When designing a plastic product, there are many materials and properties that designers can choose to determine the nature of the product. In the selection process, the relevant performance parameters are undoubtedly of decisive significance. This article describes the commonly used performance terms of plastics and some tips related to materials and processing.
Homogenous homogeneity refers to the uniform performance of all parts of the material.
It shows that both unfilled thermoplastics and thermosets have fairly uniform physical property distributions.
The thickness of the section of the injection molded product should be designed to be even to ensure uniform stress during the filling process and to reduce the possibility of the product forming voids in the cooling process.
The position of the gate and the shrinkage of the article have different effects on the physical properties in the flow direction and perpendicular to the flow direction.
Heterogeneous heterogeneity refers to the fact that the physical properties of different parts of the material and the other part or the same part are different, and the properties are not uniform.
The strength of the plastic's heterogeneity depends on how the fluid flows when filling the mold cavity.
Glass fiber-reinforced products and very fine mineral fillers enhance plastics that are heterogeneous.
Symmetrical reinforcing materials and fillers, such as glass spheres, mica and very fine minerals, perform more evenly in all directions and have the same amount of shrinkage.
The reinforced plastic has a greater shrinkage in the transverse direction than in the flow direction.
Isotropically isotropic materials have equal physical properties in all directions.
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For the design, a 0.1 in thick non-reinforced film plastic is considered isotropic.
The thicker part of the product shows more anisotropic properties due to different product density and flow orientation effects. Crystalline polymers are anisotropic at all thicknesses.
Non-reinforced plastics are close to isotropic materials.
Anisotropically anisotropic materials have different physical property values ​​in all directions.
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Extruded films and sheets have different properties in the take-up direction and in the transverse direction, and biaxially oriented films can reduce their anisotropy. The strength of the article can be increased by orientation.
Density Density is the weight per unit volume of the material and is usually expressed in g/cm3.
Tip During the injection molding process, the weight of the parts can be converted to density for checking the quality of each molded product or evaluating the uniformity between the mold and the mold during injection molding.
The weight of the part can be used as a checkpoint for quality and process control.
Elasticity is used to describe the ability of a material to return to its original shape and size after being deformed by force.
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Plastics exhibit a certain degree of elasticity at lower tensile strength (?1%).
Elasticity depends on the number and type of resin and additives.
Rubber and thermoplastic elastomers have better elasticity over a wider temperature range (50-180F).
The behavior of plastic plastic materials that cannot be restored to their original shape after release of force before the force has not been destroyed is called plasticity, but this does not refer to the flow and creep of the material.
It is suggested that the reinforced and filled resin has a lower plasticity and will break under low stress.
As the temperature increases, the thermoplastic plastic will have better plasticity.
Plastics become less brittle at low temperatures and become brittle.
Elongation is a good way to measure plasticity.
Thermosetting plastics, especially phenolic resins, have very low plasticity.
Press forming Press forming allows the material to flow under concentrated high pressure, depending on the material's plasticity.
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Press forming allows the molecular orientation of the material to increase flexibility and tear strength in the stamped area.
Semi-crystalline and crystalline resins are often stamped and formed to make part hinges.
Plastic materials such as ABS, PVC, and other amorphous resins can also be stamped, but their flexibility and tear strength are generally lower than those of engineering resins.
Stress whitening effect Because the local stress of plastic products is easy to produce stress whitening, the method of bending beyond its yield point without deformation or other methods that will not cause its deformation will also produce stress whitening.
Tip You can use stress whitening to analyze whether a product has failed or is likely to fail.
Ductility A malleable material can be stretched, curled, or stretched into another shape without destroying the integrity of its physical properties. Ductility refers to the property of a material after it has been stretched. It is usually the speed at which the material changes its shape after it is heated.
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Injection-molded and extruded products use their ductility to be assembled or modified with other parts while still hot. For example, after extruding high-hardness and high-filled PVC pipe, a connecting expansion hole is mechanically expanded at one end after the pipe is formed.
Toughness toughness is the ability of a material to absorb physical energy without failure.
It is generally suggested that the ductile material has a high elongation and the brittle material has a low elongation.
PE, PP, unfilled PVC and nylon have higher tensile elongation.
Drop Hammer Impact This is a rapid and severe impact test method that is performed on a molded, specific thickness disc.
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This is the best way to assess the toughness of a material, but it does not test all materials.
Simple beam and cantilever beam impact strength Simply supported beam and cantilever beam impact strength tests measure the ability of a material to absorb impact energy on a molded or machined specimen with notched and unnotched bars.
Tension Impact Tension impact is the determination of the plastic material's toughness after being subjected to sudden impact under stress. The test device is similar to the test instrument for the cantilever impact strength. Tensile impact test to test the impact of the material tear strength, the sample can be square, round or dumbbell-shaped test strips.
It is suggested that many engineers believe that the tension shock is more representative of the toughness of the material than the simple beam and cantilever impact tests.
Thermoplastics modified with high molecular weight or impact modifiers have good tensile impact toughness.
Glass fiber reinforced materials have higher than expected impact strength.
Thermoplastics themselves have better toughness than thermosets.
Brittleness is brittleness indicating that the resin has no toughness and ductility, and has a low elongation.
It is suggested that thermosetting plastics, especially phenolic plastics, will exhibit brittleness if they are not modified by energy-absorbing additives and fillers.
Many fillers and fiber-reinforced resins have good high-tension strength, but the tensile elongation is very low and suddenly breaks under higher stress.
The factors that affect the brittleness of materials are molecular weight and modifiers such as plasticizers, carbon black, fillers, rubber and reinforcing materials.
Many substrate resins are inherently tough and not brittle, such as PE, PP, PET, nylon, polyoxymethylene, and PC.
Notch Sensitivity Notch sensitivity is a term that describes how easily a crack propagates along a material.
It is suggested that the high elongation resin has a good ability to inhibit the notch, and the notch sensitivity is listed on the data sheet of the material as the notched Izod impact strength data.
Lubricating thermoplastics are self-lubricating and represent the characteristics of a material that is loaded under relative motion.
It is suggested that plastics with good lubricity have a small coefficient of friction in both motion and static tests.
Wear and friction When the contact surfaces of parts, gears, bearings, pulleys, etc. move relative to each other, it is necessary to carefully select materials to reduce wear.
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Material suppliers often provide information on the wear and friction of resins when applied to different mating materials and surface finishes.
In order to reduce the contact wear of parts during movement, dissimilar materials are often used. Materials with similar performance often have higher wear rates between different materials at high friction rates.
In general, fiber-reinforced plastics have greater wear than non-fiber-reinforced materials.
Nylon has a natural lubricity that can deform under load without wear.
Plastic does not follow the classic friction law.
Before selecting materials for wear applications, determine all factors in the final application environment.
shrink
Thermoplastics will become fluid and expand when heated. When cooled, they will solidify and shrink from their initial molten state.
The states, along with changes in volume and density, are called material or mold shrinkage.
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The contraction rate that suppliers usually provide is the shrinkage measured under the best injection molding conditions. This value is an average and will vary depending on the injection molding conditions and direction.
Amorphous resins have less shrinkage than crystalline and engineering resins.
During the injection molding process, the shrinkage is slightly higher in the transverse direction and at an angle of 90° to the flow direction.
If the section thickness increases, the mold and material shrinkage will increase, even shrinking in a direction perpendicular to the flow direction.
The mold designer must adjust the dimensions that cannot be controlled by the mold through the dimensions in the mold cavity. The shrinkage of each material, the position of the gate on the part, and the location of the material filling must be adjusted according to the section thickness. Injection molding conditions such as melting temperature, mold temperature, injection temperature, and pressure also help control shrinkage during production.
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