With the continuous improvement of the accuracy of injection molding processing, in addition to the correct selection of materials and the modification of selected plastics, how to correctly select the molding process conditions, a reasonable analysis of the product's comprehensive shrinkage rate and the correlation between various parameters of the processing process, And the various causes of shrinkage and the influence of processing factors on it are a very important topic. 1 Experimental section 2 Results and Discussion Bedspread Sets,Cotton Bedspreads,Quilted Bedspreads,Bedspreads And Comforters changxing sanxing textile co.,ltd , https://www.sxhometextile.com
1.1 Experimental raw materials
PP: Model 1390, injection grade copolymer PP, Beijing Yanshan Petrochemical Company.
1.2 Equipment injection machine: SZ-100/160NB, Ningbo Plastic Machinery Factory;
Mold: steel injection plate mold.
1.3 sample specifications
200mm x 100mm x 4mm plate.
1.4 Process Conditions Table 1 lists the PP injection molding process conditions.
Table 1 PP injection molding process conditions Item injection process parameters
Barrel temperature/°C 180 200 210 220
Injection pressure/Mpa 60 80 100 120
Holding pressure / Mpa 40 50 60
Injection temperature/°C 200 210 220 230
Injection time/s 3 4 5 6
Holding time /s 30 40 50 60
Mold temperature/°C 50 60 70 80
Cooling time / s 20 30 40 50
2.1 Shrinkage process and mechanism When carefully observing the injection conditions of PP samples under the combinations of process conditions listed in Table 1, it can be found that the shrinkage of the entire product is 1%-3%, and this shrinkage consists of three parts, namely the gate Pre-cure shrinkage, cooling shrinkage and free shrinkage after demoulding.
2.1.1 Shrinkage of the product before the gate solidifies
The shrinkage of the PP product before the gate solidifies largely depends on the compensation ability of the melt. As soon as the material enters the mold, the melt temperature drops rapidly due to the lower mold temperature, and the density and viscosity of the melt continue to increase. The compensation ability of the melt depends on the pressure holding pressure and the entire filling time during which the melt continues to transfer into the mold. This process continues until the gate solidifies and closes. Therefore, it can be considered that the pressure holding pressure and the pressure holding time have a great relationship with the contraction at this stage. If the holding pressure is high and the holding time is long, the shrinkage rate of the product is relatively low.
2.1.2 The shrinkage of the product during the cooling process starts from the gate and solidifies until the entire stage of demolding is the cooling stage. No other new melt enters the mold cavity at this stage, and the weight and density of the PP product no longer change. During the cooling process, the shrinkage of the product is related to the mold temperature, cooling rate and cooling time. The cooling rate is fast, the product is too loose for relaxation, and the shrinkage rate may be small. If the mold temperature is high, the cooling rate is slow, and the molecules have sufficient relaxation time. The rate may increase.
2.1.3 Free shrinkage of the product after demolding The PP product after demolding needs to stand at room temperature for a period of time because the demolded product has a free shrinkage process. If the process conditions are not properly set, some natural stress release may occur during this period of time.
Therefore, the total shrinkage we measured from the product is the sum of the above three parts. The shrinkage of plastic materials is normal, but how to control and grasp the relationship between product shrinkage and process factors is a more complex research topic, and must be analyzed and studied from the process factors that affect the shrinkage of products.
2.2 Process factors that affect shrinkage rate
2.2.1 Effect of cavity average pressure on shrinkage and stability During PP injection, the melt is filled into the cavity at a specific temperature and pressure until it reaches the gate seal. This pressure can also be referred to as the sealing pressure. . Sealing pressure plays a decisive role in the shrinkage of the product. The higher the sealing pressure, the smaller the shrinkage of the product; if the process conditions and equipment at the time of injection cannot guarantee a stable sealing pressure, it will cause the product to fluctuate and fluctuate, affecting the dimensional stability of the product.
The pressure in the cavity before sealing is determined by the holding pressure. The holding pressure can actually be regarded as maintaining the injection pressure in the holding pressure phase. If the frozen sealing time is certain, then the sealing pressure at the gate must be increased to increase the holding pressure. Increasing, the impact of holding pressure on the sealing pressure of the gate actually reflects the impact on the shrinkage rate, and the holding pressure is actually a direct reflection of the injection pressure.