Die Springs

What a die spring is, and how it differs

A die spring is a compression spring that works inside press and forming tools. Its job is to return the pressure pad, the stripper or the ejector pins to their starting position when the press opens. What separates it from an ordinary compression spring is not the job but the working condition: it has to produce a very large force over a very short stroke, and repeat that same force over hundreds of thousands of cycles.

Ordinary spring geometry cannot meet that. Space in a tool is tight — the pocket the spring sits in is a hole already bored into the tool itself and cannot be enlarged. More force has to come out of the same volume. This is why ISO 10243 fixes both the section and the dimensions: different load classes are defined for the same hole size, so a toolmaker can change the spring without changing the tool.

In practice that means the dimensions of a die spring are not a free choice — the hole in the tool has already decided them. What you choose is the load class at that size, and that is shown by the colour.

What the colours mean — ISO 10243 load classes

On a die spring the colour is not decoration; it is the load class. ISO 10243 defines four classes, each with its own colour. Two springs with the same hole diameter and the same free length will give different forces — and accept different strokes — if their colours differ.

The scale stiffens from green towards yellow. Green is the softest class and accepts the longest stroke; yellow is the stiffest, gives the greatest force, but permits the shortest stroke. Blue, in between, is the general-purpose class and the one used most often in toolmaking. Choosing a colour is therefore not a case of "stiffer is better" — it is a trade between force and travel.

Before you order by colour: ISO or JIS

This is where most field mistakes happen. ISO 10243 is not the only standard; tools of Far Eastern origin often carry springs made to the JIS standard, and the JIS colour system is not the same as the ISO one. Matching the colour of the broken spring in your hand will therefore bring the wrong load class if the tool is JIS.

So do not order on colour alone. If you know which standard the tool was built to, say so; if you do not, give the hole diameter, the rod diameter and the free length. Dimensions mean the same thing in both standards. Colours do not.

If you still have the old spring, the quickest route is to send it, or photograph it next to a ruler. We will work back from the dimensions to the load class.

How to state die spring dimensions

A die spring is defined by three dimensions: the diameter of the hole it will sit in, the diameter of the rod or pin it passes over, and its free length. In ISO 10243 notation these are written with the class code — for example "DS2 / hole diameter × free length".

One detail catches people out: the hole diameter here is not the outside diameter of the spring itself. The spring is made a little smaller than the hole so it drops in freely. Measuring across the outside of the spring and calling that the hole size leads to a spring that rattles in its pocket, or will not enter at all. Take the measurement from the hole in the tool.

The rod diameter matters for the same reason: if the spring passes over a pin, its inside diameter has to suit that pin. Free length is the length of the spring under no load, chosen to suit the clearance with the tool open.

If you do not have these three figures, a sample is enough. Because our coiling line works with no tooling dependency, sizes outside the catalogue can also be produced — there is no separate tooling to pay for.

Stock and made-to-dimension production

We hold commonly requested sizes in stock; everything else is coiled to order. Which sizes are on the shelf is confirmed at the quotation stage — if it is in stock the lead time is short, otherwise it enters the production schedule.

Heat treatment, end grinding, coating and packing have been done in our own plant since 1978, and die springs are no exception; nothing is outsourced. That shortens lead times and keeps quality responsibility in one pair of hands.

There is no minimum quantity. A single replacement and a production order for a toolroom come off the same line; the only difference is unit cost.