Plastic Shrinkage?

[robbor]

What kind of shrinkage do you guys see on you baits after they are removed from the mold? Im wondering how large i need to make a cavity to end up with a piece that is .100 Diameter. My bit is .109 , will it be enough?

[longhorn]

Not enough information for a good answer. Just a rough one...the plastic doesn't shrink in diameter uniformly....if hand poured it will usually just shrink on the poured end if the mold doesn't have a large enough sprue volume...and then it will not shrink in diameter but will be hollow in the poured end. Some pics of the mold would help.

[Delw]

All plastic shrinks differently from supplier, likewise for soft med and hard.

Use a 10% rule and you will be more than enough, generally its 10%-15% and the harder the plastic the more it shrinks. more surface area etc.

if you add salt you be as high as 20% usually around 15% or less

non salted around 10%.

now this is all surface shrinkage with has nothing to due to the shrinkage of what longhorn was talking about. that you don't have to worry about as long as you keep it filled you wont get a bubble.

Surface shrinkage is pouring plastic into a mold then pulling it out and getting a different dia on let say a stick mold

the one longhorn is describing is what happens when you fill a mold up and the plastic will be hollow inside, this is cause by the plastic NOT shrinking, but slowing filling every nook and cranny rib etc etc

while we call it shrinkage it accutally is more of a settling deal as all the bubbles some out and filling into voids.

oh yeah you run a drill bit at .109 you will be at 9% if you want .100 so your probally good.

Edited December 4, 2008 by Delw

[longhorn]

Del,

If I understand what you're saying then for example your 4 1/4" stick mold which says .350 dia. will actually come out at about .315? I've never poured in an aluminum mold but I've never experiened what you're talking about...when I de-mold my baits...even after leaving them in overnight they still are tightly clinging to the mold in full diameter. I still believe that the plastic shrinks as it cools and respectfully disagree with your explanation that it's settling...but I've been wrong before. When I said respectfully I meant it so please don't take this as anything but a technical discussion. We all respect your knowledge on making soft baits and have learned a lot from you. I'd like to hear what others think about shrinkage/and or settling. (too cold and windy to fish today)

[Delw]

Its no problem Longhorn. this is just what I found in making alum molds.

Most people can't measure a bait and get with in 1/32 on an inch. calipers or even a micrometer wont work cause just the pressure of barely touching one side with push the bait in and give you a false reading, I use and optical comparator. even laying the finished bait on a table will deform a perfectly round bait just from its own weight.

for examply lay a soft full round worm flat on the table and the side will push out slightly making in not fully round.

when I have measured them( I have one mold a guy wants perfect) I cut a 1/4 inch section out and stick a pin through it then take the pin and stick the bottom end of the pin in clay thats laying on the table.

When I do molds I allow 5-10% depending on what the bait looks like, it took me a few years to figure it out on why the baits were smaller than the mold I made, on diameters. There are some that are very critical on the Dims which in all honesty is silly cause they fish don't care nor can the measure anything.

plastic is stretchable it cools on the outside first on alum molds, it will cling/stick to the ribs on a stick bait. This is why it feels tight pulling it out. when you pull it out and let it set for a while the finished prodct will come out smaller depending on how hot it is when its pulled out of the mold. even a .0005 tool mark will make a bait stick to the sides of the mold.

if your were to take a perfectly smooth mold, like a 8 finish or better it would be easier to see and understand.

if your making a flat mold (one piece)you will only notice it on the sides and not the bottom, a 2 piece mold shrinks more and is more noticeable because its held straight up.

I have had guys tell me the stick mold was about .025-.040 off the size I have stated even though they are approx. sizes. I really believe that the type/brand of plastic you use andeven the temp. its cooked at makes a huge difference in size as far as shrinkage.

in the first post above unless he has a heavy duty drill press and measures the hole with a gage pin it wont make any difference, cause drilling a hole with a drill bit in a hand drill or sloppy drill press with automatically open the hole a few thousandths if not more.

Hope that makes sence, anyway its just what I have found over the years so I always stayed with the 10% shrinkage rule

Delw

[Spike-A-Pike]

Gents - Very interesting thread.

As a curious bystander and not trying to challenge or be disrespectful - Del, if I'm understanding this correctly, adding salt could result in shrinkage up to 20%, hard plastic 10% to 15%, and a normal plastic about 10%.

Would adding glitter also add to the amount of shrinkage likely to occur?

[Delw]

Gents - Very interesting thread.

As a curious bystander and not trying to challenge or be disrespectful - Del, if I'm understanding this correctly, adding salt could result in shrinkage up to 20%, hard plastic 10% to 15%, and a normal plastic about 10%.

Would adding glitter also add to the amount of shrinkage likely to occur?

adding salt

no not surface shrinkage, just the shrinkage/settling form the airbubbles that slowly get pushed to the top due to salt and other particles in the plastic. I guess a good way to describe this type would be like a box of ceral as its moved it packs itself in, thats why on the boxs they say some settle may occour.

when you add stuff to plastic other than plastic like salt, you get airbubbles when the airbubbles work themselves out this is what causes the bubble on the top. this doesnt have anything to due to plastic shrinkage as a surface issue.

All materials shrink when they cool off some materials shrink more then others.

you know the stuff your dentist uses when building a crown that nasty crap they fill your mouth up with and make you wait till it cures while your slobbeirng all over yourself looking like a fool?

thats stuff shrinks as well, over time it will shrink. if I remember correctly its 3-5% after 24 hours( we used to use it for making molds of radius and chamfers on parts then put in in a optical comparator to get out final dims.)

in all hoestly the fishing bait its a non issue as its such a small amount it really doesnt matter.

I have probally made this more complex than it really is or needs to be.

[Spike-A-Pike]

Thanks Del - I learned something new today thanks to your explanation.