Mixed-level Designs

[HelenaB]

Hi,

Is there any situation in which a mixed-level design is associated with a quadratic model?

It seems to me that the mixed-level L18 designs are only capable of generate main effects models, is this right? But I can't understand if for example a mixed-level full factorial or a mixed-level fractional factorial (non L18) can generate a quadratic model. 

Thanks in advance!