Click to enlarge This continuous conveyor oven includes two individual conveying systems that allow for different cure times to be processed in the same oven.

At every manufacturing plant, someone within the organization is tasked with the responsibility of process development. These employees include process and manufacturing engineers as well as a number of different individuals depending on the particular organization. In many cases, the production process will include a thermal procedure requiring an industrial oven. The processes before and after the oven are determined in great detail based on extensive product knowledge, yet often, the oven selection is made by someone with little or no specific knowledge of industrial ovens. In the course of selecting the correct oven type to complete the thermal portion of the process, the following questions often arise:

• Which oven type is best for thermal processes, batch or continuous?
• What type of oven would be best suited for this particular production requirement?

This article is intended to help answer those two questions.

Click to enlarge This gas-fired batch oven has with a horizontal crossflow air pattern. It is used to process parts loaded in racks or baskets.

Batch Ovens

Without question, batch and continuous ovens both have a place in the manufacturing process.

Batch ovens are well suited for inert atmosphere and clean room standards applications because they are closed process machines. At the time the door is closed, the heated chamber is sealed. Batch ovens also are best utilized for large parts that are difficult to convey. A batch oven operating for general processes is normally less complex and easier to maintain - and thus less expensive from an initial investment standpoint.

In most cases, however, in order to utilize batch ovens productively, more work in process (WIP) is required to create groups of parts to load the oven. (Most batch oven applications also require some type of fixture - a rack or basket - to group the parts to place them in the oven for thermal processing.) In essence, parts are removed from the production flow to be grouped for batch oven processing.

If overall manufacturing time is important, consideration should be given to the time required from the start point of production until the finished part is ready for shipment. The additional time required to group parts for thermal processing could extend the overall manufacturing time required. The batch process also requires labor for part handling: to load and unload parts to and from the basket or cart, and transfer the loaded fixtures in and out of the oven.

If the process prior to the oven cycle heats the part during production, in most cases, this heat is lost (the product cools) during the batching process prior to placing the cart or fixture in the oven. In some cases, the required batch heating time is increased because of the overall thermal mass placed in the oven. Also during batch heating, the carts or baskets must be heated every cycle as well, increasing the heat load and overall energy consumption. The batch oven will designed with the capability to heat the circulated air to the setpoint temperature, but the density of the thermal mass requires more time for the heat to actually be introduced to the part. For example, if the loaded batch oven includes multiple rows of product baskets, the individual parts closest to the heat source will heat faster than the individual parts in the center of the basket located in the middle of the thermal mass. This is because the perimeter parts are exposed to the heated air at setpoint temperature with the best air impingement on the parts at the point the heated air enters the chamber. This can be verified with temperature profile testing, which will illustrate the perimeter parts reach the target temperature before the parts located in the center of the overall mass.

In processes that require all parts to reach a specific temperature, the perimeter parts will reach the required temperature before the parts in the center of the oven load. Longer heating times overall are required due to the time required for the center parts to reach temperature, but depending on the product, this long “soak” could overheat or over-cure the perimeter parts.

Another point to consider when selecting a batch oven is that all parts in the overall mass are not exposed to matching cure profiles. The perimeter parts reach temperature faster and then soak at the setpoint temperature for the remainder of the cycle. By contrast, the center parts require more cycle time to reach setpoint temperature, reducing the soak time at setpoint temperature. In some cases, post-heating product testing shows quality differences in the specific parts depending on their location in the heated chamber.

Continuous Ovens

Click to enlarge A temperature profile taken from an indexing conveyor oven shows the temperature of six individual products as they index through an eight-hour cycle.

Generally, continuous ovens are more complex, require a higher initial investment, and call for additional maintenance. The continuous-type ovens also offer distinct advantages when compared to batch-type ovens. One advantage is that racks or baskets are not required to process the parts, and energy consumption is decreased by not heating the fixtures each cycle. Another benefit is that the labor cost associated with loading and unloading is eliminated. In addition, the work in process volume is reduced.

The continuous oven can be designed with two modes of operation. One mode is to operate these ovens continuously, as the name implies, where the parts are placed on a conveyor and moved through the heated chamber at a predetermined speed to match production rates and thermal process requirements. The other mode of operation is an indexing configuration to provide a stop-and-go type part movement through the chamber. In the index configuration, parts or small groups of parts are staged in a load position and, based on a time increment, are advanced through the oven. In this type of operation, the conveyor will move all parts in the process a specified distance and then sit at rest for a specific period of time.

Another advantage to continuous ovens is that each part is processed individually compared to basket or rack processing associated with batch processes. In continuous ovens, the parts normally are placed side by side across the width of the conveyor and receive direct air impingement from the heated supply air as they move through the heated chamber. This type of heating would be similar to the perimeter parts described in the batch process earlier. The continuous oven process allows for all parts to be heated as if they were perimeter parts in a batch process. With a continuous oven, the heating profile to which the completed parts are subjected is more consistent from part to part than parts processed in mass in batch technology. The consistency can be confirmed by profile testing to illustrate that all parts ramp to temperature and soak at temperature more equally than batch-type processes. In some cases, depending on the specific requirements for the part being processed, actual product testing shows improvements in part quality and consistency compared to batch processing.

In addition, at times the overall thermal process time can be reduced as compared to batch processes because the parts are heated more efficiently. When compared to the time required to heat parts in the center of a large mass in a batch process, the cycle time can be reduced to achieve the same quality of heating. The fact that every part receives direct heated air impingement also shortens the required time for process.

Many other features also can be configured in the design of continuous-type systems that might be difficult to add to batch ovens. These include temperature monitoring and recording along the length of the heated chamber, automatic doors at the chamber ends and temperature zoning along the length.

The continuous process oven also can include a cooling section after the heated chamber if the process requires parts cooling before the next manufacturing process. Cooling times generally will be reduced in continuous ovens as compared to the cooling times for batch-type processing. Just as the batch ovens requires longer periods of time to heat all parts because of the overall thermal mass and exposure to direct air impingement, the same restrictions are present if a cooling cycle is required. The cooling process in a batch arrangement will require more time to cool the part in the center of the overall thermal mass and the perimeter parts will be cool well in advance of the center parts. In order to cool parts in a batch oven, the heated chamber has to be cooled as well. In a continuous process, the heated portion remains at temperature and parts are cooled in the adjacent cooling section of the system. In a continuous arrangement, all parts receive direct impingement of cooling air as well, reducing the time required to cool the parts.

In conclusion, even with an overview of each type process, the ultimate question remains: Which process is best for the production of parts for a specific requirement? To answer this question, the first step is to analyze the thermal requirements for the part or parts to be processed. If the part is the same shape and size every time and the process repeats constantly, the continuous oven could be the best choice in most cases. If the parts change in physical size, weight and thermal requirements, the batch oven may be the best choice because of the range in manufacturing requirements.

Click to enlarge   This continuous conveyor oven includes a cooling zone and four access doors.

The first basic steps are to determine the temperature and cycle time requirements and production rate of the product to be processed. The next step would be to determine the size requirement for the oven to process the parts to keep pace with the required production rate. This would be calculated using the physical size data for the parts.

A batch oven would be selected based on the number of parts produced in the amount of time required to complete the oven process. In general, with slight quantity adjustments, enough parts can be produced to have the next batch ready at the completion of the oven cycle. It also may be an option to utilize multiple ovens that are smaller to change the operation to improve production timing.

The conveyor type ovens can be designed to include narrow conveyors with a long oven length or wide conveyors to reduce oven length. The best design for the application depends on the available floor space and manufacturing layout, but many options exist.

As stated, both types of ovens have a useful place in manufacturing, depending on the requirements of the product and process. When selecting the best type oven for your application, consider all the possibilities and the variations available. Take the time to factor in overall floor space availability, labor required for process, and energy consumption when making your decision. Lastly, get your oven vendor involved as early in the decision process as possible to help you choose the best oven for your application.

Links

• Davron Technologies Inc.