Choosing the Right Screw and Barrel

It cannot be overstated that the process of selecting the appropriate screw and barrel combination for your plastics application is a critical part of the success or failure of any particular machine cell. However, most people may not realize how often that this very selection process is done in error or even done incompletely.

Unfortunately, it’s more often than you might think. Hard numbers are probably a bit elusive since most companies are not likely tracking this particular metric. Just ask any seasoned sales person and they are likely to have 5 stories off the top of their head that would corroborate. It’s probably not as bad as it once was in the industry, but in many cases the barrel and screw is sometimes a secondary conversation when purchasing a machine.

Why is that?

Oftentimes manufacturers are more focused on the performance of the machine, the control(s), the clamp speed, the dry cycle speed and a whole list of other general features and benefits when shopping for a new machine. That’s not to say that they are wrong to focus on clamp speed and performance, which in reality is critical since time is money, however, it’s arguable that the screw and barrel selection were an afterthought for a number of years in the industry.

Again, it’s not as bad as it used to be. However as companies are using more and more engineered resins, the pressure to be more productive and eliminate scrap has caused said companies to extend their focus to the options and selection of the screw and barrel. That pressure to be more productive has been compounded over the past 10 years as the industry has continued to see an ever evolving makeup of the plastics/resins used in products. With that continual evolution and advancement in plastics and resins, more attention is being placed on the selection of the right components for screw and barrel assemblies to handle the newer nuances of each material makeup.

If the clamp end of the machine is perfectly tuned but you aren’t delivering a quality melt and mix material to the mold, then a fast clamp will only help you make bad parts, faster.

So what goes into the selection of the right barrel and screw?

It’s probably fair to say that many sales conversations on technical matters, including the selection of an appropriate screw and barrel, begin with the phrase “it depends”. The variances and variety that exist by industry segment, material and even general application are lengthy, to say the least. Considerations such as the size of the machine, potential for screw wear, heat, cycle time, melt temperature, color mix, recovery time and so many more variables can make for a rather complex equation.
Understanding the challenges of the application is the first step. Now aside from a shameless plug for Milacron’s rather deep background and decades of industry expertise on the matter of “understanding the challenges of practically all plastics applications”, it is worth noting that having a strategic manufacturing partner to walk you through that process; can save you in the long run. The second step would be the design of the screw.

Quick Did You Know: The actual design of the screw, and not metallurgy, become more important the larger the diameter of the screw. They have more effect on the melt process than the heater bands in some cases. Consequently, the design of smaller screws involves more wear-related material decisions, of which the conversation then becomes a metallurgy or coating issue.

Generally speaking the ratio of importance in the selection between the screw and barrel should fall more so on the screw and non-return design/selection and metallurgy.

So if you are still keeping score, the need to select the right screw and barrel is critical for any application. However, when should this critical component be clarified in the commercial conversation? In reality, it isn’t discussed as early as it should be in the build-up for a brand new machine. It is not likely that you’ll find it at the top of a new machine conversation, but it should follow close behind.

More often than not, you’ll find that the topic of screw and barrels is discussed in an aftermarket setting. Even after a machine has been running for a long period of time, many molders forget to look at newer or improved screw and barrel technology. Continual improvements in screw and barrel sizing, coatings and metallurgy have shown to improve efficiency, performance and general material handling versus potential cost and changes related to the clamp end.

As plastics molders look at updating old machinery with brand new ones, or even just looking at retrofitting some existing machinery; make sure you take a look at your screw and barrel for some much needed performance and productivity.