Roll Forming Machines The sheets used for forming, whether single or multi-layered, must have the following properties:
First, plastic memory, that is, when the tensile softening of the sheet, both the tendency to tighten the resistance against tension, as well as the tendency to stretch as much as possible. This feature allows the already molded article to reheat to the original molding temperature and it will return to the original flat sheet shape. This characteristic has an important effect on the stretching of the forming process.
Second, the hot stretch, that is, the sheet can be heated in the stretch, this feature for the shape and quality of the product has a great impact. Some can stretch 15% to 20%, while some can even be stretched to 500% to 600%.
Third, the heat intensity, that is, heating softened sheet as long as a little pressure, it will form a clear outline on the mold. On the contrary, if you need too much pressure to shape, and vacuum forming the pressure provided by the pressure difference is limited, for some subtle patterns is difficult to show.
Fourth, the molding temperature,Roll Forming Machines that is, forming a sheet to be suitable for a certain processing temperature range. Both in its heat softening temperature, easy to shape, but also with its melting temperature has a certain distance, forming a wide temperature range; not only in a smaller specific temperature range molding, the temperature is high or low, the molding easy to tear , Collapse and so on.
In order to gain a better understanding of the characteristics of the sheet for blistering, the following aspects will be analyzed.
First, the chemical composition and structure of thermoplastics
Roll Forming Machines Thermoplastics are composed of macromolecules (polymers) with a molecular chain length of 10-3 mm. These macromolecules can be linear, for example, HDPE, can also be branched, such as LDPE. Macromolecules completely disordered, we call it amorphous thermoplastics. Uniform structure of macromolecules, such as linear polyethylene or polyoxymethylene, can form part of the regular arrangement, macromolecules according to certain rules part of the crystallization, we call it part of the crystalline thermoplastics.
Amorphous and partially crystallized thermoplastics,Roll Forming Machines amorphous thermoplastics are not crystalline because of their asymmetric structure or large parenchyma, and are transparent without modification and coloring. The use temperature of amorphous thermoplastics should be lower than its glass transition temperature. Part of the crystalline plastic containing the molecular chain of the regular arrangement of the region, known as the crystallization zone. Because of the crystallization,Roll Forming Machines partially crystallized thermoplastics are usually opaque, and the transparency decreases as the crystallinity increases. The use temperature of some crystalline thermoplastics is between several and the melting point Tm.
If the sheet of HDPE is heated above the melting point of the crystal, the crystal will melt and the sheet will all become amorphous and transparent. During the cooling process, the crystals are formed again. For many partially crystallized thermoplastics, the crystallization can be suppressed by rapidly cooling the shaped sheet and the molded article, resulting in a transparent article (such as a PET bottle, a transparent PET sheet and a transparent random aggregate Propylene sheet).
Transparent thermoplastics are amorphous, but not all amorphous thermoplastics are transparent, for example, are colored and modified amorphous plastic is the case. As the molecular chain part of the orderly arrangement, part of the crystalline thermoplastic is no longer transparent, according to the different crystallinity,Roll Forming Machines the degree of transparency will be different. Amorphous and partially crystallized thermoplastics have a maximum operating temperature range.
Roll Forming Machines At temperatures below the glass transition temperature Tg (formerly known as softening temperature), thermoplastics are usually very brittle (such as ordinary polystyrene PS), and the rigidity (modulus E) and strength (σ) of the thermoplastic (Ε) increases, but the deformability (ε) increases. The material must also have sufficient rigidity at the maximum operating temperature. The coefficient of thermal expansion can be considered to increase linearly with temperature over a range of temperatures.