figuring shrinkage

[spock]

I am tinkering with a container design, I would like for it to be at least "leak resistant" if I dont end up with watertight. (I know how hard that can be). I have the same material for lid and container, and I was wondering if I should allow for the same amount of shrinkage on both.
Another way to ask is this: Will they shrink a little differently because of the different shape of lid/container?
Material is PP, and it is a thermoform container.

[Stu_M3]

Allow for one or the other component to be slack until you have shots off the mould then adjust the sizes till you get the fit you require

Stu.

[Wiseco]

It's pretty hard to predict shrinkage but as reference, check with your supplier for the techical datasheet for the shrink rate. Blow mold grade PP will shrink something near 0.010in/in to 0.025in/in so if your container is 6" tall, it will probably shrink by something like 0.100in. As it will be thermoformed, I think the shrink rate will be pretty constant everywhere.

But as Stu said, make a trial die to figure what's happen to the finish product.

[barsse]

Hi Spock
Why don´t you ask someone in the simulation software related sites for these processes,
they should know by sure in order to make the math for their softwares.

regardas

[jetski]

unless you have a thicker wall section from one to the other they should be stable. Look at these areas Water flow and location, is it equal? gating location and flow length similar? Family tool or 2 separate tools? I build tools to the shrink numbers for a multitude of materials and rarely have a problem (but then again I have been doing this for 30 years).

[Wiseco]

Originally Posted by jetski unless you have a thicker wall section from one to the other they should be stable. Look at these areas Water flow and location, is it equal? gating location and flow length similar? Family tool or 2 separate tools? I build tools to the shrink numbers for a multitude of materials and rarely have a problem (but then again I have been doing this for 30 years).

He said that is going to be thermoforming so I think all the srinkage will be the same everywhere and the cooling is mainly done with fans.