Compression molded out of oven (MDF molds?)
I have been researching on this page for a couple of weeks now and each time I am closer to know what I want to do with the information of this community.
I will make a compression machine and I have seen some different ways to use the molds:
#1. The one Dave shows in his videos: heat the oven, put the plastic in the mold and use the press built into the machine.
#2. Another technique I have seen is to heat the plastic in the open mold, then take out the mold and use a press to mold the piece.
#3. And the other is to heat the plastic in a tray and pour the half melted plastic to a mold outside and exert pressure with a machine.
While the first 2 methods are made with metal molds, in the last I have seen a video (https://vimeo.com/josecamachopd/pplastic) where they make MDF molds.
I wanted to know, according to your experience, the pros and cons of these different types of finishes. I suppose that in the technique #3 the molds are more economical, and in addition, you can heat the plastic in a conventional oven (not the PP machine).
What can you tell me about these techniques, in terms of finished products and production times or something else.
Not sure. MDF is fabricated at temperatures comparable to the plastic temperature. I don’t know what is released at that point. If you look at section VI of a typical MSDS http://www.hardwoodweb.com/architectural/docs/MSDSUltraStock.pdf it talks about some of what can be released during thermal decomposition (without specifying a temperature). I would research it a bit before a lot of use.
As far as using it, I’ve used wood molds with my injection machine. I use aluminum tape on the contact surfaces to avoid sticking, and make sure all the thicknesses are robust enough to handle the pressure. For a limited number of runs it works well.
Heating MDF might indeed not be the healthiest way to go, at least not indoors, without ventilation and a mask, but the same goes for working with the plastics so the point could be made that if using MDF as a mould is not safe nor is melting the plastic!
Combining the techniques (MDF with tape/foil) also might improve both.
I can’t use MDF in an oven, it catches on fire and off-gasses some really nasty smelling stuff. I have used MDF for molds outside the oven and it works reasonably well but only for a few uses. They tend to break down under pressure and then don’t apply pressure equally unless you are using a very large press with steel plates on top and bottom that are larger than your mold. If you try using c-clamps or a vice, the MDF bows and finished product is uneven.
Steel forms are much more reliable and consistent. I use them in a compression oven with good success. I can apply a lot of pressure, don’t need to worry about the mold melting or catching fire, and I can reuse/improve the mold over and over again.
I’ve never seen HDPE melt to the point you can “pour it”. It will burn long before it turns liquid. I’m guessing whatever was poured is something entirely different then recycled plastic, maybe a two part plastic resin?
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