(Q): A company is moulding parts that have to be stress-free to 1200 psi. The dilemma is how to know whether what we are seeing now is a very small stress crack or simply a "shadow" in an area with ports, curves, etc. Is there a way to highlight stress fractures that will clearly identify them as such? Any help would be greatly appreciated.

(A): Cracks on plastic parts can be detected with penetrating liquids, as is done with metal parts. Another technique that is particularly suited to plastics is to take advantage of plastics' sensitively to environmental stress cracking immersions in a stress-crack promoter for the concerned plastic will induce crack propagation and show cracks present on the part. With amorphous plastics, which are more sensitive to chemicals, it may even be possible to choose a chemical that will lead to a colour change along the cracked area. Some semi crystalline engineering materials with very high chemical resistance (such as PPS) call for exposure to combined effect of high temperature and a suitable chemical

(Q): There is a need for a thermoplastic material that can be moulded and used as an ashtray. Does anyone know of a suitable material?

(A): Ashtrays for passenger cars have been moulded for many years in glass-reinforenced PA or PPE/PS. In spite of the higher raw material costs (when compared with thermoset phenolics), they lead to a comparable finished product cost due to the expenses associated with finished and rejects due to breakage of thermosets. Thermosets provide a much better surface finish. However, this is hardly noticeable on many designs of ashtrays for passenger cars because the ash compartment is generally covered by grained lid or front panel matching the interior trim.

(Q): When do your tools need cleaning?

(A): Residues, generally the plastic or a byproduct, often cover surfaces in the form of a film, as is the case on screws. In over injected hot runners and clogged-up filters, the molten plastic solidifles and completely fills up all the hollow spaces. Cleaning-resistant plate-out forms on the surface of injection moulds, caused by release agents, outgassing, or product residues. In some cases, corrosion develops as a result of aggressive chemical compounds. But the tiresome task of cleaning moulds is often performed with antiquated methods like wire brushes, solvents, and open flames, which not only constitute a hazard for staff but frequently damaged sensitive mould surfaces as well. Sharp-edged, lightweight, flexible thermoset granules cut what are primarily organic deposits off the hard metal surface. Only such exposed surfaces as mould halves or screws can be cleaned. Much soiling caused by such plastics as polyethylene, urethane, or nylon can be removed rapidly and with no damage to the surface of the mould.

Water-based cleaning

Cleaning in an alkaline ultrasonic bath is particularly suitable for removing exuded additives as well as lime and corrosion residues. The advantage of this method is that soiling can be rapidly removed from sensitive mould halves, ejector units, and small parts, including inaccessible undercuts and holes. The mould halves, which can weigh up to 3,000 kg, are hung from a crane in a heated basin, which is irradiated by ultrasonic generators mounted on its sides. Cleaning process takes between 15 and 30 minutes.

Courtesy: Mr. P.M. Jariwala, Kolsite Industries