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By Design: Polypropylene part design, Part 1

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

The commercial production of polypropylene in the United States began in 1957. That was the same year that I started working in the plastics industry. Polypropylene and I have now become senior citizens of the industry. As a result, I have a fondness for the material. It has been interesting to watch a new material be introduced, develop, and find its place in the plastics industry.

Polypropylene's (PP) place in the industry is second only to polyethylene. Approximately 15 percent of all of the plastic produced in this country is PP. In large quantities, PP sells for a published price of $.33 to $.38/lb. With a density of only .903 g/cu cm, PP is the lightest weight of any of the standard plastics. On a volume basis, PP costs only $.01 to $.013/cu in. This makes PP the lowest-cost common plastic material suitable for the injection molding process. PP is a large-volume, lightweight, low-cost plastic material that is well established in the industry.

This material's large volume and low cost caused it to be categorized as a commodity plastic along with polyethylene, polyvinyl chloride, and polystyrene. This classification is, in my opinion, a mistake. There is no scientific definition of a commodity plastic. To most plasticians the words commodity plastic connote a low-cost, large-volume, low-performance material. But PP can be a high-performance engineering material. Glass-fiber-reinforced PP has a tensile strength in the same range as nylon.

Many applications that were molded using engineering plastics are now being converted to PP. For example, nearly all automotive interior trim parts have now been converted from ABS to PP. Many design engineers mistakenly classify PP as a low-performing commodity material and overlook its ability to perform as an engineering plastic.

Over the years, many different types of PP have been developed for special applications such as coatings, fiber, filaments, film, thermoforming, extrusion, and injection molding. All of the PPs can be divided into two types: homopolymers and copolymers. The homopolymers are favored for their average lower cost. The copolymers are various combinations of PP and ethylene. The copolymers are chosen for their improved melt strength, clarity, and impact strength.

As the percent of comonomer increases, tensile strength, stiffness, heat deflection temperature, and hardness decrease while impact strength increases. Within the various types of homopolymers and copolymers, the primary distinguishing characteristics are molecular weight and molecular weight distribution. In general, the higher-molecular-weight PPs are more resistant to flow but exhibit improved physical properties.

These are easy-flow materials with melt index ranges of less than 1 to more than 35 g/10 min. The production of fibers and film are the two largest markets for PP. Injection molding ranks third in the amount of PP processed. PPs are easy materials to injection mold at low temperatures and pressures. Molders must, however, take into account that PP is a semicrystalline material. A properly injection molded part is 50 to 60 percent crystalline. The degree of crystallinity has an effect on the physical properties of a molded part.

As the percent of crystallinity increases, there is a corresponding increase in the material's density, tensile and flexural strength, mold shrinkage, and heat and chemical resistance. Impact strength and transparency decrease. Mold shrinkage becomes less uniform. The formation of PP's crystalline structure is relatively slow. Rapidly cooling a part in the mold results in a reduction in crystallinity. Minor changes in mold cooling conditions can have a drastic effect on a part's size and physical properties. Also, rapidly cooled PP parts continue to shrink long after they are removed from the mold.

Polyethylene (PE) and PP compete for many of the same applications. There is overlap, but PP is chosen when the application requires a little bit more stiffness and temperature resistance than that provided by PE.

PP components range in size from micromolded medical and electronic parts weighing less than a gram to a 9.8-lb minivan interior trim part that measures 87 by 26 inches.

Designing with PP
Part design requirements for the different types of PP are the same with the exception of wall thickness. PP's wide range of melt indices (MI) must be taken into account. Not all PP can be molded in the same size and thickness part. A 2-MI PP may not fill a part designed to be molded in a 10-MI PP. See below for other design considerations:

  • Wall thicknesses can be as thin as .004 inch for small parts. This is pushing the limits, and a better minimum wall thickness is .030 to .040 inch. Large PP filter plates have been successfully injection molded with 3.5-inch-thick walls, but this is an exception. Considering PP's crystallinity and high mold shrinkage factor, the maximum wall thickness should be limited to .250 inch.
    Variation in wall thickness greater than 10 to 15 percent of the part's nominal wall thickness must be smoothly blended from thick to thin.
  • Radiusing the corners of PP parts improves melt flow while producing a stronger part with less molded-in stress. The minimum inside corner radius on a PP part should be at least 25 percent of the part's wall thickness. The stiffness and especially the impact strength of a PP part can be improved by increasing the size of the radiuses up to 75 percent of the part's wall thickness.
  • Draft angles and a good polish are important on PP parts due to their stiffness and high mold shrinkage factors. A molding draft angle of 1º/side is recommended on inside surfaces that shrink onto cores in the mold. A minimum draft of .5º/side is normally adequate on outside surfaces that shrink away from the cavity. Larger draft angles may be required on deep-draw parts, or those with a lot of geometry. Larger draft angles are always desirable as they result in parts that are easier and, therefore, less costly to mold.
  • Projections of all types can be incorporated into PP parts. Their thickness at the junction with the part's nominal wall should be limited to 50 percent of the part's wall thickness. In cases where appearance and the absence of sink marks is critical, the thickness of projections can be reduced to 40 percent of the part's wall thickness.
  • Depressions, or holes, of any size and shape can be easily molded with PP. The inside corners of holes should be radiused to minimize molded-in stress. The easy-flow properties of high-MI PP allow the molding of very small holes without the core-pin bending problems associated with low-MI PP or other harder-flow plastics. With good venting and proper molding conditions, good-looking, strong weldlines can be produced.
  • Tolerances are the same for all types of nonfilled or reinforced PPs. A 1.000-inch-long PP part with a .125-inch thickness can be molded to a commercial tolerance of +/-.007 inch. Longer dimensions require an addition of +/-.005 in/in. A fine tolerance would be +/-.0043 inch for the first inch plus +/-.003 in/in for each additional inch. The commercial tolerance can normally be achieved by any competent injection molder with no cost penalty. The fine tolerances usually result in longer molding cycles and increased cost. In some instances, even smaller tolerances can be achieved but only by mutual agreement between the molder and customer. The ideal tolerance is always the largest tolerance that produces a functional part.
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