Thermoplastic structural foams have been around for several decades now. We all know that, at the same part weight, the stiffness of the structure increases in proportion to the reduction in plastic density. And that at the same part thickness, the weight decreases in proportion to that same reduction in plastic density. Injection molders and extruders have developed new equipment and methodologies to produce structures with uniform density foam and structures with skins. Even blow molders and thermoformers have produced foam parts.
So, why do rotomolders have such a difficult time foaming? After all, we start with powder that is sinter-melted. And as everyone will attest, our biggest problem is getting rid of the bubbles! So, why do we struggle?
In an ANTEC paper (ANTEC 2000) and a RETEC paper (Cleveland 2002), I’ve detailed the dichotomy between the rotomolding process conditions and the conditions necessary to produce controlled bubble nucleation and growth. Furthermore, I’ve described the entire field of foam technology in my 1996 book, Thermoplastic Foams, unfortunately now out of print. So I will not go into these details in this note.
Instead I want to focus on how I believe the rotomolding process needs to be changed in order to produce quality foam parts.
To begin with, consider the various ingredients in the recipe. In general everyday rotational molding, we like a polymer that has relatively little melt elasticity. We know from results of many research papers that elasticity slows the sinter-melting process. Unfortunately, for foams, we need a fair amount of elasticity. The polymer must resist the internal gas pressure of the growing bubble. If it doesn’t, the foam will collapse. I may return to this point later.
Then the blowing agent needs to be properly selected. The blowing agent used in rotational molding is a pure chemical that decomposes to produce the blowing gas at a very specific temperature range. It is imperative that the blowing agent decompose in a temperature range greater than the melt temperature range of the polymer. If the blowing agent decomposes while the sintering step is in progress, the gas will simply escape to join the mold cavity air. I know that many people are using 4,4′-oxybis (benzene-sulfonyl) hydrazine, OBSH or OT, for polyethylene. I prefer azodicarbonamide or AZ. The decomposition temperature for OBSH is 160oC and the gas yield is only 125 cm3/g, whereas the decomposition temperature for AZ is 195oC and the gas yield is 220 cm3/g. The higher decomposition temperature gives me a temperature “cushion” for LLDPE and HDPE. And the higher gas yield means that I don’t need as much blowing agent, which helps reduce the cost.
And the third element is the rest of the ‘stuff’ that we add to the mix. One must be very careful of certain colorants, especially carbon black or iron oxide pigments. These additives can act to adsorb the blowing gases or complex with the chemical blowing agents, thus reducing the blowing agent efficiency. Sometimes dramatically.
Now that brings us to another aspect, the nature of the powder. In other disciplines, such as injection molding, the finely divided blowing agent powder is sometimes added directly to the polymer pellets at the hopper of the injection-molding machine. Although this is not recommended, it is done to determine the optimum level of blowing agent needed to achieve a given density reduction. In rotational molding, the blowing agent must be compounded into the polymer prior to grinding. It is the only sure way of getting uniform dispersion of the sometimes very sticky blowing agent powder.
The nature of the foamed structure depends on its end-use. Foams, in general, have crappy surfaces. [For cosmetically desired surfaces, a two-step process is recommended. Here I'll only discuss the single-foam process, I'll deal with the 'two-step' in a later technical note.] Does this mean you cannot make an acceptable structure by simply pouring compounded powder into the mold’ It means that you’re going to work a lot harder to achieve a quality surface. And you’ll probably never achieve the surface of an unfoamed rotationally molded part.
Okay, so the only things we need to do is compound the blowing agent into the polymer, grind it to the right size, and drop it in, right? Wrong! For some reason, the “instant gratification” or “instant results” mentality pervades the rotational molding industry. Remember the injection molder? And the extruder? These guys worked dozens of years to perfect their process. In those years, they invented dozens of processes, most of which were not very successful.
I’ve consulted with several rotomolders who insist that “you’ve gotta make it work! Now!” When I recommend process changes they are aghast at my recommendations. I’ve even had a couple refuse to pay me when I told them how to make good quality one-step foam. Their response to me? “That’s ridiculous!”, “That’s stupid!”, “We can’t afford another second on cycle time!” And so on.
So, since many of you won’t hire me or even pay me to help you make quality one-step foam, I’ll give you my approach for free! So listen up!
First, invest in quality tooling. Sheet metal just won’t hack it. One plugged vent tube and the internal gas pressure will turn your part into a sphere. Then increase the wall thickness of your mold. A lot. And replace that inane Teflon tube with the fiberglass that you call a vent. Use a pressure control valve arrangement so that you can control the inner mold pressure.
Wait! You’re just starting! Then make certain that you are using AZ rather than OBSH with LLDPE. And that you have no gas-absorbing pigments compounded in. And use a blowing agent that liberates nitrogen, not carbon dioxide! After all, 79% of the air already in the mold cavity is nitrogen! And then there’s this thing about concentration gradients! Never mind! Just use AZ! It works!
Now we get to the process changes. Run an empty mold into your oven. Measure the time-dependent mold temperature with a hand-held infrared pyrometer. Record the time when the mold temperature gets to 180oC. Now program your oven to hold that temperature for several (or many) minutes. And program your mold inner air pressure to 10 to 15 psi (or whatever your mold is designed for).
Now program your oven to increase the measured mold temperature to 210oC at the rate of 5oC/minute. Now have your oven hold that mold temperature for several (or many) minutes. [Note that I am saying the mold temperature, NOT the air temperature!] Finally, program your system to remove the arm and mold assembly from the oven to quiescent (still) air for several (or many) minutes.
With this profile stored, you are now ready to run your first part. Obviously you will need to play with the hold times, “the several (or many) minutes” things I mentioned earlier. And you’ll also need to play with the cooling sequence. If you hit the hot foam too quickly with cold ambient temperatures, it will collapse. Just like momma’s cake or your first attempt at a soufflé.
Now if you’re like most rotomolders, you’re already screaming at my suggestion of lengthening the already-absurdly-long oven cycle time. “That’s ridiculous!. that’s stupid!” And you’re screaming that your customer won’t stand for the increased cycle time cost or the added expense in the polymer. And you’ll probably rail at me for even suggesting that you have the blowing agent compounded into the polymer. Or that you spend money for more robust molds. Or pressure control valves. “We just can’t afford it!”
And, like the other rotomolders who have ignored my suggestions, you’ll struggle to produce parts without uneven quality surfaces, coarse cell structure, nonuniform density reduction, breakthroughs, blow-outs, and huge voids.
But, if you really want to make quality foam structures, I’m available to guide you through these steps. For a consulting fee, of course.
[In my next epistle, I'll discuss doing the 'two-step.']