Can You Cook Plastic Too Long?

You have to obey the manufacturer's instructions - unless you want to take the risk, yourself, of the moulded pieces falling apart.

Personally, I haven't go a clue. It may work, but there'll be no come-back if they all collapse in a couple of years - it'll be on your head.

Hello OleTechGuy

As JohnDG states above, best to use the combination of Roto-oven and plastic pellet/powder manufacturer's advice.

Generally Roto-ovens use Thermoplastics such as Polyethylene, Polystyrene, and these don't have a problem when exposed for a long period to slightly above melting point.

Often too, the objects are thick and require longer heating to ensure correct melting, and so the cross-linking can properly take place on the cooling cycle.

As JohnDG said, you don't want a product which parts fall out of, later, do you.

Thermosetting plastics are different, of course, because they require a one-shot heating/cooling process.

Kind Regards....

Thanks for the help. I have started working in this business and was looking for more info. When I asked the same questions of my co workers they had no answers other than "thats what we have been doing for 20 years". I'm more of an analyitical type so I like to know whys and whats happening.

I did some testing where I actually measured the temp in the moulds. The plastics mfg says that the melting point is 260 F. During the normal cycle we reached as high as 380 F. I then did a part where I went to 260 F and shut off the oven. The temp rose for another few min before starting to level out at 280 F, then dropped. When it again cooled to 260 F I started the cooling cycle. I removed the part when the inside temp was 160 F. The part looked the same, except that the inside (which you can't normally see) was not as glossy as the longer hotter one. The plastic had melted and fused just fine, and the outside finish was to the eye just the same. The part also was a little bit thicker. Without waiting 10 years, I'd like to know if the cooler part chemically is as good, better or the same as the hotter one?

The other contributors commented on the supplier having suggestions, well the suppliers was not very helpful.

I would also like to know, when the plastic is hot (> 260F), if it were to come into contact with ambient air, would that cause any reactions that might generate bubbles?

Thanks so much for your help, it is very hard to find anyone to ask about this stuff.

Thanks;

OleTechGuy

"...the inside (which you can't normally see) was not as glossy...The part also was a little bit thicker."

I think the anwers lie in your observations. I would liken the rough inner surface to asphalt before the steamroller, thicker but with less strength.

On the one hand, a smooth inner surface is easier to clean.

On the other hand, a rough surface can be more abrasion and cut resistant.

So, cleaning aside, if your part is thick (strong) enough minus the roughness, no worries.

Otherwise, go back to Plan A.

I think you got lots of comments about the finer points from several sources but no specif answers. Here we go: The problem is heat transfer, so the middle is not melted completely and thus not glossy nor consolidated (thicker part = lower density). This is perfectly fine if the part meets other functional requirements. If you need to worry about long term structural (fatigue) performance, you are in trouble not because of the molding but because you have chosen wrong plastic and the wrong process. As Rotomolded parts go, your seems fine. Aussie guy can probably tell you the thickness limitation of Rotomolding...cheers mate

Damn drago, 10 points for keeping us to form, fit and function.

Roto moulding is a very special process for plastics. The main advantage is that you can do a large hollow item with out a core and with a consistent wall thickness. This is where it beats injection moulding and blow moulding. It is a very small niche market.

The wall thickness is determined by how much plastic powder is in the mould and how long you expose it to the correct heat. As the heat soaks through the mould it liquefies the plastic powder that is directly in contact with the mould. As the mould is rotating the powder that hasn't yet melted falls/rolls on to the next bit of the mould or the already plasticised plastic. In this way it gradually builds up the thickness of the plastic until all the powder is in the moulding. This where the prototyping comes in to determine how long the required heat soaking and subsequent cooling time should be.

It is also possible to use a half mould. Heat the half mould evenly to the required temperature. Dump a large quantity of plastic powder into the mould so that a far thicker layer than would be required is achieved. Let the heat soak into the plastic powder to achieve the required plastic thickness then invert the mould to remove excess powder and cool the mould.

This was a technique we used for making plastic kayaks. By making them in top and bottom halves a fortune was saved in shipping costs.

The dies that we used were usually cast aluminium that was highly polished. The main reason for this was the thermal conductivity.

The oven temperature was as per the plastic manufacturer's specification and the timing was the critical factor. By increasing the temperature you may gain a little in cycle time but you run the risk of burning the plastic and increase the need maintenance on the dies. Tooling cost are usually cheaper for rotational moulding so it can be used for short runs that would not be cost effective for injection moulding.

Depends to some degree upon whether the Plastic is a thermoplast or a thermoset. Most roto molding I have met prefer the thermoplast. These contain a plasticizer which is a liquid with a high boiling point but is volatile. Using thermplast you need to have the temperature rise to a point at which the solid plastic dissolves into the plasticizer. In this way the viscosity will drop on initial heating and then increase as the dissolution takes place. Once complete dissolution is acheived you are at an end point and the mold can be cooled. Heating for too long can cause the plasticizer to volatilise too high a temperature will speed up the loss. Best temperature can be evaluated by putting a blob of plastic onto a preheated plate and after a short time to be determined experimentally tilt the plate to 45 degrees the run down distance will give measure of the time needed for the plastic solution to have formed as it will get more viscous. Over-heating of any plastic is inadvisable, a waste of energy & time.