In an example, a method includes receiving, at a processor, object model data representing at least a portion of an object to be generated by an additive manufacturing apparatus by fusing build material. Using a processor and from the object model data, a property diffusion model for the object in object generation may be determined. Using a processor and based on the property diffusion model, a manufacturing boundary object shell around the object and encompassing an external volume may be determined. The shell may have a variable thickness determined so as to include build material for which, in generation of the object, the property modelled in the property diffusion model has a value which is predicted to conform to a predetermined parameter.