Dear Jaap Schut,

shrinkage is the recovery of the material or its relaxation from the input thermo-stresses encountered during processing. I thnik instead of looking for a blend with the lowest shrinkage upon processing, it will be better and much easier to look for the operating processing conditions that will lead to a product with the lowest shrinkage. In this situation, you can use polymer grades with low molecular weights, you will find low shrinkake but you will loose other properties mainly the mechanical ones. Regards

Thanks for your help, Abdelkader! I really appreciate it. We are aware of the thermal history/processing conditions effect, but I also know about low-shrinkage PP grades that are offered (blends, copolymers, filled, nucleated etc...). Time, temperature and pressure are obviously important parameters for processing any polymer, but also filler type, level, morphology, as well as polymer type/grade. Your tips wrt MW are indeed on that last mark, which has my specific interest. Thank you again. Best regards, Jaap

Dear Jaap Schut,

no thanks as far as this area is for discussing and exchanging idees and opinions. I would like to add the following that I am sure that it is a very basic for you. Just to extend my previous answer.

Shrinkage is the result of the ability of the chain to be oriented with the ability to store elastic energy which will be used for the chain to recover and regain back an arrengement with high entropy and low enthalpy. This conditions of orientation and storing energy are possoble only for high molecular weight chains. Entaglements (which occur beyond a certain critical molecular weight) play a dramatic role for storing the input energy and for the recovery.

If the end-use of the product does not require high properties, than low molecular weight polymers (with MW lower then the onset of entanglements) will I think have low shrinkability. Regards

Shrinkage is contraction or volume decrease under constant form.  With isotropic material this can be estimated using de pvT relationship.  Warpage is a change in form under constant volume.  Deformation is the sum of both.  Linear shrinkage is another concept of high importance in injection mold design.  Materials with high filler content shrink less than unfilled.  Fiber reduce volume shrinkage but material   becomes anisotropic.  It means it shrinks differently depending on direction.  In injection molding the molecular orientation during filling and packing can be reduce with processing condition, and location and number of gates during mold design.  The use of simulation packages like Cadmould, Moldex3D, Sigmasoft, Moldflow can help on this, but the material data are critical.