain range according to the properties of the wire to ensure that the extruded wire is in a state of melt flow. The test shows that the nozzle temperature should be controlled at about 230℃. ℃。

The temperature of the molding chamber will affect the thermal stress of the molded parts. Too high temperature is helpful to reduce thermal stress, but the surface of parts is easy to wrinkle. However, if the temperature is too low, the quenching of the wire extruded from the nozzle will increase the thermal stress of the molded parts, which will easily lead to warping and deformation of the parts. Due to the rapid cooling speed of the extrusion line, the latter layer will start to accumulate only after the cross section of the previous layer has been completely cooled and solidified, which will lead to the weak bonding between layers and the tendency of cracking. The experiment shows that the temperature of the forming chamber should be set to be 65438 0 ~ 2 lower than the melting point of the extruded wire. ℃。

3、? Influence of extrusion speed: extrusion speed refers to the speed at which molten wire is extruded from the nozzle, and the volume of extruded wire per unit time is directly proportional to the extrusion speed. Within the range of reasonable matching with the filling speed, with the increase of extrusion speed, the cross-sectional width of extruded wire gradually increases. When the extrusion speed increases to a certain value, the extruded silk will stick to the outer conical surface of the nozzle and cannot be processed normally.

4? Influence of interaction between filling speed and extrusion speed: the filling speed should match the extrusion speed. If the filling speed is faster than the extrusion speed and the material is not filled enough, the wire will be broken and it will be difficult to form. On the other hand, the filling speed is slower than the extrusion speed, and the melt accumulates on the nozzle, which makes the material distribution on the molding surface uneven and the surface will be convex, which affects the molding quality. Therefore, the filling speed and extrusion speed should be matched within a reasonable range and meet the following requirements:

5. Influence of delamination thickness: Delamination thickness refers to the thickness of each section in the forming process. Because each layer has a certain thickness, there will be a step phenomenon on the surface of the molded entity, which directly affects the size error and surface roughness of the molded entity. For FDM process, it is impossible to completely eliminate step phenomenon. Generally speaking, the smaller the thickness of delamination, the smaller the steps produced on the solid surface and the higher the surface quality, but the required delamination and molding time will be longer and the processing efficiency will be lower. On the contrary, the greater the thickness of delamination, the greater the steps generated on the solid surface and the worse the surface quality, but the processing efficiency is relatively high. In order to improve the molding accuracy, post-processing such as grinding and polishing can be carried out after solid molding.

6? Influence of molding time: the molding time of each layer is related to the filling speed, the area of the layer and the complexity of the shape. If the area of the layer is small, the shape is simple and the filling speed is fast, the formation time of the layer is short; On the contrary, it took a long time. In machining, only by controlling the working temperature of the nozzle and the forming time of each layer can high-precision formed parts be obtained. After repeated experiments, it is concluded that when processing some entities with small cross section, it is often difficult to form because the forming time of one layer is too short, because the upper layer has not had time to cure and form, and the next layer will pile up again, which will cause? Collapse? And then what? Painting? Phenomenon. In order to eliminate this phenomenon, in addition to using a smaller filling speed and increasing the molding time, cold air should be blown on the current molding surface to force cooling, so as to speed up the curing speed of materials and ensure the geometric stability of molded parts. When the forming area is large, we should choose a faster filling speed to reduce the forming time, which can improve the forming efficiency on the one hand and reduce the cracking tendency of the formed parts on the other. If the molding time is too long, the section of the previous layer has been completely cooled and solidified, and then the next layer begins to accumulate, which will lead to the weak bonding between layers.