I'm considering making a dish antenna (130mm diameter) from a plastic injection moulding with vacuum metalisation. Three major issues: Dish profile needs to be very accurate ideally no more than +/-0.2mm deviation from the intended shape Needs to operate between -40 to +80degrees C so needs good dimensional stability over this range Needs to be low-cost Any ideas on a suitable material choice?
Don't think you need anything too exotic or expensive. polyethylene and polystyrene are not hygroscopic. Polystyrene is very easy to mould I am told. Mind you I suppose with vacuum metalisation you have sealed it against water .... maybe. 0.2mm tol on a 130mm dia part sounds fair (assume it is very simple shape) Of course these are not robust plastics. There is a very good moulder near Stansted that could help - www.[B]bktooling[/B].co.uk
.2mm repeatabilty over 130mm is certainly doable, but .2mm accuracy over that distance is a little tougher. Make sure any ribs are less than .6 times the thickness of the face. You might want to look at a glass filled bulk molding compound used in compression molding, not quite as cheap as injection molded and less regrindable I think, but very good dimensional stability due to the long fiber glass. If it must be injection molded pay attention to how it is gated. If there is a dead spot in the center of the antenna then it could be gated there to get even filling.
I believe you'd find that dimensional stability would be improved by the use of structural foam with glass fill. This is the kind of material used for heavy duty crates with open cell rib structures. Of course gas assist and glass fill can be done by many injection molders, but there are also specialty molders for structural foam, and I'd suggest you contact one of those before you commence actual design.
Thanks to you all for the help. This has certainly given me a few ideas. I have sent out drawings for this part to be made in a variety of ways including moulding. To be honest I'm hoping to deep-draw this part from aluminium because I think it will turn out be the cheapest way. I'll see how costs and tolerances look for all the options, but you've certainly given me some great ideas and a few things to consider if the injection moulding route looks favourable. That said, I would really love to do an injection mould with vac-metalisation for maximum coolness (mmm... just imagine that shiny mirror finish), but we'll see.....
Looks like we need more metal for thermal management anyway so the likelyhood is that it'll be a casting.
