Designing Plastic Parts for Assembly

1	Understanding Plastic Materials

1.1	Basic Resins

Polymers are divided into two major groups: thermoplastics and thermosets.

Thermoplastic resins are formed from individual molecular chains, which have a linear structure and exhibit no chemical linkage between the individual molecules.

Thermoset resins have molecules that are chemically linked together by crosslinks and form a sort of network structure.

1.1.1

Thermoplastics

A major characteristic of thermoplastic polymers is that they repeatedly soften when heated and harden when cooled. The molecules are held together by intermolecular forces, such as van der Waals. During the molding process, when heat and pressure are applied to the thermoplastic resin, the intermolecular joints break and the molecules move, changing positions in relation to one another. In the holding phase of the molding cycle, the molecules are allowed to cool, doing so in their new locations. The intermolecular bonds are restored in the new shape.

Thermoplastic polymers are ideal for recycling purposes because of their ability to rebond many times over (Fig. 1.1).

When heated, the individual chains slip, causing plastics to flow. When cooled, the molecular chains are strongly held together once again. There are practical limitations to the number of times the material can be heated and cooled, depending upon the thermoplastic being used.

Polycarbonate, nylon, acetal, acrylic, thermoplastic elastomers (TPEs), and polyethylene are examples of thermoplastics.

1.1.2

Thermosets

Thermoset polymers undergo chemical change during processing.

During the molding process, when thermoset resins are heated and cured, crosslinks form between molecular chains (Fig. 1.2). This reaction is also called a polymerization reaction. When reheated, these cross bonds prevent individual chains from slipping. Chemical degradation occurs if more heat is added after the cross bonding is complete. Therefore, applying heat and pressure cannot remelt thermosets, and they cannot be recycled.

1.2	Basic Structures

1.2.1

Crystalline

Crystalline polymers are orderly, densely packed arrangements of molecular chains (see Fig. 1.3). These molecular chains have the appearance of a shoestring when magnified many times under a microscope. The highly organized regions show the behavioral characteristics of crystals.

It should be noted that complete crystallinity is seldom achieved during polymer processing. There will almost always be some amorphous areas left in the part. During processing, parts cool from the outside in, so the skin of the part is the area most likely to lack the necessary crystallinity. Many crystalline polymers achieve a degree of crystallinity of only 35 to 40 %, even under ideal processing conditions. That means that slightly more than one-third of the component structure becomes self-organized.

Some typical examples of crystalline polymers include acetal, polyamide (nylon), polyethylene (PE), polypropylene (PP), polyester (PET, PBT), and polyphenylene sulfide (PPS).

1.2.2

Amorphous

Polymers having amorphous structures represent a disordered or random mass of molecules (Fig. 1.4). A typical noncrystalline or amorphous structure tends to give the resin a higher elongation and flexibility. It will also exhibit higher impact strength than would a crystalline structure.

Some examples of amorphous polymers are acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN), polyvinyl chloride (PVC), polycarbonate (PC), and polystyrene (PS). Table 1.1 highlights various properties of semicrystalline polymers compared to amorphous polymers.

1.2.3

Liquid Crystal Polymer (LCP)

Liquid crystal polymers (LCP) are generally considered a separate and unique class of polymers. Their molecules are stiff, rod-like structures that are organized in large parallel arrays in both the molten and solid state (see Fig. 1.5). This parallel organization of molecules gives LCP characteristics similar to both crystalline and amorphous materials.

1.2.4

New Polymer Technologies

1.2.4.1

Inherently Conductive Polymers (ICP)

Polymers, since their inception, have been known as insulators both electrically and thermally. In the last few decades a number of suppliers have tried to make polymers conductive by using metal fillers or reinforcements. The result of improving polymer conductivity has been marginal.

The discovery of inherently conductive polymers (ICP) has changed all that. Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa were awarded the Nobel Prize for Chemistry in 2000 for their pioneering work related to ICPs.

The above scientists, as well as many others, discovered that adding or subtracting atoms to or from polymers allows the plastic to become electrically or thermally conductive. This process, known as doping, typically removes or adds conducting electrons, leaving a polymer with some positive or negative charges. Dopants are chemical substances that either supply additional electrons to conduct a charge or, alternatively, take electrons away to create what are known as holes or places in a molecule that conduct the charge by accepting electrons. Having fewer electrons that remain in the polymer, they will be capable of moving more openly, allowing conduction.

The most promising ICPs are polyacetylene, polyaniline (PAni), and polypyrrole (PPY). They can be added to known polymers, such as acrylics (known as poly methyl methacrylate or PMMA), poly vinyl chloride (PVC), polyproylene (PP), and others, to make them conductive.

1.2.4.2

Electro-Optic Polymers (EOP)

Electro-optic polymers (EOP) are distinct resins, but they do have some overlapping characteristics with ICPs (inherently conductive polymers). Upon the application of an electric field, EOPs exhibit optical characteristics: they glow. The effect is due to the large molecules making up the polymers. When voltage is applied it raises the molecules’ electrons to higher energy levels, after which they drop back to their original energy levels, emitting light in the process (electroluminescence). Richard Friend and Jeremy Burroughes from Cambridge University, England, developed the first electroluminescent polymer, PPV (polyphenylene vinylene). They showed that sandwiching a thin layer of resin between a pair of electrodes—one of which was transparent—made the polymer glow.

There are a number of polymers emitting various colors. For example, green is emitted by PPV, red by PT (polythiopene), and blue by PF (polyfluorene). When an electric charge (typically low voltage of 3 to 5 volts) is applied, the benzene electrons of each polymer are excited. Then the benzene electrons, returning to their original state of energy levels, emit light in a color specific to their resin, which is vibrant and soft.

The simplified manufacturing process consists of an electrical conductor laid down on a carrier foil, glass, or plastic. Then a thin LEP (light-emitting polymer) layer is applied, which is less than 1 µm (0.00004 in.) thick. Finally, another electrode is deposited on top and the display is realized (see Fig. 1.6).

It should be noted that display stability and performance is greatly improved when sealing...