One of the issues faced by Sanitaryware manufactures using Battery casting systems is that of mould breakage, slip leakage and a deteriorating mould quality well before the expected life span of 80 to 100 casts per mould. Most often the casting staffs are labeled as the culprit due either to lack of skill or carelessness of procedures. After all the internet is now full of videos showing the various processes at different plants, your company probably has the same system so it must be operator error. Perhaps not, the yield achieved is not displayed with the video or photo. Battery Machines can be purchased as a base system or enhanced with automatic slip circulation, slip casting, slip draining and now common firming cycles. The following is worth consideration:
Wind the clock back to when Items of sanitary ware were hand cast and standing individual on the casting bench. Certainly, a mould would rarely burst during filling with slip.
If a mould did happen to leak it could be traced to a number of reasons. Loose clamps, a caster error and learnt from.
Poor quality moulds from a damaged case (mother mould), a problem lived with until the case could be repaired or replaced. At least a traceable reason and potential solution.
A leaking mould due to initial overfilling of the funnel (slip receptacle). The solution caster education or smaller funnels that however required a top up after the initial fill.
There is no doubt that Battery casting systems have enhanced output and greatly reduced the labour intense process of mould and clay ware handling. However the systems have drawbacks especially when gravity tanks are employed to cast the system and maintain a slip feed.
Many factors influence the efficiency of the Battery casting system, for now two aspects are highlighted and both are beyond the casters control as they are part of the process supplied to them.
Most systems are clamped end to end by a single screw clamp at one end. This is often hydraulic with the machine tightened to a pre determined pressure. The caster has no other contribution especially on simple Close Couple pans of 4 mould parts. The system is slightly open to question is the product is Back to Wall requiring a 5th mould part to form the back and trap-way. Was the machine closed end to end before or after the 5th part is installed? Either way caster influence is obliterated if factor 2 below is not taken into consideration.
Air bags have become a popular clamping method and again the system is outside the casters control so long as the pre determined air bag pressures are adhered to via the pressure valve reading on the airbags.
It is therefore a feasible conclusion the moulds are not kept clean at the mould joints! After all when new they did not leak or not so bad? As a reasonable and possible scenario the mould making , case-making and mould installation team are not influential to the problem .Refer to article “Consistent mould quality”.
Slip casting and pressures incurred:
Slip properties can vary slightly on day to day basis . Controlling the Thixtropathy, Fluidity and Pint weight is a major part of slip production. This varying slip is supplied to a machine with a consistent monitor by way of Gravity tanks with fixed level probes. Casters/ supervisors then try to calculate the cast time for that day using the figures supplied by the laboratory or readings from the Test pot. There is also a visual observation available, viewing the air tube from the air manifold to the mould furthest from the Gravity tank feed. If observed and the level in the tube marked , over a period of say 10 casts a scale will be produced that will collate very closely to the figures supplied from the laboratory.
Due to the Density of slip there is a pressure drop over a given length of pipe-work, This can be observed by viewing the air pipe from air manifold of the first mould and last mould in the Battery line. The following sketches using various pipe line layout highlight this behaviour. Important to note that a pressure of 3 PSI to 3.25PSI(.207 to .224bar) on a plaster of Paris mould should not be exceeded. Burst , cracked ,leaking moulds or internal clay cracks will be the result of this oversight.
This sketch shows a single end feed and the reduced slip level through the moulds. The longer the machine the greater the pressure drop.
Battery castin moulds
By filling the machine from a centre point the slip level in the Gravity tank can be reduced. The fill rate (time) may be affected, using a larger diameter feed pipe will resume adequate flow rate. It will also be affected by the volume of slip required to fill an individual mould.
With all the variables to consider , machine length, slip volume, filling time, multiple mould parts, slip property variation , mould height, together with a balanced feed (important) throughout the machine length gravity tanks are not a fit one fit all solution. The high and low point of the sensor probes are best at 20mm on /off with a slow open reaction by air restrictor and a rapid close of the supply valve. This will calm the action of slip pressure surge and reduce pin-holing caused by slip vortex. Its worth to remember that whilst filling the mould is subject to air pressure, once full it becomes hydraulic pressure which is far more destructive to the mould.
A motorised valve usually pneumatic with an adjustable pressure sensor and a central feed to the casting manifold is perhaps a more suitable control. There are a few fail-safes required contact www.sanitaryware.org or myself for an in -depth installation and description.
A final point to consider is that of air pressure for assisted draining of excess slip.
Dependant on the slip quantity and size of mould for example a one piece WC the mould may well be at its limits of pressure acceptance (MOR) A further pressure is applied by air to the mould for the draining cycle. Its common to see pressure gauges reading 0.2 bar! Until the run off valve is actuated and the slip begins to move this is an additional although temporary pressure that the mould is subjected to. And it does easily exceed the recommended maximum pressure of3 PSI to 3.25PSI (.207 to .224 bar). Casting and air pipe often do come loose at this point leaving slip pools up and down the machine. Perhaps the pressure is not that high but it will cause the moulds to flex! This can cause a fissure at the point where hollow section meets solid cast and often is not traceable however the result is unexplained and inconsistent clay cracks.
Ideal is to have two pressure settings on the air manifold, one for actual draining bearing in mind it only requires air to replace slip to avoid the hollow sections collapsing. After all old fashioned hand casting did not have air assist to drain. A high volume of air creating high velocity is not good for the internal hollow sections, its easy to cause rippling or wreathing on internal hollow sections that distort during firing leave a lumpy uneven surface on the fired piece.
The second pressure setting can be 3.25 PSI or .224 bar forming a firmer hollow section.
The use of blower fans for the draining and firming cycle should be used with caution. It is important that the air velocity is controlled as well as the air pressure which can vary depending on how many machines are using the system at one time. Accurate pressure control valves are essential to avoid internal cracks and lumpy uneven hollow sections.
It is possible to monitor the casting and draining process and its effects on the moulds. Place engineers dial gauges on suitable stands at various points throughout the battery line . The dial gauges should show no movement if the system is set up correctly.
High pressure casting eliminates all of the above issues , its expensive and suitable for large volumes of a particular item.
Low pressure systems can produce a wider range of items with the newly developed Cell casting method perhaps being the ultimate for manufactures product requirement.
If you have any queries leave a comment below.
The moulds are filled via the gravity tank, mains pressure used only for circulation
are the moulds feed by the gravity tank directly or the moulds feed from the main slip source and the tank used to control the slip level ?