Combined Extruder and Injector

Hello everyone, I have a design which combines the extruder and injector. The extruder design is modified so that the screw can move back and forth, thus functioning as an injector piston.



Please provide feedback about the feasibility of such a design.


when my team were making the compression moulds, we found that the shredded plastic took up much more volume compared to when they were molten and moulded


so this leads to another advantage of using the screw to ‘pre-compress’ the plastic in the heated pipe:
in the original injector, the shredded plastic is poured straight into the heated pipe, in the form of irregular and poorly packed solids, and the piston has to compress the plastic as it melts to squeeze out all the air between the shredded plastic before actually extruding it outwards, and as such the actual volume of plastic is a fraction of the working volume of the injector (area of pipe hole x stroke length = volume)…
but if the plastic is already compressed by the screw and it is already in a liquid state before it is injected, then 100% of the working volume of the injection action is filled with plastic, and although my design has a shorter stroke length , it may actually deliver more plastic and be able to make bigger injection moulds

its better to ask a local engineering workshop directly to design something like this for you, because they would know if they are able to make it using their capabilities

they might also give you suggestions for improving the strength , usability and accuracy of the part you are making

Are you able to design a nozzle that can extrude a little plastic pipe? about 5mm internal diameter and 1mm thickness, I would like to extrude pipe for a nice project to recycle plastic. Thank you in advance.

in the scenario of a precious plastic injector immediately following an injection operation, some plastic would likely have flowed back up along the gap between the solid rod and the pipe wall, and if the rod is then pulled back up for the next injection operation, it will carry this plasic to an unheated part of the pipe


but in the case of using the screw as in my design, there is no need for the screw to egress as its fully ingressed position is the same as it would be in an extruder

the mechanical advantage of the motor gearbox will keep it in control of the rotation of the screw

the lever at the back can be extended to give more leverage, the cantilever converts the vertical force to horizontal force while adding even more leverage due to the shorter stroke length
so it should easily equal or exceed what is possible with the original injection machine

the function of the non return valve is simply to stop backflow during the high pressure injection, a cursory search online shows that not every plastic injection machine is equipped with it anyway, the viscosity of molten plastic should not allow it to easily flow back along the helix of the screw, especially with the thin depth at the ‘metering zone’ in the front of the screw , and the fact that the plastic will have to travel a much further distance back along the screw helix instead of simply exiting into the mould

furthermore if you are comparing the quality of the machined screw to the apparently loose-fitting solid rod used in Dave Hakken’s video , its likely that the screw would give a better seal, depending on the quality of the rods you can source




@carmatic. A few questions…how do you keep the screw from rotating during injection?  How much force do you think you can exert horizontally?  Most manual presses are quite small. Typically one needs at least 5000-6000psi injection pressure for plastic. I think you should read more on why the non-return valve is located at the end of an injection screw before you delete it from your design. Also, your design will take quite a bit of time to build so I hate for you to spend a lot of time to build a boat anchor. Again not trying to be negative here but possibly help,you avoid a lot of heart ache. I like your ideas. They just need a little physics refinement.

i tried to make it similar to an industrial plastic injection machine in this video, but using the extrusion screw from the ‘quest for the ulitmate extrusion screw‘ , so there is no valve at the tip of the screw

interesting! what was you motivation behind that ? I mean, do you use this on a daily base,… ?

what software are you using to design and animate?

This is a cutaway view of the ‘plunger’ , showing how the pointed end of the shaft meets the cap of the pipe, thus allowing the pipe to apply longitudinal force  upon the shaft without causing excessive friction for rotation

The shaft is supported by the 6000 bearing

This is a view of the rear part where the horizontal ‘plunger’ is

It is constructed of 4 parts, a 6000 bearing holding the shaft, a 26mmx30mm pipe which holds the bearing,  a 30mm square bar which holds the pipe , and a pair of metal plates which connects to the cantilever at the rear.

The 30mm square bar of the plunger sits on a longer 30mm square bar, which acts as a rail for the plunger

This is a view of the arrangement of the sprocket, bearing, and hex shaft