Top Ten Design Tips
Páginas: 9 (2007 palabras)
Publicado: 26 de octubre de 2012
TOP TEN DESIGN TIPS
Transformación y Selección de Materiales II
Top Ten Design Tips
1. Comparison of materials 2. Material selection 3. Wall thickness 4. Ribbing 5. Gate positioning 6. Cost-saving designs 7. General assembly technology 8. Welding technology 9. Tolerances 10. Check list
1.Comparison of Materials
Plastic is not a metal
Aunque muchos plásticos todavía se derivan de losmetales, las propiedades de estos son muy diferentes ya que a los plásticos se les agregan aditivos para cambiar sus propiedades.
Different basic material characteristics
Metals have higher density, maximum service temperature, rigidity/ strength, thermal conductivity and electrical conductivity, While plastics have greater mechanical damping, thermal expansion elongation at break andtoughness
Temperature and time dependence of deformation characteristics
Factors influencing component properties
The characteristics of plastics are not purely material properties. The basic property level of a plastic component can be changed by various factors (e.g. UV radiation, see Fig. 3) right up to the point of serviceability.
The nearer the service temperature of a material is to itsmelting point, the more the material’s deformation behavior will be temperature- and timedependent. Most plastics exhibit a change in their basic mechanical properties at room temperature or on exposure to short-term stress. Metals, on the other hand, usually display largely unchanged mechanical behavior right up to the vicinity of their recrystallization temperature (> 300 °C).
2.MaterialSelection
The Right Choice
There is no such thing as a bad material-just the wrong material for a particular application. It is therefore essential for designers to know the properties of the competing materials inside out and to test them all carefully in relation to the factors affecting the injection molded part.
Conventional thermoplastics
Fillers and reinforcing materials
Effect ofmoisture
Amorphous
• Polystyrene • Acrylonitrilebutadi ene-styrene • Polymethyl • methacrylate • Polyvinyl chloride • Polycarbonate
Semi- Cristalyne
• • • • • • Polyoxymethylene (Polyacetal) Polyamide Polyester Polyethylene Polypropylene
Thermoplastics are supplied in unreinforced, glass-fibrereinforced and mineral- and glass-spherefilled forms. Glass fibres are used primarilyto increase strength, rigidity and service temperature; minerals and glass spheres have a lower reinforcing effect and are employed mainly to reduce warpage.
Some thermoplastics, especially PA6 and PA66, absorb moisture. This may have considerable effect on their mechanical properties and dimensional stability. Particular attention should be paid to this property in material selection. 3.Wall Thickness
As much as necessary – as little as possible
In designing components made from engineering plastics, experience has shown that certain design points arise time and again, and can be reduced to simple design guidelines. One such point is wall thickness design, which has an important influence on component quality.
Effect on specific component criteria
Changing the wall thicknessof a component has a significant effect on the following key properties: – component weight – achievable flow lengths in the mould – production cycle time of the component – rigidity of the moulded part – tolerances – quality of the component in terms of surface finish, warpage and voids
Ratio of flow path to wall thickness
Flexural modulus as a function of wall thickness
The flexuralrigidity of a flat sheet is determined by the materialspecific elastic modulus and the moment of inertia of the sheet cross section. If wall thickness is automatically increased to improve the rigidity of plastic components without any thought given to the consequences, this can very often lead to serious problems with partially crystalline materials. In the case of glassfibre-reinforced materials,...
Transformación y Selección de Materiales II
Top Ten Design Tips
1. Comparison of materials 2. Material selection 3. Wall thickness 4. Ribbing 5. Gate positioning 6. Cost-saving designs 7. General assembly technology 8. Welding technology 9. Tolerances 10. Check list
1.Comparison of Materials
Plastic is not a metal
Aunque muchos plásticos todavía se derivan de losmetales, las propiedades de estos son muy diferentes ya que a los plásticos se les agregan aditivos para cambiar sus propiedades.
Different basic material characteristics
Metals have higher density, maximum service temperature, rigidity/ strength, thermal conductivity and electrical conductivity, While plastics have greater mechanical damping, thermal expansion elongation at break andtoughness
Temperature and time dependence of deformation characteristics
Factors influencing component properties
The characteristics of plastics are not purely material properties. The basic property level of a plastic component can be changed by various factors (e.g. UV radiation, see Fig. 3) right up to the point of serviceability.
The nearer the service temperature of a material is to itsmelting point, the more the material’s deformation behavior will be temperature- and timedependent. Most plastics exhibit a change in their basic mechanical properties at room temperature or on exposure to short-term stress. Metals, on the other hand, usually display largely unchanged mechanical behavior right up to the vicinity of their recrystallization temperature (> 300 °C).
2.MaterialSelection
The Right Choice
There is no such thing as a bad material-just the wrong material for a particular application. It is therefore essential for designers to know the properties of the competing materials inside out and to test them all carefully in relation to the factors affecting the injection molded part.
Conventional thermoplastics
Fillers and reinforcing materials
Effect ofmoisture
Amorphous
• Polystyrene • Acrylonitrilebutadi ene-styrene • Polymethyl • methacrylate • Polyvinyl chloride • Polycarbonate
Semi- Cristalyne
• • • • • • Polyoxymethylene (Polyacetal) Polyamide Polyester Polyethylene Polypropylene
Thermoplastics are supplied in unreinforced, glass-fibrereinforced and mineral- and glass-spherefilled forms. Glass fibres are used primarilyto increase strength, rigidity and service temperature; minerals and glass spheres have a lower reinforcing effect and are employed mainly to reduce warpage.
Some thermoplastics, especially PA6 and PA66, absorb moisture. This may have considerable effect on their mechanical properties and dimensional stability. Particular attention should be paid to this property in material selection. 3.Wall Thickness
As much as necessary – as little as possible
In designing components made from engineering plastics, experience has shown that certain design points arise time and again, and can be reduced to simple design guidelines. One such point is wall thickness design, which has an important influence on component quality.
Effect on specific component criteria
Changing the wall thicknessof a component has a significant effect on the following key properties: – component weight – achievable flow lengths in the mould – production cycle time of the component – rigidity of the moulded part – tolerances – quality of the component in terms of surface finish, warpage and voids
Ratio of flow path to wall thickness
Flexural modulus as a function of wall thickness
The flexuralrigidity of a flat sheet is determined by the materialspecific elastic modulus and the moment of inertia of the sheet cross section. If wall thickness is automatically increased to improve the rigidity of plastic components without any thought given to the consequences, this can very often lead to serious problems with partially crystalline materials. In the case of glassfibre-reinforced materials,...
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