White Goods are a special class of consumer products
Heavy consumer durables such as Air-conditioners, Refrigerators, Washing Machines, Microwave Ovens, Water Heaters, etc. which are large, difficult to move and generally fixed in a place, are classified as White Goods. These appliances usually have substantial electricity requirement that necessitates special wiring to supply higher current than standard electrical outlets. White Goods accomplish routine housekeeping tasks like cooking, washing laundry, food preservation, etc. and are put into continuous use, throughout their lifetime. White Goods are expensive and signify long term investment for the end-users. Thus, these appliances are expected to be robust in structural, tensile and impact strength, fire resistant and have good looks. too.
Manufacturing Challenges for White Goods
Injection Moulding and Injection Blow Moulding are the prime processes for manufacturing most of the components used in White Goods and typical materials used are- Acrylonitrile-Butadiene- Styrene (ABS), Nylon (PA), Polycarbonate (PC), Polypropylene (PP) and Polystyrene (GPPS). However, producing high quality White Goods with superior finish and desirable properties like structural, impact and tensile strength, poses some unique challenges to manufacturers.
Hygroscopic Nature of Engineering Plastic
Engineering polymers mentioned above have high affinity towards moisture, including Nylon, ABS and Polycarbonate. Polystyrene, does not normally absorb much moisture, but is able to carry significant moisture on its surface when exposed to liquid water. Improper drying of hygroscopic resins can cause loss in tensile strength, surface defects like surface cracking, internal cracking, surface lines, bubbles, etc.
The injection moulding process involves injection of plastic melt into the mould and cooling the melt in the mould to take the desired shape. Water Chillers are used for quick cooling of plastic melts and to reduce the cooling time in a particular cycle. However, when chilled water is used for cooling the plastic melts, the moisture present in air in the vicinity of chilled mould surface condenses, leading to formation of water droplets on the mould surface (mould sweating). Mould Sweating is undesirable, as the condensed water droplets tend to drip on the machine bed and seep into the mould, which causes rusting and severely affects the life of expensive mould and machine bed. Further, condensation in the core and cavity of the mould will lead to defects in the moulded component.
Injection Moulding is a fast process and is used to produce large numbers of identical items continuously which makes handling of resins in the shop floor a complex job. Employing labor to handle resins manually is open to human errors like spillage of resins and delay in changeover of material. Feeding material continuously requires constant monitoring which becomes more tedious manually.
Drying Parameters : • Drying time • Air flow • Temperature • Dew point
While temperature, drying time and air flow are dependent on the type of resin, there is a huge difference in quality of molded component if the dew point can be maintained at minimum –40ºC. Dew point plays a very critical role in creating a lower vapour pressure around the surface of the resin. This lower vapour pressure forces the moisture in the molecular chain to dissipate out to its surface.