THE PULP MOULDING PROCESS

THE PULP MOULDING PROCESS

In the normal pulp moulding process, the stock preparation system is very similar to a small paper mill. There is a pulper that beats the waste paper or pulp bales to a uniform consistency with water to yield a 4% (solids) consistency pulp stock. To this mixture is added rosin and wax that usually forms about 1% of the solids content of the final pulp stock. This pulp is pumped into a storage chest called the raw stock chest. It is then pumped through a high density cleaner, and a deflaker or refiner to eliminate fibre bundles or lumps of unpulped paper and then it is dumped into a refined stock chest. The pulp slurry is pumped to the machine on demand from the machine control centre. Aluminum sulphate solution is automatically metered into the stock line by a metering pump. White water is also added to the stock and is automatically metered with flowmeter proportioning and recording system. The level of stock in the moulding machine recycling chamber is automatically controlled by means of a differential pressure cell and level controller. This level controller automatically adjusts the magnitude of flow of white water and stock that has been proportioned above. A "white water" chest stores the process water and this is reused at the pulper for dilution to 4% consistency mentioned above. This system is the most modern and automatic to be installed in a pulp moulding plant. The automatic level and flow controls regulate constant weight and quality. If you are going to be using waste sludge as your primary source of material, only a small amount of waste paper would be pulped to add to the waste sludge. The waste sludge would enter the building and be pumped to the pulper by pipe. Thus, the need for transportation and storage of raw materials would be reduced considerably over the normal situation. The wet forming section consists of a series of wire screen covered wet forming dies mounted on a moulding drum, and a matching set of transfer dies mounted on a transfer mechanism.

The forming die is made up of rigid corrosive resistant metal and consists of many component parts that are drilled with small drainage holes and covered by pre-formed stainless steel screens. The matching opposite transfer die has no screen and is drilled with small holes.

The forming die, mounted on the moulding drum is rotated at a uniform speed, immersing it in a taper flow vat containing the pulp slurry of about 1% consistency of fibres in recycled water. The taper flow vat moves the stock at the same relative tangential speed as the dies to reduce wash off of fibres. A vacuum system attracts the fibres to deposit on the forming die screens as the suspending recycle water is drawn through the screen and drainage holes. The fibres orient in a mechanically interlocked layer of fibres to form a moulded fibre product and the suspending water is recycled through the closed system to serve as dilution water for the 1% consistency at the re-circulating vat and is also used at the pulper to be mixed with waste papers to form more pulp supply.

When the matching transfer die comes into contact with the freshly formed fibre product on the forming die, there is a gentle puff of air from the forming die and a vacuum in the transfer die that gently lifts off and transfers the delicate wet formed products (about 75% water at this stage) onto the transfer die.

The transfer consists of a series of transfer dies rotating about a central shaft and mounted on a series of pivot shafts. These are designed to locate, orient and mate the transfer dies in perfect synchronization by pivot and orientation cams, with the movement of the wet forming dies, and to remove and transfer the weak wet moulded products by vacuum at a high speed and then deposits them on dryer conveyor trays by an alternate gentle air puff through the transfer dies when they are above a matching dryer conveyor tray.

This conveyor carries these wet products through a drying oven where most of the balance of water content of the products is evaporated off by means of re-circulating hot air until the products have only 8% moisture left.

The drying section consists of a direct-fired natural gas (or No. 2 oil) re-circulating air dryer with a multiple pass conveyor carrying the wet products through the heated dryer tunnel. An endless chain conveyor system conveys them through in the dryer tunnel while re-circulated hot air is blown from the top through the layers. The dried products are then discharged to an unloading conveyor where they are automatically sorted, stacked, counted and wrapped. These bundles are placed on pallets and a forklift truck is used to place them in storage ready for final shipment.

The cleaning solution and high-pressure water die cleaning systems allow the moulding dies to be cleaned in place on the machine without any of the normal cumbersome and time-consuming die removal. The stainless steel wire mesh screen and nickel bronze dies allow the rescreening to last about four times as long as is that associated with bronze screens.