Martin Frings
12 September 2024

How does the weight and thickness of a stamper affect how it performs on press?

We were recently asked this question by a pressing plant who had been diligently weighing all their stampers for many years as a matter of course without ever really knowing why they were doing so.

In this guide, I’ll try to explain what an ideal stamper should weigh – and seeing as the weight is directly proportional to the thickness of a stamper, why stampers are the thickness that they are.

Why should pressing plants monitor stamper weights?

As a stamper manufacturer, we measure the average weight over a number of stampers and use it to fine tune our plating programs and to ensure we are delivering consistent stampers.

But for a pressing plant there are two main reasons to weigh your stampers:

• To help fine tune your press settings. 

If you use stampers from multiple sources – or if you just use one galvanics facility, but the output is inconsistent – then you may be faced with a scenario where stamper weights vary a lot from job to job.

Here’s a real life example: after completing a job with stampers that weighed circa 220 grams, the next job on had stampers that weighed circa 150 grams.

Almost every job requires a little bit of fine tuning of press process times. It is unlikely you will be able to press a 25 minute-per-side classical record record with the same setting as a 10 minute 45rpm dance track – but generally this would involve a few tweaks and not a radical recalibration.

But a major recalibration will be needed if you were switching from 220 gram stampers to 150 gram stampers – and vice versa. This is an extreme example, but knowing you have a significant discrepancy between stamper weights before the stampers are on the press should save a lot of time and head scratching.

• To check for any discrepancies between the two sides.

Let’s suppose that as a result of inconsistent process control at the galvanics facility, there is a big variation in stamper weights between the ‘A’ and ‘B’ sides. For example, the ‘A’ side is 220 grams, and the ‘B’ side is 150 grams.

In this instance it would be near impossible to press a good record.  By pre-weighing the stampers, the stampers can be rejected prior to loading, and you avoid wasting both the time and material trying to press a record. 

At Stamper Discs we keep any discrepancy between the two sides to <10grams. We recommend that stampers with a discrepancy of more than 20grams should be rejected.

What should an ideal stamper weigh?

This depends on the size of the stamper – its diameter – which in turn is dictated by the press profile you are using.

Here’s how to work it out for yourself:

The density of nickel is 8.907g/cm³

Therefore a piece of nickel with an area of one decimeter square (referred to as ‘dm²’), which is 1 micron thick, will weigh 89.07 milligrams (‘mg’). (A square decimetre is 10cm x 10cm; a micron is a 1000th of a millimeter; a milligram is 1000th of a gram.)

If you know the size of the stamper (or any metalwork part), and its thickness, you can calculate the weight as follows:

(89.07/1000) x part area in dm² x desired thickness = weight of part in grams

The area of the part is calculated from: 3.142 (pi) x Radius² .  So if you have a 14” lacquer the diameter of the part converted to millimeters is 356mm – you arrive at an area of 9.96 dm².

Thus our calculation for the weight of a 14” unformed stamper at 240 micron thick looks like this:

(89.07 x 1000) x 9.96 dm² x 240 microns = 212.9 grams

When calculating weights for formed stampers remember that you need to work from the diameter before forming (your stamper supplier should be able to help you with this)… and also make allowance for the centre hole.

A stamper with a 324mm formed diameter will have an unformed diameter of 327mm.

And the deduction for a centre hole is circa 1.4g.

So using the information above, you should be able to calculate that a stamper with a formed diameter of 324mm, made from an unformed diameter of 327mm, at 240 microns thick, will weigh 178.14g.

Why is a stamper 240 microns in thickness?

The range of 230 to 250 microns in thickness has been settled on by most of the industry as a happy medium for pressing. 

The thinner a stamper is, you can get a quicker cycle time on the press.  But the stamper has a considerably shorter lifespan. 

The thicker the stamper is, it will last longer on press – but the trade off is that it increases the cycle time and can lead to issues such as non-fill.

It is also worth noting that the ductility of nickel peaks at 250 microns¹ and there is no benefit in exceeding this – it would only slow down the press cycle time.

In theory you could go thinner than 230 microns or thicker than 250 microns but to radically change from this range would require a different mold. Most molds on presses (including Toolex/Pheenix, Viryl and Newbilt) are designed with a 240 micron clearance between the centre bushing/centre pocket and clamping ring/mold shoulder.

Using a significantly thinner stamper would mean that the centre bushing will be sunken on the mold creating a small lump in the middle of the pressed record which would mean they wobble around on the cooling spindle and may warp.  It would also mean that the clamping ring doesn’t fit perfectly and moves around allowing debris beneath the stamper.

You would also introduce problems by using a stamper thicker than 250 microns, as the centre bushing would not seat properly into the stamper and would protrude above the stamper in the mold. This would risk piercing the record label which would not only be unsightly, but would also allow a chance for PVC to get between the stamper and the bushing, putting undue stress on the bushing with every cycle and potentially creating a nightmare scenario like this…

In summary…

Stamper weight is a quick and easy check to make sure that nothing unexpected is going on and that things are in the right ballpark. It means you can reject any large anomalies before they arrive at the press and thus save operator time.

But don’t get too worried about variations of a few grams. Plating is imperfect, and electrical circuits in different plating cells can have slightly different level of resistance. Stamper manufacturers make allowances for ‘current inefficiency’ in their plating calculations, but this can vary fractionally from plating cell to plating cell.

And remember that although you may have your 324mm formed diameter stamper at the ideal weight of 178 gram… but that doesn’t necessarily mean those 178 grams of nickel are evenly distributed. You could have a stamper with an thick outer diameter and a thin centre. Thickness uniformity is a whole other article…