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Lightweight Design worldwide

, Volume 10, Issue 6, pp 34–41 | Cite as

Highly Dynamic and Homogeneous Heating of Organo Sheets

  • Mesut Cetin
  • Christian Herrmann
  • Stefan Schierl
Production Organo Sheets
  • 184 Downloads

The infrared heating technology developed by KraussMaffei with an intelligent hardware and software solution enables fast and homogenous heating of fiber-reinforced thermoplastic semi-finished products. To evaluate the heating process, the company has developed a methodical approach for determining the heating quality of organo sheets.

Components made of fiber-reinforced thermoplastics are very much in fashion. Component manufacturers are placing increasing importance on thermoforming continuous-fiber-reinforced semifinished products and on other lightweight construction technologies, such as the Resin Transfer Molding (RTM) process, as well as on short-fiber injection molding. High strength values with an extremely light weight and short cycle times are among the chief requirements for fiber-reinforced thermoplastic components.

Despite the many advantages of IR heating technology, heating organo sheets is associated with a couple of challenges.

Injection molding of thermoplastic plastics is particularly well suited with regard to the requirements mentioned above because it enables high levels of design freedom, short cycle times as well as the implementation of continuous fiber structures and metallic inserts.

The Fiber Form technology developed by KraussMaffei combines the injection molding process with thermoforming continuous-fiber-reinforced thermoplastic semifinished products, known as organo sheets. These are large-format semifinished products usually made up of glass, carbon, aramid or composite fiber layers embedded in a thermoplastic matrix, for example polyamide (PA) or polypropylene (PP).

The manufacturing process can be divided chronologically into the process steps for heating up, thermoforming and over-molding. The first process step of heating up the organo sheets is of central importance for the entire process. The heating of organic sheets above the melting temperature of the thermoplastic matrix can be carried out by contact heating, convective heat transfer in a convection oven or by infrared (IR) radiation. From a cost-effectiveness standpoint and with regard to high productivity, IR heating technology has proven its value in both research and industry. The major advantages of IR technology compared to heating in the convection oven are lower investment costs and a higher power density of the IR emitters, which leads to significantly shorter heating times.

KraussMaffei uses an overriding control system that can take multiple pyrometers into account.

Despite the many advantages of IR heating technology, heating organo sheets is associated with a couple of challenges. Fast heating with conventional control systems frequently leads to overheating and a non-uniform temperature distribution on the component surface, because the reaction times of standard controllers are too long for the highly dynamic heating behaviour of the IR emitters. In addition, using IR heating technology in a closed system requires intelligent thermal management, because the repeated heating processes feed thermal energy cyclically not only to the organo sheet, but also to the IR oven. The thermal energy leads to a continuous temperature increase within the IR oven, which prevents heating operations from being static and thus reproducible. During production, this can cause overheating of the oven and thus scrap or even bring production to a standstill. Therefore, heating with IR emitters requires an intelligent hardware and software solution tailored to the FiberForm process.

Intelligent Heating of Organo Sheets

With the target of fast and uniform heating of the organo sheet, KraussMaffei has developed an IR heating technology specifically for the FiberForm process. Its outstanding features include intelligent hardware and software solutions as well as complete integration into the MC6 control system for injection molding machines.

Compared to conventional, commercially available IR heating technologies for fiber-reinforced thermoplastic semi-finished products, KraussMaffei uses a closed IR oven system for this application.

Requirements-based energy dissipation that compensates for the high energy input via the IR emitters ensures a static energy state of the system before and during series production, which brings about a reproducible heating process in a closed system.

The general structure of the KraussMaffei IR oven for heating fiber-reinforced thermoplastic components is displayed in Figure 1. For vertical positioning of the IR oven above the fixed (mold) platen (FP), the connection to the injection molding machine is made using the standardised Euromap 18 interface. This enables the IR oven to be retrofitted into existing injection molding machines. To increase energy efficiency and to comply with existing safety requirements, the IR oven is completely insulated. In addition, ventilation flaps and other patent-pending measures enable controlled dissipation of energy from the IR oven. The listed advantages can be used independently of the component geometry, because the modular oven concept permits the oven size to be adapted to the component with great flexibility. In addition, the number and positioning of the pyrometers can be configured easily, for example for a product change. There are two options for the transfer route of the heated organo sheet from the IR oven into the mold. After the sliding door is opened, the heated organo sheet can be passed from the IR oven into the injection mold (from the front) or by opening a flap at the bottom of the oven housing (from the bottom). The type of oven opening used depends on the automation concept.
Figure 1

General structure of the vertical KraussMaffei IR oven (© KraussMaffei)

The KraussMaffei control system ensures fast, uniform and synchronous heating up of the organo sheets and reliably prevents overheating of the organo sheet surface. The difference between this control system and conventional ones is in the number and assignment of the controllers as well as in the control system itself, which is illustrated below in the example of the IR oven with a heating surface size of 750 x 750 mm (nine IR emitters).

Conventional IR heating panels with control usually have only one pyrometer (Pi), which is made available as a measuring element for the controlled system. For this purpose, the controlled system combines all nine radiators to create one heating zone (Zone 1). During the heating process, the control variable, the surface temperature of the organo sheet (TACT-P2), is measured by pyrometer P2 and compared to the control variable, the target temperature (TTAR), Figure 2. The comparison provides the control variation used by the controller and the actuator to calculate the control variable, the output of the emitter. Pyrometers 1, 3 and 4 are used solely for surface temperature measurement and are not considered by the control system.
Figure 2

Commercially available control systems for heating fiber-reinforced thermoplastic semi-finished products (© KraussMaffei)

In contrast to commercially available control systems, the KraussMaffei control system can take multiple pyrometers into account and assign these to multiple zones. With regard to the example on the IR oven with the heating surface size of 750 mm × 750 mm (nine IR emitters), all four pyrometer temperatures (P1 to P4) are made available as the control variable for the controlled system, where different IR emitters are assigned to each individual pyrometer. This results in a total of four zones, Figure 3, each of which has its own control loop and is additionally incorporated into an outer control loop. Through this system (outer control loop and inner control loop), the zones are not only controlled separately (own temperature in the inner control loop), but also take into account the heating behaviour of the other zones (outer control loop). The outer controller takes the heating rates of all zones into account, resulting in a synchronous increase in the rise of the temperature curves. This synchronism causes all zones to reach their setpoint temperature at the same time.
Figure 3

KraussMaffei control system for heating fiber-reinforced thermoplastic semi-finished products (© KraussMaffei)

If we compare the result of the heating process of both control systems on a fiber-reinforced semifinished product with a size of 670 x 490 mm, with a vertical hanging organo sheet and a heating setpoint temperature of 225 °C, it becomes evident that conventional control systems have substantial shortcomings in both the heating time and in the heating process, Figure 4. The temperature curves measured using pyrometers 1 to 4 show strong deviation and different heating rates, resulting in heating of the organo sheet that is not uniform, Figure 4 (top). A synchronous and uniform heating behaviour is provided by the KraussMaffei control system Figure 4 (bottom). The specially developed intelligent control of the individual zones, separately and relative to each other, calculates the control variables for the different IR emitters, ensuring that the material is heated synchronously across all zones.
Figure 4

Comparison of the heating curve for an organo sheet: top with a conventional control system, bottom with the KraussMaffei control system (© KraussMaffei)

Another advantage of the KraussMaffei control system is evident in the heat-up time, which is shorter than that of a conventional control system. This has a positive effect on the overall cycle time.

Thermographic measurements are a great way to evaluate the heating quality of a organo sheet. Thermography is an imaging process for displaying the surface temperature of objects. In this process, the amount of heat emitted by an object is detected by an IR camera and temperature values for the object in the camera’s field of view are calculated taking into account the emission coefficient of the measurement object surface [1]. KraussMaffei defines the heating quality of composite sheets by using the homogeneity/heterogenity of the temperature distribution on the surface of the organo sheet and the deviation of the temperature values from the heating setpoint temperature. The thermographic measurement is carried out after the heating process, for example outside the IR oven, before the handover to the mold. Due to basic principles of physics, evaluation of the temperature distribution over the entire surface is not useful. Border areas or cutouts are more strongly exposed to natural convection. This leads to faster cooling of the organo sheet in these areas compared to the central area. In light of these facts and because no evaluation methods have been in place to date, KraussMaffei has developed an easy-to-use evaluation method that makes it possible to evaluate and compare the heating quality of composite sheets of nearly any geometry and size using key figures. In doing so, depending on the thickness of the organo sheet, the bore diameter or the geometry of cutouts, a measurement surface is defined on the organo sheet that can be used in the evaluation of the heating quality. Individual measurement fields are positioned on this measurement surface. The number and size of the measurement fields results from the geometry and size of the composite sheet.

Subsequently, the temperature distribution in the individual measurement fields is analysed and tested to see whether the temperature values are within the defined tolerance range. In the next step, the average temperature values of the individual measurement fields are used to determine the key figures for evaluating the heating quality. Firstly, common statistical methods are used to analyse the scatter of the average temperature values of the individual measurement fields to evaluate the homogeneity/heterogenity of the temperature distribution over the entire organo sheet surface. Secondly, the deviation of the average temperature values of the measurement fields from the specified temperature set value is considered. In doing so, the tolerance range defines the requirement for the heating quality. It is necessary to consider that the defined temperature set value has to be corrected according to the cooling rate and the time of the measurement. The lesser the temperature fluctuation between the individual measurement fields, for example the more homogenous the temperature distribution, and the lesser the difference of the average temperature values of the measurement fields to the temperature set value within the tolerance range, the better the heating quality. The dimensionless key figure for the heating quality Hq can be defined according to Eq. 1. In doing so, the two key figures for homogeneity and the deviation from temperature set value are considered equivalent. Depending on the requirement, however, a weighting of the key figures can be carried out. To enable comparisons of the heating quality of various composite sheets, the two key figures are set in relation to the respective specified tolerance range. Therefore, a lower key figure stands for better heating quality.
The heating quality of the KraussMaffei hardware and software solutions for IR heating technology can be determined in accordance with Figure 5, Table 1 and Table 2, in accordance with Eq. 1. At a specified heating setpoint temperature of 225 °C, a defined temperature tolerance range of ± 5 % (TTol max = 236.25 °C), a cooling rate of 6 K/s and a temperature measurement 4 s after the end of heating, we have a key figure for the heating quality of 0.36 for the KraussMaffei control and of 1.02 for the commercial control system.
Figure 5

Thermographic measurement of organo sheets; left: KraussMaffei control, right: commercial control (© KraussMaffei)

Table 1

Comparison of the heating quality between the KraussMaffei control and commercial control for heating fiber-reinforced thermoplastic semi-finished products (material: Tepex dynalite 104-RG600(x)/47 %, 0.5 mm) (© KraussMaffei)

KraussMaffei-Control

Commercial Control

T1

 

203.5

201.7

T2

 

203.0

193.8

T3

 

199.3

192.8

T4

 

199.2

187.4

T5

[°C]

202.0

200.1

T6

 

204.3

192.1

TSetpoint

 

225.0

225.0

TSetpoint, corrected

 

201.0

201.0

TTol max

 

236.25

236.25

tEA

[s]

4

4

vc

[K/s]

6

6

Hq

0.36

1.02

Table 2

Parameters of the test setup (© KraussMaffei)

Parameters

Value / data

Width of the organo sheet [mm]

490

Height of the organo sheet [mm]

670

Thickness of the organo sheet [mm]

0.5

Material of the organo sheet

Tepex dynalite 104-RG600/47%

Thermographic camera

Optris PI 400 (62° objective)

Distance to the measurement object [mm]

1300

Ambient temperature [°C]

23

Emission value

0.97

Heating set temperature [°C]

225

Control of the heating emitters using a single pyrometer Figure 4 (top) has been proven to result in lower heating quality. The thermographic measurement shows a highly inhomogenous heating pattern, Figure 5 with a maximum temperature difference of the measuring ranges of 14.3 °C. Compared to this, the maximum temperature difference with the KraussMaffei hardware and software solution is just 5.1 °C.

The heating quality is very important for the forming and injection molding process, as is shown by the cooling-off curves in Figure 6 and Figure 7. These show that the cooling has a nearly linear curve at a cooling rate of 6 K/s and the temperatures of the individual areas do not equalise during cooling. However, a homogenous temperature distribution is indispensible for good thermal forming and a good positive plastic bond. From this, we can infer that the heating process is of fundamental importance for the quality of the overall process. An inhomogenous temperature distribution on the organo sheet surface (poor heating quality) can no longer be corrected (linear cooling behaviour) and can have a negative effect on component quality in the forming or injection molding process.
Figure 6

Cooling behaviour of the heated composite sheet with commercial control (© KraussMaffei)

Figure 7

Cooling behaviour of the heated composite sheet with KraussMaffei control (© KraussMaffei)

Summary

The heating process of composite sheet is central process step of FiberForm technology. The lack of commercially available oven solutions and the lack of evaluation methods for the heating quality were reasons for developing the KraussMaffei IR heating technology.

The specially developed IR heating technology is characterised by intelligent hardware and software solutions and has been proven to demonstrate fast, homogenous and synchronous heating of composite sheets, Figure 5. The KraussMaffei IR heating technology is a major component of product-oriented manufacturing solutions for series production of fiber-reinforced components [2] and is fully integrated into the KraussMaffei MC6 injection molding control system. This permits connection to Plastics 4.0 products from KraussMaffei, such as DataXplorer. This gives the user a means of not only simple adjustment and monitoring of the heating process on the machine, but also transparent documentation and analysis of all quality-oriented process data, such as heating curves of the composite sheets, using DataXplorer. One application example was introduced at K 2016: The data of DataXplorer are assigned to each specific component using a QR code — this enables seamless tracking of all product and process data for each component. No less importantly, the heating zones can be assigned flexibly to the existing pyrometers Figure 4 (bottom) to implement simple adaptation of the oven for another product or a changed geometry. All heating parameters are stored along with the injection molding parameters, enabling simple startup after a product change.

References

  1. [1]
    Bernhard, F. (Ed.): Handbuch der Technischen Temperaturmessung (Handbook of Technical Temperature Measurement). VDI-Buch, Berlin: Springer Vieweg, 2014Google Scholar
  2. [2]
    Cetin, M.; Herrmann, C.; Fenske, S.: Automatisierungskonzepte zur Herstellung faserverstärkter Thermoplastbauteile (Automation concepts for manufacturing fiber-reinforced thermoplastic components). In: lightweight.design 10 (2017), No. 2, pp. 38–43CrossRefGoogle Scholar

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2017

Authors and Affiliations

  • Mesut Cetin
    • 1
  • Christian Herrmann
    • 1
  • Stefan Schierl
    • 2
  1. 1.KraussMaffei Automation GmbHOberding-SchwaigGermany
  2. 2.KraussMaffei Technologies GmbHMünchen-AllachGermany

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