PVC -- It's Great for Inventions

Step 5: Using molds

A mold is a shape that is used to create another shape. In the case of the toilet paper roll holder shown below, the hole at the end of the central pipe served as the "female" part of the mold. The ball of a ball peen hammer served as the "male" part of the mold. Between the two of them, they forced the flat plastic into a domed shape. When the PVC cooled, it hardened again.

The dome locks into the end of the pipe section upon which the roll of toilet paper spins.

Comments

[blowbyblown]

 can pvc be melted down and poured into a mold though?

[fishdirt]

Yeah but it's a high melt. IOW you need a steel die mold. I've been hunting alternatives but can't find any. 350 F is the melt point. You do it in a double boiler pot or with a heat gun until it becomes clear. Another alternative is to buy hot melt vinyl (Vinyl is PVC and vice versa). Vinyl fumes are highly toxic. A safer way, if you have the cash, is smooth on polyurethane resins. It's a soft, rubbery type plastic as well and not as fragile as the acrylic resins.

[Thinkenstein]

There are techniques for casting PVC in molds, or we wouldn't have PVC faucets, etc.  Probably in a home workshop one would not be successful, for lack of special equipment.   Perhaps it needs high pressure and absence of oxygen?

I talked with someone at a PVC factory once about getting second quality pipe cheap.   He said that any defective product just went back into the process again and was extruded anew.  

Anyway, I wouldn't attempt trying casting like that.  You might set it all on fire by accident. 

[blowbyblown]

 well i live on a farm, so i've got plenty of room to not to burn anything down

[Thinkenstein]

I was thinking more about the plastic burning.  Someone wrote that once it catches fire it can continue burning without oxygen, and would be hard to put out.  I've never had that experience, but it pays to be cautious.  Burning PVC makes some nasty fumes. 

[mettaurlover]

Plastic doesn't continue burning without oxygen in my experience. The only thing that I've had do that to me was thermite and that's because it's supposed to do so.

[Thinkenstein]

I tend to agree with you. It doesn't seem like anything would burn without oxygen. I thought that perhaps chlorine in the PVC might be the oxidizer, to explain the supposed burning. The thermite brings its own oxygen to burn.

[mettaurlover]

Yeah, though oxidization, by definition, requires oxygen to occur.

[Thinkenstein]

Maybe. Quite likely. I vaguely recall that in a chemical reaction it has something to do with which one provides the free electrons as to which one is considered to be the oxidizer. That is something like a 45 year old vague recollection from high school chemistry. I am probably wrong.

[mettaurlover]

Yep, you are-oxidation is called that because it always involves oxygen. Burning is just another name for that process.

[Thinkenstein]

From my dictionary: "Oxidation reduction, n. Chem. a chemical reaction by which electrons are transferred or shared by atoms forming a compound." As I recall, the oxidizer need not be oxygen. Of course it can be. -- Maybe "burning" is an oxidation reduction reaction that always involves oxygen. Maybe it isn't. Maybe chlorine could be the oxidizer in PVC burning, the same as iron oxide is the oxidizer in a thermite reaction with aluminum. -- Anyway, I'm no pro chemist, but I do vaguely recall that detail; that the oxidizer need not be oxygen. -- The important question is whether or not PVC can keep internally reacting under heat in the absence of outside oxygen once a reaction is started. The one who first mentioned that sounded like one who knew. It's an experiment I don't really want to run myself.

[mettaurlover]

True, but that generally assumes that oxygen is present as it is in almost every case except for a vacuum, in which case you shouldn't be trying to oxidize anything in the first place.

[Thinkenstein]

Anyway, it is misleading to non-chemists to use "oxidation" for a process that doesn't always use oxygen as the oxidizer. It's part of specialist jargon for chemists. -- That reminds me of an aluminum house siding salesman who guaranteed to all his customers that the siding would never rust. -- It would oxidize of course, but it wouldn't rust, since technically rust is iron oxide, not just any oxide. It was misleading in a way, which leads into business ethics and advertising.

[mettaurlover]

Exactly. I'm just coming out of a chemistry class from before the summer and have this stuff fresh in my mind.