Rapid injecTion Molding - .Rapid injecTion Molding Mechanical ... (the original plastic explosive)

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  • special issue

    Technologyof Material

    Plastic Materials Page 3

    Choosing the Right ResinPage 5

    in This issue:

    2006 Issue 1 The Protomold Company, Inc.

    Rapid injecTion Molding

    Mechanical PropertiesPage 4

  • > executive view

    Ive been reading a book entitled American Plastic: a cultural history. No doubt, youll see it reviewed in a future issue of this journal; however, unlike some of the other books weve reviewed, this one is less about plastic itself, and more about its impact on, and integration into, our lives. But while the book focuses on what plastics do, author Jeffrey Meikle acknowledges the importance of what plastics are.

    In this issue of the journal, we focus on the material aspects of plastic resin: the characteristics of various formulations and how we use them to meet a virtually endless range of human needs. Plastics have come a long way from their low-tech, labor-intensive beginnings with the 19th century introduction of celluloid. That material began as cotton, was processed into paper, and treated with nitric acid to yield crude-but-useful nitrocellulose plastic. It was largely

    a hit-and-miss process a risky one as well, since over-treatment with the acid yielded, instead of plastic celluloid, hazardous guncotton (the original plastic explosive). But even when the treatment went as planned, the resulting material was not much easier to work than the horn, bone, ivory, and tortoiseshell it replaced.

    Today, we have thousands of pure resins, resin compounds, and processing methods, allowing us to easily and cost-effectively produce an almost limitless array of materials and objects. Plastics are no longer poor imitations of natural materials. On the contrary, they have superseded the limitations of materials that can be grown, raised, or mined and are limited only by our ability to define a need.

    In their almost limitless incarnations, they package our food, encase our electronics, lighten our cars, and replace our body parts. They can be nearly as light as air or heavy as iron; opaque as lead or transparent as glass; rigid as stone or flexible as a leaf, but with so many functional options to choose from, the choice of material can be as demanding as the design itself. The opportunities are vast, but so is the complexity of the selection process.

    Protomolds rapid injection molding gives designers the opportunity to actually test drive resins in a finished prototype to make sure they are getting the functionality they want before finalizing production specifications. This issue presents an overview of some of the options, shares the experience of other designers, and points readers toward sources of in-depth information. It is, of course, impossible to do more than scratch the surface of such a vast field, but we hope we can, in some small way, expand your options and simplify your choices.

    Brad cleveland President & CeO bradc@protomold.com

    What comes out of the mold depends on what goes in

    > Materials

    Monomers are fairly boring little molecules. But start linking them into chains called polymers and all sorts of interesting things start to happen. Plastics are polymers. They come in a great variety of forms and offer a vast array of traits. Like those other polymers called proteins, what plastics do is determined largely by how they are structured.

    The first factor in determining the nature of a plastic is its component elements. The most basic plastics polypropylene and polystyrene for example are made only of hydrogen and carbon. Add chlorine, nitrogen, fluorine, or oxygen (or silicon, phosphorus, or sulfur) and you can create plastics like polyvinyl chloride (PVC), Nylon, polyester, Teflon, and more. But constituent elements are only the beginning.

    The degree of branching of the polymer chain affects melting point and tensile strength. Cross linking increases strength and can prevent melting entirely. And crystallinity can affect both melting point and opacity. Move beyond the basic chemical and structural characteristics of individual resins, and you can compound multiple resins and additives to offer whole new ranges of capabilities. Fillers can enhance bulk, weight, strength, or other characteristics.

    The point is that the capabilities of plastic are, for all practical purposes, unlimited. If what you need

    doesnt exist, someone will compound it for you or may already be busy developing it. In a very real sense, the challenge isnt getting what you want; its defining what you want. Here are some of the many possible variables to consider:

    Mechanical properties for parts subjected to stress and deformation include:

    n Strength

    n Toughness

    n Elasticity

    n Hardness

    Thermal properties for parts that will be subjected to extremes of temperature include:

    n Melt temperature (range)

    n Heat deflection temperature under load (DTUL)

    n Coefficient of linear expansion

    n Thermal conductivity

    n Specific heat capacity

    Electrical properties for parts used around current or electromagnetic fields include:

    n Resistivity

    n Dielectric constant

    n Dielectric strength

    n Arc resistance

    Chemical properties include:

    n Resistance to a variety of chemicals

    n Resistance to stress cracking

    n Flammability

    n Toxicology

    Other potentially significant properties include:

    n Resistance to weathering and radiation

    n Fire behavior and smoke development

    n Water absorption

    n Density

    Mechanical properties are addressed in more detail in a separate article in this issue of the Journal. Other properties will be discussed in future issues.

    the Material world

    If what you need doesnt exist, someone will compound it for you or may already be busy developing it.

    Plastic Materials: the View from 15,000 meters



  • > Plastic on Plastic

    Hi, Professor Plastic here. Recently someone asked me to name the strongest plastic I could think of. Im rarely lost for a snappy answer, but

    strength is a general term covering

    a variety of properties and a subject to which entire engineering courses could be devoted, so I simply explained that:

    n Two sets of strength measurements are most commonly used for plastics. Tensile is pulling, as in pulling on a rope until it stretches and ultimately breaks. Flexural, is bending, as in bending a pencil until it curves and breaks.

    n Modulus is a measure of stiffness. Two commonly used types of modulus are tensile, which is resistance to stretching, and flexural, which is resistance to bending. An easily-stretched thermoplastic rubber like TPE has a low tensile modulus.

    n Yield is the point to which materials can be stretched or bent and still return to their original shape. Beyond this point, they are permanently deformed. This is the yield point or elastic limit.

    n Yield point and breaking point can vary independently. A brittle material elongates or bends very little before it breaks. An elastic material can stretch or bend a lot without breaking and still return to its original shape. A ductile material can also

    stretch or bend without breaking but does not return to its original shape.

    n There are two types of strength: yield and ultimate. Yield strength is the amount of stress required to bring a sample to its yield point. Ultimate strength is the amount of stress required to break it.

    n The above measures apply energy to a material relatively slowly. Impact resistance measures material behavior when energy is applied quickly (as it would be in a crash or from a sudden blow). It measures the amount of energy the material absorbs when it is broken by a blow from a pendulum hammer.

    n Then theres hardness, which measures resistance to permanent surface indentation, and abrasion, which measures loss of material to scraping or rubbing.

    n Finally, there is high temperature strength, which is strength at a standard elevated temperature.

    So, in picking a resin, you have to consider: how much stress it will have to handle, whether the stress will pull or bend the material, how gradually or suddenly the stress will be applied, whether the material will need to resist rubbing or indentation, and whether it will have to withstand heat. Protomolds resin guide (www.protomold.com/designguidelines/resininfo) can give you a general idea of the material properties of various resins, but for detailed information, you can consult the books or sites mentioned elsewhere in this issue.

    Of course, there are other resin properties to consider, but those are for a future issue.

    > case study

    Electric utilities have always been concerned with the quality voltage, harmonics, and disturbances of the power they supply, and are more so now with the increased demand by sensitive electronic equipment. Power Monitors Inc. (PMI) is a leading supplier of instruments for evaluating power quality, and one of their most

    popular products is the Flex CT. This flexible probe is wrapped around single or bundled wires to measure, by induction, the characteristics of current in the wires and feed that information to a scanner for analysis.

    We needed a new, smaller flexible probe for use in tight spaces, says Manager of Hardware Engineering Glen Shomo. One of the challenges was the connector that holds the ends of the flexible probe in place while measurements are taken. Alignment has to be very precise and, because of the high voltages, it has to be operable by a user wearing protective gloves.

    Shomos team selected a self-locking, two-part des