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The supramolecular structure of polymers has a very obvious influence on the injection molding conditions and the performance of the product. Polymers can be divided into crystalline and amorphous according to their supramolecular structure, and the molecular chains of crystalline polymers are arranged in a regular manner, while the molecular chains of non-crystalline polymers are arranged in an irregular and amorphous manner. Different forms show different process properties.
Generally, crystalline polymers have heat resistance and high mechanical strength, while non-crystalline polymers are the opposite. Polymers with simple molecular structure and high symmetry can form crystals, such as PE, etc., and although the molecular links are large, the intermolecular forces are strong and can also form crystals, such as POM, PA, etc. Polymers with large molecular chain rigidity are not easy to crystallize, such as PC, PSU, PPO, etc.
The criteria for evaluating the crystalline morphology of polymers are crystal shape, size and crystallinity.
The effect of polymer crystallinity on product properties
For example, the density of PP with 70% crystallinity is 0.896 kg/m3, and the density increases to 0.903 kg/m3 when the crystallinity increases to 95%.
Tensile Strength High crystallinity and high tensile strength. For example, the tensile strength of polypropylene with 70% crystallinity is 27.5MPa, and when the crystallinity increases to 95%, the tensile strength can be increased to 42 MPa.
Impact Strength The impact strength decreases with the increase of crystallinity, such as 70% polypropylene, its notched impact strength is 15.2 kgf.cm/cm, when the crystallinity is 95%, the impact strength decreases to 4.86 kgf.cm/cm.
Thermal properties The increased crystallinity helps to increase the softening temperature and heat deflection temperature. For example, polypropylene with 70% crystallinity has a heat deflection temperature of 125°C under load and 151°C at 95% crystallinity. Stiffness is one of the conditions for the release of injection molded products, and higher crystallinity will reduce the cooling cycle of the product in the mold. Crystallinity will bring vulnerability to low temperatures, such as 55%, 85% and 95% of isometric polypropylene, and its embrittlement temperature is 0 °C, 10 °C, and 20 °C, respectively.
Warpage The increase of crystallinity will reduce the volume and increase the shrinkage, and the crystalline material is more prone to warping than the amorphous material, because when the product is cooled in the mold, the difference in crystallinity due to the difference in temperature causes the difference in crystallinity, so that the density is uneven and the shrinkage is unequal, resulting in higher internal stress and warping, and reducing the stress crack resistance.
Gloss: An increase in crystallinity will increase the compactness of the product. The gloss of the surface of the product is improved, but the transparency is reduced due to the scattering of light waves due to the presence of spheroids.
Source: Micro Injection Molding
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