Chapter One Basic Knowledge of Plastic Packaging (3)

7. What is the melt flow rate (melt index)? What does it mean for plastics processing?
A: The melt flow rate (MFR) is at a certain temperature and pressure. The resin melt passes through a standard capillary tube. Melt flow rate is an important reference for selecting plastic processing materials and grades, which can make the selected raw materials better adapt to the requirements of the processing technology, and improve the reliability and quality of the molding products.
Melt Flow Rate of General Extrusion Hard Products: MFR
Extrusion hollow products: MFR 0.5
Extrusion strapping products: MFR 0.5 to 4
Extrusion film products: MFR 6-10
Extruded monofilament products: MFR 0.5 to 4
Extrusion biaxially oriented products: MFR 1 to 4
Extruded sheet: MFR 0.1 to 0.5
Extruded wire and cable: MFR 0.1 to 1
Hollow high-gloss containers: MFR 1 to 2
Injection products: MFR 1~2
Injection thin-wall products: MFR 3~6
Vacuum forming: MFR is 0.2-0.5
Coating: MFR 9-15

8. What is creep? How does it affect the quality of plastic products?
Answer: When the creep batch polymer material is in the glass state, its deformation gradually increases with time under the effect of the stress-invariant condition. This is an inherent characteristic of polymer materials. According to this feature, all molded polymer materials, such as long-term loading, will not only deform, but will also exhibit viscous flow and even rupture. At the same time, if the processing process is not properly controlled, there will be a large internal stress in the product, and this characteristic will also cause deformation or even cracking of the product during long-term storage or application. Therefore, this characteristic must be taken into account during the processing and use of plastic products (especially under heavy loads).

9. How to understand the solubility parameter? What does it mean for plastics processing?
A: The dissolution of substances is a chemical phenomenon. People begin to use the universal rule of “similarly compatible” to the “polarity similar” dissolution theory (ie, high polarity solutes are soluble in highly polar solvents and low polarity solutes. Soluble in a low-polarity solvent) to constantly understand the nature of the substance's dissolution. In order to embody the concept of abstract polarity size, the concept of "solubility parameter" was proposed.
The solubility parameter is a characteristic value of various polymers, which is a quantitative reflection of the polarity and agglutinative energy of the polymer chain, that is, the solubility parameter is equal to the square root of the agglutination energy per mole of the polymer unit.

δ=(ΔE/Vm)1/2= (△Hm-R·T/Vm)1/2

Where: δ - solubility parameter E - agglutination energy Vm - molar volume △ Hm - molar vaporization heat R - gas constant T - thermodynamic temperature The compatibility of the polymer can be batch polymer and polymer The compatibility between the two can also be used to approve the compatibility between the polymer and other materials. The solubility parameters can fully characterize this compatible or compatible relationship. Understanding this concept has important guiding significance and practical value for the plastics processing industry. For example, plastic blending modification, plastic plasticizer selection, selection and synergistic effect of various additives in plastics processing, coloring of plastics, surface printing, solvent and coating preparation, etc., almost all need to be guided by this theory. (See Table 1-3 to Table 1-5).

Table 1-3 Solubility Parameters of Some Polymers (δ)

Table 1-4 Solubility parameters of commonly used plasticizers (δ)