From: Michael Tsenter (nesttech@pacbell.net)
Date: Fri Apr 18 2003 - 08:51:08 EEST
Hi Andrew:
I have to confirm that I agree with you regarding most of your statements.
Vacuum casting process by itself can not and will not assure 100% reliability in
production of high quality urethane castings. You are absolutely correct that
waiting for the machine to mix & de-gas resins, and then waiting for the resin
to set for several hours is no way to run a production outfit. Furthermore,
resins with long pot life will attack the mold thus shortening the mold life.
However, there are situations when the presence of vacuum (or negative pressure)
during filling of the mold is extremely useful. These situations are common
when you are dealing with very thin wall parts that have numerous intricate
details, and at the same time the resin has high viscosity. In these cases that
presence of vacuum during the mold filling will insure a more thorough filling
of the mold minimizing large voids. Once the mold is filled, it can then be
pressurized in order to eliminate any remaining small bubbles due to air in the
resin, gas from resin reaction, or other air-trap areas that were not vented.
Many of our Protomix machine users inject the resin into the mold using
cartridges or meter-mix machines while the mold is sitting under vacuum. Once
the resin fills the mold, the vacuum is immediately released and the mold is
transferred into the pressure tank where it is pressurized to 60-80 PSI. This
process is extremely fast since filling of each mold only takes seconds.
Production rates of 100-300 urethane castings per day are typical at job shops
that follow these methods while using fast setting urethanes (10-15 minute
de-mold time).
Best regards,
Michael Tsenter
-- NEST Technologies, Inc. 3849 Ridgemoor Drive Studio City, CA 91604 USA Tel: 818-761-6500 Fax: 818-761-6116 e-mail: nesttech@pacbell.net web site: http://www.nesttechnologies.com Andrew Miller wrote: > > I have had better luck with the reverse approach. Instead of forcing the > > material into the mold suck the air out of the mold and the pour material > > will fill the empty space. First, you put your pour material into an > > oversize container and place it in a vacuum chamber.Then you place > > your RTV mold in the chamber, pour in the cleaned pour material and draw a > > vacuum on it. > > This approach works fine if you are using low viscosity LONG pot life > materials.....I like the idea of using an old fridge though...clever.. > > Pressure casting is the only way to make high quality pieces from an RTV > mould in a short timeline... > > We do a huge amount of urethane casting and have found that it is just not > economically feasible to vacuum cast parts if you can only do 2 parts a day > because you are either waiting for the machine to mix and degas or you're > waiting 6-8 hours for demold.. > > Our average mould cycle here is a little over an hour > > One cycle equals Prepared mould --> shoot Material--> > Pressurize -->Depressurize -->Open and demould --> clean and prep mould --> > Prepared mould ready to shoot again. > > We have some moulds/resins we can turn around in 35 Min. > > 10-12 parts from a mould per day is not unusual for us > > We've worked with more than 30 different resins and 10 different silicones > We can now consistently produce 80-100 pcs per mould with minimal flash and > excellent dimension retention. > > Big trick is not to entrain any air into the material as you mix it.... > which means static mixers and cartridge or meter/mix equip (I can point you > to all the cheapest places for that stuff too if you need it) > trust me $2-$3 in disposables for each part will save a TON in time and > finishing. > > anyway...if anybody wants to know the ins and outs of pressure casting..call > me...we have been through the learning curve three ways to Sunday and > back....I can save you some grief... > > Andrew Miller > www.aminnovationsinc.com
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