Material selection for an application can sometimes prove to be very difficult.  But what happens if you identify a material then find that it can be supplied as a homopolymer or a random copolymer.  Is there a difference?  The answer is YES!

Without getting too complicated with explanation:

A homopolymer has very similar molecular chains (homogenous) therefore creating consistency with regards to shrinkage.  As the homopolymer cools, it creates much tighter, uniform crystalline sites.

A Random Copolymer has molecular chains that are dissimilar where branching and length can vary.  This creates less uniformity in the melt and tends to decrease shrinkage.  As the random copolymer cools, the crystalline sites are broken up due to the interference of the random branching of the molecules which breaks up the crystalline sites.  This results in non-uniform crystallinity throughout the polymer which translates into less shrinkage.

Now that we have a distinction between the two, there can be some understanding as to how each material might perform differently when packed into a cavity. 

 

If the cavity is thick and chunky and cannot be packed out sufficiently, the homopolymer will tend to shrink more and may cause sink marks or even worse, voids.  This is where the random copolymer might be more applicable.

Likewise, if the part is not terribly thick and chunky but the gate size tends to freeze off prematurely, the same situation will occur.  Having a gate that freezes off prematurely (undersized) doesn't help either situation.

On the flip side of the coin, if gate sizing is optimal for the part and the part can be packed out sufficiently, the homopolymer will crystallize more uniformly during cooling and not create any sink or voids.  The random copolymer will shrink even less and not create any sink or voids either.  This is how dimensional stability is maintained.

In order to keep a part dimensionally stable, the process will need to be stable as well.  This includes being able to select the appropriate material, application, gate size and process.

In essence, choosing a suitable polymer for the application can be further improved upon if there is an understanding of how the polymer responds to the process. 

 

FEA analysis can identify and expose these differences between materials allowing the user to select the most suitable material before purchasing it.
 

Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs and please visit our website at www.BozillaCorporation.com.