Thermoplastic

Polyethylene (PE) is the world's highest-volume thermoplastic, made by polymerizing ethylene. Semi-crystalline, chemically inert and inexpensive, it is processed by injection, extrusion, blow molding and rotomolding. The PE family includes several grades with very different properties. ## Main families - HDPE (high-density): 0.94 – 0.97 g/cm³, stiff, opaque; caps, drums, pipe - LDPE (low-density): 0.91 – 0.94 g/cm³, flexible, translucent; film, soft packaging - LLDPE (linear low-density): high tear strength; stretch film - UHMWPE (ultra-high molecular weight): up to 6 M g/mol; gears, prosthetics, armor - PEX (crosslinked): PEX-A; hot-water pipe ## Key properties - Excellent chemical resistance (acids, bases, salts, water) - Food-contact approved (FDA, EU 10/2011) - Service temperature: -50 to 80 °C (HDPE), -70 to 60 °C (LDPE) - High permeability to oxygen and aromas (no barrier) - Non-hygroscopic (no drying required) ## HDPE molding parameters - Melt temperature: 200 – 280 °C - Mold temperature: 20 – 60 °C - Shrinkage: 1.5 – 3.0 % (high) - Speed: moderate; very fluid resin tends to flash - No pre-drying ## Common defects Significant warpage from directional shrinkage, visible weld lines (PE is hard to self-weld at the flow front), waxy odor during processing, and degradation above 300 °C with smoke.

Polymer is a macromolecule formed by the repeated covalent bonding of many small units called monomers. It is the molecular foundation of all plastics, rubbers, fibers and many biological materials (proteins, cellulose, DNA). ## Classification by origin - Natural: cellulose, starch, proteins, natural rubber, lignin - Synthetic: PE, PP, PVC, PS, PET, PA, PC, ABS… (most of the market) - Semi-synthetic: rayon, cellulose acetate, modified natural rubber derivatives ## Classification by architecture - Linear: straight chains (HDPE, PA 66, PS) - Branched: chains with branches (LDPE, ABS) - Crosslinked: PE-X, vulcanized rubber, thermosets - Dendritic: tree-like structures (specialty) ## Classification by thermal response - Thermoplastics: melt and reshape reversibly (PP, PE, PA, PC) - Thermosets: chemically cured, do not remelt (epoxy, phenolic) - Elastomers: flexible, recover shape after deformation (rubber, TPE) ## Classification by composition - Homopolymers: single monomer type (PE, PP-H) - Copolymers: two or more monomers (ABS = acrylonitrile + butadiene + styrene) - Blends: two polymers physically mixed (PC/ABS, PA/PPS) ## Key properties governed by structure - Molecular weight: stiffness and processability - Molecular-weight distribution: process window and toughness - Crystallinity: stiffness, opacity, shrinkage - Chain polarity: chemical resistance, adhesion, transparency

Polypropylene (PP) is a semi-crystalline thermoplastic obtained by polymerizing propylene. It is one of the most widely used commodity plastics in the world: packaging, closures, automotive parts, carpet yarn, medical nonwovens and outdoor furniture. Its balance of properties and price makes it dominant in injection molding. ## Key properties - Density: 0.89 – 0.92 g/cm³ (lowest among commodity plastics) - Melting temperature: 160 – 175 °C - Continuous service temperature: up to 100 °C - Excellent chemical resistance (acids, alkalis, polar solvents) - High flex-fatigue resistance (used for living hinges) ## Commercial grades - Homopolymer (PP-H): rigid, transparent, ideal for packaging - Random copolymer (PP-R): improved transparency and low-T impact - Block copolymer (PP-B / impact copolymer): high toughness for automotive - Talc-, glass-fiber- or glass-bead-filled grades for technical parts ## Molding parameters - Melt temperature: 200 – 280 °C - Mold temperature: 20 – 80 °C - Mold shrinkage: 1.2 – 2.5 % (high, must be compensated in CAD) - Moderate injection speed to avoid flow marks - No pre-drying needed (non-hygroscopic) ## Common defects Warpage from directional shrinkage, flow marks on cosmetic parts, visible weld lines, contamination by PE (causes delamination), and degradation when processing recycled material without stabilization.

Regrind is plastic material recovered by grinding runners, sprues, defective parts or purges, then blended with virgin resin and reintroduced into the process. It is a key sustainability and cost-reduction tool in injection molding. ## Why use regrind Recovers the ~20 – 30 % unavoidable scrap from cold runners and cuts raw-material cost by 5 – 25 %, with a lower carbon footprint. On non-critical parts, pure or near-pure regrind is fully viable. ## Typical proportions (regrind/virgin blend) - Cosmetic / engineering parts: 10 – 20 % - Non-visible structural parts: 20 – 50 % - Internal / non-critical: 50 – 100 % - Some resins (PVC, PE): up to 100 % in approved applications ## Regrind system equipment - Granulator: rotating blades, sizing screen - Pellet size: 3 – 8 mm for homogeneous mix with virgin - Beside-the-press: granulator next to the machine, regrind returns to the hopper via blower - Magnet + metal detector: mandatory to avoid screw damage - Volumetric or gravimetric blender: meters regrind and virgin in a defined ratio ## Limitations and issues - Cumulative thermal degradation each pass (lowers viscosity, properties) - Cross-contamination with another resin causes delamination or breakage - Yellowing or graying on unpigmented resins - Property loss in multi-generation regrind - FDA / medical / automotive restrictions forbid uncertified regrind ## Disallowed applications FDA food-contact, class II/III medical devices, structural crash parts in automotive, and certain children's toys under specific regulation.

Thermoset is the polymer that, during processing, undergoes a chemical crosslinking reaction (cure) that creates permanent covalent bonds between chains. Once cured it cannot be remelted; reheating only degrades it. ## Fundamental difference vs. thermoplastic | | Thermoset | Thermoplastic | |---|---|---| | Processing | One-time (chemical cure) | Multiple thermal cycles | | Recyclability | Difficult (grind as filler only) | Easy (regrind) | | Structure | 3D crosslinked network | Independent chains | | Scrap reuse | Not reprocessable | Reprocessable | | Thermal resistance | Up to degradation | Up to Tm or Tg | ## Commercial thermoset resins - Phenolic (PF, Bakelite): the first synthetic resin, still in use - Epoxy: adhesives, coatings, structural composites - Unsaturated polyester (UP): glass-fiber, gel coat - Vinyl ester: improved polyester, chemically and mechanically - Melamine (MF): tableware, laminates - Urea-formaldehyde (UF): wood particle board - Polyurethane (PU): foams, RIM - Cured silicone elastomer: seals, vulcanizates ## Forming processes - Compression molding: classic, simple, slow - Transfer molding: more complex, better quality - Thermoset injection molding: special machines with cold barrel - RIM (Reaction Injection Molding): two liquid components react in the mold - Pultrusion: continuous profiles with fiber - Hand layup / lamination: large parts by hand ## Advantages - Very high thermal resistance (epoxy: 200 °C; phenolics: 300 °C) - Excellent dimensional stability - Superior chemical resistance - No creep under load (unlike thermoplastics) - Good electrical insulation ## Limitations - Not recyclable at end of life - Long cure time in some processes - Brittle without fiber reinforcement - Risk of residual monomers (formaldehyde, styrene) during cure