Influence of the hottest process conditions on the

Influence of process conditions on the properties of injection molded parts

injection molding process not only determines the shape of the molded part, but also its material properties. Figure 1 shows the size and distribution of spherulites produced at the center of the sample under study under reference production conditions. In the case being discussed, the spherulite diameter is about 18mm

insufficient understanding of the importance of morphology

it is the overall morphological structure of plastic parts that determines the end use performance in each case. This means that not only the external performance of plastic parts is conditional on technology, but also the internal performance. This also means that the thermomechanical treatment of molding compounds, such as the pressure, temperature and shear rate in the processing process, determines the material structure (morphology) of the plastic parts produced. The external and internal performance of the product determines the final performance of the product. Therefore, the shape of plastic controls the shrinkage and shape of plastic parts, and also determines the use performance of materials. The performance of plastic parts is not necessarily the best performance for a certain shape of plastic parts, such as high mechanical strength, ideal hardness and good wear resistance. On the contrary, the ideal shape will inevitably produce a stable plastic shape without "cutting corners" to increase the market share of enterprises, but it is also a shape that cannot be changed again

once the relationship between process and final quality is established, it becomes clear that high-quality plastic parts can only be obtained by optimizing the process control of injection molding. This control focuses on form, and also includes the recording and control of state variables. Therefore, stable process control requires the process to adapt to the processed plastic. It is based on thermodynamic factors. For semi crystalline thermoplastics, crystallization kinetics must also be considered. However, there is no general understanding of this in the plastic processing industry, and this idea needs to be inculcated into people

Research on processing POM

taking the flowable copolyformaldehyde (POM) as an example, people have studied and discussed the influence of process conditions on the final properties of plastic parts made in the injection molding process

in order to do this research, according to din/iso 527 standard, 5A type tensile template (sectional area 4) is made in the double cavity mold at different injection speed and pressure × 1mm2）。 The mold cavity is filled by a pin gate located on the side of the plastic part. The mold is equipped with a pressure sensor near the gate, which can measure the pressure change at the central shoulder of the drawing template. In all tests, the mold temperature for the production of plastic parts is set at 95 ℃, because this is the temperature recommended by the raw material manufacturer for high-quality plastic parts. The molding compound is processed on an injection molding machine with a mold locking force of 220kn. The diameter of the screw is 18mm, and the temperature of the nozzle on the machine is 210 ℃

Figure 1: micrograph of spherulite structure in the center of the sample

the research results reveal the clear relationship between the internal and external properties of the plastic parts on the one hand, and the relationship with the selected process conditions on the other hand. Although the plastic parts produced have almost the same mechanical strength, there are obvious differences in the deformation capacity of different sample material structures. This can be seen clearly from the different tensile strains at fracture, and the corresponding mandrel is replaced, especially the different tensile impact strength obtained in each case. The injection speed is normalized to 20cm3/s, and the tensile impact strength can be reduced by 55%, while the measured changes in weight and shrinkage are only 2.5% or 15%

the necessity of material centered quality management

the supplementary research on different samples by dynamic thermal difference method (DSC) only highlights the subtle differences in morphology at the beginning. In other words, on the whole, the studied template has uniform crystallization. This result can also be observed in cooling (crystallization heat) and in the secondary heating stage (melting heat after homogenization of the previous process)

from the crystallization process measured by dynamic differential calorimetry, it is obvious that when the injection speed increases step by step during polymer processing, the crystallization heat at lower temperature increases from -74j/g to -97j/g. This implies that the POM material used will change due to processing. The change of molecular weight and molecular weight distribution of the material improves the curing performance of different materials (the overall crystallization is more or less consistent), so it promotes the formation of different structures (forms), which is proved by the ratio of peak height to peak width, which is reduced from 2.7 to 1.6

in this case, observing the overall crystallization of different samples only by DSC (first step heating) will lead to incorrect evaluation of quality, because there is no difference here. Only when the structure is evaluated on an indirect basis will this highlight the existing differences

microscopic observation of thin sections (about 10mm) of different samples under polarized emission light shows that the structures of plastic parts made under the control of different process conditions are very different. When the injection speed increases, the thickness of the non spherical outer layer that can be observed decreases sharply from 102mm to 30mm, and the remaining structure also experiences changes (Fig. 2). Therefore, the sharp drop in hardness of plastic parts made at high injection speed can also be attributed to the undesirable morphological changes in materials. The structure shown on the right of Figure 2 is related to the extremely brittle deformation performance of the plastic parts studied. The impact strength of tensile impact test is 100% lower than that of the plastic parts with the same structure shown on the left of Figure 2

Figure 2: effect of processing on morphological structure

effect of processing on rheological properties

processing POM that is easy to flow at different injection speeds obviously has a far-reaching impact on the rheological properties of materials. The increase of polymer shear degree causes the gradual chain decomposition of micro molecules, which is accompanied by the change of melt fluidity (pressure transmission in the mold), and then the crystallization kinetics changes. The DSC curves that have been measured and discussed represent these ongoing processes. Rheological investigation also confirmed this performance. The rheometer used is uds200 model. Take 100mg of sample, and the rheometer will withstand the temperature of 210 ℃ and 0 The logarithmic increasing shear rate within the range of S-1, with an interval width of 0.1mm

these results prove that when the injection speed is increased, the decomposition of polymer in processing is intensified. The average molecular weight of the polymer changes according to the processing conditions, which can be clearly seen from the decrease of zero viscosity when the injection speed increases

conclusion

the reported research shows that different injection speeds have a significant impact on the final quality of injection molded parts made of POM material. The rheological properties of plastic parts change with shear, which plays a role in POM melt in processing. This change is attributed to the decrease of molar mass and the adjustment of molecular weight distribution, which is proved by experimental data. As the melt cools down during the formation of plastic parts, the rheological properties of POM melt, together with the actual melt temperature and melt pressure, determine the crystallization kinetics. This means that completely different structures and highly variable service properties can change with processing conditions

for copolyformaldehyde, which is easy to flow, embrittlement is obvious and can provide a range of economical graphene materials, which will reduce the impact strength of thin-walled plastic parts by 50%. This research result is basically consistent with that observed in practice. The phenomenon discussed is less common for polyoxymethylene with general fluidity

therefore, it is impossible to make a clear description of the quality of plastic parts without considering the shape. Therefore, the plastic production process with the latest technology needs to make your work no longer flawed, and a certain form of quality management is required. It first monitors the quality of the internal performance of the plastic parts (such as monitoring the pressure curve of the mold cavity), so as to ensure that the obtained products are of high quality on the whole, assuming that the internal performance is also conditional on the external performance

therefore, the research results have shown that in order to achieve preventive quality management, the application of material centered process monitoring and subsequent process control is desirable and indeed necessary in the future. Only in this way, it is possible to greatly avoid the potential damage to plastic parts caused by insufficient performance caused by processing operations

further research is needed to define the ideal structure, which will ensure the production of very thin-walled or very small plastic parts and realize the final stability of plastic parts