Applying Established Oven Profile Techniques to Rotomoulding

Transcription

1 Applying Established Oven Profile Techniques to Rotomoulding Monitor, Understand & Optimise your Process Dr Steve Offley Oven Product Manager Datapaq Ltd, Cambridge, UK The following paper reviews the application of well established temperature profiling techniques used extensively over the last 20 years to the emerging Rotomoulding market. Technical developments made in core markets such as paint finishing, electronic assembly, food processing, heat treatment and ceramic manufacture are described with direct reference to the demands and benefits of the Rotomoulding application. Use of profiling to understand, optimise and validate Rotomoulding cycles will be discussed. Oven Monitoring Industrial Need Oven monitoring, or profiling as it is often described, has become common place in industry today. Whether monitoring the temperature of a car body, as it travels through a convection paint cure oven, or monitoring the core temperature of a chicken fillet through a spiral steam oven, the desire for accurate product temperature through process has never been greater. Determining product temperature and or the environmental temperature of a thermal process helps understand the workings of the process. With such knowledge, the process can be optimised, and a reference can be set against which process control can be monitored. Profile data collected regularly from any process gives the tool to guarantee quality, optimise productivity and when problems occur, as by definition at some point they will, perform rapid fault finding and validate any corrective action undertaken. Profiling Principle The basic Temperature Profiling principle is really a very simple concept. As illustrated below in figure 1, a temperature data logger is sent through a process with the product being processed. Thermocouples are attached to the product at user specified locations, or within the working environment of the product/oven, which measure and store collected data at regular intervals. Protection from the process whether being heat, cold or water is provided by a thermal barrier which surrounds the datalogger. Post process, collected temperature data is retrieved from the datalogger and transferred to a computer, where custom software provides profile review and analysis. Figure: 1 Basic Temperature Profiling Principle In most situations oven profiling is done as an ongoing process control strategy, collecting process data independently of the processes own control mechanisms. Performing a run once per day or shift and analysing temperature data at its completion (30 to 60 mins after start of run) in most cases satisfies such requirements.

2 Where such post run analysis becomes problematic is in processes such as ceramic firing where the profile may take up to 2-3 days. Waiting 3 days to confirm that your kiln is operating correctly is a major disadvantage which in recent years has been eliminated by the development of real time analysis systems. Temperature measurements taken inside the kiln are transmitted live back to the PC via a RF signal, where the product /environmental profile can be seen gradually building over time. With such technology instant decision making can be performed and, if appropriate on the spot process adjustments made. With the many benefits of profiling discussed previously it became quickly evident that the application of Rotomoulding could greatly benefit from the same type of approach. Curing a polymer in a mould has many chemical and thermodynamic similarities to that of curing a paint or powder film on a metal substrate. The rotational movement of the rotational machines also creates the same monitoring challenges as a conveyor oven or a rotating batch oven, in which hardwired options employing trailing cables can be difficult. Environmental issues of heat and possibly water as part of the cooling process also mimic the challenges faced in the food industry where protection from steam, water and even cooking oil are required. For this reason early in 2005 Datapaq launched the RotoPaq system. Combining the tried and tested technologies applied in other industries the RotoPaq system provides a reliable work horse for regular monitoring of processes and as an ideal development tool for optimising new rotational mould oven recipes and mould design. Datapaq RotoPaq System The key components of the Datapaq RotoPaq system are detailed in the following section. Datalogger Taq21 Providing 10 measurement channels (type K) and RF compatibility the Tpaq21 provides the state of the art profile logger. With an accuracy of ±0.3 C and sampling down to 0.1 s the system guarantees the highest resolution and accuracy profile possible. Fitted with a rechargeable NiMH battery it is capable of operating continuously for 40 hours using RF (barrier performance permitting). To guarantee data integrity, data storage is used as a backup to RF operation. Profile data can be collected in cases where RF operation is not feasible for what ever reason. Other loggers like the EasyTrack are available without RF capability (See Fig 1). Telemetry RF Fitted internally within the TPaq21 is a radio transmitter working on MHz frequency approved by Euro EN The radiated power of 10 mw allows transmission over 200 m in free space. Receiver antenna options include standard or UGEF aerials. Figure 2: Tpaq21 Figure 3: Telemetry Kit Thermal Barriers For standard dry Rotational moulding processes the TB5000-RP has been selected as being the optimum thermal barrier in terms of size, weight and thermal performance. The barrier provides dual protection via a ceramic wall insulation packing the inner faces of the barrier. This material interestingly is also used in an aeroplane flight recorders black box. This insulation hinders the conduction of heat from the oven to the logger which operates safely and accurately up to 70 C. The secondary protection is supplied by two metal heat sinks. These cases are filled with a phase change material which operates at room temperature so does not need freezing prior to use. Once the inner barrier temperature reaches 58 C the heat sink chemical phase changes from a solid to a liquid. This reaction takes time over which the temperature is maintained safely at 58 C. The TB5000-RP is supplied with fixing brackets which can be used to mount temporarily or permanently the barrier to the Rotational moulding machine.

3 Figure 4: TB5000-RP Weight: 7.0 kg Size: 130 x 190 x 292 mm Thermal Protection 100 C 14.5 hrs 200 C 4.5 hrs 300 C 3 hrs For rotational mould processes incorporating water cooling of the mould the TB5016 thermal barrier is recommended. The barrier fittings allow the use and interchange of exposed junction thermocouples. Figure 5: TB5016 Weight: 9.0 kg Size: 118 x 200 x 398 mm Thermal Protection 100 C 17.0 hrs 200 C 5.0 hrs 300 C 3.0 hrs Although not designed for 24/7 process control, during lengthy method development or process optimisation work the barrier must provide continuous protection. Obviously by the cyclic nature of Rotomoulding process incorporating heating, cooling and reloading phases the temperature is never constant. From Figure 6 it can be seen that the TB5000 can manage extended operation periods without problem. With a peak process oven temperature of 240 C the system completed an 8 hours processing period. After this period the logger had reached a safe 57 C only. Figure 6: Thermal performance of TB5000-RP in process. Shift of 8 hours / 9 cycles. Figure 7:TB0046 Thermal Protection 35 C Weight: 2.4 kg, Size: 70 mm x 195 x 240 mm For polymer suppliers the value of a profile system other than for R&D activities is as a tool to allow validation of customer processes. For this purpose real time analysis is not compulsory and often requires only one cycle to be measured. This being the case a barrier like the TB0046 (Figure 7) could be used with the 6 channel Datapaq 9000 logger. Size, shape and weight lends itself to easy transportation, even as hand luggage on a domestic or international flight.

4 Thermocouples Choice of thermocouple relies heavily on process conditions and or intended placement of the thermocouple. For process temperatures below 265 C PTFE cables can be used reliably giving flexible yet robust operation. Above 265 C, thermocouple cable should be either glass fibre or mineral insulated. The type K thermocouple supplied is specified to ANSIMC96.1 special limits of error. Up to 300 C this gives a measurement accuracy of the system of ±1.5 C or better. For general use exposed junction thermocouples are the most universal, they can be taped or mechanically fixed to the location where measurement is needed. Such probes can be used for oven, environment, and mould surface or polymer temperature measurement. For mould external surface measurement (ferrous) it is feasible to use magnetic attachment probes. These probes can be placed over the exterior of the mould to check for uniformity of heating. This will obviously have a knock on effect on the uniformity of heating of the polymer skin internally within the mould. (a) (b) (c) Figure 8: Exposed Junction Probes (a) PTFE (up to 265 C) (b) Glass Fibre (up to 700 C) (c) Mineral Insulated (up to 1200 C) (a) (b) Figure 9: Magnetic Probes (a) Oven Environment (b) Mould Surface Software Collecting temperature data is obviously only a part of the profiling story. The data needs to be interrogated and analysed to give the complete process picture. Profile software is used to interpret data quickly, easily and efficiently and allows important process decisions to be made either live from a RF signal or with the complete downloaded data file. Analysis functions helpful in the optimisation or process control task include: Raw data Max Temperature and Time Reached Time above Temperature Slopes Temperature Rise and Fall times Datapaq Value Index of Cure (see later) Overlay / Tolerance Curves Process Alarms Figure 10: Datapaq Insight Profile software

5 Profile Benefits Getting the cure process correct in the Rotomoulding process is critical to the quality of the product both cosmetically and physically as illustrated in Table 1. Table 1: Common Polymer Cure Problems Over Curing Degradation of resin Degradation of release agent Product sticks to mould Discoloration Productivity low throughput Too Slow Cooling Poor impact resistance Poor release from mould Crystalline structure Under Curing Product quality - strength Pin holes Bubbles Rough inner surface Too Fast Cooling Warped Product Amorphous Structures Non uniform Cooling Warped product Poor release from mould Profile Analysis Converting raw data into meaningful information Table 1 illustrates the essential need for process temperature monitoring in the Rotomoulding application. Figure 11 illustrates the RotoPaq profile system in operation collecting such data and a typical profile trace that can be obtained. Fig 11: Typical Generic Rotational Moulding Set-up and Profile Trace obtained from a Rotomoulding customer located in France.

6 Oven Environmental Monitoring Measurement of the oven environmental temperatures gives a base line to the day to day operation of the oven itself. Most oven temperatures are governed by a control thermocouple. Placement of such probe can affect significantly the temperature which the mould experiences since its location if at the outlet may measure temperature after some degree of cooling. From experience I have seen an oven set-point at 200 C yet the mould experience an air temperature of up to 250 C. If the oven set-point is varying by definition the mould heating characteristics will also change. Monitoring the set-point temperature with time may show a slow deterioration in performance. This data may highlight the need for service before catastrophic failure and or predict when product quality may start to be affected. Mould Mapping Within a uniformly heated oven the temperature of the inner surface of the mould is governed by the thermal mass (dimension and material) and heating characteristics of the mould itself. This is particularly an issue for large complex moulds where structural supports may also act as heat sinks drawing heat away from the mould wall. Hot or cold spots in the mould need to be avoided to prevent problems in differential curing of the polymer and associated warping problems. Mould mapping can be performed as part of method development strategies by placing thermocouples either externally on the mould or even internally on the inner mould surface. As you can see in figure 11 thermocouples 3, 4 and 5 Figure 12: Patch Probe placed on the mould surface give different profiles. Thermocouple 5 being at the axis of the mould due to the higher mass of metal in that region is shown to heat up significantly slower than top and bottom surfaces. For mapping of the inner surface of the mould it is advised that Patch probes be used. Fitted with their own adhesive patch they can be stuck easily and efficiently to the mould surface. For such investigation there is no requirement for polymer so the probes can be retrieved post testing. Polymer Cure Optimisation To assist with cure confirmation in the paint and powder coating industry, Datapaq has developed an Index of cure calculation known as the Datapaq Value (See Figure 13). The calculation is based on the first order Arrhenius relationship. Three independent cure schedules Cure Temperature) which from physical testing such as differential scanning calorimetry have been proven to give the same level of cure 100 Cross linking are used in the calculation. From these values an exponential fit is applied to the data giving the time temperature cure relationship over the complete process temperature range. Using this relationship every temperature data point making up the profile can be converted to an equivalent time at a known cure schedule. For example 10 s at 205 C may be calculated to be equivalent to 13 s at 200 C. Performing this operation over the whole profile irrespective of its shape allows total cure to be assessed against the relationship. If the sum of the time temperature contributions gives a Datapaq value of 100 this means the profile gives identical cure to heating the coating for 200 C for 10 mins (Mid Target Cure Schedule). A value over 100 indicates over cure and under 100 under cure. Obviously for general sintering processes of standard Polyethylenes where no actual chemical reaction occurs the Datapaq Value is redundant. Where the interest lies with such calculation it is suggested is in the development of new novel Rotomoulding materials which involve some degree of chemical reaction associated with polymerisation, cross linking or catalysed reactions involving cross linking agents such as Peroxide. During process optimisation studies and cure recipe set-up, essential for all new products, monitoring the Datapaq value could provide a very accurate way of optimising cure conditions. For such studies the patch probe or exposed junction probe would be fitted to the inner mould wall and the thermocouple junction positioned so that it was located at the centre of the polymer skin. This may mean that the probe needs to cut out of the product or sacrificed but gives excellent data to base R&D activities on.

7 k = A e -E/RT k = the reaction rate constant A = frequency factor (constant) E = activation energy R = gas constant T = Temperature Min = Temperature at which cross linking activated Max = Temperature polymer damage occurs Three Cure Schedules C C C Temperature C Max (Damage) Index of Cure 100 Min (no Cure) Time (mins) DPV = (100/t o )(e -(A)(Tx-To) )(t x ) DPV Datapaq Value A Arrhenius constant calculated for exponential line Tx Temperature under investigation tx Temperature under investigation To Temperature taken from exponential ISO Cure Line 100 (Mid Point) 200 C to Time taken from Exponential ISO Cure Line 100 (Mid Point) 10 mins Figure 13: Datapaq Value Index of Cure Polymer Cure Optimisation To illustrate the application of Datapaq value see figure 13 showing the calculated Datapaq Value for two different shaped profiles profiles. Figure 14: Datapaq Value change with Profile Shape Conclusion Discussions made in this paper suggest very strongly that many of the established profiling techniques routinely applied to the paint and powder coating industry can be directly applied to the principles of Rotomoulding. Application of profiling can help significantly with process optimisation, control and validation of this application guaranteeing product quality, optimum productivity and as a result satisfied customers.