Wood for moulding

Respect for the environment, natural resources and materials are the requisites for a new range of wood plastic composite materials that also offer enviable mechanical performance

Moulding wood: this is the idea that inspired Plasticwood, a company from Verona with a well established track record in manufacturing WPC (i.e. wood plastic composites) which are suitable for extrusion processing and the production of extruded profiles which are then marketed as a valid alternative to the same articles in wood. Currently demand for more eco-sustainable products is growing and as a result the company has decided to diversify its offer by introducing composites suitable for injection moulding, which have been developed using natural fibre.

sprea_web«Of course ecology is a priority – says the Company owner Dino Sprea – but we also have to take the needs of the processing industry into consideration, because more than anything else these materials need to be easy to process for manufacturers who are used to working with more traditional thermo plastics. Currently these compounds are used in the production of flower pots, drip trays and household goods, but they are beginning to attract the attention of designers whose agenda is geared more towards environmental values, particularly in the production of chairs and other furnishing items. However ecology is sometimes just a pretext because the typical patterns assumed by wood flour at the end of the production process, gives each piece a decidedly unique and special finish. Items made from WPC really do have the ‘flavour’ of wood with none of its issues. They are particularly sought after because they are hardwearing, long-lasting and resistant to corrosion and mildew. They also keep their mechanical properties even after they have been exposed to different weather conditions for long periods of time»

Compounds for injection moulding
The different grades used in injection moulding are made up of a polyolefin-based thermoplastic matrix with different flow indexes modified with vegetable fillers in quantities that vary from 30% to 50%. The correct compatibility and cohesion between natural fibre and polymer is obtained by the addition of special additives. «These additives not only favour the dispersion of the filler in the polymer matrix, but also improve the mechanical properties of the composite – Dino Sprea points out –. The compounding process is carried out using a twin-screw counter-rotating extruder. One of the most important variable in this stage is the temperature: if this is not correctly monitored the natural component tends to burn, which in turn gives rise to negative effects such as a noticeable alteration in the colour of the material and problems such as extruder malfunctions».

The range of products
One outstanding product from this range is PP40 SCD, a polypropylene product filled with 30-50% of fir-wood flour from carefully chosen, renewable sources. The vegetable-based materials come from sources that have never been treated with varnishes or chemical products. The choice of fir-wood is dictated by its colour, which is very light, and offers continuity in production by preventing variations in the colour of the natural fibre. The grain size of the flour varies according to the type of article in production: finer flour makes the compound more fluid and therefore particularly suitable for injection moulding even when the item in question has very thin walls (up to 3 millimetres). Generally speaking this type of WPC has a high flow index, an excellent balance between mechanical and thermal properties and high flexural strength. The ease with which it can be processed is also an important factor given that it can be injection moulded like any normal thermoplastic material making it versatile and easy to use.

1 Impact resistance (Charpy test – ISO 179-1) of some WPC materials produced by Plasticwood
2 Flexural and tensile strength test values (ISO 178 and ISO 527 respectively) of some grades of WPC produced by Plasticwood
3 An example of the grain size distribution of wood flour used in the production of WPC

Process conditions
Before being processed WPC requires a drying pre-treatment so that the end result will give the best possible aspect and prevent surface defects sometimes caused by residual moisture. The engineers at Plasticwood recommend the following conditions:
• press head temperature: approx. 160 °C;
• injection point temperature: approx. 175/180 °C;
• mould thermoregulation: 40-60 °C;
• screw temperature: approx. 160/165 °C (at higher temperatures the material could become overheated due to friction);
• medium-high injection speed.
At temperatures above 190/200 °C the material could degrade and the wood flour could take on a brownish tinge. These materials are processed using traditional injection machinery. «So for those operating in the field of thermoplastic moulding no further investment in new machinery will be necessary» says Dino Sprea.

PP40 SCD grade 
Specific gravity at 23 °C ISO 11831,064 g/cm3
MVR – Melt Volume Rate (190/10) ISO 113329 cm3/10’
Water absorption D1037 (ASTM)less than 1%
Flexural modulus (2 mm/min) ISO 178/A/23.490 MPa
Flexural yield strength60 MPa
Tensile modulus (ISO 527-2/1A/1)1.740 MPa
Tensile strength at break32 MPa
Elongation at break5%
Charpy impact test (without notch) at 23 °C ISO179-1 1 and U13 KJ/m2




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