This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

NEW! Free Purging Cost Analysis App

Routsis Training has developed a new Purging Cost Analyzer app to help scientific injection molders compare the cost of different purging methods. This free tool serves as a companion to the purging strategies discussed in our Purging for Scientific Molders online training series.

  • Automatically calculate costs associated with purging compound, machine downtime, labor, production resin, scrap & rework
  • Compare as many different purging compounds and/or methods as you like
  • Print a detailed cost analysis of your chosen purging compounds and methods
  • Export your cost analysis data to a spreadsheet (CSV format)
  • Use your smartphone machine-side to conveniently record purging data

This app runs on any modern computer or handheld device with an internet connection.

Click here to access the software.

Never Empty the Barrel

The only time you should empty the barrel of your machine is when you are preparing to remove the screw.


When discussing purging with most injection molders, the first step they follow is to empty the barrel of material. This practice is much less common in both extrusion and blow molding, but for some reason is commonplace with injection molders.


Once you empty the barrel it becomes a hot air oven which bakes all left over material directly to the screw. This also removes all pressure in the barrel when the next material or purging compound is introduced, thus eliminating positive conveyance until the barrel is completely full again. The only good reason to empty a barrel of material is immediately before the screw is to be pulled.

Best Practice:

You should always keep the barrel full of material at all times. The best practice is to use a mirror to view the feedthroat and stop purging once the screw flights are fully visible. At this time, introduce the new material so that the barrel remains full of material at all times.

You should always ensure that the barrel is full of safe, heat stable material whenever the machine is to be shut down or stopped for any period of time. If you are running an unstable material, you should always have a safe, heat stable, material nearby available for use at a moments notice.

There are many other important aspects to proper and efficient purging, but maintaining a full barrel is critical.

Excessive Injection with Multi-Cavity Molds

You should always transfer from injection to packing with all cavities short-shot.


Let’s say a mold is imbalanced, the injection speed is 4 IPS and the mold is filled to 95% full. The first cavities finish filling at one speed and the last cavity to fill would fill faster than the first cavities. Why would it do that?

My Response:

What was being discussed is that if the screw is moving at a constant speed (such as 4in/s) then there is a constant amount of material being forced into the mold. In the case of a 1″ dia screw, this would be ~3 cu-in/s

In the case of a 4 cavity mold, all 4 cavities are receiving 25% of this flow or ~.75 cu-in/s.

Once the first cavity fills, the remaining 3 cavities are receiving 33% of the flow or ~1 cu-in/s.

Once the second cavity fills, the remaining 2 cavities will split the flow and get ~1.5 cu-in/s

Following this logic, the last cavity to fill will receive the full 3 cu-in/s.

Additional Thoughts:

During this process, the injection pressure is increasing, viscosity is shifting, and the mold cavities are packing with injection pressure for differing amounts of time before transfer. When the screw bounces back at transfer, there will also be a differing degree of back flow from each cavity causing more cavity to cavity variations and shot to shot inconsistencies.

If you transfer when all cavities are short, then they will all complete filling and pack with the same pressure. This gives you the highest potential to get equal dimensions across all cavities. This also reduces your clamp tonnage requirements as well as make the requirements unchanging over time.

Improving the Strength of Ribbon

Increasing the rate of production is an easy way to increase the linear strength of an extrudate.


We are blending 3 different types of PE as well as adding rubber to produce a decorative ribbon. Should we also add a grade of PP to increase the product strength?

My Response:

Blending more materials would only weaken the material. When you blend different grades of semi-crystalline polymers, you will decrease the overall strength of the polymer. This occurs because the different grades will actually hinder the semi-crystallinity of the primary material. If you increase the amount of the primary material in your blend, you will likely increase the strength of your final extrudate.

More strength would also come from faster production as you will increase the orientation in your extrudate.



How is Clamp Tonnage Adjusted?

There are two types of systems used to generate tonnage, Hydraulic Cylinders and Toggle Linkage Systems. Many toggle systems used hydraulic cylinders to actuate them, but the actual tonnage is applied through the mechanical advantage of the linkage system.


How is clamp tonnage adjusted?

My Response:

On a toggle clamp, the tonnage is generated by the mechanical linkages and the
stretch of the tie bar. The movement of the rear platen (die height) adjusts
the clamp tonnage on this machine.

On a cylinder clamp, there is no toggle, just one or more hydraulic cylinders
holding the mold closed. The adjustment of the hydraulic pressure adjusts clamp
tonnage on this machine.

Additional Comments:

On a cylinder clamp, the tonnage is determined by a direct relationship between the hydraulic pressure and the clamp tonnage. ie. Max pressure = Max tonnage.

On a toggle clamp, the tonnage is determined by an indirect measurement of the tie bar stretch using a strain gauge on one or more tie bars. ie. Max strain = Max tonnage.