Plastics - better than metal?

Plastics vs. Metals

From an engineering point of view, plastics differ from metallic materials in the following ways:
- Plastics tend to have pronounced non-linear time-dependent stress/strain behavior. Below the yield point, metals typically have elastic behavior which is nearly linear.

- Instead of a single melting point, plastics typically have a
Glass Transition Temperature (TG). Metals typically have a clearly-defined melting point.

- Plastics in general exhibit pronounced creep. Metals undergo creep to a certain extent, but for most engineering configurations their creep is insignificant.

- Plastics tend to degrade or denature (due to heat) rather than corrode within a typical atmosphere. Of course, chemical degradation can occur when reactive chemicals are present. Plastics impregnated with organic fillers can be subject to bacterial infestation. Metals can corrode even in a benign atmosphere from reaction with oxygen and water, but are not affected by bacteria.

Plastics vs. Plastics

AMORPHOUS POLYMERS: e.g., PVC, Polystyrene, Acrylic, ABS, PPO, PC, Polyetherimide have
- a wide melting range,
- low shrinkage after molding,
- better impact and lower chemical resistance than crystalline polymers,
- moderate heat resistance,
- dimensional stability,
- and superior cosmetics of outer surfaces.


CRYSTALLINE POLYMERS: e.g., PPS, PBT (Valox), Polypropylene (PP), have
- a sharply-defined melting point,
- high shrinkage after molding,
- low impact strength,
- high heat resistance,
- good fatigue endurance,
- good lubricity; wear resistance,
- good chemical resistance,
- and the ability to flow in thin-walled sections.

Selection Methodology - which plastic should i use?
The methodology of selection of polymer material

Experience

What are similar products made of ? When a product is conceived, it is good practice to see what has been done in the past to gain some knowledge of successes and failures. Failure analysis data is often useful in selection of the right polymer. This information can be gathered from other design and manufacturing engineers, mold builders, and the molding plants where such parts are produced. These sources are also good at providing valuable input on raw materials that are fairly new on the market that may lack substantive documentation from the manufacturer.

Criteria

The search for materials is based on subjective criteria as outlined above as well as objective criteria for the current design. The objective criteria include:
º Required criteria such as flammability, chemical resistance, temperature resistance (both high and low), electrical, humidity etc.
º Regulatory requirements, such as UL, FCC, FDA etc.
º Cost. Cost is not simply cost per unit weight; rather it is cost per unit volume since it is the volume of the part that stays more or less fixed for a given mold.
º Environmental compatibility. Includes the ability to recycle the polymer, pollution, and energy demands.

Analysis

Stress analysis can be done for features under stress. These include snaps, latches, screw-bosses, load bearing elements etc. Classic hand calculations and finite element analysis (FEA) can be useful, but the design engineer must be aware that plastics have characteristics partly of solids and partly of viscous liquids, so that classical Hookean engineering formulas cannot be used with confidence.

Processing and the Skin-Effect

Molding and extrusion of plastics alters their properties so care must be taken to look at similar parts. Processing typically induces high anisotropy and non-homogeneity to a plastic. It is difficult to produce a structurally accurate model of the part since a molded part will be anisotropic and non-homogeneous. Most molded parts have a surface "skin" devoid of filler and often crystalline and therefore highly directional. Fortunately, directionality and non-homogeneity typically impart added strength to a plastic structure. Processing-induced differences are most pronounced in crystalline plastics such as nylon, acetal and polyethylene. The differences are less significant in amorphous materials such as polycarbonate, polystyrene, and ABS.

Prototyping

Prototypes parts can be made for those features that are highly stressed or difficult in some other way. These prototype parts can be produced in small prototype molds that just produce say one design of latch and a snap. Thus a hard to design latch can be validated, without building a whole mold. As far as the whole part is concerned, rapid prototyping methods (such as stereo-lithography) can be used to make full sized models or scale models for the purpose of visualization.

Design Validation

The ideal way to mock up a part for design verification testing is to mold the part in a "soft" steel mold. This is more expensive than a resin or aluminum mold, but thermal properties most resemble those of a hardened tool steel mold and the part properties will resemble the production part. Paramount to the similarity is the cooling rate of the plastic as it is injected. For non-structural parts, an aluminum or even a resin prototype mold will suffice.

Testing

Once some promising polymers are chosen based on the above criteria, they should be tested. The features with the greatest weaknesses should be tested against known criteria/challenges. These could include drop tests for structural challenges, dielectric strength for electrical voltage challenge, latch cycling for fatigue challenge, etc. This will gain valuable time and experience as the mold is being scheduled for fabrication. Any testing prior to mold-build start will be advantageous from both cost and schedule point of view.



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