HDPE survives impacts that shatter other plastics, resists chemicals that dissolve them, and costs less than most alternatives. These properties explain why it dominates applications from fuel tanks to cutting boards. Yet the same characteristics that make HDPE versatile create specific molding challenges. Its high shrinkage, crystalline nature, and sensitivity to cooling conditions demand understanding rather than just parameter adjustment. Molding HDPE successfully requires working with its material science, not against it.
What Makes HDPE Different
High-density polyethylene gets its name from the linear molecular chains that pack tightly together, producing higher crystallinity and density than LDPE or LLDPE. This molecular architecture determines every processing behavior and end-use property.
Crystallinity in HDPE typically ranges from 60 to 80 percent, meaning most of the polymer organizes into ordered, repeating structures rather than the random tangle of amorphous materials. This high crystallinity produces HDPE’s characteristic stiffness, chemical resistance, and relatively high melting point compared to other polyethylenes.
The crystalline structure also drives HDPE’s high shrinkage. When molecules organize into crystals during cooling, they occupy less volume than in the random melt state. More crystallinity means more shrinkage. HDPE shrinks 1.5 to 4 percent, considerably more than amorphous materials like ABS (0.4 to 0.7 percent) or polycarbonate (0.5 to 0.7 percent).
Molecular weight distribution varies among HDPE grades. Narrow distributions produce more consistent properties but may be more sensitive to processing conditions. Broad distributions process more easily but with less predictable performance. Melt flow index (MFI) provides a practical measure: low MFI grades (under 1 g/10min) suit blow molding and extrusion; injection molding grades typically run 4 to 35 g/10min.
Compared to LDPE (highly branched, lower density, more flexible) and LLDPE (short branches, intermediate properties), HDPE provides the best combination of stiffness and chemical resistance in the polyethylene family. It sacrifices some impact resistance and flexibility for these advantages.
Key Properties for Molding Applications
HDPE’s property profile suits demanding applications where chemical exposure, impact, or outdoor weathering would defeat other materials.
Mechanical properties combine moderate stiffness with excellent impact resistance. Tensile strength ranges from 21 to 38 MPa depending on grade. Flexural modulus runs 0.8 to 1.5 GPa, providing rigidity without brittleness. Impact strength remains high even at cold temperatures, unlike many plastics that become brittle below freezing.
Chemical resistance is exceptional across a broad spectrum. HDPE resists acids, bases, alcohols, and most solvents. It fails only against strong oxidizers and certain chlorinated or aromatic hydrocarbons. This resistance makes HDPE the material of choice for chemical containers, pipes, and fuel system components.
Temperature limits extend from approximately minus 50°C to plus 80°C for continuous use. Short-term exposure to higher temperatures is possible, but prolonged heat causes creep and softening. HDPE’s usefulness in cold environments, where many plastics crack, distinguishes it for outdoor and freezer applications.
Density range for HDPE spans 0.94 to 0.97 g/cm³. Higher density correlates with higher crystallinity, greater stiffness, and higher shrinkage. Grade selection balances these property tradeoffs against application requirements.
FDA compliance for food contact is available in appropriate grades. HDPE’s chemical inertness and ability to withstand cleaning procedures make it suitable for food containers, cutting boards, and food processing equipment. Specific grades carry regulatory approvals; not all HDPE is food-grade.
Industrial applications demand similar material integrity. Contract molders processing HDPE for material handling applications, such as pallets, bins, and crates, require large tonnage injection molding capability to produce these high-volume components efficiently.
Environmental stress crack resistance (ESCR) varies significantly among grades. Some HDPE formulations resist stress cracking in aggressive environments; others fail prematurely. Applications involving sustained loading combined with chemical exposure require grades specifically optimized for ESCR.
Processing Parameters
HDPE’s forgiving nature makes it one of the easier materials to mold, but optimization still matters for quality and efficiency.
Melt temperature range spans 200°C to 280°C, providing considerable flexibility. Lower temperatures reduce cycle time but may cause flow problems in thin walls or long flow paths. Higher temperatures improve flow but extend cooling time and risk degradation. Most HDPE molding runs between 220°C and 260°C.
Mold temperature significantly affects crystallinity, shrinkage, and surface finish. Cold molds (10 to 30°C) produce fast cycles but lower crystallinity, potentially reducing chemical resistance and stiffness. Warmer molds (40 to 60°C) allow more complete crystallization, improving properties but extending cycle time. High-gloss surfaces require warmer molds.
Injection speed can run relatively fast because HDPE’s low viscosity accepts high shear rates without degradation. Fast injection helps fill thin sections and long flow paths. Very fast injection may cause jetting if gates are poorly designed.
Drying requirements are minimal. HDPE absorbs virtually no moisture from the atmosphere. Pre-drying is unnecessary unless material has been contaminated with water from outdoor storage or condensation. Contaminated material should be dried at 80°C for 2 to 3 hours to prevent splay.
Purging between materials requires thorough screw cleaning. HDPE’s low processing temperature makes it a good purging material for higher-temperature resins. When switching from HDPE to another material, purge with the new material until no white streaks remain.
Design Considerations
HDPE’s high shrinkage and crystalline behavior demand specific design approaches that differ from engineering plastics.
Shrinkage rates of 1.5 to 4 percent require generous mold dimensions and careful tolerance analysis. Non-uniform shrinkage is common: thicker sections shrink more than thin sections, and crystallinity differences between fast-cooled and slow-cooled areas create differential shrinkage. Predicting final dimensions precisely is difficult; prototyping or iterative mold adjustment may be necessary.
Wall thickness should be as uniform as possible to minimize differential shrinkage. HDPE tolerates thicker walls than some materials because its impact resistance prevents cracking from residual stress. However, thick sections still create sink marks and extend cycle time. Coring out thick areas improves both quality and efficiency.
Draft angles of at least 1 degree per side are essential. HDPE’s shrinkage pulls parts tightly onto cores, making ejection difficult without adequate draft. Textured surfaces require 1.5 degrees or more. Deep draws may need 2 to 3 degrees.
Warpage tendencies result from HDPE’s sensitivity to cooling rate differences. Faster-cooled surfaces develop lower crystallinity and shrink less than slower-cooled surfaces, causing the part to warp toward the slower-cooled side. Symmetric mold cooling and uniform wall thickness minimize warpage. Post-mold fixtures may be necessary for critical flatness requirements.
Common Defects and Solutions
HDPE-specific defects trace back to its crystalline nature and high shrinkage. Recognizing these patterns speeds troubleshooting.
Warpage is the most common HDPE defect. When one side of a part cools faster than the other, differential crystallization creates differential shrinkage, and the part bows toward the warmer side. Solutions include balancing mold cooling, reducing mold temperature difference between core and cavity, extending cooling time, and post-mold fixturing.
Sink marks appear opposite thick sections because HDPE’s high shrinkage cannot be fully compensated by pack pressure. Reducing wall thickness disparity, increasing pack pressure, lengthening hold time, and gating near thick sections all help. Some sink marks may be unavoidable with certain geometries.
Flow lines from crystallization during fill appear as surface streaks following the flow pattern. Higher melt temperature delays crystallization and reduces these marks. Warmer mold temperature and faster injection speed also help. Some flow line visibility may be inherent to the material.
Surface gloss variation occurs because crystallinity affects surface reflectivity. Areas with different cooling histories have different crystallinity and therefore different gloss. Uniform mold temperature and consistent wall thickness minimize this effect.
Application Examples
HDPE’s property combination creates dominance in specific application categories.
Containers and caps leverage chemical resistance, impact strength, and low cost. Detergent bottles, household chemical containers, and industrial packaging rely on HDPE’s ability to survive shipping abuse while containing aggressive contents.
Material handling products like pallets, crates, and bins use HDPE for durability in rough service. Impact resistance prevents cracking from forklifts and drops. Chemical resistance allows cleaning with industrial detergents.
Automotive fuel systems exploit HDPE’s fuel resistance and impact properties. Fuel tanks, filler necks, and fuel line components withstand both the chemical environment and crash loading requirements.
Outdoor furniture benefits from weathering resistance, though UV stabilizers are necessary for long-term exposure. HDPE maintains properties across temperature extremes and resists mildew.
Industrial components including gears, bearings, and wear strips use HDPE’s low friction and chemical resistance. Self-lubricating properties reduce maintenance in material handling equipment.
HDPE Grades and Additives
Selecting the right HDPE grade matches material capability to application requirements.
Injection molding grades have higher melt flow index (typically 4 to 35 g/10min) than blow molding or extrusion grades. Higher MFI provides easier flow in molds but lower melt strength. Ultra-high MFI grades (above 25) fill thin walls easily but may have reduced impact strength.
UV stabilizers protect against outdoor degradation. Unstabilized HDPE chalks and cracks within months of outdoor exposure. Properly stabilized grades withstand years of sun exposure. Black grades with carbon black provide excellent UV resistance.
Impact modifiers improve toughness, especially at low temperatures. Rubber-modified HDPE maintains impact strength in freezer applications or cold climates where standard grades might become brittle.
Nucleating agents increase crystallization speed and uniformity. Benefits include faster cycles, more consistent shrinkage, and improved clarity in natural (unpigmented) grades. Nucleated HDPE represents a processing upgrade for high-volume applications.
Slip and anti-block additives reduce friction between part surfaces. Important for stacking applications like lids and containers. These additives migrate to the surface after molding.
HDPE offers a combination of performance and economy that few materials match. Processing it successfully requires understanding its crystalline nature and designing around its shrinkage characteristics. The material’s forgiveness during processing should not obscure the need for proper design and mold engineering. Get those right, and HDPE delivers reliable performance at an attractive price point.
Sources
- Modern Plastics Handbook, McGraw-Hill.
- Rosato, Donald V. “Plastics Engineered Product Design.” Elsevier, 2003.
- HDPE Resin Supplier Technical Data Sheets (LyondellBasell, Chevron Phillips, ExxonMobil).
- Plastics Technology. “Processing Guide: Polyethylene.” https://www.ptonline.com/
- Society of Plastics Engineers. “Polyethylene Processing Guide.”