In 1933, scientists at Great Britain's Imperial Chemical Industries found traces of a unique substance inside a pressure vessel. This waxy, white discovery became the predecessor to the polyethylene (PE) material that would one day grab the lion's share of the plastics market. In World War II, all of this new material became earmarked for military use. PE provided electrical insulation, which grew scarce as enemy forces raided the Pacific's rubber plantations. After the war, PE entered the commercial market at $5.00/lb. Huge quantities of PE are now only 32 to 56 cents/lb. Today, PE is the most abundant volume material—roughly one-third of the plastic material used in the country.
Polyethylene is an entire family of unique materials with similar characteristics. Low- and medium-density polyethylene (LDPE and MDPE) were the first materials developed. In 1957, high-density polyethylene (HDPE) was discovered, and in 1965, linear low-density polyethylene (LLDPE) was introduced in Canada. Crosslinked polyethylene (XLPE) and ultrahigh-molecular-weight polyethylene (UHMWPE) are newer family members but are hard to injection mold. The family's positive attributes include their low cost, low weight and good impact and chemical resistance. Many satisfy FDA and NSF requirements. In addition, PE materials are thermally stable and easy to process at low temperatures and pressures. On the other hand, their limitations include their relatively high mold shrinkage factor, inadequate stiffness and low temperature resistance. Despite these shortcomings, PE is the leading material in the plastics industry.
A PE with a 0.191 to 0.925 g/cu cm density is considered an LDPE. Those with densities ranging from 0.941 to 0.959 are classified as HDPE. The densities of MDPE and LLDPE materials have values between these two. Generally, increases in PE density are accompanied by increases in tensile strength, stiffness, heat deflection temperature, hardness, surface gloss, mold shrinkage, permeation and chemical resistance and decreases in elongation, impact strength and environmental stress crack resistance.
All PEs are made up of lengthy molecular chains of carbon and hydrogen. HDPE molecules are linear with only a few side branches while LDPE molecules have many side branches—approximately 10 times more. As a result, LDPE molecules get entangled because of entwining side branches, and this intermolecular entanglement is responsible for LDPE's greater elongation and impact strength. LLDPE molecules also have many side branches, but these branches are shorter and display an organized arrangement along the length of the molecules. As a result, the physical properties of LLDPE generally fall between that of HDPE and LDPE.
Because they have few side branches, HDPE molecules are able to align close to each other. This tight molecular arrangement promotes the formation of crystalline structures in the material. When molded correctly, HDPE is 70-90% crystalline. In contrast, LDPE molecules do not align as tightly because of the presence of many side branches. Properly molded LDPE is only 45-65% crystalline. In general, increases in crystallinity result in increases in the material's density, tensile and flexural strength, mold shrinkage factor, and heat and chemical resistance and decreases in impact strength and transparency.
The way that a material is molded can alter the part's degree of crystallinity. Molten PEs possess no crystalline structures. Crystals re-form, however, if the PE part is permitted to gradually cool in the mold. Conversely, if the part is cooled rapidly, the crystals will not have sufficient time to re-form. Even slight modifications in the molding process can significantly affect the molded part's degree of crystallinity and physical properties. These varying degrees of crystallinity are responsible for the discrepancy between batches of injection-molded PE parts.
All PEs possess good melt flow properties, but some family members flow more easily than others do. The shorter the molecule, the easier the melt can be injected through small gates and narrow cavities. HDPE molecules are roughly 50 times longer than LDPE molecules. Therefore, LDPE material can be molded into big, thin-walled parts, but this material's relatively low physical properties may compel injection molders to seek higher-density PE.
Most design engineers make the mistake of proportioning all PE parts the same, regardless of which PE will be used for the part. This is unwise because each PE family member reacts differently to the injection molding process. Engineers must consider the following factors when designing parts with the various PEs:
Wall thickness considerations supersede all other concerns. How thick the part should be molded depends on the product's functional requirements and molding concerns. However, function should override ease of molding because a poorly functioning product will not sell.
Radiusing PE part corners leads to better melt flow and reduced molded-in stress. Sharp corners concentrate stress. Rounded corners are stronger and can better withstand impact type forces. A PE part's inside radius should be at least 25% of the part's wall thickness. Parts are at their strongest with an inside radius equaling 75% of the thickness. Because of its high impact strength, LDPE parts can be made with radiuses at the low end of the range. In contrast, HDPE parts must have radiuses close to the maximum value.
Draft angles are recommended on all molded parts because they lessen ejection forces and reduce the cooling portion of the molding cycle. However, HDPE's slippery surfaces permit part molding without draft. With highly polished surfaces, LDPE will be better able to stick to cores and cavities. Often, a light matte finish or a liquid honed surface allows better release from the mold, which, in turn, reduces the molding cycle.
Projections of all kinds can be integrated into PE parts. Because of the high mold shrinkage factor of all PEs, the thickness of stiffening ribs, bosses, gussets and other projections must be kept no higher than 50% of the part's nominal wall thickness. Thicker projections result in unallowable wall thickness where the projection and the nominal wall meet. This greater thickness promotes sink marks, molded-in stress, warpage and lengthier cycle times. HDPE's higher mold shrinkage factor is especially problematic.
Depressions or holes can be easily molded in all PE parts. The plastic's superb flow properties make attractive, strong weld lines. However, because of its greater crystallinity, HDPE produces more weld line problems than LDPE. Polyethylene allows low injection pressures, which, in turn, permit the molding of tiny holes without the core pin bending problems that commonly occur during the molding of harder-flow plastic materials.
Tolerances are hard to define because they are influenced by many interrelated factors. Generally, a one-inch-long part with a 0.125-inch thickness should be kept within +/- 0.0080 inch and +/- 0.0085 inch with LDPE and HDPE, respectively. A fine tolerance for LDPE is +/- 0.0045 and +/- 0.0070 for HDPE.
By taking these factors into consideration, stronger PE parts can be designed and produced economically.
Source: By Design: Polyethylene Part Design
Glenn Beall
Injection Molding Magazine, April 2002
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