When designing a plastic product, there are many materials and properties that the designer 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 performance terminology commonly used in plastics and some tips related to materials and processing.
Homogeneity homogeneity refers to the uniformity of performance across all parts of the material.
It is suggested that unfilled thermoplastics and thermosets have a fairly uniform distribution of physical properties.
The cross-section thickness of the designed injection molded product should be uniform to ensure uniform stress during the filling process and reduce the possibility of the product forming pores during the cooling process.
The location 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 a difference in physical property values ​​between a part of a material and another part or the same part in different directions, and the performance is uneven.
It is suggested that the heterogeneous behavior of the plastic depends on how the fluid flows when filling the mold cavity.
Glass fiber reinforced products and very fine mineral filler reinforced plastics are heterogeneous.
Symmetrical reinforcements and fillers, such as glass spheres, mica and very fine minerals, are more uniform 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 property values ​​in all directions.
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For the design, a 0.1 in. thick non-reinforced film plastic is considered to be isotropic.
The thicker portions of the product show more anisotropic properties due to different product densities and flow orientation effects. The crystalline polymer is anisotropic in all thicknesses.
No reinforced plastic is close to an isotropic material.
Anisotropic materials have different physical property values ​​in all directions.
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The extruded film and sheet differ in performance in the winding direction from the transverse direction, and the biaxially oriented film can attenuate its anisotropy. The strength of the article can be increased by orientation.
Density is the weight per unit volume of material and is usually expressed in g/cm3.
It is suggested that the weight of the part can be converted into density during the injection molding process to check the quality of each molded product or to evaluate the uniformity between the mold and the mold during the injection molding process.
Part weight can be used as a checkpoint for quality and process control.
Elastic 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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The plastic exhibits a certain elasticity at a lower tensile strength (≦1%).
Elasticity depends on the amount and type of resin and additives.
Rubber and thermoplastic elastomers have better flexibility over a wide temperature range (50-180F).
The plastic plastic material cannot be restored to its original shape after releasing the force until the force has not reached the damage, which is called plasticity, but this does not mean 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 will have a better plasticity.
Plastics have a lower plasticity at low temperatures and become very brittle.
Elongation is a good way to measure plasticity.
Thermosetting plastics, especially phenolic resins, have very low plasticity.
Press forming is based on the plasticity of the material, which allows the material to flow under concentrated high pressure.
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Stamping can impart molecular orientation to the material, increasing flexibility and tear strength in the area where the stamp is formed.
Semi-crystalline and crystalline resins are often stamped to form the hinges of the part.
Plastic materials such as ABS, PVC and other amorphous resins can also be stamped, but their flexibility and tear strength are generally lower than engineering resins.
The stress whitening effect is easy to produce stress whitening due to the local stress of the plastic product, and the method of bending out beyond the yield point or other methods that do not cause deformation can also cause stress whitening.
Tips can use stress whitening to analyze whether a product has failed or may fail.
The malleable malleable material can be stretched, crimped, or stretched into another shape without destroying its physical properties. Ductility refers to the property of a material after it has been stretched, usually the rate at which the material changes after being heated.
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Injection molded and extruded products utilize their ductility to assemble or modify products with other parts while still hot. For example, the extruded high-hardness and high-filled PVC pipe is mechanically expanded at one end to form a connection expansion port after the pipe is formed.
Toughness toughness is the ability of a material to absorb physical energy without failure.
It is suggested that generally 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 fast and violent impact test method that is performed on a molded wafer of a specific thickness.
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This is the best way to assess material toughness, but not all materials.
Simply supported beam and cantilever beam impact strength The simply supported beam and cantilever beam impact strength test is the ability to measure the impact energy of a material with a notched and unnotched spline on a molded or machined specimen.
Tensile impact Tensile impact is a test instrument that measures the toughness of a plastic material after it is suddenly subjected to an impact under stress. The test device is similar to the tester for the impact strength of the cantilever beam. The tensile impact test measures the impact tear strength of the material. The sample may be a square, round or dumbbell test strip.
It is suggested that many engineers believe that the tensile impact is more representative of the toughness of the material in practice than the simply supported beam and cantilever beam impact test.
Thermoplastics with high molecular weight or modified by impact modifiers have good tensile impact toughness.
The glass fiber reinforced material has a higher impact strength than expected.
Thermoplastics themselves have better toughness than thermoset plastics.
Brittle brittleness means that the resin has no toughness and ductility and has a low elongation property.
Tips Thermosetting plastics, especially phenolic plastics, exhibit brittleness if they are not modified by energy-absorbing additives and fillers.
Many filled and fiber-reinforced resins have good high tensile strength, but tensile elongation is low and can be suddenly destroyed under higher stress.
Factors affecting material brittleness are molecular weight and modifiers such as plasticizers, carbon black, fillers, rubbers 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 suppress the notch, and the notch sensitivity is listed on the material data sheet as the notched Izod impact strength data.
Lubricative thermoplastics are self-lubricating and represent the property of the material to withstand loads during relative motion.
It is suggested that plastics with better lubricity have a lower 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, careful selection of materials is required to reduce wear.
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Material suppliers typically provide information on the wear and friction of the resin when applied to different mating materials and surface finishes.
In order to reduce the contact wear of parts during exercise, dissimilar materials are often used. Materials with similar properties often have higher wear rates between different materials at high friction rates.
In general, fiber reinforced plastics have greater wear than fiber reinforced materials.
Nylon has natural lubricity that can be deformed under load without wear.
Plastics do not follow the classic friction law.
Before selecting materials for wear applications, determine all the factors in the final application environment.
shrink
The thermoplastic will become fluid and expand when heated, and will solidify and shrink from the initial molten state upon cooling, from liquid to solid.
The state and accompanying changes in volume and density are referred to as material or mold shrinkage.
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The shrinkage typically provided by the supplier is the shrinkage measured under optimal injection conditions. This value is the 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 is increased, the amount of shrinkage of the mold and material will increase, and even in the transverse direction perpendicular to the flow direction will be higher.
The mold designer must adjust the dimensions that cannot be controlled by the mold by the dimensions in the mold cavity. The shrinkage of each material, the position of the gate on the part, and the position of the material filling must be adjusted according to the thickness of the section. Injection molding conditions such as melting temperature, mold temperature, injection temperature and pressure also help to control the amount of shrinkage during the production process.
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