Is Polyurethane Foam Repeatable?

Are there any uncontrolled variables? I can think of:

- temperature of mix entering mold

- storage temperature of constituents prior to mixing (would affect dissolved air and possibly shot temperature)

- mold temperature

I could imagine that a small jacket temp change or change in the temp gradient inside the mold contents could have a large effect on the relative speeds of polymerization and foaming

We did some diffrent testing yesterday, at diffrent temperatures. What you are saying confirms these results.

We are injecting the foam at room temperature but the mold is set to 125°. We did some testing at lower temperatures and got a better skin with less air bubbles.

However when i "disect" the part along the length, i have diffrent cell structure from one end to the other. The fill side has a small cell, yet the rise side has large air bubbles. This is consistent on all temperature tests.

Any other suggestions? I appreciate all the responses.

That sounds as if you have insufficient mixing inside the mold, particularly towards the end of the fill. Sometimes that can be controlled via the location of the inlet; however, many product shapes are effectively impossible to make even by mixing, so you need to use other techniques.

If the fill is heated as it enters the mold, this could cause the initial fill to be hotter during expansion than later sections - however, you say this happens at all mold temperatures (including room?), in which case it is not so likely.

Examples of techniques that can help even out bubble sizes include using a relatively small outlet vent (or even a tube) so that there is reasonable pressure at the top of the mold before and during setting. This need not significantly reduce the rate of fill. If you can orient the mold during filling to minimise the vertical depth, this too can help. Turning the mold upside-down once it is full may help a little, but possibly not enough to notice (because once surface tension and/or wall thickness between bubbles become important the pressure inside larger bubbles will be lower than that inside smaller ones).

Another thing that can help is to vary the rate that the material enters the mold - but I suspect your equipment is not capable of this.

Good luck.

heat not controled correctly with cause the bubbles.

Without detail it's hard to comment sensibly, but it sounds as if either you not adequately controlling the temperature and other conditions (e.g. mold surface coating), or you are operating outside the region where the system can work. Obviously, you can't expect to apply the same nominal conditions for different charges or mold sizes, so I'm assuming that the variations you are seeing are between injections into identical molds and under nominally identical conditions.

Here are some examples of specific possibilities that would exceed operating ranges:

Overheating the mix could accelerate setting to the point where the viscosity is changing markedly during the injection.

Most forms of injection acceleration can result in increased bubble size, but depending on formulation the effect of increased temperature can be extreme. This is liable to cause variation in surface finish between different points on the product. (You don't report this specifically, however)

The surface relies on the flow conditions. If entry speed is too rapid, you will get spraying rather than flow. If this is the problem, part of the solution might be to use a larger input orifice rather than increased pressure.

Other brief notes: Mold surface finish can have a surprising impact on visual repeatability. Might you be looking for a uniformity of finish that is usually achieved using a lower-foaming pre-charge to coat the surfaces?

http://www.sae.org/mags/AEI/3779

The process is repeatable however the number of intertwined parameters is significant. Let me know where you are located and I might be able to get you some onsite help with your process.

Michigan

Hi Fellow Michigander: Without changing the mold can you vary the fill rate? I was wondering if the higher heat input (which it picks up from the mold surface) into the first material entering the mold is causing it to expand more and have larger air bubbles as it goes on to fill the far side of the mold. Start fast and slow down?

GOD'S BLESSINGS Bill 80 miles North of hell