Mould cleaning

Many release agents build up on the metal mould surface and can form hard deposits combined with compound degradation products. This leads to the need for frequent mould cleaning. 

An injection machine can be equipped with a two-sided shuttle table using either 1 or 2 complete moulds. Applications are in production using a common top mould plate and two identical lower plates. Products with metal inserts or which require unloading and mould cleaning times as long as cure times are best suited to this process. 

Modem shuttle machines can be set up with two complete moulds so that approximately 2/3 of the cure time is under full clamp pressure in the main station. The clamp pressure is then reduced and the closed mould is shuttled out to continue the cure in the side station where it is opened for product removal, mould cleaning and insert loading. While one mould is in the side station the other is injected and receives the first part of its cure. With a special mould opening device on the shuttle table the machine becomes as effective as a two-station machine. 

Frequent mould cleaning is essential in long-run production to ensure good product appearance. The cleaning frequency is dependent on the application of the components being manufactured, the type of compound being processed and the temperature of vulcanisation. 

Salt baths can also be applied to mould cleaning. They are efficient in use but have the hazards associated with high temperature caustic chemicals. 

Mould cleaning is required because of the chemical nature of the polyurethane process. The objective in developing the Foam Film technology was to eliminate the tedious, time-consuming manual operations, which must be performed on the moulds as part of a discontinuous polyurethane foaming process. 

At least 24 aggregate particles are placed in a steel mould in a single layer with their flattest surface down. The interstices between the aggregates are in-filled with fine sand to approximately three quarters of their depth. The rest of the mould is in-filled with epoxy resin, and a metal plate, which has been precoated with grease, is placed on top. After the resin hardens, the specimen is removed from the mould, cleaned thoroughly from any sand and transferred to the abrasion machine. 

Because the RIM materials enter the mould as low viscosity liquids, they accurately reproduce the mould surface. This characteristic is an advantage when a stained or textured surface is desired for aesthetic reasons. Secondly, both mould halves must be closely fitted to avoid excess leakage and fiash of the low-viscosity liquid mixture. This close-fit requirement must be balanced with the need to dispel all the air in the mould within the 5 s or less fill time. Some provisions for the release of mould air must be made. Also mould release must be applied to the mould surface to effect release of the parts, and mould cleaning is required to control flash removal and polymer build-up. 

There is a wide variety of cleaning systems on the market catering to different abrasion requirements. As the expanding gas creates considerable noise at the tip of the nozzle, ear protection and sound insulation are mandatory. Customised systems are also available for various purposes, e.g. automatic tire-mould cleaning. 

Some systems allow access to nozzles and cleaning of the insulation chamber without removing the mould from the machine, e.g., STRACK, Spear System, Husky, Mold-Masters. The mould cleaning procedure is shown in Figure 10.1 ... 

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