A moulded dimension is the tool dimension minus shrinkage, and shrinkage depends on resin, wall thickness, packing pressure, cooling and how the flow ran. That chain is why moulding tolerances are looser than machining tolerances and why some dimensions on the same part are far easier to hold than others.
Where the variation comes from
The tool itself, made to a fraction of the part tolerance and wearing over its life. Shrinkage, which varies with resin batch, moisture and process settings. Flow direction, since filled resins shrink less along the flow than across it. And ejection, which can deform a part that is still warm. A quoted tolerance is the sum of all of those, not just the toolmaker's accuracy.
Which dimensions are cheap to hold
Dimensions formed entirely within one half of the tool are the easiest, because they depend only on that steel. Dimensions that cross the parting line add the closing accuracy of the tool. Dimensions that cross a side action add that mechanism's repeatability. Where a tight tolerance is needed, designing it to live inside one tool half is nearly free and makes the difference.
Filled resins and warp
Glass filled grades shrink less along the flow direction than across it, so a flat part warps into a saddle unless the design and the gating manage it. This is the single most common cause of a moulded part failing a flatness requirement, and it is a design and tooling problem rather than something the moulder can dial out at the machine.
Buying tight tolerances honestly
Say which dimensions are critical and let everything else run to the general moulding tolerance for that resin. Expect the toolmaker to leave steel on those features so the tool can be adjusted after first samples. Expect a sampling loop. And expect an inspection cost per lot, because a tolerance nobody measures is not a tolerance.
Questions people ask about injection molding tolerances
How tight can injection moulding hold?
Looser than machining, and it depends heavily on the resin, the wall thickness and whether the dimension crosses the parting line. Ask the moulder for their standard tolerance table for your resin.
Why is my flat part warped?
Usually uneven shrinkage, most often from a filled resin shrinking differently along and across the flow, or from uneven wall thickness and cooling.
Which dimensions are easiest to hold?
Those formed within a single half of the tool. Anything crossing the parting line or a moving action inherits that mechanism's variation.
Can tolerances be tightened after the tool is made?
Sometimes, because steel can be removed but not easily added. That is why toolmakers deliberately leave cavities slightly full for the first samples.