There are several technical benefits of extruded composites tailored in multilayer structures - and not least important from cost savings point of view ! See here for 2-rotor CONEX-extruder for multilayer WPC-structures !

While a single layer profile unnecessarily consumes e.g. color, UV and other expensive additives beneath the surface layer, which alone needs to be protected, a multilayer profile or panel structure produced by 2-rotor CONEX-extruder can split the costs of additives to less than half as they are placed only in the front rotor material formulation giving the product outer surface layer. Material cost savings in multilayer product structures e.g. in decking boards or other equal volume products in the construction industry provide improved competitiveness and healthy margins for profitable business !

Furthermore the encapsulated product core in a multilayer structure would typically utilize recycled or waste plastics which can be also in lowest cost mixed color grades to reduce the material costs to minimum. 

It is technically advisable to use more plastics e.g. in levels say 30-35% at the surface material layer formulation for improving the weathering quality and performance of the product - but in the core layer one may use more lower cost fibres and reduce the quantity of higher cost plastics down to 20-25%.

SAVINGS IN MATERIAL COSTS

The example calculation below shows for a solid decking board 140x28mm (3,5 kg/m) material costs;

Material unit prices used in the calculation (Eur/kg) :                fibre 0,2, virgin plastics 1,5, recycled plastics 0,65, processing aid 4,5, coupling agent 5,5, color pigment 10, UV-stab. 19

Note; both single layer and multilayer boards have exactly the same material formulation at board surface ! 

• single layer A profile 4,02 Eur/m
• multilayer A_B_A profile 2,40 Eur/m
• material cost savings 1,62 Eur/m equals -40,2%

THIS MEANS 1,62 MEur SAVINGS IN MATERIALS FOR EACH MILLION METERS OF WPC-PRODUCTION !

As in extruded composites material costs represent 70..75% of all manufacturing costs (OPEX), the overall savings gained in manufacturing multilayer A_B_A decking boards vs. single layer are ca. -30%. 

ADVANTAGES IN MATERIAL LOGISTICS

• core layer can utilize such plastics waste (mixed, hybrid, contaminated, etc.) which are not adequate for the surface layer due aesthetics or properties
• core layer can utilize various types of plastics waste available from alternative sources and thus reducing dependency of supplies and avoid possible sudden shortcomings 
• core layer gives freedom and adds flexibility in short lead times to adopt new waste materials coming available at marginal or no cost

TAILORING PROPERTIES

• high fibre content at core (up to 80%) and less at surface (ca.60%) where more plastics is needed
• waste and/or lower grade recycled plastics at core and higher grade recycled plastics and/or virgin plastics at surface
• simple material formulation at core with less or no expensive additives enables to focus on tailoring the surface formulation and building on properties
• cooling of product core at higher fibre content is quicker and makes it easier for dimensional stability when less plastics is present meaning shorter lines and higher line speeds

COMPARING MATERIAL COSTS OF SINGLE LAYER AND MULTILAYER WPCs

Calculating flexural rigidity of a multilayer solid profile A_B_A sandwich structure in bending: