Chapter 2: go shonaike & sg advani: Advanced Polymeric Materials Structure and Property Relationships, crc press, Boca Raton, 2003




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Chapter 2: GO Shonaike & SG Advani: Advanced Polymeric Materials - Structure and Property Relationships, CRC Press, Boca Raton, 2003.

In-process monitoring for control of closed mould techniques

for the manufacture of thermosetting matrix composites


John Summerscales,

Department of Mechanical and Marine Engineering, University of Plymouth

Plymouth, PL4 8AA, United Kingdom.


It is the aim of this chapter to review the technologies available for monitoring the progress of the processes employed for manufacture of continuous fibre-reinforced thermosetting matrix composites in closed mould techniques. The manufacturing techniques of especial interest are usually referred to by the generic names Liquid Moulding Technologies (LMT) [1] or Liquid Composite Moulding (LCM) [2], and more specifically as Resin Transfer Moulding (RTM) [3, 4] or Resin Infusion under Flexible Tooling (RIFT) [5]. A variety of other acronyms have been used for these processes [see 6]. The techniques may also be applicable to other composite manufacturing processes (e.g. compression moulding, vacuum bagging and autoclave cure).


The physical changes which take place during composite manufacture include heating and cooling, pressure changes and fluid flow. Each of these can influence the performance of the finished component in a variety of ways. Insufficient heating may result in a low glass transition temperature and consequent creep under stress. Inadequate pressure or flow may result in high levels of porosity or large voids. On-line sensor systems, which do not compromise the integrity of the finished component, would be a useful step towards improved quality in composites manufacture. The chapter concludes with a brief survey of advanced process control and optimisation techniques.


§1 Temperature


Elevated temperatures are often used in LCM processes to promote more rapid flow of the resin and shorter cure cycles. A balance must be struck between the reduction in resin viscosity (and hence more rapid flow) and a faster cure (and hence a shorter gel time) in order to achieve an optimised mould filling cycle. Thermal management of the process may include either injecting cold resin into hot moulds or injecting hot resin into cold moulds. The rate of heat transfer between the mould walls (and the preloaded reinforcement) and the resin may be critically important to the success of the process. The resin flow and cure of epoxy resins in RTM and RIFT, and hence heat transfer, are relatively slow processes. However, for unsaturated polyester resins in these processes, and especially for processes such as Structural Reaction Injection Moulding (SRIM), there may be a significant generation of heat (exotherm) as the polymerisation/cross-linking reaction proceeds.


1.1 Thermocouples [7]

A thermocouple is a sensor for measuring temperature. Two different metals/alloys in contact produce a small voltage at a given temperature. This voltage is measured and interpreted by appropriate instrumentation. Thermocouples are available in different combinations of metals (calibrations). The most common calibrations are summarised in Table 1 with the standard limits of error (SLOE: the percentage of the temperature being measured expressed in degrees Celcius, referenced to 0ºC).
Table 1: Common thermocouple calibrations [8]

Calibration

Couple

Temperature range (ºC)

SLOE (%)

B


Pt – 30%Rh : Pt – 6%Rh

800-1700

±0.5

E

Ni- 10%Cr : Cu - Ni

0-900

±0.5 or ±1.7

J

Fe : Cu - Ni

0-750

±0.75 or ±2.2

K

Ni – 10%Cr : Ni – 5%Al - Si

0-1250

±0.75 or ±2.2

R

Pt – 13% Rh : Pt

0-1450

±0.25 or ±1.5

S

Pt – 10% Rh : Pt

0-1450

±0.25 or ±1.5

T

Cu : Cu - Ni

0-350

±0.75 or ±1


Each calibration has a different temperature range and working environment. The maximum temperature varies with the diameter of the wire used in the thermocouple. The following criteria should be used in selecting a thermocouple and sheath: temperature range, chemical-, abrasion- and vibration-resistance, precision/accuracy and installation restrictions. The American Society for Testing and Materials (ASTM) has published a manual on the use of thermocouples in temperature measurement [9].


Thermocouple probes are normally available in one of three junction types:

  • In the exposed junction, the junction extends beyond the sheath to give a fast accurate response. The sheath insulation is sealed where the junction protrudes to prevent penetration of moisture or gas which could cause errors. Exposed junctions are limited to non-corrosive and non-pressurised applications.

  • In a grounded probe, the thermocouple wires are physically attached to the inside of the sheath. This probe gives an intermediate response time in the absence of lag due to the thermal mass of the sheath.

  • In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is relatively slow, but the thermocouple wire is physically insulated from the thermocouple sheath.

The response time for a thermocouple is defined by the time constant: "the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilise at 100% of the change value" [7].


1.2 Thermistors [10]

Thermistors are thermally sensitive resistors and have either a negative (NTC) or a positive (PTC) resistance/temperature coefficient. Manufactured from the oxides of the transition metals (manganese, cobalt, copper and nickel), NTC thermistors are temperature dependent semiconductor resistors with a range of -200°C to + 1000°C. They are supplied in glass bead, disc, chips and probe formats. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity. The excellent combination of price and performance has led to the extensive use of NTCs in applications such as temperature measurement and control, temperature compensation, surge suppression and fluid flow measurement.


Wen et al [11] reported that cross-ply continuous-fibre polymer-matrix composites were found to be thermistors due to the decrease in electrical resistivity with increasing temperature. The resistivity was the contact resistivity between cross-ply laminae. The activation energy of electrical conduction was up to 0.12 eV. Each junction between cross-ply fibre groups of adjacent laminae was a thermistor, while the fibre groups serve as electrical leads.


1.3 Infrared thermocouples [7]

Infrared thermocouples receive the heat energy radiated from the target object and use the thermo-electric effect to convert that heat passively to a millivolt electrical potential. The devices are self-powered, using the incoming infrared radiation to produce a signal through thermoelectric effects. The signal is subject to the non-linearity inherent in the thermal physics of radiation. Infrared thermocouples normally work in a limited part of this non-linear response. They are usually accurate to within 2% or 5% of the values from a conventional thermocouple over their working range.


Temperatures may also be measured using a variety of other techniques including resistance probes, optical fibres or thermo-chromic coatings (e.g. liquid crystals or paints). However, some of these techniques have limited applicability to on-line monitoring and control.

§2 Pressure [12]


The International atmosphere is defined as 1.013 bar (101.3 kPa or 14.7 lbf/in2). Typical process pressures may be from –1 atmosphere (~-1000 mbar) for a “rough” vacuum up to 7 atmospheres in autoclave cure of thermoset matrix composites in an autoclave. A pressure measurement system normally comprises three main components:

  • pressure transducer

  • excitation power supply

  • signal processor

There are four principal designations for pressure as indicated in Table 2.


Table 2: The principal pressure designations.

Designation

Imperial

Measurement

Gauge (the most common type)

PSIG

pressure with respect to the local atmospheric pressure

Sealed

PSIS

pressure relative to one atmosphere at sea level regardless of local atmospheric pressure

Absolute

PSIA

pressure relative to a perfect vacuum

Differential

PSID

pressure difference between two input pressures


The three primary considerations in selecting a transducer are the working pressure, working temperature range and the chemical environment. The recommended range for a pressure transducer is 125% of the normal working pressure providing that the proof and burst pressure ratings give an adequate safety margin. The transducer diaphragm is usually of silicone (greater accuracy but limited to low pressures and dry gas media) or stainless steel (high pressures, superior corrosion resistance and wide media compatibility).


Millivolt systems are of relatively low cost as the signal conditioning functions are remote from the actual sensor. This allows the transducer to be relatively small in size. These transducers are compatible with most strain gauge and load cell instrumentation. Millivolt systems require a regulated power supply. Amplified voltage systems contain instrumentation grade amplifiers within the transducer. They are commonly used in laboratory applications and electrically noisy environments. They are generally compatible with process controllers and computer interface systems. Current loop systems are particularly well suited to applications where long distances and high noise immunity are required. This is achieved by building a 4-20 mA current transmitter into the transducer. Wire runs may be over 300 m (1000 ft) with virtually no signal degradation. These systems are often selected for use in the process industry and interface directly with industrial process controllers, most computers and data acquisition systems.


There are two types of calibration equipment. A precision dead-weight tester provides an accurately known signal to the system for comparison. Electronic digital gauge standards are also available. Generally calibration should be traceable to a national standard.


§3 Viscosity and flow rate


It is critically important in the processing of most polymer matrix composites that the resin flows. The resistance to flow is measured as viscosity, normally expressed in terms of a relationship between an applied shearing stress and the resulting rate of strain in shear. The optimum viscosity depends on the manufacturing process. The unit for viscosity is Pascal seconds (Pa.s) although older texts use poise (1 Pa.s = 10 poise). It is normal to quote viscosity for fluid resin systems in mPa.s or centipoise (the numerical values in these units are identical).


In liquid composite moulding (LCM) processes, e.g. resin transfer moulding (RTM) and resin infusion under flexible tooling (RIFT), the resin flows long distances in comparison to most other processing techniques. Rudd et al [1] suggest that the most significant practical limitation on the suitability of a resin system is imposed by viscosity. Resins with extremely low viscosity may be unsuitable for LCM processes as they may result in high porosity or gross voidage. Becker [13] quotes an upper limit for viscosity in RTM of 800 mPa.s. The non-injection point (NIP) is defined as a viscosity of 1000 mPa.s [14]. At this viscosity, the flow front is effectively stationary at the low pressures used in LCM processes. High-performance resins for RTM (e.g. PR-500 or RTM-6) may be solid at ambient temperatures and require heating to reduce the viscosity to a level suitable for the process.


For vacuum-bagging of wet lay-up carbon fibre/epoxy composites, Stringer [15] suggested that cure should be allowed to progress (dwell) until the viscosity is in the range 75-165 poise (7500-16500 mPa.s) before the vacuum is applied. If vacuum is applied at low viscosity (<75 poise), then high levels of voids result. If vacuum is applied at higher viscosity (>165 poise), then low void levels and low fibre volume fractions (<49%) result. In the defined viscosity range, the void content was low (<2%) and the fibre volume fraction was up to 58%.


In-line viscometers are available commercially. Reference to specific devices below is purely an indication of the equipment currently available and should not be taken as an endorsement of that product or of its suitability for use with resin systems. Indeed, one system is described as "ideal for food and dairy industry applications". Conversely, the exclusion of a product does not imply that it is unsuitable for use with resin systems.


The Nametre Viscoliner® [16] consists of a transducer and an electronic controller. The transducer is “electromagnetically driven into torsional oscillation and is servo-controlled at a constant torsional amplitude of oscillation of 1 µm”. A change in viscosity of the material surrounding the probe results in a change in the amount of electrical power required to maintain a constant oscillation amplitude. This change in power is a mathematical function of the fluid viscosity. The Viscoliner systems are able to measure viscosities in the range 0.1-1,000,000 mPa.s, with calibration (traceable to NIST standards) covering up to five continuous decades of viscosity


Cambridge Applied Systems [17] viscometers use patented technology with one moving part, a piston, driven electromagnetically through the fluid in a small measurement chamber. Proprietary circuitry then analyses the piston travel time to measure absolute viscosity and monitor temperature. The total two-way travel time is an accurate measure of the fluid absolute viscosity to within a ±1% specification.


The Brookfield SST100 Process Viscometer [18] uses a co-axial cylinder measurement geometry for viscosity at defined shear rates. It has a viscosity range: 350-250,000 mPa.s and shear rate range: 7.5-225/s.


A specific apparatus, the Vibrating Needle Curemeter (VNC) [19], monitors the viscosity of the fluid resin system and early stiffness changes after gelation. A steel needle, suspended in the formulation, is vibrated along the vertical axis by an electrodynamic vibrator and resistance to needle movement is recorded. The shape of the trace is dependent on the driver frequency. This technique is normally confined to the laboratory bench rather than being used in-situ during the process.


§4 Flow front position
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