PS PMMA PET PC Transparent Injection Molding
Due to its significant advantages such as light weight, good toughness, easy molding and low cost, plastics are increasingly used in modern industrial and daily life products, especially in the fields of optical instruments and packaging industries, where the substitution effect of plastics is becoming more and more obvious. However, to achieve high transparency, excellent wear resistance, impact toughness, and fine control of cost and process, in-depth research and optimization of plastic composition, injection molding process, equipment and molds are required.
Currently popular transparent plastic materials on the market include polymethyl methacrylate (PMMA, commonly known as acrylic or plexiglass), polycarbonate (PC), polyethylene terephthalate (PET), transparent nylon AS and polysulfone PSF. Among them, PMMA, PC and PET are the most widely used. Next, we will take these three plastics as examples to explore the injection molding process of transparent plastics.
Process characteristics of PMMA
PMMA, that is, polymethyl methacrylate, has a high viscosity and relatively poor fluidity. Therefore, high material temperature and high injection pressure are required during the injection molding process. It is worth noting that the injection temperature has a more significant effect on the fluidity of PMMA, while increasing the injection pressure helps to improve the shrinkage of the product. Its melting temperature is 160℃, while its decomposition temperature is as high as 270℃, which makes the adjustment range of material temperature relatively wide, thus having good processability. In order to further improve fluidity, we mainly optimize from the injection temperature.
In addition, PMMA performs poorly in terms of impact resistance and wear resistance, and is prone to scratching and brittle cracking. To overcome these defects, we can consider increasing the mold temperature and optimizing the condensation process.
Process characteristics of PC
PC, that is, polycarbonate, also has the characteristics of high viscosity and melt temperature, and its fluidity is relatively poor. During the injection molding process, we need to use a higher temperature (approximately between 270-320℃) to ensure that it is fully melted. Although the material temperature adjustment range of PC is narrow, its processability is slightly insufficient compared to PMMA. The injection pressure has little effect on the fluidity of PC, but due to its high viscosity, a larger injection pressure is still required. At the same time, in order to prevent the generation of internal stress, the holding time should be as short as possible.
Since PC has a large shrinkage rate and stable dimensions, but the product has large internal stress and is prone to cracking, we prefer to improve its fluidity by increasing temperature rather than pressure. At the same time, by increasing the mold temperature, optimizing the mold structure and performing post-processing, we can effectively reduce the possibility of product cracking. In addition, when the injection speed is low, defects such as ripples are prone to occur at the gate, so we need to control the temperature of the radiating nozzle separately and ensure that the mold temperature is high enough and the runner and gate resistance is as small as possible.
Process characteristics of PET
PET requires a higher temperature (approximately between 260-300℃) during the molding process, and the material temperature adjustment range is narrow. However, once melted, its fluidity becomes very good. It is worth noting that PET often needs to add an anti-draft device to the nozzle to prevent poor flow. In addition, the mechanical strength and performance of PET are not high after injection, and need to be further improved through stretching process and modification. Therefore, during the injection molding process, we need to accurately control the mold temperature (hot runner mold is recommended) and ensure that the mold temperature is high enough to prevent product warping or deformation. At the same time, we also need to pay attention to avoid problems such as poor surface gloss and difficulty in demolding.
PET product
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