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November 2010

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Thermoplastics | design feature

Jac Gofers of Promolding describes the challenges that were met in the design, development and manufacturing of an innovative full polymer aircraft catering trolley

Plastic makes its mark in aircraft catering The project

later advances targeted on weight saving led to the use

Airlines are

The design of the traditional aluminium panel aircraft

of aluminium sandwich panels. The focus was on

expected to

trolley had remained unchanged for more than 50 years,

making the aluminium stronger so that the panels

increasingly

despite airlines’ requests for trolleys with less weight

could be constructed of thinner walled sheets. This

switch from

and less maintenance. In 2006, Helm and Lambrecht

reduced the weight to 22­–23 kg per cart. Compared with

metal to

illustrated the importance of weight saving. They

the previous weight of 35 kg, this was a good result, but

thermoplastics

calculated that 100 kg less weight saves 4.5 million

not the limit of what can be achieved.

litres of fuel and 1 million tonnes of CO2 over the

To decrease the mass yet more, there are basically

lifetime of an aircraft. Product development and

two options: make the walls even thinner or change the

plastics processing company, Promolding then

materials. Both approaches were tried. Making walls

undertook a complete redesign; its concept was an

thinner resulted in lighter trolleys, but they were no

innovative thermoplastic polymer trolley that would

longer robust. It is easy to dent the thin-walled panels

save weight and avoid extensive maintenance. Three

and once dented, the trolleys are less appealing or, even

years later, the product was enthusiastically received in

worse, no longer usable. There is a limit to how far you

the market. This article charts the design and manufac-

can go with thinning the sheets, and foil was not doing

turing journey to this successful outcome.

the job.

From massive aluminium to aluminium-foil panels

Trials for plastic

The establishment of the commercial aircraft industry

combining aluminium with thermoset sandwich panels.

created an opportunity for specialised companies to

The aluminium panels were exchanged for thermoset

design a standardised trolley that would serve meals

panels without modifying the design. This resulted in

and beverages onboard in a convenient way. The

lighter, more expensive trolleys, but the drawback was

development of these trolleys stabilised with carts

that the panels were hard to repair.

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typically made from thick-walled aluminium panels,

In other cases, companies made hybrid trolleys,

Another issue occurred when those trolleys under-

which meant that each cart weighs approximately 35 kg.

went certification tests. Because the design had not

Although some airlines still use these heavy trolleys,

changed, the door locking mechanism still functioned on

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November 2010 | injection world

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design feature | Thermoplastics

The complete design of

ance and toughness. PEI performs better for stiffness

the thermoplastic

and strength. The designers chose PPSU (Radel R7700)

trolley

made by Solvay Advanced Polymers rather than PEI (Ultem) because l modulus and yield stress are not significantly different l the stiffness and strength of the product can be improved by a clever design l the durability and image of the polymer trolley must under no circumstances be limited by the possibility of fracture due to impact or a lack of chemical resistance l improvements in chemical resistance and impact properties cannot be reached by modifications to the design.

Once the material was selected, the company started to design the trolley based on the properties of PPSU. The task was divided into body design and door design; both projects ran in parallel and both ended in a patentable design.

For the body, the extremely high loads and test demands led to a double-walled rectangular extruded tube with inner ribs in a configuration determined using finite element analysis. In this way, the tray and drawer holders could be extruded in the same process in one part.

For the door, the main challenge was the innovation l dimensions are according ATLAS standard

of the locking mechanism. Instead of the existing

l inner space is to have a minimum of seven standard

construction of three interconnecting points where the

drawers

local tensions would make the plastic fail, it was

l the maximum allowable load is 100 kg, which

necessary to spread the load over more points. This

includes the weight of the trolley.

ended up in a multiple 14-point interconnecting slider

Authorities bound:

that is operated through an ergonomically located grip

l the trolley is to meet all air safety regulations,

handle on the left top of the door front.

particularly those regarding strength B flammability,

After the design of those two important components,

smoke, toxicity and heat release and fire containment.

the focus turned to the hinge of the door; the door-

Manufacturing bound:

frame, which has to interface with the extruded body;

l the trolley is to be recyclable, cradle-to-cradle

and the assembly of all parts and components. This

l be mass producible

ended in the completed design that is shown above.

l be low maintenance.

This POD placed extremely high demands on the

During the design phase, Promolding realised that the

and choice of materials. The company’s research and

equipment required to extrude and inject these large

development department intensively researched

parts with thin walls (10 mm) was not available within

materials applicable for these applications. It reached

the company. After a search for outside manufacturing

the conclusion that basically only two types of polymer

expertise, it partnered with Bemis Contracts Group

would be candidates: PEI (Ultem 9075/9085) and PPSU

(Sheboygan Falls, Wisconsin, USA), which had the

(Radel R7700). The choice between PEI and PPSU was

facilities and the courage to take on such a demanding

based on four factors:

task. When entering into discussions about the work

l elastic modulus (stiffness)

with the Bemis engineers, it became clear that

l strength (yield stress)

extrusion of the body as one part (a rectangular tube)

l chemical resistance

was too risky. Promolding had to redesign this part into

l toughness/impact properties.

two identical C-shaped parts that were to connect on

PPSU performs better in terms of chemical resist22

Production is the hard part

construction and therefore on production technology

injection world | November 2010

bottom and top. In that way, manufacture was challengwww.injectionworld.com

design feature | Thermoplastics

Fill time = 5.128(s)

Last but not least In the aerospace industry, an important step is to certify the product with the regulations and this is a time consuming, complex and costly process. It took more than one year to fulfil all tests and obtain feedback from the authorities, but the result was the first thermoplastic aircraft trolley in the world that complies with the most rigorous tests in the regulations. It is capable of carrying 135 kg of contents and withstanding all forces.

This certification proves that the design and the production technology delivered a robust product that meets requirements for l lightweight l cradle-to-cradle recyclability

Scale (10 in)

Using 10 drop gates, the flow is fairly balanced

l high impact resistance (not sensitive to denting) l ergonomics l low maintenance.

In addition, this plastic trolley can carry radio frequency identification (RFID) tags, which an alumini-

ing, but less risky. The other parts for extrusion, that is,

um trolley cannot. This feature was not a design

the top tray, door filler panel and connecting H-profile

parameter, but during the design and pre-production

were not difficult and did not need any modifications.

phase it became an increasingly important attribute.

Making the extrusion die proved challenging. An

Airlines want to improve their catering logistics and

extrusion die for PPSU RADEL R7700 [for these dimen-

control their assets and RFID is indispensable for this.

sions (90 cm wide, legs of 14 cm legs, double walls with

Moreover, because it is possible with RFID to track and

wall thickness of 1 mm and inner ribs of 0.5 mm) had

trace the trolleys all over the world (assuming that the

never been made. It presented Bemis and the mould

necessary infrastructure is installed), leasing compa-

maker with an extremely challenging job, which they

nies are interested in leasing these plastic trolleys to

completed with a new mould concept in due time. The

the airlines, which offers airlines an option to exchange

surprises came only when the mould was fired for the

capital expenses with operating costs.

first time. Although the melt came out flowing in a well

The design and manufacture of a thermoplastic

divided and balanced way, the material set so fast, that

aircraft catering trolley was not an easy project. The

the calibrator had to be placed extremely close to the die,

whole team ran into technical challenges that were not

in fact, a mere half an inch away. After the first runs, it

foreseen. The undertaking shows unique co-operation

took the team more than one year to finalise the set-up

between developer, manufacturer, raw material supplier

for reproducible and stable processing conditions with

and market players. With a joint effort the project

this extremely difficult to process material. Thanks to

succeeded in bringing innovation to a conservative

help from Solvay, the process of subsequential runs was

market.

taken to the final result: an extruded PPSU part to design specification.

For the door, the locking mechanism, the hinge and

We dare to say that this will be the turning point for airlines, which will increasingly move from metal to thermoplastics. The first proof of this is the development

the interfacing to the body, Promolding designed two

of an innovative new galley that Airbus has created,

injection moulded parts. Although these seemed to

known as SPICE. Promolding has been asked to develop

cause less trouble than the extrusion parts, there were

the plastic boxes that are to be applied in this concept.

still some interesting challenges. Because of the long

http://videos.airbus.com/video/iLyROoaf89yy.html

flow length at wall thicknesses of less than 1 mm at

Jac W.C.A. Gofers is managing director of Promolding, based in The Netherlands, â?™ www.promolding.nl

some spots, the company found in the mould flow simulations that it had to take care of a considerable amount of runners and gates (Figure 6). There were shorts in filling, air traps, burning spots and too many

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Reference

visible weld lines, which eventually led to the number of

1. H. Helm H. and U. Lambrecht, The Potential Contribution of

gates, as shown. Assistance from Solvay and Bemis

Light Weighting to Reduce Transport Energy Consumption,

resulted in control of the injection moulding process.

International Journal of Lifecycle Assessment (2006).

injection world | November 2010

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