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Precision profile extrusion

Modified on Saturday, 31 January 2015 02:28 AM by mpieler Categorized as Consultants Corner
Holding close tolerances on rigid PVC and other rigid materials profiles has been a problem as long as profiles have been extruded. The expanding use of rigid plastics in building products has put pressure on the industry to come up with extrusions with tighter dimensional specifications than ever before because of the requirement for the profiles to fit together in a system to make a variety of products. The most rigid requirements are demanded by window manufacturers who are fitting together from 8 to 9 up to 20 profiles to manufacture double-hung, side slider and hinged windows which must meet government specifications for air and water infiltration. The windows must be tight enough to resist infiltration yet loose enough to function easily for the consumer.

The European markets have been supplied with an ever increasing complexity of window and door systems for over 25 years. Pressure for tight tolerances came early. These pressures were answered by European extrusion machinery manufacturers with the process which, I call dry sleeve calibration. Acceptance of dry sleeve calibration in the American market has been slow, primarily because of the tooling cost and the lack of compatibility of U.S. downstream equipment with the process. In addition only large requirements for a profile would justify the expense on capital purchases and maintenance of dry sleeve calibration systems; volumes approaching those like pipe and siding. Volumes required by the window industry and the requirement for precision justify dry sleeve calibration.

The dry sleeve calibration system for precision profiles consists of a single or twin screw extruder, an extremely well designed streamline extrusion die, a series of vacuum calibrators (similar to a sizing sleeve in a vacuum tank) 2, 3 or 4 which are mounted on a specially designed calibrator stand to supply specifically metered vacuum and cooling water to the calibrators; air cooling and radiant heating stands to keep the profile straight; a puller with pulling force capability from 2 to 4.5 metric tons, and a reliable cutoff saw and stacking system.

The key to the dry sleeve calibration system us design of the extrusion die, its size relationship to the calibrator and the size of the calibrator which is designed to anticipate the shrinkage of the specific compound being extruded. The extrusion die is typically constructed of stainless steel and designed to afford maximum internal stream lining. No dead spots or eddys can be tolerated. The die is designed to distribute the material to fill all the walls and internal webs uniformly. Often webs are not just for structural strength, but to accomplish balancing of the flow of material to the die orifice opening. Flow directors and die lands are designed into a series of plates in the die to effect streamlining of the die and pressure drops which ultimately succeed in a uniform volume of material flow to all portions of the die orifice opening. The desire to minimize the die swell and drawdown of a given compound is treated by allowing the die swell to occur mostly inside the extrusion die, having a final melt restriction internal to the die and a die lip land orifice larger than the restriction. The material expansion typical of an elastic mass released from compression occurs in the final die land and delivers melt to the calibrator, which is positioned ‘300" to .750" from the extrusion die orifice opening. Die swell control and proximity of the calibrator to the extrusion die have a predictable effect on the shrinkage and straightness of the final profile be cause such treatment minimizes strains in the part due to drawdown and non-uniform cooling. A typical main frame window profile extrusion die would only be 1.5% to 2.5% over size of the final dimensions of a profile, allowing for little or no drawdown of the extrudate prior to its entry into the calibration sleeve. As the main frame requires the most footage in a window system the highest obtainable output consistent with keeping tolerance is desirable. The higher outputs are obtained through use of multiple calibrators each one in turn cut smaller on its ID and finally to a dimension which allows the part its final shrinkage into the dimensional specification.

The dry calibrator sleeve design is critical in that it must deliver a means of thermal energy removal from the extrudate uniformly to assure straightness and dimensional uniformity in the final product. Location of holes and slots on the internal path of the calibrator is critical to assure surface contact of the extrudate to cooling surfaces in a pattern consistent to a given profile, its wall thickness and areas of maximum tolerance.

By the same token location of cooling coring in calibrator is critical to uniform cooling. The orator table usually has multiple sources of vacuum and water so that each source can be varied in a way to assure optimum performance in obtaining extrusion rate and precision dimensional tolerance.

Most recent calibration systems which represent state-of-the-art use four calibrators, and, with a very complex profile and wall thicknesses of .120"-.135 (4-4.5MM) obtain line speeds of 8’ to 9.75’ f.p.m. (2.5 to 3 meters per min.) of rigid PVC.

The typical custom profile extrusion firm is not likely to jump at the chance to purchase a $150,000 to $350,000 profile extrusion line because of the low volume nature of most of their business. Emerging markets in the building industry do represent volumes of business and precision requirements which justify the cost of the extrusion system.

- Louis Fow Jr. & Floyd Bush, PLASTICS ASSEMBLY CORP.

See also:
  • Answers - preheating feedstocks for extrusion
  • Performance of twin screw extruders
  • PVC testing
  • Structural changes in PVC due to extrusion

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