Читать книгу Packaging Technology and Engineering - Dipak Kumar Sarker - Страница 36

Glass (plain glass or soda glass) is manufactured at 1580 °C in a furnace with a mixture of cullet (broken glass), fine sand, lime (CaO), and soda (NaO). The molten glass is then pressed, drawn, blown, and shaped into the desired form (Figure 2.3). After forming into bottles the glass is crimped for effective container closure. The bottles are annealed at 460 °C to aid removal of internal stresses, then allowed to cool without defect, fissure, or crack formation. Coloured glasses are blended to give the same continuous output of glass.

Paper is a highly recyclable commodity like glass, steel, and aluminium. The Kraft and sulfite/sulfate processes for making paper are the most common forms of paper fibre manufacture. Paper is manufactured from Kraft‐, mechanical‐, or sulfite‐treated wood chips and then formulated with fillers and whiteners such as kaolin and gums, e.g. poly(vinyl alcohol). After dilution to about 4% solids by weight and formation by the Fourdrinier process (see Figure 5.2) on a mesh, from the deposited aqueous slurry, the samples are wet pressed. Following on from basic sheet‐making the ‘wet’ mass is dried at 120 °C to a final moisture content of approximately 6% w/w (Figure 2.3). Calendering, the process of making a uniform thickness by pressured successive rolling follows (at 20 kPa–40 MPa), terminated by sizing. Rolls of finely graded 0.5 mm thickness processed paper of 2.5 m spool width, 2.5 m spool diameter, and a supporting roll core of 0.25 m can weigh 8 tonnes. The crude paper is then ready for the final step, which is grading for application depending on its coarseness and degree of refining.

Product quality tests form an essential part of maintaining the consistency of the raw materials produced. For metals this would be reflected in the chemical distribution and ratios of minor elements or impurities, such as copper, arsenic, or sulfur [11]. Metal quality is also routinely evaluated through mechanical indices such as tensile strength and hardness as well as sheen and lustre. Glass quality would be defined by uniform transparency and shading and freedom from wall variation thickness, micro‐fissures (air‐tightness), bubbles, blisters, cracks, or erroneous moulding. Paper quality is usually estimated in terms of tear and crease resistance.