Skip to content

Frequently Asked Questions

Does the company undertake projects?

Yes, the philosophy of the business is to solve process problems for the polymer industry, principally via the use of rheology.

This may involve anything from testing incoming materials for variation and implementing a QA programme, to increasing the line speed of a cable coating extruder using process simulation techniques in conjunction with material data.

In many cases, the produced materials have gone on to be impact tested in order to establish their toughness and fitness for purpose.

Back to top

What is rheology?

Rheology is the science of deformation and flow taken from the Greek, rheo to flow and ology the science of. The definition is sufficiently broad as to encompass all flow regimes from solid to liquid.

Back to top

Why is rheology important?

Polymer production processes such as injection moulding and extrusion exploit deformation and flow in order to form useful products. A polymeric material often begins the conversion process as either a pellet or a powder, whereupon it is heated and deformed into a more useful shape-a product.

If we can understand the way in which the material deforms, or more importantly, resists deformation (viscosity), we can optimise the process.

Back to top

What is rheometry?

Rheometry is the science of reproducing deformation and measuring the consequences on materials of interest.

Back to top

What is a rheometer?

A rheometer is an instrument used in the science of rheometry. A rheometer reproduces deformation under controlled conditions representative of those found in real production processes (temperature and deformation rate) and measures the consequences.

The consequences are usually conveyed in terms of stress and viscosity from which a host of rheological parameters can be further computed.

Back to top

What is a capillary rheometer?

A capillary rheometer is a specific type of rheometer which computes rheological parameters from pressure measurements taken prior to the flow of material through a small capillary.

The rheometer typically uses a speed controlled piston to drive material from a heated bore through a small (1mm diameter typically) capillary. Pressure measurements are made at each piston speed using diaphragm transducers in the bore prior to entry of the capillary die.

Back to top

What is viscosity?

Viscosity is resistance to deformation and can be sub-divided into shear and elongation. Shear viscosity is envisaged as the resistance of material laminates sliding past each other in a flowing fluid.

Since polymers in the melt phase are often visco-elastic, they also stretch, and the resistance to stretching is termed the elongational or extensional viscosity. Both viscosity measurements are important as they are sensitive to different aspects of molecular architecture.

Back to top

What is elasticity?

Most polymers are visco elastic, thus when they stretch in the direction of flow, they subsequently attempt to return to their original condition. This is achieved via restoring forces established in the direction of deformation.

The magnitude of these restoring forces, measured as first normal stress difference (N1), is a measure of the elasticity. A consequence of elasticity is die swell, where material attempts to return to its original condition after exiting a die and becomes different to the die size.

Back to top

What is batch to batch variation?

Most feed polymer is assumed to be identical, however sometimes one batch can be distinctly different to another. This is not normally problematic provided the variation is understood and expected, if it is not it needs to be measured and its consequences on the process predicted.

Back to top

What is process simulation?

As the name suggests, most polymer production processes can be simulated, that is the die geometry and operating conditions can be recreated in a virtual environment and the consequences of change predicted. In this way processes can be optimised with little real process trialing.

Back to top

What processes can be simulated?

Cable extrusion, wire coating, spiral dies, coathanger dies, multi-layer film extrusion, co-extrusion, pipe & profile extrusion and side fed dies. Via the use of the 3D simulation software, almost any flow geometry can be recreated and simulated.

Back to top

I work for an extrusion company, how can rheometry help me?

Rheometry can bench mark your polymers and establish whether there is any batch to batch variation. Is your process under control? Could you improve your incoming material QA procedures by using tighter rheological measurements?

How has the batch to batch variation occurred, has the supplier sent you the wrong grade or is there something wrong with your process? Does a new grade suddenly not work like the old one, is there a recipe change you don't know about?

Are you suddenly getting surface distortions or sharkskining? Do you want to increase your output? Could you achieve this with our current extruder or do you need to upgrade?

Rheometry has been used in all of the above typical cases with real manufacturers, so if you have encountered any of these problems it is likely rheometry can help.

Back to top

I work for an injection moulding company, how can rheometry help me?

Obtaining and formatting Moldflow viscosity data. Examining the effects of new material viscosity data on the resulting Mouldflow simulations.

Back to top

My polymer is a confidential development

The business works with many developmental systems, confidentiality is assumed in all cases.

Back to top

What sort of polymer can be tested?

Essentially almost all thermoplastics used in production. PE, LLDPE, LDPE, HDPE, PP PC, PA, EPDM, PET, PS, PSu, polymer blends, polymer alloys, filled polymers, bio-polymers and many, many more. If it can be melted and a pressure can be measured on driving the material through the die, characterisation by rheometry stands a good chance of being successful.

Back to top

Can non-polymers be tested?

Yes, many non-polymeric materials can be tested on a capillary rheometer, such as food stuffs (chocolate, mayonnaise, and pasta (spaghetti),), ceramics and oils (mineral, silica). As above, if a pressure can be measured prior to the material being driven through a die system, rheometry stands a good chance of success.

Back to top

Materials selection exercises

Can be undertaken using the vast experience gleaned over many years spent testing materials and simulating a variety of different processes. The company has helped many businesses to source alternative materials.

Do you want to change your material, or can the current one be used more efficiently? Who are the manufacturers? How much does it cost? What chemical resistance does an alternative polymer have?

Back to top

Can you set up a QA system on incoming material using rheological measurements?

Yes, a whole host of measurements can be used to monitor the integrity of either your incoming or produced polymer. Extensional viscosity has been used recently by a client in order to track molecular weight distribution changes in their polymer where MFI and shear viscosity was insensitive.

Shear viscosity is used by another client to monitor any changes in molecular weight of their incoming polymer.

Back to top


01299 251 914

or use our contact form

Our postal address:
Polymer Testing Ltd
Holly House
DY11 7TE  UK