Flat Enough for the Parting Line
Where flatness earns its keep
A mold base closes on its plates, so any bow shows up at the parting line as flash or light seams. For typical mid-size mold bases, holding flatness in the range of a few hundredths of a millimetre over the plate surface is the practical target.
Tighter than that usually buys nothing: clamping force, thermal expansion during moulding and plate stress relief all move the geometry after assembly anyway.
How flatness is checked
Standard practice measures the plate on a surface table with a dial indicator, sweeping a grid of lines and reporting the worst peak-to-valley. Machine certificates should state the method, not just a number.
If your moulds run thin-wall or high-cavitation layouts, ask for the flatness report per plate rather than a blanket certificate value.
FAQ
Does thicker plate automatically stay flatter?
Not necessarily. Heat treatment and stress relief matter more than raw thickness; a poorly relieved thick plate can bow worse than a well-processed thinner one.
Should I specify flatness tighter than the standard?
Only for specific parting-line-sensitive projects. Blanket tight tolerances raise grinding cost and lead time with little part-quality gain.
Can flatness be corrected after delivery?
Minor correction by regrinding is possible, but repeated regrinding thins plates and changes assembly stack-up. Prevention at machining stage is cheaper.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.