Let’s start with the scenario in which you know the part weight. I like to work in grams but most machine specifications are in ounces. If the part, runner (cold), and sprue together weigh 164 g, the equivalent is 182.2 cc of PP (at 0.90 g/cc density), or 6.16 oz (1 cc = 0.0338 oz). Assuming you have a barrel capacity of 8 oz, you might be tempted to conclude that you have plenty of shot volume. Unfortunately this is where things start to unravel.
First, what if the machine-capacity specification in ounces is for one material, often polystyrene, and you are running polypropylene? (Note: Many machine specs today quote barrel capacity both for PS and HDPE.) There is an important density difference between these resins. At room temperature the density of PS is 1.04 g/cm3 and PP has a room-temperature density 0.90 g/cm3. You might then think, “Okay, for my 6.16 oz of PP you can take the ratio of densities 1.04/0.90 or 1.16 and arrive at a required barrel capacity of 7.14 oz.” With an 8-oz barrel you’re still okay, right?
Unfortunately, this is when Mr. Murphy usually knocks with his lesson that what can go wrong usually does. He’ll remind you that these are room-temperature densities, and in molding we deal in melt-temperature conditions. Sorry, there’s another step of complexity: dealing with melt densities.
When melted, the polymer molecules are farther apart and density decreases. The problem is that melt-density data is harder to come by. It is not on many material specification sheets and you have to do a bit of research to find it. (Plastics Technology’s PLASPEC Global Material Datacenter is one place to look.)
For PS the melt density is 0.945 g/cc and for PP it is about 0.74 g/cc. This changes the ratio for our calculation; it is now 0.945/0.74 or 1.28. Now we need a minimum of 6.16 x 1.28 or 7.88 oz of shot capacity. We are still (just barely) within our 8-oz barrel capacity, but unfortunately Mr. Murphy is still lurking.
Consider our check valve: It leaks! Is it practical to try to use 98% of the barrel capacity (7.88/8.0 oz)? Not in my book, and you need some cushion and room for decompression—especially for hot runners. I recommend shot-sizes of 25% to 70% of barrel capacity. If we add all this up, plus a 10% safety factor for check-valve leakage, cushion, and decompression and to ensure uniformly melted plastic, we need a barrel with at least 11-12 oz capacity.
But we’re not done yet. We have to think about plasticating capacity. PP is a semi-crystalline resin and it melts differently, perhaps more stubbornly, than amorphous resins. Generally, semi-crystalline resins stay hard up to their melting point and often require twice as many BTUs to melt as amorphous resins. This double whammy makes semi-crystalline resins significantly harder to melt uniformly than amorphous ones. You don’t want to use too much of your barrel capacity, or you’d be running the risk of having unmelted or partially melted solids in the melt stream. Warpage, shrinkage, and physical properties would be significantly affected.
Once again, you do not want to underestimate this detail of properly calculating shot size vs. barrel capacity. Too much is at stake. Send me your email address and I’ll send you a spreadsheet to do this calculation. You plug in ounces or grams and resin melt density, then it calculates the suggested barrel capacity in ounces.
The next task will be to determine if your screw can melt this shot within the quoted cycle time. That’s for the next column.
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