Everything you always wanted to know about screws

In an earlier column, I promised to talk about the heart of an injection molding machine, the screw. What’s so important that the lowly screw warrants a whole column? Like most aspects of plastics processing, it’s about money. But first, some screw basics.

The screw rotates inside the injection molding machine’s barrel and performs three basic functions. The first function is to introduce resin pellets into the barrel. The second is to “compress” and shear the pellets in the heated section of the barrel, which melts and homogenizes the resin. The third involves metering the melt into proper quantities so the resin can be shot into the mold. Simple, right?

“Universal” screws work well with a wide range of straight polymers when using injection molding machines with moderate speeds, particularly when molding non-critical parts out of commodity resins. However, there are limits to using such “universal” screws, especially when used with exotic materials, or with higher machine throughput rates.

Materials are a critical factor when selecting the right screw. Heavily filled materials lower costs, but what’s the driving force behind filler choice? The answer is cost, cost and cost, and maybe compatibility with colourants or additives.

But abrasiveness is another key consideration because highly abrasive fillers can be hard on screws, which rely on a close and consistent fit in the barrel for proper plasticization and melt homogeneity. The resin itself also plays a role; heat and shear make the melt, but they also decompose some of the resin chemistry, which can result in some pretty corrosive compounds.

A good way to prevent screw abrasion and corrosion is by choosing a screw made of steels with a higher alloy content (stainless grades are good, but expensive) and/or hard chroming of the screw’s surface. It’s also important to set up the machine to use the minimum heat and shear necessary for a smooth melt.

In a perfect world — meaning a new press is bought for a single material and process — the screw’s length, which is expressed as a ratio of its diameter to overall length (L/D), surface treatment and profile are matched to the resin and to the required throughput rates. For most small- to medium-sized job shops, that’s a pipe dream, so it’s all about matching the job to the machines on hand.

Switching materials or even cranking up the press speed can be limited by the screw’s design. If the resin you’re using evolves gases during the plasticization process, you’ll likely need a screw that feeds, compresses, decompresses and vents them, and then recompresses and meters the melt. The extra venting function changes the effective L/D, which changes the polymer’s residence time in the barrel, which will then require different speeds and heat settings.

Is a screw change the answer? Maybe, but remember that the screw and barrel are a matched assembly, so if wear is a factor, the barrel needs to be checked carefully. Screws can be rebuilt and re-chromed economically, but pulling a screw on all but the smallest presses requires a gentle touch and significant machine downtime.

The screw must be supported along its length during removal and installation. It must never be allowed to “hang” in the barrel. It should be crated and supported on wooden V-blocks, and never handled with chains or forklifts without protection.

My favourite way to destroy a screw is to start up a press with solidified resin in the barrel. By doing so you can snap a screw like a toothpick. Modern machines are smart enough to prevent this kind of abuse, but it’s an example of what can happen if you’ve forgotten to chug your Monday morning litre of coffee.

Stay tuned for next month where I’ll talk more about screws, then I’ll head straight down the barrel towards the mold.