Plastics : Shrinkage control during production of plastic products

Introduction:

Shrinkage control is the process of predicting how much a plastic part will shrink as it cools from its molten state, then adjusting the mold design to compensate for this shrinkage so the final part meets exact size specifications.

In depth explanation:

When plastic is heated and injected into a mold, it's in a liquid state and fills the entire mold cavity. As it cools and solidifies, the plastic naturally contracts and becomes smaller - this is shrinkage. Different plastics shrink by different amounts, typically ranging from 0.3% to 3% of their original molded size.

The challenge:

The challenge is that this shrinkage isn't uniform. 

Thick sections cool slower and shrink more than thin sections. Areas with fiber reinforcement shrink less than pure plastic areas. Complex geometries create stress patterns that cause uneven shrinkage, potentially warping the part.

How shrinkage control works:

Engineers use material databases and simulation software to predict shrinkage patterns before manufacturing begins. 

They calculate how much each area of the part will shrink, then deliberately make the mold cavity larger by exactly that amount.

Example:

If a plastic gear needs to be exactly 100mm diameter, and the plastic typically shrinks 2%, they'll make the mold cavity 102mm so the cooled part shrinks to the perfect 100mm size.

Advanced shrinkage control:

It involves adjusting processing parameters like injection pressure, cooling time, and mold temperature to minimize unwanted variations. 

Some manufacturers even use different shrinkage compensation values for length, width, and thickness dimensions of the same part.

Conclusion:

This precision is crucial in industries like automotive and electronics, where parts must fit together perfectly despite being made from materials that naturally want to change size as they cool.