In part 1 of this series, we talked about the history of polypropylene as well as its most important properties. In part 2, we will be focusing on how polypropylene is relevant in regards to the injection molding world.

Polypropylene is a very useful plastic for injection molding. In addition to the conventional plastic applications, polypropylene also lends itself well in fiber applications, which give it even a wider range of uses that goes beyond just injection molding, including: ropes, carpets, upholstery, clothing, and the like. Polypropylene has gained popularity since it was first synthesized in the 50s. According to some reports, the current global demand for the material generates an annual market of about 45 million metric tons and it is estimated that the demand will rise to approximately 62 million metric tons by 2020. The major end users of polypropylene are the packaging industry, which consumes about 30% of the total, followed by the electrical and equipment manufacturing, which uses about 13% each. Household appliances and automotive industries both consume 10% each and construction materials follows with 5% of the market. Other applications together make up the rest of the global polypropylene consumption.

POLYPROPYLENE AS AN INJECTION MOLDING PLASTIC

Polypropylene is a very popular material in the injection molding world; this popularity is due to it being a very ‘moldable’ material. Its versatility, wide range of uses, and unique properties are among those reasons. Polypropylene is easy to mold despite its semi-crystalline nature, and it flows very wellbecause of its low melt viscosity. This property significantly enhances the rate at which you can fill up a mold with the material. The rate should not be set too high, however, to ensure good surface finish and to avoid defects such as voids, weld lines, and flow fronts. The melt temperature of polypropylene can be anywhere between 200oC (392oF) to 250oC (482oF), although it can go higher depending on the grade of polypropylene and the kind of additives it contains. Shrinkage in polypropylene is about 1% but can vary based on a number of factors, including holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives.

POLYPROPYLENE AS A CNC MACHINABLE PLASTIC

Polypropylene has gained a reputation as a material that cannot be machined. This is because it has a low annealing temperature, which means that it starts to deform under heat. Because it is a very soft material in general, it requires an extremely high skill level to be cut with precision; Creative Mechanisms has been successful in doing so. Our teams can use a CNC machine and cut the polypropylene cleanly and with extremely great detail.

In addition, we are able to create living hinges with polypropylene that have thickness as little as .010 inches. Making living hinges is a difficult endeavor on its own, which makes using a difficult material like polypropylene even more impressive.

POLYPROPYLENE PROS AND CONS

Polypropylene, like every other polymer, has advantages and disadvantages that make it more suitable for certain uses than others.

Advantages of polypropylene:

• It is a relatively inexpensive material
• It possesses high flexural strength because of its semi-crystalline nature
• It has a low coefficient of friction • It is very resistant to moisture
• It has good chemical resistance over a wide range of bases and acids
• It possesses good fatigue resistance • It has good impact strength
• It has a good resistance to electricity and is thus a good electrical insulator
• It is more easily repaired from damage

Disadvantages of polypropylene:

• It has a high thermal expansion coefficient which limits its high temperature applications
• It is susceptible to UV degradation • It has poor resistance to chlorinated solvents and aromatics
• It is known to be difficult to paint as it has poor bonding properties
• It has a high flammability
• It is susceptible to oxidation

Despite its shortcomings, polypropylene is a great material. It has a unique blend of qualities that aren’t found in any other material, which makes it an ideal choice for many projects.

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 For a comprehensive and updated discussion of polypropylene, read here.