Mete,I'm not an engineer.I had machine shop class in high school.Got a job as shop gopher in an evil corporation R+D dept.Was asked to do some simple machine jobs.Those evil corporatist decided if they invested a little patience and training in me,they could exploit me to make profit.
Dad Gum if they did not turn me into a Moldmaker,Modelmaker,Test apparatus builder,...a general "What we need is...." Make it guy.
Along the way I was trained by GE Plastics experts,John Kleese,a brilliant molding science consultant,W D Mold Finishing in Downers Grove,Ill,ParametricsTechnologies Pro-Engineer,Hansvedt EDM,....and maybe a few more.They paid me more than the younger Engineers.
Then I worked for a major Industrial Controls outfit.I wore a few hats there. from manuallathe hand in Limited Production processing quality issues..the "non conforming " parts.
So,no,I'm not an engineer.Just a High School Shop guy.And via those evil,profiting corporatists with non-union shops,I was given opportunity,training,a career,paid for a house,raised a family,Ii have a car,pickup,boat,home and a pension.And no debt.I had decades of paid health care and was up to 4 wks vacation..Oh,yeah!!I was exploited!!!I made them some money.Naive,masochistic pawn that I am,I actually take some Pride in that.
To answer your question,some thermoplastic polymers are crystalline
olypro,Poly E,Acetal,Poly carb,,and,as they cool,crystals are formed.They typically have higher shrink rates...ballpark .016 to .027 per in.The quality of crystal formation and molecular orientation play a part in the success/failure of the product.Good example,the living hinges molded into MTM ammo boxes.
So,gate location /polymer flow influences how the spaghetti molecules are oriented...the "grain",and the cooling system designed into the mold and processing parameters determine the crystalline structure.
The amorphous resins,like ABS,PVC,etc,typically have a lower shrink rate.In some,but not all,materials,these are the solvent bondable materials...like plumbing pipe.
What may happen:Based on inadequacies/failures of an existing part,or bean counters looking for 12 cents or $1.12 cost savings in a part to remain competitive in the market and keep the doors open....
A team gets the job of re-engineering the mfg process for a part.
They have a budget,a deadline,and criteria.
Some are better than others,but typically,the engineer is a product manager who designs nothing ...is an administrator who may research vendors,comply with agencies,etc.
The CAD designers design the parts.They know Pro-Engineer,or AutoCAD,or whatever design and doc software the firm uses.
In the same fashion I learned molding,they learn to apply the tech of the mfg process to their designs...example,uniform wall sections,radii,etc.
Some are better than others.
Guys like me turn ideas into something that can be held and tested.
Which happens.
They have a lot of meetings.The budget is spent or over run.The deadline looms.
The product passes tests,generates data.Someone declares "Success!"
The champagne flows,the bonuschecks are cut.
Then R+D throws it over the fence to Production.
The product goes to the field.There will be problems.They get solved,or they don't.
But look at the history of the Liberty Ship,the B-17,the P-38,the M-16...The 1903...
These endeavors evolve from dismal to great...via being defeated by greater and greater things.
I'm a 1911 guy,but I'm saving for an S+W M+P ...a compact.
Have not made my mind up,Shield,M+P Compact 45...still thinking,but I like the product,and Apex is solving problems...and S+W is paying attention.
It might well be that a sear or hammer hooks are not ideal to a given mix of MIM.
Civil war Hard Tack would break teeth,yet it's made from pretty much the same stuff as your Grannie's breakfast biscuits.
MIM can be as diverse as baked goods.Some is better than others.
Or metamorphic rock.Yes,some MIM sucks,but that is like compring the cast crap at Harbor Freight to Ruger's castings.
World class race cars and motorcycles have sintered gears in the transmissions.re they MIM?I do not know,but its all powdered metal technology.
I'm pretty sure the top line Henkels chef knives...the Cera Max or something like that...series,are a powdered metal blade.Sears and knife blades need to hold edges.
It just occurred to me,reading Clyde Baker's old book on Modern Gunsmithing.and the J.M.Pyne stories in Gun Digest,etc in High School,
had something to do with forming the "Ore" inside me,that my later mentors figured they could work with.