Introduction of injection molding extrusion grade PVDF

Introduction of injection molding extrusion grade PVDF resin

Polyvinylidene fluoride (PVDF) mainly refers to vinylidene fluoride homopolymer or copolymer of vinylidene fluoride and other small amounts of fluorine-containing vinyl monomers. It is a chain crystalline polymer.

The alternating arrangement of CH2- and CF2- groups in the PVDF structure makes PVDF resin have the characteristics of both fluororesin and general resin, showing extremely high chemical and thermal stability, and excellent resistance to radiation and adverse climatic conditions. Compared with PTFE, FEP and PFA, it has higher mechanical strength and toughness, and can be used for a long time in a wide temperature range (-40 to 130°C) and basically maintain its excellent performance.

Application of injection molding extrusion grade PVDF resin
PVDF is one of the general resins with the best comprehensive performance among fluororesins. It has the strongest toughness, low friction coefficient, strong corrosion resistance, aging resistance, weather resistance, and good radiation resistance among fluoroplastics. It is the second largest fluoroplastic after polytetrafluoroethylene (PTFE). The relative density of PVDF resin is 1.75-1.79, the glass transition temperature is -35℃, the brittle temperature is below -60℃, the melting point is 165-175℃, the thermal decomposition temperature is above 350℃, the difference between the melting point and the decomposition temperature is large, the thermal stability is good, and it is easy to process. PVDF resin is a highly crystalline polymer and can be processed by the processing method of general thermoplastics. No lubricants and stabilizers are required during processing. Its mold shrinkage rate is large, and its products can be subjected to mechanical processing such as sawing, planing, drilling, grinding and turning. It can also be welded and surface metallized. The excellent physical and chemical properties and good processing properties of PVDF make injection molding and extrusion grade PVDF widely used.

Injection Molding

Injection molding is one of the most commonly used processing forms of PVDF resin, which can be used to process various types of containers and plates. Generally, the injection molding pressure is 80-110MPa, the barrel temperature is 220-290℃, the nozzle temperature is 180-260℃, the mold temperature is 60-90℃, and the molding cycle is 40-60s.

Injection molding conditions are closely related to the product shape and the resin brand used. During injection molding, the mold temperature should be high rather than low. High mold temperature is conducive to filling the mold with materials, the product appearance is good, and the internal stress of the product is smaller than that of low mold temperature.

Molding pressure is related to mold design and product shape. Molding shrinkage is related to molding pressure and product wall thickness. Attention should be paid during molding. Because PVDF melt viscosity is high and product shrinkage is large, screw pre-plasticizing injection machine is generally used. The aspect ratio is usually 20-24, and the compression ratio is between 2 and 3. Injection molds generally use high-quality mold steel. The balance of flow channel configuration should be considered during design to ensure that the material is evenly distributed in the mold. The shape of the diverter is preferably circular, because the circular diverter can minimize the loss of material pressure and temperature reduction, ensuring the stability of product size. A cold material well should also be set at the end of the diverter.

Extrusion type

Extrusion processing of PVDF resin can produce various products such as plates, rods, tubes and films. The general barrel temperature is 205-260℃, and the die temperature is 220-275℃. Single-screw extruders are often used in daily life. When used to process PVDF powder, PVDF-based composite materials, PVDF masterbatch or PVDF plastics with fillers, twin-screw extruders can be used.

The thermal conductivity of PVDF melt is low, about 1/2 of PTFE, PFA and FEP. The viscosity is high and decreases with the increase of shear rate. The diameter of the extruder barrel is mostly 30-65mm, the screw aspect ratio is 24, 28 and 32, and the compression ratio is between 2 and 3. The screw should be of gradual type, the metering section length should be longer, and the depth of the screw groove is related to the diameter of the extruder. The head should be processed into a streamlined shape to ensure that there is no dead angle of flow. It also requires high polishing and nitriding.

Injection molding can be used to produce pumps, valves, pipe fittings and packing towers, etc., and extrusion molding can be used to produce rods, plates, tubes, films, hoses, monofilaments and wire insulation layers, etc.; extrusion molding can be processed into plates, hoses, sheets, monofilaments, wire and cable sheaths and special-shaped products, etc. In addition, PVDF resin can also be blended and modified with other resins, such as PVDF and ABS resin blended to obtain composite materials, which are widely used in construction, automotive decoration and home appliance housings.