Calculating Direct Machine Downtime Penalties from Moisture Induced Part Defects
Calculate downtime penalties by summing unabsorbed press hourly burden, loaded labor, scrapped resin mass, purge cycles, and hot runner clearing expenses.
PET
Meaning
Polyethylene terephthalate is a semi-crystalline thermoplastic polyester used extensively in rigid packaging and synthetic fibre production. The molecular structure of pet consists of repeating units of ethylene glycol and terephthalic acid linked by ester bonds. This chemical backbone allows the material to function as an effective barrier against oxygen and carbon dioxide, a property that governs its dominance in the manufacturing of beverage containers.
Industrial synthesis happens through polycondensation, resulting in a polymer that demonstrates high thermal stability and mechanical strength.
Thermal Processing
Injection moulding of this resin requires precise control over the melt temperature and residence time to prevent thermal degradation and the formation of acetaldehyde. Because the polymer is hygroscopic, any moisture content exceeding a few hundred parts per million will trigger hydrolytic degradation, causing a sharp drop in intrinsic viscosity and final part strength. Processing variables like screw speed and back pressure dictate the degree of polymer shear, which directly affects the distribution of molecular weight across a batch.
Moulders adjust these parameters to ensure the material flows evenly through complex tool geometries, avoiding localized overheating that might compromise the structural integrity of the finished component.
Material Economics
Procurement strategies differentiate between virgin resin, which offers consistent processing performance, and regrind material, which introduces variability into the production loop. Virgin pellets possess a stable molecular weight distribution that allows for predictable cycle times and high clarity in transparent parts. Blending regrind into the feedstock creates a cost advantage but requires tighter monitoring of melt flow rates, as repeated thermal cycles tend to shorten the polymer chains and weaken the material.
Successful management of the regrind ratio involves balancing the lower price point of reclaimed material against the risk of reduced impact resistance or yellowing in the final product.
Quality Specification
Datasheet values provide a baseline for physical properties such as tensile strength, elongation at break, and glass transition temperature measured under controlled laboratory conditions. Practitioners evaluate these nominal values against the actual performance of a moulded part to verify the efficacy of the cooling and filling sequences. A deviation in the crystallinity of the part, often caused by inadequate cooling times in the tool, leads to increased haze and poor dimensional stability.
Variations in the thermal history of the material during moulding indicate a divergence from the original specification that often manifests as brittle failure under mechanical stress.