It is more durable and like a rubber material comparing to the white beaded EPS foam. It is white coloured foam available in two different weight densities: 1.3 Ib/ft3 and 1.9 Ib/ft3.

Because of it belongs to a type of molded foam, it has a higher density at the surface than the core of the material. Such variation in density distribution causes the hot-wire cut EPP foam sheets being slightly heavier than the sliced sheets.

EPP beaded foam is also a type of ‘shape memory foam’ that it returns to its original shape after being distorted mechanically.

Despite that EPP foam was originally developed for impact absorption (e.g. bumpers) in the automotive industry, it is widely use today for building the wing and fuselage sections of RC combat aircrafts.

Unlike the EPS foam made model aircrafts, EPP foam models are not suitable for covering with balsa sheets. EPP foam made model aircrafts are often to be covered with tape or a low temperature heat-shrink finishing (e.g. Ultracote).

However, EPP foam does has a waxy feel that it really needs to be sprayed with 3M adhesive before applying finishing materials such as the fiberglass reinforced tape or plastic firm.

Overall, EPP foam stands on its own probably as the toughest foam that is excellent in withstanding compression, but weak in that for tension loads (i.e. it will tear in tension and requires strapping tape for reinforcement). It is also not as easy to mould due to its high toughness.

Extruded Polystyrene (XPS) foam
It is often identified by the trade name ‘Styrofoam’ though it is also being known in the UK as ‘Blue foam’ or in France as ‘floormate’ or ‘roofmate’. It is available in a variety of colours like blue, gray, orange, green and pink colours.

It has a fully closed-cell structure (i.e. which means that each cell that makes up the structure is completely closed off from adjoining cells which prevents it from behaving like a sponge) that helps the foam to resist moisture penetration and water retention better than other types of foam materials. Such closed-cell structure also contributes to the long-term durability and strength of the foam material.

When the foam is used for building wing cores of model aircrafts, this closed-cell structure is giving the item a much finer trailing edge. Meanwhile XPS foam is lightweight (i.e. available in density of ~ 1.0 Ib/ft3) and can easily be well covered by painted finishing. However like many other foam materials, XPS foam is combustible.

Expanded Polystyrene (EPS) foam
It has a partially closed-cell structure that allow water infiltrate through capillary action and water retention in the foam’s interstitial voids.

Compared to the XPS foam, it is heavier (i.e. with density of ~ 1.3 Ib/ft3) and does not cover as well with painted finishing. However it is cheaper than the XPS foam.

When use for building wing sections of model aircraft, EPS foams are suitable for covering with wood veneer (i.e. balsa sheets) due to the fine trailing edge required for vacuum-bagged wings is difficult to produce with this type of foam. Continue reading