I saw these on a styrofoam product the other day and realized that I had seen them before. Not all styrofoam has them and I had to do a little searching to find a photo (here.
Any ideas?
Something to do with release of the molded material from the mold?
Could be to keep multiple sheets slightly apart when stacked so they don’t suck into each other when you try to lift one off the pile.
Just a guess, but maybe it’s to help if you glue it to something (rough surfaces adhere better than smooth).
Note that this sort of packaging material is actually expanded polystyrene, not Styrofoam. And this page that describes the manufacturing process offers a clue.
Another step in the production process which is unique to FPM [Fabricated Packaging Materials, the company whose website I linked to] is the perforation process. Immediately after coming out of the mold, the surface of each block is perforated with thousands of tiny holes. The purpose of this step is to help moisture to escape the freshly molded block. The molding process leaves a smooth skin on the outside of each block and the perforator allows the moisture, a result of the steam, to more easily escape from the block.
So perhaps what you are seeing are the result of these perforations.
There is possibly tiny rods that reach almost to the inner surface of the mold, as the mold opens they push the part out.
The answer seems to be that they are gas vents, in particular vents to allow steam to be injected into the mould.
I was rather surprised by this, but the story seems to be this:
Polystyrene foam is made by creating a solid polymer that has a blowing agent incorporated into it (often during the polymerisation process.) The solid product may be made into small beads or flat sheets. In either case, when heated the material softens and the blowing agent expands and makes creates the foam - increasing volume about 40 times.
Sheet material can be used directly in some moulds, or expanded to make thick sheets. But if you want to make a complex moulding, like form fitting packaging, beads are used, and are blown into the mound, and then steam is blown in through all the little vents (and we assume some vents allow for gas escape) and the beads expand and stick together. Which explains why the foam is clearly granular, and why there are all those funny rings of dots.
The actual vents are made as a separate commodity item for use in moulds, which is why they are all very similar, and have a noticeable ring around the dots. Also why sometime you don’t see dots, but little lines. Still jets, but a slotted vent design.
Since the polystyrene is never actually molten, or under much pressure, there isn’t an issue with material flowing out through the vents. The surface of the expanding beads just butts up against the vents and gets a little imprint of the holes in the vents.
I wonder if the polystyrine beads used in beanbags and “stuffed” animals is the same beads used in molding, repurposed?
Same thought occurred to me. I suspect the answer is yes. At least they probably are made from the same feedstock.
I have seen, from time to time, manufacturing processes where a flat, lightweight, but unwieldy component is removed from a mould via suction cups.
I wager the polystyrene is still warm and pliable and slightly expands into the suction cups, leaving behind the raised bumps as the piece fully sets.
Have a look here: Mold Design & Build
Pics clearly show the vent holes in the moulds used to allow air to escape as the partly expended beads a re blown in, and allow steam to be injected to finish the expansion and complete the part.
Possibly made from the same beads, but they would be hard and sharp little bits resembling sand before being expanded at all.
Hard, but not necessarily sharp - they are typically tiny spheres of actually quite uniform size - incredibly dangerous as a slip hazard if they are spilled on a hard flat surface. I remember getting to play with these in Physics class at school - a scattering of them inside a flat tray makes a nearly frictionless environment for flat objects to be pushed about - sort of like an air hockey table (we were studying forces, collisions etc)
The stuff I got to touch reminded me of sand but maybe ‘sharp’ isn’t a good description. I don’t know what it’s purpose was but it sounds like what you worked with was a bit finer in size. There are many manufacturing processes that use the stuff in different ways. The processes are more complicated than they sound in simple summaries, there are multiple expansion and fusion phases, all sorts of ways to affect the density, and extruded products that are much stronger materials than the expanded (beaded) material that is so common. The pictures in the OP remind me of minimum density material where they don’t want to use excess ,material to be compressed to a smooth hard finish on the exposed surfaces.
I think there are pellets of different shapes and sizes, but the ones in the common expanded polystyrene blocks where you can see it’s made of spheres (and indeed the stuff that becomes beanbag fill) start off as little balls
Francis, great answer! Thanks.