For Paper-Pulp Packaging

January 2017 • Volume 24, Number 1 Finding the CORRECT DRYER for Paper-Pulp Packaging By Mike Grande, Wisconsin Oven Corp. Follow us on: PH Cover 0117-2.indd 1 1/25/17 10:01 AM Ovens Photo courtesy of Mecsmart Molded-fiber parts are carried through the oven on a mesh belt conveyor. Finding the CORRECT DRYER for Paper-Pulp Packaging Once formed, molded-fiber packaging must be dried in a hot-air convection oven with specific characteristics. By Mike Grande, polystyrene (EPS) and vacuum-formed Yet, it was not until the late 1980s, with Wisconsin Oven Corp. PET and PVC. It has gained popularity increased environmental awareness re- in recent years because it is made entirely garding consumer packaging, that the olded paper-pulp packag- from recycled materials and can be recy- market gained a strong foothold. More ing material — also re- cled again and again. The raw ingredients recently, molded-pulp packaging with a ferred to as molded fiber are simply paper and water; no binders or thin plastic lining — often called “paper M — is a fast-growing, envi- additives are required. No wastewater is bottles” — has been used in wine, beer ronmentally friendly segment of the pack- generated in the manufacturing process and some other drink containers. aging market. It is manufactured using re- because any excess water either evaporates cycled paper products such as paperboard, or is returned to the system. For these rea- Manufacturing Molded newsprint and other post-consumer and sons — and due to its landfill friendliness Paper-Pulp Packaging post-industrial waste. Molded paper-pulp — paper-pulp packaging is considered a The raw material (paper waste) is dumped is used to produce fast-food drink trays, sustainable packaging material. into a vat, and warm water at 109 to 149°F egg cartons and protective packaging for Molded paper-pulp packaging was (43 to 65°C) is added. The warm water consumer and industrial products as well invented in 1903 by Martin Keyes, who swells the fibers, causing them to break as disposable plates, bowls, plant trays and was awarded a patent for the process. In apart. The mixture is stirred thoroughly clamshell containers, among many others. 1920, additional patents were granted for until the paper fibers are unbound and be- Molded paper-pulp is an attractive al- molded-pulp packaging designed for light come a slurry-like pulp. ternative to foam packaging, expanded bulbs, fruit, radio tubes and other objects. The pulp is formed into the finished- 2 Ovens packaging shapes using custom-designed a final shape to the part and creates a matter the type of mold, the suction from one- or two-part molds. The upper mold smooth finish on the bottom side. the vacuum and the pressure generated by is lowered into the pulp mixture, and a Traditionally, paper-pulp molds have the mold-halves bind the fibers together. vacuum is pulled on the rear of the mold been aluminum or bronze shapes into After forming, the wet parts contain to draw the water out while, at the same which a multitude of small passages approximately 70 to 75 percent water and time, forcing the pulp to conform to the have been machined to provide porosity 25 to 30 percent fiber by weight, and they mold’s shape. The upper mold then is re- through which to draw the vacuum. A must be dried. The molded parts are placed moved from the vat, and air pressure is stainless steel mesh then is laid on top. onto a conveyor and passed through a hot- used to remove the part from the mold. More recently, 3-D printing technology is air convection drying oven to remove the When two-part molds are used, the up- used to produce plastic molds that provide moisture. After leaving the oven, the final per mold is pressed into the lower mold, certain advantages — among them, re- product contains 6 to 8 percent moisture, which squeezes out excess water, imparts duced lead time and design flexibility. No which is required to prevent breakdown Molded-fiber parts are removed from the molding machine and placed on the oven conveyor. 3 Ovens and flaking of the material. veyor. The air is discharged from the ducts control, meaning the oven is divided in After drying, the molded pulp products at velocities of 3,000 to 5,000 feet per the direction of travel into perhaps three are trimmed if necessary and can undergo hour via air nozzles that are located along or four separate heating and recircula- additional processing as required. For ex- the width and length of the ducts. The tion systems. A 120-foot (36.5-meter) ample, they can be made waterproof with hot air impinges on the wet shapes, using long oven, for example, might have four a spray or dip coating of wax, or printing air turbulence to remove the microscopic 30-foot (9.1-meter) zones, each with its can be added. boundary layer of moisture that continu- own heating and recirculation system. ously forms adjacent to the surface of the Each individual zone can be operated at Drying Ovens for shapes during evaporation. Without suf- a different temperature, and it is common Paper-Pulp Packaging ficient impingement, this boundary layer for the first zone — where the incoming After the shapes exit the molding process, will retard further drying. Steps must be molded parts are the wettest — to be set they are passed through a hot-air convec- taken to ensure the air velocity is sufficient at a higher temperature. Progressively tion oven or dryer. Because molded paper- for drying but not so great as to cause the lower temperatures are used in each suc- pulp products usually are formed at a high parts to move or levitate. It is common to cessive zone to avoid burning or overdry- production rate, a typical conveyorized use variable-frequency drives (VFDs) on ing the material. paper-pulp dryer must be fairly large to the blower motors to prevent this. Several conveyor styles are used in provide sufficient drying time. A 5-to-7- In molded paper-pulp drying applica- molded-pulp drying ovens. The most du- feet wide conveyor and a length of more tions, it is important that the oven has rable is a chain-edge mesh-style belt. It has than 100 feet is common for these units. sufficient heat input to evaporate the high two drive chains located on the edges of Typical drying times can range from 6 to volume of water being processed. It is the conveyor, and a woven mesh belt is sus- 60 minutes, depending upon the thick- not unusual, for example, for a dryer to pended between them. The molded parts ness of the parts. evaporate 3,000 pounds (1,400 kg) per are carried through the oven on the mesh. Paper-pulp drying ovens use convec- hour of water from molded parts, which This design requires a higher initial invest- tion technology to evaporate the moisture is equivalent to 360 gallons (1,360 liters) ment but has good longevity. Also popular from the molded parts with heated air. per hour. For this reason, these ovens nor- are flat wire belts and Teflon-coated fiber- The ovens have pressurized ducts located mally are gas fired with heat input ratings glass. Each style has certain advantages. above (and below if necessary) the parts of several million BTU per hour. Further, Also, it is critical that the oven has suf- as they travel through the oven on a con- pulp dryers usually have several zones of ficient exhaust to remove the moisture- Most pulp dryers are large, multiple-zone units. After leaving the oven, the final product contains 6 to 8 percent moisture. 4 Ovens This three-zone, gas-fired oven is used for drying molded-fiber packaging. In molded paper-pulp drying applications, it is important that the oven has sufficient heat input to evaporate the high volume of water being processed. laden air from the heating chamber. If the moisture is not exhausted at a sufficient rate, the environment in the oven will be- come saturated and will prevent drying of the paper-pulp shapes. In addition, excess moisture can condense on the oven inte- rior and cause rusting of the oven body. As an energy-saving measure, the hot exhaust air from the oven can be run through a heat exchanger to preheat the water used in the pulp or elsewhere in the factory. A strategy to reduce the exhaust rate — and, therefore, gas consump- tion — is to include a humidity sensor to detect the moisture level in the oven. Then, the humidity sensor output is used to control the exhaust rate via a variable- frequency drive on the exhaust fan. By properly adjusting the humidity setpoint, the oven will exhaust only the volume of air required to achieve part dryness. Otherwise, the exhaust system must be adjusted to accommodate the highest production rate and will waste energy when the oven is running at lower production. Mike Grande is a sales manager/senior applications engineer with Wisconsin Oven The chain-edge mesh-style conveyor belt has two drive chains located on the Corp. The East Troy, Wis.-based company edges of the conveyor. A woven-mesh belt is suspended between them, making can be reached at 262-642-3938 or visit the it a durable choice for drying molded pulp-paper packaging. website at www.wisoven.com. © of Process Heating 2017 5.

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