When manufacturing food packaging plastic containers, the processing materials used are required to have the following characteristics: low volatile odor, low tongue odor, heat resistance to withstand cooking temperature, barrier properties up to one year or longer shelf life, and good Molding processability. The composite sheets meeting the requirements of this characteristic include: impact-modified PP, a multilayer composite sheet in which an EVOH barrier layer is sandwiched.
There are two main problems that are often encountered: uneven wall thickness distribution, poor odor and mouthfeel. In the process of processing such a composite sheet, the sheet is extruded from an extruder and transferred directly to a thermoforming station by a conveyor belt. The reason for the instability of the wall thickness of the product is that the melt strength is not high. Where the sheet is not supported, such as from the exit of the extruder die, where the belt is between the thermoforming stations, the sheet will exhibit excessive sag. The sheet deformed immediately after it exited the extruder die, resulting in instability of the wall thickness of the article.
At the same time, it is necessary to draw attention to the strange odor problem that occurs after the sheet is extruded. It is speculated that the sheet may be in the stage of overheating and scorching, which is the main reason for the odor and taste odor of the product. Attempts to eliminate scorch and excessive sag problems by improving extrusion process conditions are unlikely to succeed.
In this way, attention is paid to the properties of the raw materials, and the characteristics of the PP material used in the thermoforming process are required to have a high molecular weight and a low melt flow rate (MFR) in the melt stage. The processing plant has already submitted the MFR rating requirements for the material to the raw material supplier. However, after we examined the MFR of the raw material pellets, it was found to have exceeded the MFR requirement. The MFR requirement is 0.5 g/10 min and the MFR of the material has been detected to have exceeded 1.0 g/10 min. Fig. 4 shows that the tendency of the PP sheet to swell increases as the MFR increases.
Further in-depth research found that the resin supplier did not go through the compounding workshop in the production process, that is, the process of mixing the impact modifier into the resin material for mixing. In order to increase production capacity as much as possible, all materials are extruded at high temperatures. As a result, the material is inevitably degraded and burnt.
The correct solution is: all raw material MFR indicators need to be less than 0.6g/10min, the new mandatory technical specifications stipulate that the mixing plant should be mixed under low melt temperature conditions, changing the extrusion process conditions to prevent material from happening. degradation. After adopting the above improvement measures, the raw materials obtain reasonable melt strength, the wall thickness of the product is evenly distributed, the material is no longer subject to overheating and scorching, and 100% of the molded parts pass the odor and taste test requirements.
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