Polymer Testing 21 (2002) 313–317www.elsevier.com/locate/polytest

Test Method

PVT testing of polymers under industrial processingconditions

Sekhar Chakravorty*

The Materials Centre, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK

Received 25 June 2001; accepted 8 August 2001

Abstract

The development of a PVT (Pressure–Volume–Temperature) equipment for measuring polymer properties at indus-trial processing conditions is described. Results of rapid cooling (200°C/min) and high pressure (20–160 MPa) isobarictemperature scan tests in HDPE and PP are presented. The development of a new test cell that allows PVT measurementson thin samples (�2 mm thick) has been reported with typical results from a commercial grade PP material. A newgeneration of industrially relevant fast cooling PVT data on 10 widely used polymers have been collated on CD thatare aimed to benefit designers and engineers in the polymer processing sector. The database may also aid furtherdevelopment of the commercial plastics processing software packages. 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Shrinkage, warpage and sink marks are importantproblems of plastic injection moulded products. Ther-mally induced volume shrinkage of the part during coo-ling and thermal stresses produced during filling andpacking has been related to these defects [1]. For aninjection moulder to take on premium work requiringtight tolerances with increased confidence and reducedscrap rates, an improved understanding of the shrinkageand warpage behaviour in plastic components may berequired. The ability to predict behaviour accurately canbe expected to provide the moulder with the benefits ofreduced cycle times, reduced time to market and reducedscrap rates with consequent commercial benefits.

Commercial software packages, such as Moldflow/C-Mold, are available which have the capability of pre-dicting the shrinkage and warpage behaviour in plastic

* Tel.: +44-(0)20-8943-6919; fax:+44-(0)20-8943-6046.E-mail address: sekhar.chakravorty@npl.co.uk (S.

Chakravorty).

0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0142-9418 (01)00089-7

components. Fig. 1 shows a typical 3-D mesh display ofa hubcap, on which volume shrinkage predictions weremade at slow and fast cooling rates (at 10°C/min and at260°C/min). At slow cooling rate, the predicted shrink-age was�1.2% in the central region as compared to�1.8% in the outer region, whereas at the fast coolingrate, the predicted shrinkage at the centre was�5% ascompared to�3% in the outer region. A difference ofup to 400% in shrinkage distribution prediction value inthis component is obtained due to the cooling rate effect.Accurate prediction is, therefore, only possible whengood quality credible data relevant to industrial pro-cessing conditions are made available to the softwarecompanies. This is also particularly true for a moulderlaunching new products to ensure mould design is rightfirst time. In addition, it is also important to be able tooptimise processing conditions and to modify the choiceof polymer with ease each time requirements change.

Plastics processing, such as injection moulding, is arapid, high pressure process where both cooling rate andpressure play critical roles for the final componentdimensions. The degree of crystallinity in semi-crystal-line polymers depends on the rate of cooling, molecularstructure, shear and pressure history, presence of nucleat-ing agents etc. Properties of semi-crystalline polymersare particularly sensitive to the rate of cooling. Under

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Fig. 1. 3-D mesh display of a hubcap from Moldflow(diameter 410 mm, thickness 4–5 mm).

fast cooling rates, the transition to the crystalline phaseoccurs at lower temperatures [a downward shift of�100°C in transition temperature due to fast cooling ina polyamide has been reported [2]]. The degree ofchange from slow cooling rates to fast cooling ratesdepends on the molecular structure, the degree to whichthe material shrinks, and also due to the change inmaterial density. Changes induced by crystallisation, inparticular, have been found to significantly influenceshrinkage and warpage values. Several other factors suchas visco-elastic behaviour of polymers, variable mouldtemperature profile due to non-uniform cooling, and mol-ecular and fibre orientations, also contribute directly toactual shrinkage and warpage values in moulded parts[1].

Pressure is known to induce molecular order and shiftpolymer melting points (Tm), crystallisation temperatures(Tcrys) or glass transition temperatures (Tg) to higher tem-peratures. It is, therefore, important to understand thepressure dependence of Tm, Tcrys and Tg from both practi-cal (i.e. machine settings) and theoretical (i.e. free vol-ume, compressibility effects) viewpoints [3].

Cooling rate plays a critical role in polymer processingbecause materials experience rapid cooling when trans-ferring from the melt to the solid state [4]. Like pressure,cooling rates also influence the parameters (Tcrys and Tg)but in this case, their positions are shifted to lower tem-peratures. Since most Pressure–Volume–Temperature(PVT) data on polymeric materials are only availableunder slow cooling rates (maximum 10°C/min) whichrarely occur in commercial injection moulding, there is

a need to introduce fast cooling rate effects into bothPVT measurements and modelling software.

2. Results and discussion

The demand for accurate and relevant data by thedesigners, process engineers and software manufacturerswho evaluate the relationships between PVT behaviourof polymers at high pressures and at high cooling ratesis increasing. Until recently most of the experimentalPVT data in the literature were reported either in equilib-rium state [5] or at slow (maximum 10°C/min) coolingrates ([4,6]) that are far from being the actual conditionsexperienced during polymer processing. In addition, thesample size has also not been given proper considerationin the reported PVT results, hence, the data publishedcannot be related directly to the actual component partthickness.

In response to these measurement problems, theNational Physical Laboratory (NPL) in the UK hasdeveloped a rapid cooling PVT equipment (jointly withSWO/Haake GmbH) by incorporating a number of noveltechnical features. The measurement of critical PVTparameters is now possible, for the first time, at fast coo-ling rates (up to 300°C/min), at high pressures (up to250 MPa) and at high melt temperatures (up to 420°C)with the measurement capability down to sub-zero tem-peratures (to �100°C) on samples of thickness down to2 mm in iso-baric and iso-thermal conditions, bringingthe NPL PVT testing close to industrial polymer pro-cessing conditions [7,8].

An example of the fast cooling PVT plots from a highdensity polyethylene (HDPE) is shown in Fig. 2 undera range of pressures between 20–160 MPa and at a coo-ling rate of 200°C/min (the temperature, in this case, wasmeasured in the metal barrel, �1 mm away from the meltsurface in a 7.8 mm diameter test cell). The isobaric coo-ling plots show the typical “S” shape at all given press-ures indicating phase transition regions. These regions(and hence, the shape), however, become sharper in slowcooling tests where the crystallisation rate is relativelyslow (Fig. 3). Likewise, semi-crystalline polymers wouldshow phase transition regions more clearly than amorph-ous materials since the density would increase with crys-tallisation. In general, the onset of crystallisation greatlyvaries between about 250°C to 120°C in most polymers(150°C to 120°C in HDPE) due to differences in materialtypes as well as applied pressures and moderately depen-dent on the cooling rates. Increasing pressure increasesthe onset temperature of crystallisation. The end of crys-tallisation in most polymers takes place between the tem-peratures 50°C and 100°C (50°C–70°C in HDPE). Theshrinkage due to melt compressibility is generally foundto be 10–20% in polymers (�10% in HDPE) with thehighest values obtained in TPE, PS, ABS and PVC. The

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Fig. 2. Fast controlled cooling PVT test results to sub-zero temperature: HDPE, cooling rate 200°C/min, sample thickness 3.9 mm,isobaric temperature scan runs.

Fig. 3. Slow cooling PVT test results in HDPE. Cooling rate 5°C/min, sample thickness 3.9 mm, isobaric runs.

overall shrinkage in the test samples is found to varybetween 20–25% in all polymers including a change of10–15% in value due to crystallisation.

In order to assess the correct shrinkage values in thespecimen, it was necessary to ensure that the entire samplebecomes solid in rapid cooling tests before the test is over.This led to further development of the NPL equipment,which enabled it to operate under sub-zero temperatures(i.e. down to �100°C) from the melt temperature (see Fig.2) at a constant cooling rate of up to 300°C/min and atpressures between 30 and 250 MPa. This particular

improvement is also expected to indicate the extent of anytemperature gradient present in the rapidly cooled testsamples, thus benefiting the PVT measurement from betterunderstanding of the morphology and related shrinkagebehaviour in polymeric materials [9,10].

A new design of the test cell based on piston/cylindertype geometry (Fig. 4) has been developed at NPL whichenables PVT measurements to be carried out on thin annu-lar samples (�2 mm thick). The PVT data thus generatedwould be closer to typical component part thickness. It isalso expected that any temperature gradient present in the

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Fig. 4. Test cell sealants for 2 mm thick annular sample (inthe centre).

thicker test samples would be drastically reduced in thiscase. Results of PVT tests on 2 mm thick samples of poly-propylene (PP) at 400 bar pressure are shown in Fig. 5at various cooling rates using the newly developed testcell. The typical “S” shape of the isobaric cooling plotsindicate the onset and the end of crystallisation regions inthis material and show a maximum difference of �5% inshrinkage value (specific volume) compared to the dataobtained from similar PVT tests on 3.9 mm thick samplesin a 7.8 mm diameter test cell (Fig. 5).

3. Conclusions

The NPL PVT equipment is the only one of its kind inthe world that had been modified to make measurementsof critical PVT parameters close to injection moulding

Fig. 5. PVT test results in 2 mm annular and 7.8 mm diameter cells, polypropylene, 400 bar pressure, isobaric runs.

conditions. This facility is expected to help the plasticsindustry to optimise processing parameters. A new gener-ation of industrially relevant fast cooling PVT data on 10widely used polymers is now available on CD, whichcould benefit further development of the computer simul-ation software packages such as Moldflow. Engineers anddesigners in the polymer processing sector may gain betterunderstanding of the shrinkage and warpage behaviour intheir products from these new industrial PVT data.

Acknowledgements

The author would like to thank the members of theIAG on “Measurement Methods Relating to the Pro-cessing of Plastics” for their kind support and advice andto the following staff members of NPL for their help anduseful discussions: C.S. Brown, P. Alder, L. Sharma andC. Hobbs. The work reported here was carried out underthe EID “Measurements for Processability of Materials(MPM) Programme” fi nanced by the United KingdomDepartment of Trade and Industry.

References

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[2] M. Mahishi, C-Mold News 10 (4) (1998) 4.[3] S. Yang, M. Ke, ANTEC’93, 1993, p. 2182.[4] R.Y. Chang, C.H. Chen, K.S. Su, Polym. Eng. Sci. 36 (13)

(1996) 1789.

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[5] P. Zoller, D. Walsh, Standard PVT Data for Polymers,Technomic Co, 1995.

[6] S. Chakravorty, C.S. Brown, NPL report DMM (D)262,June 1995.

[7] C.B. Hobbs, C.S. Brown, NPL report CMMT (A)163,February 1999.

[8] S. Chakravorty, NPL report CMMT (A)244, December1999.

[9] S. Chakravorty, Plastics and Adhesives News newsletter,issue 6, NPL, 2000.

[10] S. Chakravorty, Plastics and Adhesives News newsletter,issue 7, NPL, 2001.