How to mould an exceptionally flat disc

[doctorplastic]

I would appreciate some advice on the right material and tooling approach for the production of an exceptionally flat disc. The disc will be about 160mm diameter by about 2mm thick. The disc surface must be very accurate (from a polished mould) and perfectly flat. The disc will also have a small central spigot on one side about which it will be spun at about 4000rpm. It should have minimal runout.

Any advice will be gratefully recieved.

 

[patprimmer]

Almost any amorphous material should meet your stated criteria, however I expect that you will disclose more requirements as the thread unfolds

Materials that spring to mind are polycarbonate, ABS, Noryl, crystal styrene, acrylic, PVC

Regards
pat

 

[Soupcat]

As Pat has said, almost any amorphous material should meet your stated criteria. Try to select a material which has a comparably low shrinkage value to minimise warpage and try to gate the parts at one end preferably with an edge tab or fan gate.

Regards

Kieran Fegan

http://www.moulddesigner.com

 

[patprimmer]

As I understand it a spigot is the male portion to locate a hole

I agree 4000 rpm is a lot, but it depends on the material chosen, load and time as well.

I did state that I expected more critera to gradually evolve. It normally does when the initial question is ver vague, general or siplistic

Regards
pat

 

[doctorplastic]

The spigot referred to can also be known as a 'boss'. Essentially it is a small raised cylinder protruding from the adjacent surface (on one side only). In this case the cylinder has an outside diameter of 12mm (inner diameter 9mm)and has a height of 12mm. This is the feature about which the disc will spin at 4000 rpm. I note the comments about friction. Ideally material selection will overcome this issue, otherwise we may have to consider insert moulding a steel pin into the spigot.

I am also interested in your thoughts as to where to incorporate the feed/gate on this mould/mold. We are planning on a single impression mould. We need to fill the cavity quickly because no distortion is allowable whatsoever. It is crucial that material mass is evenly dispersed across the component to maintain its balance whilst spinning at high speed.

Your responses are much appreciated so please keep them coming!!!

 

[patprimmer]

It's starting to sound a bit like a big CD.

PTFE Filled PC could possibly do the trick.

I would diaphram gate into the spiggot internal surface, with the material flow aimed at the cylindrical wall of the spiggot rather than at the opened cavity of the disc.

I would keep the wall section of the cylindrical portion equal to the wall section of the disc, with generous and concentric radii at the transition.

I would not fill it to fast, as that would require excessive pressure, possibly ressulting in overpack near the gate, but not as much at the extremities.

I would use a hot mould, medium melt temp, medium back pressure (so as to get a homegenious melt) and a slow to start injection speed, ramping up to about 80% full, then slowing down so as to prevent overfilling and blowing the mould opened. As the speed dropped off, I would still keep the pressure up, so that the mould packed in a controlled but firm manner. It is essential to maintain consistent and uniform pack, and consistent and uniform mould surface temperature.

It will be necessary to take care in the thermodynamics of the mould at design stage.

The moulds and techniques used for the manufacture of CD's should give a good starting point

Regards
pat

 

[Pud]

Is cost per part an issue? If not, you could try a LCP - some of them have virtually zero shrinkage (and hence low warpage). In addition they are usually high strength/modulus. Also flow well so low pressures needed to fill. Needs good drying as polyester-based.

BTW, pat has said it all, but it cannot be emphasised enough that tool thermodynamics (i.e. cooling patterns) are going to be critical - i.e. uneven tool surface temps=uneven surface!

Rgds

Harry