chillroll basics

2000 PFFC PEER REVIEWED PAPER OCTOBER 20001

2000 PFFC PEER REVIEWED PAPER

A BASIC UNDERSTANDING OF CHILL ROLLS

Michael PuhallaNew Castle IndustriesNew Castle, PA 16107

Abstract

Historically there has been little information published on the basic principles of chill roll design, construction andfunction in the extrusion process. Most presentations focus on polymer processing relating to screw and die design.The purpose of this paper is to provide a basic understanding of chill roll design and function.

Introduction

The primary function of a chill roll is to act as a heat transfer and finishing device in the extrusion flat sheet, castfilm and coating operations. Although the processes are different, the design criterion, which must be considered, isthe same. The key design principles considered are, roll load (PLI), face deflection and heat removal rate. Howeverthe functional aspects of the roll varies considerably from process to process. Roll construction, materials andsurface finish are predicated by the process and product. All of these factors must be considered in order to achievea properly designed and functional roll.

Design

The first of the primary design principles to be discussed is roll load often termed PLI (pounds per linear inch). Thedefinition of PLI is the amount of load applied to web by the actuating system. The actuation system used in thisdiscussion is either air or hydraulic. To determine the PLI, first the area of the actuating cylinders must bedetermined. The following formulation (1) is used to determine the cylinder area.

A = (πD2)/4 (1)A = cylinder area (in2)D = cylinder diameter (in)

The cylinder area, system pressure and roll face width are used in equation (2) in determining roll PLI.

PLI = (2 x A x P)/L (2)PLI = roll loading (pounds per linear inch)A = cylinder area (in2)P = system pressure (p.s.i)L = length of load on roll face (inches)

Note: If the actuating system is not direct, but has a mechanical lever arm, a force multiplier might be required.

The determined PLI is an estimate only; the actual load in the nip in fact is slightly different because of the fluidnature of the polymer. However, the accuracy of this number is sufficient and has been historically accepted andused.

Once the load across the roll face factored, the deflection of the roll shell can be found. For the purpose ofdiscussion, only a double shell spiral baffle roll construction is used. (See Figure 1) To determine the deflection ofthe roll assembly, the following equation (3) is applied.


γ = 5 x PLI x L4 (3) 384 x E x (Io + Ii)L = length of load on roll face (inch)E = modulus of elasticity (p.s.i.)Io = moment of inertia outer shell (inch4)Ii = moment of inertia inner shell (inch4) Io or Ii = π[(D4) – (d4)]/64

D = outside shell diameter (inch) D = inside shell diameter (inch)

The deflection calculation assumes a uniformly loaded beam supported on two ends. If the roll was not a doubleshell construction, the equation would contain only one moment of inertia. Noted in the equation, longer rolls of thesame diameter will have greater deflection, which increases by the fourth power. To compensate for the increasedroll face length, the diameter of inner and/or outer shell needs to be increased. Not only must the mechanicalaspects be considered, but also thermal which are paramount in achieving a functional roll.

Figure 1. Double Shell Spiral Baffle Roll.

Heat Removal Rate

A primary purpose of a chill roll is to remove heat from the product prior to any downstream operations. Several keyfactors are required to determine the heat removal or cooling capabilities of a roll. They are, type of polymer beingextruded, maximum output of the extruder, melt temperature at the exit of the die, the desired temperature off theroll and the specific heat of the polymer. These are then used in the following formula (4).

Q = ∆T x Qn x Cp (4)Q = heat removal (Btu per hour)∆T = temperature differential on and off the roll (Fo)Qn = output (pounds per hour)Cp = specific heat (Btu/lb/Fo)

The calculated heat removal rate provides what heat can be removed for a specific set of process conditions.Therefore, before making a significant process change, like increasing the total rate, heat removal rate must beconsidered. The heat removal rate is one of two important thermal factors. The second is coolant flow rate.


Coolant Flow Rate

The coolant flow rate impacts the roll’s ability to remove the required heat and temperature variation across the rollface. A chill roll is similar to a car radiator, sufficient flow must occur for proper cooling or catastrophic enginefailure will happen. Equal to this would be product sticking to the roll face. Therefore the coolant flow rate isimportant to a chill roll, however this important parameter is often overlooked. Because of the importance of propercoolant flow, it’s recommended that a flow meter be installed on the inlet side of the roll. Using the meter, the actualflow rate can be compared to the calculated flow for the determined heat removal rate. The calculated flow rate isthe requirement to raise the coolant 1o F in temperature. The calculated coolant flow rate is found using equation(5).

GPM = Q/k (5)GPM = flow rate (gallons per minute)Q = heat removal rate (Btu per hour)K = conversion factor (500.4)

The coolant flow rate found in equation (5), is then used to find the theoretical temperature variation across the rollface. This determined by formula (6).

∆T’ = GPM/GPM’ (6)∆T’ = theoretical temperature variation (Fo)GPM = calculated flow rate (gallons per minute)GPM’ = measured flow rate (gallons per minute)

A larger than expected temperature variation on an existing roll may indicate restricted flow in the roll or coolantsystem. Often times a large variation in roll face temperature will not permit the product to properly release from aroll. These locations are often termed hot spots. Hot spots are a result of a blockage in the cooling passages of a roll.The restrictions can be removed by an internal acid flush or by removing the outer shell and cleaning the internalpassages. The second repair is costly because a new outer shell is required.

The presented thermal calculations provide useful guidelines on the cooling capabilities of a chill roll; but they donot account for line speed, product wrap angle or thickness. These parameters influence the true cooling ability ofthe roll but require much greater analysis than the intent of this paper.

Figure 2. Cooling Can Roll

Construction

There are three types of common chill roll construction; double shell spiral baffle (Figure 1), cooling can (Figure 2)and single shell (Figure 3). The cooling can is probably the oldest design and produces poor mechanical and thermalabilities. The single shell design has improved mechanical and thermal abilities. The double shell spiral baffledesign provides the most effective mechanical and thermal functions. The internal spiral baffles provide support to


the outer shell but also improved coolant flow characteristics. The baffles can be designed to provide increasedcoolant velocity through the roll, which improves cooling efficiencies.

Figure 3. Single Shell Roll

Material

The typical chill roll is manufactured from carbon steel (i.e. A106, C1020). Carbon steel provides sufficient physicaland thermal properties and is readily available in either tube or plate form. Shell materials come in three forms,seamless tube, rolled and welded plate, and forged. Seamless steel pipe is the most common form of material usedfor a shell ≤ 24” in diameter. Rolls requiring a shell >24” in diameter are rolled and welded steel plate. A forging ofany size can be made; however because of their expense, they are often limited to rolls requiring a seamlessconstruction >24” in diameter.

Because non-heat treated steel is soft, rockwell Rc 20 to 25, its common to harden the outer roll face. The hardeningof the roll face will provide protection against high contact loads, which result in roll face deformation. Theprotection of the shell face is frequently done with 420 stainless steel or flame hardening. The stainless steel isoverlaid onto the shell via sub arc welding. The final hardness of the stainless steel is rockwell Rc 48 to 52. Themethod of flame hardening a roll face is less frequent. This method provides an equally hard surface, but mostmanufactures are not equipped to do it. Secondly, the case depth of the flame hardening is usually less than thestainless steel This reduces the ability to resurface the roll multiple times. However flame harden rolls are used inengraved rolls where durability is necessary.

Beyond the protection from destructive loads, protection from corrosive elements maybe desired and/or a goodfinish is a must. Historically industrial hard chrome plating has been the material of choice in achieving thesedesired results. Chrome plating offers several key advantages. The first is low cost when compared to othermaterials like nickel. Chrome plating is commonly available service and because of its use in many other products,production cost are low. Chrome plating also provides good corrosion resistance. Lastly, because of its hardness,980 Vickers, chrome can be easily manipulated to produce various durable roll surface finishes. Although chromedoes provide good corrosion resistance, its does not match nickel or ceramic. Chrome has microscopic pores, whichpermit corrosive materials to pass through to the substrate. Once this happens the substrate under the chrome canoxidize causing the chrome to lift or etch. Nickel its not pores like chrome providing improved corrosion resistance.When processing resins like PVC or PVDF nickel or ceramic coating should be considered.

Finishes

Rolls must not only help size and cool the product they must also provide a surface finish. The surface finish on aroll can be highly polished or rough like sand paper. Each finish provides a desired outcome and purpose. Finishesare expressed in the form of a number followed by a suffix. The suffixes are Ra, Rms, and Rq. The suffixes indicatea particular surface roughness scale and are defined in standard ANSI B46.1-1978 s. Although these scales are foundin the same ANSI standard they are not necessarily correlated. The scale for Rms and Rq are equal, but not to Ra.The Ra scale is approximately 1.10 times different from a Rms or Rq measurement. For example, a 10 Rms equals


11 Ra. The numerical value provides a reference number correlating to a particular surface finish, like wire gage sizedoes to wire diameter. The higher numerical value the rougher the surface. Using these indicators a certain rollfinish can be defined.

The four common roll finishes are, polished, matte, release and engraved. The polished or mirror finish measures .5Rms. The mirror finish provides a smooth polish look to the product. This finish is obtained by a series of precisiongrinding and polishing operations. The mirror finish roll is often found in the production of sheet products. Thematte finish ranges from 30 to 200 Rms. This finish is similar to sand blasted corroded steel. The matte finish is usedfor releasing products from the roll or to provide a textured surface. A release finish is used for releasing productfrom the roll as the name implies. Two common release finishes are gloss and pocket release. The gloss finish has anappearance similar to the matte, but appears to be glossy as though it was clear coated. Paper coating processorsoften use a glossy finish to provide a release and to obtain a smooth printable surface. The pocket release provides aunique finish to the product when released. The primary use for this finish is in coating process. The last andprobably the most expensive finish are engraved. An engraved finish provides a specific repeatable pattern on to theroll and is applied via acid or mechanical engraving process. A “green” roll is supplied to the engraver. A “green”roll is completely manufactured with the exception of the engraved pattern. The outer shell on the roll is typicallyforged or seamless pipe. Variations in the surface, like a weld seam, tend to produce a slightly different pattern.Because the pattern is applied to a steel shell, flash platting with chrome is recommended.

Geometry

Although the engineer may have properly designed the roll, the manufacturer must pay particular attention to certaincriteria. The key criteria are total indicated run out (T.I.R.) and face straightness. Total indicated run out is the totaleccentricity of the roll body to the bearing journals (Figure 4). The bearing journals are were the roll’s bearings aremounted. This mounting surface must be concentric to the roll body or the roll will rotate eccentric. Excessive T.I.R.leads to gauge variation. To produce the proper T.I.R. the manufacturer must finish the roll face using the bearingjournals as the reference surface. The definition of straightness means the roll face does not taper from one end toother. A severely tapered roll face will produce product thick on one end and thinner on the opposite edge.

Figure 4.

Although designers and manufacturers take great strides to provide a proper functioning roll, they cannot design aindestructible roll. Many calls have been placed claiming a roll was designed or built improperly. But upon furtherinvestigation, the roll was used in production and damaged. Frequent causes for a roll to lose its geometry areallowing cold product to pass through a closed nip. Stalled rolls are twisted by the torsional force excerted on thenby the roll drive system. These excerted forces are brief, but can and will exceed the design capabilities of the roll.


Conclusion

A basic understanding of how a roll is designed, its function and construction can greatly enhance ones knowledge.This fundamental knowledge can aid in decisions when purchasing a new roll, evaluating a new product or processand trouble shooting the next related problem.

References

1. Richard Palmer, “Roll Design A Review Of The Basics”, ANTEC 1999, Volume 1, pages 304-3082. Machinery’s Handbook 22nd Revised Edition, Industrial Press Inc.

This paper was accepted for abstracting and publication in "the PLACE" in the October issue of Paper, Film & FoilCONVERTER.

chillroll basics

Documents