PT ZONE: Calibration Tables
Improved Dry Vacuum Calibration Tables
Dry vacuum calibration tables were developed in response to the need to hold complex plastic profiles to very tight tolerances while they were being cooled in the extrusion process. Tables were developed to hold the calibration tooling needed to produce tight tolerances at high output rates and to allow for the easy changeover from one part to another. Although the calibration tooling is needed to achieve this, it is very expensive and alternate methods have been developed to increase rates without building longer and longer calibration tooling. Tables had to be modified to be able to handle the alternate cooling methods.
The calibration tooling can be made from aluminum for better heat transfer but it is normally made from stainless steel for better life due to the abrasive nature of filled plastics rubbing over the polished surfaces. The internal surface is cut in the shape of the desired profile and highly polished for low drag resistance. Cooling channels are cut into the tooling for flow of the critically important cooling water. In addition, channels are cut into the tool for vacuum to draw the plastic part out against the calibrator wall to make good contact to ensure cooling and obtaining the proper dimensions. Generally the tool is built to be “dry” meaning that no water touches the extruded profile in the calibrator. Some calibration is built to actually introduce a small amount of water or allow leakage of cooling water to act as a lubricant between the part and the metal surface. This can also improve the cooling efficiency.
The initial calibration tooling will smooth the surface of the hot plastic material as it first enters the tooling. The primary job of the calibration tooling is to cool the part as it is controlling the size and shape of the plastic. The length of the calibration tooling will vary with the line speed of the extruded part, the complexity of the profile, and the dimensional tolerances required of the profile. Increasing any of the factors will increase the required length of the tooling. Calibrators are typically built in sections of 4 to 15 inches in length for ease of manufacture and handling. They are then used in sets to achieve the needed length of calibration needed for the profile either with or without gaps between each calibration block. Calibration of 4 feet or more is not uncommon in complex window profile lines.
Since the primary purpose of the calibration tooling is to cool the plastic as it is being held in shape, it is critical to have water channels through the tooling in the proper location for uniform cooling and then have adequate water flow to maintain the desired processing temperature. Typically chilled water that is maintained at 50° - 55° F is used to circulate through the tooling. Sometimes it is desirable for the first calibrator to be slightly warmer than the rest to better impart a smooth surface to the plastic and to reduce drag caused by shocking the plastic with the initial cooling. This warmer temperature in the first calibrator is generally achieved by adjusting the flow of water going into that first calibrator, however a temperature controlled unit can be used to assure consistent temperature.
Dry vacuum calibration tables have been developed and are offered by many companies that offer a convenient base on which the calibration tooling can be mounted. They generally provide a heavy duty frame with the vacuum and water pumps along with all the necessary plumbing, including filters, heat exchangers, etc., along with necessary controls. They allow for simple connection to modular calibration tooling so that it can be changed out easily. The tooling is mounted on some type of rail system for consistent alignment with itself. The table usually incorporates a tray system under the mounting rails to catch any leaking or stray water.
Alignment of the calibration tooling to the extrusion tooling is critical so movement of the table is controlled by allowing adjustment of the positioning side to side and up and down. These linear movements are typically achieved by a hand wheel driving a gear system although a powered drive system can be used. Movement of the table toward and away from the extruder is usually driven because of the magnitude of the change that is needed.
An auxiliary tank is usually mounted on the calibration table after the initial calibration tooling in order to offer additional cooling for the profile. These tanks are typically 6 to 12 feet long. They are made to hold forming plates that continue to hold the part straight while the applied vacuum holds the part out against the forming plates to hold the size and dimensions. They are made to immerse the part in water with turbulent mixing to break up the insulating layer of water around the skin of the part. The tank itself is designed for water to be introduced at the front end of the tank and the vacuum is applied at the downstream end of the tank drawing the water through the tank. Turbulence is usually created by the placement of holes in the forming plates. Holes all around the part create some turbulence but alternating plates with holes above the part and below the part increase turbulence and water flow across the part, increasing cooling efficiency.
These types of tanks require a lot of water movement to achieve the turbulence required for good cooling efficiency. That water is being drawn out of the tank by the vacuum applied at the downstream end of the tank. This requires the use of liquid ring vacuum pumps that can handle both the air needed to pull a vacuum along with the water that is being introduced for cooling and has to be sucked out of the tank. However, the more water that the pumps need to move reduces their efficiency to pull a vacuum which is their primary purpose. Therefore, larger horsepower pumps and more of them are needed to make this system work. Typically a 10-hp pump would be required for each 6 to 8 feet of auxiliary tank in addition to the vacuum requirements of the calibration tooling. In many high output applications 10, 20 or even 30 feet of auxiliary tanks are needed to achieve the desired cooling. All of these liquid ring vacuum pumps running at low efficiency because they have to pull so much water create a larger capital expenditure up front as well as higher on-going operating and maintenance costs.
A better solution is to separate the water from the air so that each can do it’s intended job. The air is needed to draw a vacuum while the water is needed for cooling. The use of a high intensity spray from nozzles that surround the part all the way down the tank provide the necessary volume of cold water for cooling without the need of excessive volumes just to create turbulence. The intensity of the spray of cold water onto the surface of the part breaks up the layer of heated water that can slow down cooling. This volume of water drops to the bottom of the tank where it can easily be removed separately from the vacuum port. With this configuration, the vacuum pump needs to handle a significantly lower volume of water and can therefore be much more efficient. In fact a liquid ring pump may not be required allowing the use of a more efficient and lower horsepower Regenerative pump.
Early tables that utilized this technology had the drawback of having a fixed length of rail section for the dry calibration to allow for the specialized auxiliary tank. A new generation of hybrid dry calibration tables are being made that separate water pumping and vacuum systems and offer variable lengths to install calibration tooling. This adds the versatility that most processors require. This versatility can include adjusting spray intensity in different sections to optimize cooling as required, or allowing for different levels of vacuum or even different water temperatures in different sections of the tank.
In conclusion, these new dry vacuum calibration systems can offer the control of dimensions and size that end users have come to expect at higher rates and lower energy costs that processors are seeking. New calibration table designs make this both possible and convenient.