Dimensional Stability

Contraction (Shrinkage) is the dimensional reduction a molded part undergoes as it cools from melt to solid and reaches room temperature. It is an inherent property of each resin and must be compensated at mold design time by scaling the cavities. ## Types of shrinkage - Volumetric: occurs during cooling inside the mold, partly offset by hold pressure. - Linear mold shrinkage: measured 24 h after demolding, the catalog value in %. - Post-shrinkage: continues for up to a week or more, especially in semi-crystalline resins. ## Typical values by resin - PP: 1.2 – 2.5 % - HDPE: 1.5 – 3.0 % - PA (Nylon): 1.0 – 2.5 % - POM: 1.8 – 2.5 % - ABS: 0.4 – 0.7 % (amorphous, very low) - PC: 0.5 – 0.7 % - PS: 0.3 – 0.6 % ## Factors that affect shrinkage Wall thickness, mold temperature (higher T → more crystallinity → more shrinkage in semi-crystalline), hold pressure, hold time, flow orientation, and reinforcement (glass fiber cuts directional shrinkage by 50 – 70 %). ## Related issues Warpage from uneven directional shrinkage, sink marks in thick areas with poor packing, and internal voids.

Moisture is water held by a plastic resin — both on the surface of the pellet and absorbed inside it. In injection molding it is the single most common cause of cosmetic and strength defects, because at melt temperature that water turns to steam and can chemically attack the polymer. It is distinct from ambient humidity (water in the air) and is quantified as moisture content (% or ppm). ## Why it matters - Hygroscopic resins (PA/nylon, PC, PET, PBT, ABS, TPU) actively absorb water from the air; molding them wet causes hydrolysis — the water breaks polymer chains, permanently lowering strength and toughness even if the part looks fine. - Cosmetic defects: steam at the flow front leaves splay (silver streaks), bubbles, voids and poor surface. - Process noise: moisture changes apparent viscosity shot-to-shot, so a wet lot will not repeat like a dry one. ## How it is controlled Dry the resin before molding to the supplier's target on the material data sheet — typically in a desiccant dryer (not just hot air) for hygroscopic grades, at the specified temperature and time. Keep dried material in a closed hopper, limit exposure after drying, and re-dry regrind, which re-absorbs water fast. Non-hygroscopic resins (PE, PP, PS) only carry surface moisture and usually need little or no drying. ## Related terms - See also: moisture content, humidity, dryer, virgin resin, regrind ## Why is moisture a problem in injection molding? At melt temperature the water flashes to steam, causing splay, bubbles and voids; in hygroscopic resins it also triggers hydrolysis, breaking polymer chains and permanently reducing the part's strength. ## What is the difference between moisture and humidity? Humidity is water vapor in the surrounding air; moisture is the water the resin actually holds on and inside its pellets. High humidity is what drives a hygroscopic resin to pick up moisture. ## How do you remove moisture from plastic resin? Dry it before molding — hygroscopic grades need a desiccant dryer at the data-sheet temperature and time; keep the dried resin in a sealed hopper and re-dry regrind, which reabsorbs water quickly.

Thermoplastic is a polymer that softens and re-melts when heated above its melting or glass-transition temperature and solidifies again on cooling — with no permanent chemical reaction. This reversibility is what enables injection molding, extrusion and mechanical recycling of most plastics. ## Thermoplastic vs. thermoset - Thermoplastic: linear or branched chains without chemical crosslinks. Melts and can be remolded (PP, PE, ABS, PC, PA, PET, POM). - Thermoset: crosslinks chemically during cure (phenolic, epoxy, melamine resins). Cannot be remelted; reheating only degrades it. ## Classification of thermoplastics - Commodity: PP, PE-HD/LD, PS, PVC, PET → high volume, low cost - Engineering: ABS, PA (nylon), PC, POM, PMMA, PBT → improved mechanical properties - High-performance: PEEK, PPS, PSU, PEI, LCP → high service temperature, high cost - By structure: amorphous (PC, PS, ABS) vs. semi-crystalline (PP, PE, PA, POM) ## Processability Almost any thermoplastic can be injection-molded, extruded, thermoformed, blow-molded and roto-molded. Semi-crystalline grades require precise mold temperature to control crystallinity; amorphous grades tolerate wider windows. ## Recyclability and reuse Thermal reversibility allows scrap (regrind) to be ground and reprocessed up to 20 – 30 % mixed with virgin resin with no major property loss, depending on the polymer. Additives, cross-contamination with other resins, and accumulated thermal degradation limit the number of cycles.

Viscosity is a fluid's resistance to flow. For a polymer melt it governs how easily the resin fills the mold: high viscosity means stiff flow that needs more pressure, low viscosity flows easily but can flash. ## Polymer melts are shear-thinning Unlike water, a thermoplastic melt is pseudoplastic (shear-thinning): its viscosity drops as the shear rate rises. Faster injection speed shears the melt more and thins it, which is why fast fill can need less pressure than slow fill. Viscosity also drops as temperature rises (barrel temperature). ## What changes viscosity - Temperature: higher melt temperature → lower viscosity. - Shear rate: higher injection speed → lower viscosity. - Molecular weight / grade: higher MW (lower melt-flow index) → higher viscosity. - Moisture and degradation: can lower or raise it unpredictably. ## Why it matters Viscosity sets the fill pressure, the process window and gate/runner sizing. In scientific molding a viscosity curve (relative viscosity vs fill speed) finds the speed where the melt is least sensitive to small changes, for a more robust process. (Note: lab relative viscosity is a different, dimensionless ratio used to grade resins like PA.) ## Related terms - See also: melt, injection speed, barrel temperature, injection pressure, relative viscosity ## What is viscosity in injection molding? It is the melt's resistance to flow; it drops with higher temperature and higher shear (injection speed) and sets how much pressure is needed to fill the mold. ## Why does melt viscosity drop at high injection speed? Polymer melts are shear-thinning: more shear untangles and aligns the molecules, lowering viscosity — so a faster fill can need less pressure. ## What affects melt viscosity? Melt temperature, shear rate (injection speed), the resin's molecular weight / melt-flow index, and moisture or thermal degradation.