What Is The Foam PVC Sheets Foaming Principle?
Date:2022-7-25 Author:hcscrew
Foam PVC sheets are also known as chevron or Andi sheets. The Seluca process is used to make it. With superior physical properties, high precision, and low thickness errors, its surfaces are durable, smooth, and rigid. There are strict requirements for moulds, fittings, workmanship, and raw materials. It can be manufactured with ordinary foaming moulds and is relatively easy to control.
01 How To Produce Foam PVC Sheets
It is common to produce foam PVC profiles, sheets, and pipes through extrusion, which involves heating a chemical blowing agent (CBA). Chemically, CBA is either an organic or inorganic compound. In the production of rigid foamed PVC products, physical blowing agents (PBAs) are generally gases or liquids with low boiling points.
02 Foam PVC Sheets Foaming Principle
In the barrel of the extruder, the chemical foaming agent decomposes into a gas that dissolves in the molten polymer. Extruders have a high melt pressure, so gas can dissolve in polymer melts. A successful foaming process requires the polymer melt to exit the head before bubbles form. Dispersing the gas from the barrel to the head requires high pressures along the flow path. As a result, the screw and screw head design as well as the temperature control are subject to certain demands. A lower head temperature increases both melt viscosity and pressure, reducing premature foaming.
As the melt leaves the head, the sudden drop in pressure causes the gas to become supersaturated. Thus, bubble holes nucleate almost instantly as a result of phase separation. Bubble hole nucleation is irregular and may be caused by solid residues from the decomposition of CBA, pigments, fillers, etc., within the melt. Vesicles grow faster as pressure decreases, and then slowly as pressure increases. The melt viscosity increases as the melt cool, preventing vesicles from growing. So that the bubble structure can be frozen quickly, the melt must be frozen quickly and passed through a cooling and shaping unit. The distance between the head and the shaping unit, as well as the cooling temperature of the shaping unit, affects how the final product looks and feels.
In most cases, the formulation determines the morphology of the foamed product. The size and uniformity of bubble pores are directly related to the type of chemical blowing agent. It is usually necessary for most applications to have a closed cell structure with a fine distribution of bubbles. Bubble size is also affected by the rheological properties of the polymer. It will not be possible to obtain a low density foamed product if the polymer viscosity is too high. Low viscosity and melt strength of the polymer leads to pores rupturing or collapsing. The PVC melt strength is dependent on the molecular weight of the polymer and the use of polyacrylate processing aids.
03 Conical Screws Structure for Foam PVC Sheets
Today, twin screw extruders are the most popular equipment for making PVC foam sheets. In particular, conical twin-screw extruders offer significant production advantages. PVC foaming requires a conical twin-screw barrel structure that is not capable of mixing the melt effectively with the foamed board. For this reason, the screw barrel for PVC foaming is a new, optimised design.
PVC foaming screw sections include feed, compression, metering, venting, and mixing. The mixing section has a spiral mixing head for mixing. There are four sections on the screw: a compression section, a metering section, a venting section, and a mixing section.
PVC foaming extrusion barrels have bimetallic grooves in the feed section. Creating these grooves increases the barrel’s internal friction. This in turn enhances the extruder’s plasticising capacity. It is possible to extrude foam PVC sheets exponentially more efficiently and with better quality by using the barrel’s SKD inlay bushing process.
Additionally, HUACHEN’s foam PVC sheets screw barrel has been optimised in many patented details. PVC foaming raw materials are more abrasive than ordinary PVC products, so foam PVC sheets screw barrel must be replaced more frequently than ordinary PVC products.
Summary
The above is about foam PVC sheets and the special screw barrels for foam PVC sheets. Should you have any questions regarding the production of foam PVC sheets or technical support and quotations for special screw barrels, please do not hesitate to contact us.
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