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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.

Glenn Beall

April 1, 2002

9 Min Read
By Design: Polyethylene part design

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 1933, scientists at Great Britain's Imperial Chemical Industries noted traces of a waxy, white substance on the inside of a pressure vessel. This was the precursor to the polyethylene (PE) material that would one day dominate the plastics industry. During the second World War, this new material was rushed into service as electrical insulation, which became scarce as enemy forces overran the rubber plantations in the Pacific.

During the war, all PE was allocated for military purposes. Following the war, PE became available for commercial use at a price of $5.00/lb. Large quantities of PE now sell for only $.32 to $.56/lb. Today PE is the largest volume material accounting for approximately one-third of the plastic material used in the U.S.

Polyethylene is actually a whole family of similar materials. The first materials were low- and medium-density polyethylene (LDPE and MDPE). High-density polyethylene (HDPE) was discovered in 1957 and linear low-density

polyethylene (LLDPE) was introduced in Canada in 1965. Crosslinked polyethylene (XLPE) and ultrahigh-molecular-weight polyethylene (UHMWPE) have now joined the family, but these two materials are difficult to injection mold.

Each PE is unique, but like any family, all of its members have similar characteristics. On the positive side, all PEs are low in cost, light in weight, and have good impact and chemical resistance. Many meet FDA and NSF requirements. Of special interest to injection molders is PE's thermal stability and ease of processing at low temperatures and pressures.

On the negative side, PEs are limited by their relatively high mold shrinkage factor, lack of stiffness, and low temperature resistance. In spite of these limitations, PE has captured a giant share of the market.

Molecular Structure Dictates Properties
Any PE with a density of .191 to .925 g/cu cm is classified as an LDPE. A material with a density of .941 to .959 would be HDPE. The MDPE and LLDPE materials have densities between these two. In general, as the density of a PE increases, tensile strength, stiffness, heat deflection temperature, hardness, surface gloss, mold shrinkage, permeation, and chemical resistance also increase; elongation, impact strength, and environmental stress crack resistance decrease.

All PEs are composed of long molecular chains of carbon and hydrogen. The HDPE molecules are basically linear with few side branches. The LDPE molecules have approximately 10 times more side branches than HDPE. These side branches become entangled with the branches on other molecules. This intermolecular entanglement accounts for LDPE's increased elongation and impact strength. The LLDPE's molecules also have a lot of side branches, but these branches are shorter and arranged in an orderly manner along the length of the molecules. The physical properties of LLDPE are, in general, between HDPE and LDPE.

The lack of side branches on HDPE allows these molecules to align close to each other. This tight packing of the molecules encourages the formation of crystalline structures in the material. Properly molded HDPE is 70 to 90 percent crystalline. The side branches on LDPE do not allow the molecules to pack tightly together, and this discourages crystallinity. A properly molded LDPE has crystallinity in the range of 45 to 65 percent. Generally speaking, as crystallinity increases there is a corresponding increase in the material's density, tensile and flexural strength, mold shrinkage factor, and heat and chemical resistance. Impact strength and transparency decrease; mold shrinkage is nonuniform.

The degree of crystallinity of an injection molded part can be changed by the way the material is molded. Molten PEs do not have crystalline structures. If the PE is allowed to cool slowly in the mold, the crystals reform. If the material is cooled quickly, the crystals do not have time enough to reform. Minor changes in the molding process can have a significant effect on the degree of crystallinity and the physical properties of a molded part. These changes in the degree of crystallinity account for some of the batch-to-batch variation in injection molded PE parts.

All of the PEs are known for their good melt flow properties, but some members of the family flow better than others. The HDPE molecules are approximately 50 times longer than LDPE molecules. The shorter the molecule, the easier it is to inject the melt through tiny gates and restricted cavities. Large, thin-walled parts can be molded in LDPE; however, this material's relatively low physical properties may favor the use of a higher-density PE.

Designing with PE
Most design engineers proportion all PE parts the same, independent of which member of the family will be used to mold the part. This is a mistake, as each member of the family responds to the injection molding process in a different way.

Following are a few factors to consider when when designing with the different PEs:

  • Wall thickness determinations override all other considerations. Thickness is determined by the functional requirements of the product and molding considerations. Functional requirements must, of necessity, take priority over ease of molding. If the product does not function properly no one will buy it and there will not be any molding problems. Always remember that nothing happens until somebody buys something.It is difficult to be definitive about the maximum and minimum allowable wall thickness for PEs. Micromolding and thin-wall molding have redefined the minimal allowable wall thickness. Setting aside these two special molding techniques, LDPE can be molded as small parts with thicknesses of only .010 inch. HDPE parts are difficult to mold with thicknesses of less than .020 inch. These are the minimum thicknesses for these two materials; however, a better thickness would be in the range of .030 to .040 inch.Wall thicknesses of up to 3 inches have been compression and flow molded with PE. These materials are, however, at their best with thicknesses of .250 inch or less. Their high mold-shrinkage factor and low thermal conductivity make thicker walls costly and more difficult to mold. The lower mold shrinkage factor and reduced crystallinity of LDPE make it the better of the two for thick-walled parts.Once a wall thickness has been chosen, every effort must be made to maintain that same wall thickness throughout the part. Thicker walls stay hot longer and shrink more than thinner walls. The high mold shrinkage of PE is troublesome in


    this regard. An acceptable wall thickness variation in a PE part would be 10 percent. A 15 percent change in wall thickness begins to cause nonuniform melt flow and variations in mold shrinkage. In instances where greater thickness variations cannot be avoided, the thick to thin walls must have a gradual transition in thickness.

  • Radiusing the corners on PE parts improves melt flow and minimizes molded-in stress. Sharp corners are stress concentrators. Rounded corners are stronger and more resistant to impact type forces. The minimum inside radius for a PE part should be equal to 25 percent of the part's wall thickness. Smaller radiuses result in increased levels of molded-in stress. Maximum part strength is achieved with an inside radius of 75 percent of the part's thickness.The highly branched LDPE molecules have excellent impact strength, and they can be designed with radiuses near the low end of the radius size range. The lower impact strength of HDPE requires radiuses near the maximum side of the range.

  • Draft angles reduce ejection forces and minimize the cooling part of the molding cycle. Draft angles are desirable on all molded parts. However, the slippery surfaces on HDPE allow many parts to be molded without draft. With HDPE the best results are achieved with polished cores and cavities and molding draft angles of 1/2 to 1º per side.The softness of LDPE requires that these parts be thoroughly cooled to develop strength enough to resist the force of ejection. A molding draft angle of 1º per side is beneficial. Highly polished surfaces encourage LDPE to stick to cores and cavities. In many cases a light mat finish or a liquid honed surface improves release from the mold. This, in turn, allows the parts to be molded on shorter cycles.

  • Projections of all types can be incorporated into PE parts. The high mold shrinkage of all PEs dictates that the thickness of stiffening ribs, bosses, gussets, and other projections be limited to 50 percent of the part's nominal wall thickness. Thicker projections produce an unacceptable wall thickness at the junction of the projection and the part's nominal wall. This increase in thickness encourages sink marks, molded-in stress, warpage, and longer cycle times. The higher mold shrinkage factor of HDPE is more troublesome in this regard than LDPE.

  • Depressions, or holes, are easy to mold in all PE materials. The excellent flow properties of PE produce good-looking, strong weldlines. The increased level of crystallinity of HDPE creates more weldline problems than with the same part molded in LDPE.The low injection pressures that are possible with PE allow the molding of very small holes without the core pin bending problems associated with the molding of the harder-flow plastic materials.

  • Tolerances are difficult to quantify as they are dependent on many interrelated factors. As a general guide, a 1-inch-long part with a .125-inch thickness can be held to +/-.0080 inch with LDPE and +/-.0085 inch with HDPE. A fine tolerance would be +/-.0045 and +/-.0070 inch, respectively. The larger tolerance of HDPE is due to the material's higher crystallinity and mold shrinkage factors.The general tolerance can normally be achieved by any competent injection molder with no cost penalty. The fine tolerances normally 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 best tolerance is always the broadest tolerance that produces a functional part.Designing parts to accommodate the subtle differences in the various members of the PE family results in stronger parts that are more economical to produce. This improved molding efficiency may be just what you need in order to survive next year's 5 percent mandatory cost reduction.The individual design details mentioned in this article are reviewed in greater detail in By Design, February 1999 through August 2001 IMM.

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