What Is Back Pressure in Injection Molding?

RJG expert Trainer / Consultant, Jason Robinson, talks about the importance of back pressure in injection molding and how to use it to make better plastic injection molded parts.
Read more about back pressure on our blog: rjginc.com/wha...
#moldsmart #injectionmolding #materials #plastic #resin #systematicmolding #howto

[@DJPLAST2]

Back pressure increases the friction on the screw, basically from the inside out, as opposed to the heater bands from the outside in. This is very prominent in the extruder machine. Extruders have water cooled screws, just for this reason. Extruders will run after start up on frictional heat only. Extruders also have cooling fans on the heater bands, to dissipate too much heat generated by friction. I speak with over 40 years in the plastic business, both molding and extrusion. I am also an injection mold maker.

[@isaakeverson9083]

I ran single and twin screw extruders for six years, never once saw fluid cooled screws; it makes sense but i wonder what size screws you worked with?

[@uktgujhgvukhyjbj3619]

what do you suggest me to start in ? wich product ?

[@lolbored801]

Worked with one guy who would always change the backpressure. Didn't matter what the issue was. Flash, short shots. Air bubbles. It didnt matter he always changed the back pressure first. I would have to go in behind him and revert the changes. He claimed he had 15-20 years experience. My manager thought he was an amazing tech. I don't miss working there at all. Lol

[@rjg]

We understand your pain! The struggle is real! Hopefully he see this video! 😅

[@madjack821]

My boss would have a conniption if he saw us doing that 😂 I can hear him yelling in his Huckleberry Hound voice “whatn tha hell y’all doin?!?”

[@gambit1216]

Another great video explaining a fundamental portion of the injection molding process.

[@rjg]

We're glad you enjoyed it! Thanks for watching!

[@Randaddy25]

I just had an issue with HIPS, where I kept getting trapped gas in the barrel, leading to bubbled parts. I increased the back pressure, in an attempt to exhaust the gas through the hopper. I don't have any formal training, so you'll have to excuse me if anything I say is wrong. But, it seems to have worked. I have a ring of splay I'm trying to work out, next. Great video!

[@rjg]

Thanks for sharing and for watching!! 😁

[@sshashlan]

that was extremely helpful, Thank you for sharing

[@rjg]

You're welcome!

[@JosephMcFadden-nq4bd]

Good job, enjoyed your story telling to convey knowledge

[@notyouraveragegoldenpotato]

Ahh perfect. Building a machine myself and this was a question ive had that hasnt been well answered or addressed. Thanks

[@rjg]

Awesome!! We're so happy it was helpful to you! Thanks for watching!

[@33Karandeep]

Amazing video!!!

[@rjg]

We're so glad you enjoyed it! Thank you for watching!

[@robertleffler4286]

Good video like always, 👍 nice facility also was just there earlier this year 😀

[@rjg]

We're so happy you enjoyed it! And that you got to visit our facility! We hope to see you again soon! 🙂

[@manpreetsaini3467]

Please make a detailed video that how a molding machine works …like each step …injection pressure ,injection speed ,injection position in barrel ,screw recovery back pressure suck back

[@rjg]

That's a great idea!! We'll add it to our list. Thanks!! 😁

[@ianscianablo8507]

Hi. Any in depth intros to molding and how the different machine parts work? I'm still very new to this and I have a job in a mold room. Still a bit Greek to me. Thank you!!1

[@selvam65koodalingam]

Thank you sir.

[@SuperLslslslsls]

Thanks! We have a big PVC item, not filling all the time, I will try this.

[@rjg]

Good luck! Let us know how it goes!

[@user-fq1fz2vs9f]

You are master who courage to share your mistaken.

[@himalaysharma8]

Can you please give me a brief view on Injection Pressure and Hold pressure?

[@inquisitive980]

Machinery manufacturers like to boost the recovery rates of their machines in terms of oz/second. Therefore they design their screws for maximum recovery. This can lead to unmelted resin, poor mixing with colorants….. Back pressure will certainly help this situation.

[@mustafashapaan7141]

first thanks for your amazing content. second i have a question about the importance of Simulation of injection molding using Moldflow. do you use simulation for your products ? can you just run a DOE to figure out the best machine parameters for the plastic part without using any software ? and what about warpage and cycle time in moldflow a reverse shape is made to account for warpage that is something we cant change using machine parameters, so please tell me about the significance of simulation in the plastic business is it worth it or not ? thanks

[@rjg]

Thanks, Mustafa! We're glad you enjoyed it! Simulation is a very useful tool to save time and money. Yes we use and offer flow simulations as a service. A simulation can be done to an existing mold/part or it can be performed in the very early stages of part and mold design. Multiple iterations of part geometry, mold design, and processing parameters can be simulated before money is spent on mold construction. Warpage , cycle time, knit line formation, and strength can all be analyzed. So yes, a DOE can be performed during simulation. The money spent on simulation has huge advantages and can save thousands of dollars by finding potential issues before part designs are finalized and molds are constructed.

[@antonioreyes4842]

Great video. Just wondering why there is 2ejection cycles instead of one?.... lol

[@kinetic85]

My best guess is that it is because the product might not fall of at first ejection :)

[@dp6824]

BP is more heat more speed ( and better residency time ) if you need , you will sacrifice time for good parts in certain situations also , injection time will get faster adjust accordingly

[@tanveerkhan9221]

Can you please elaborate on gate freeze off point

[@kennethcoppins1842]

You forgot the real reason for Back Pressure - air will travel to the area of least resistance - no bp - air goes to the front of the screw

[@RogueLifeClothing]

I think still have some ice scrapers

[@CnC_pro]

Hello sir, can I talk with you about injection molding machine

[@majith9035]

Holding not working how to check

[@pro-plastics908]

Would you test part weight consistence for fill only parts.

[@Menyuamhmoobmekas]

Yes, weigh fill only parts and record them. That weight should be the part technicians use without any holding for troubleshooting.

[@payel410]

I need a job

[@payel410]

Hallo sir

[@user-mk4nm8yl3u]

So after that semi confusing information you advise to not adjust backpressure? All that nonsense about 'drying the material' via the back pressure setting was just added confusing drivel too. Hygroscopic material needs to bd suitably prepared and conditioned pre moulding, end of. Backpressure is purely an adjustable parameter to ensure consistency in melt preparation. Multiple factors can add inconsistency, wear, polymer, additives and environmental changes. You should use scientific optimisation to determine the ideal backpressure for each and every product you trial/run. Once the process is optimised and passed IQ there is no reason to adjust backpressure (or anything else for that matter) as any defects witnessed at this stage during normal production will stem from an underlying issure that needs revolving NOT by adjusting machine settings. Setters/techs - Dont piss about with machine settings if production was running fine beforehand, examine route cause first. Only once everything has been tripple checked should you consider batch to batch material variation and make minute changes to settings while informing your process engineer.

[@rjg]

Andrew, I think we agree on this. We teach a scientific, systematic methodology to find the optimal BP. My stories are there to provide context from my mistakes early in my career. Over my years of instructing, I have found that some people need a direct to the point piece of information and some of us like a story or analogy to drive the point home. Us molders are a tight knit community, we have to pass on what we are learning to the next generation. Thank you for the feed back. - Jason

[@user-mk4nm8yl3u]

​@rjg I'll be honest Jason, RJG are highly regarded in the UK as a training centre, that's why I watched the video. Are you related (same company)? My comment was almost to distinguish if you were a professional outfit and by your reply, you are. I appreciate you taking the time to reply and it seems we very likely are of the same thoughts. There's many ways to get your point across, I guess for me if you'd explain the optimisation through systematic study first then discussed past wrong doings I may have felt differently, but that's just my opinion, not necessarily the right one.

[@rjg]

@@user-mk4nm8yl3u Yes, we are the same company! We have offices around the globe, including the UK. We appreciate you watching and providing feedback! We certainly take it all into consideration. We hope you have a great day!

[@downbelowu1928]

This isn’t true, you are making the plastic hotter. Explain concentrate color mixing of the resin then? If you are just putting more material in the shot why does the color all of a sudden mix? You raised the friction and rotations it takes to get to your shot size and now have raised the melt temp. Basic physics, friction creates heat. Set your barrel temps at 400, they will increase on their own especially on the 3rd or 4th extruded zone with back pressure. It drools Bc you have heated the plastic up and now have changed the viscosity of the polymer. Your grandmother and your mom have products I molded in their kitchen. I have an rjg certificate, it doesn’t work in the real world and I’m questioning some of these statements.

[@rjg]

Thank you for the comment. I would like to address a few of you points for you to consider. 1. The industry used to think that the plastic had a dramatic increase in temperature when BP was increased. But with modern instrumentation , Thermocouples in the nozzle and in the cavity, we have learned that the increase is modest at best. If the plastic temp is directly measured with a pyrometer rather than the reading on the barrel thermocouples, you can usually see that the barrel reading does not always correspond to the actual plastic temperature. 2. I did state in the video that increasing the BP will improve color mixing, although this is not usually the best option to improve color mixing. 3. I hear the statement about the real world a lot, usually this means that the machinery is in poor condition. This can make us believe a lot of false truths because we are seeing the result of poorly maintained equipment. Another point to keep in mind is that equipment is evolving rapidly. The molding machines I use today are not always the same as the machines I used 20 years ago. We as plastic processors learn something new everyday. It is healthy to challenge and be challenged in what we believe to be true , this has to be done with data. This is a small knit community that seems to be shrinking. Lets try to lift each other up and encourage each other to be better. I would be happy to try and come to your plant and spend a day looking at some of these topics if you are interested. Thanks! -Jason

[@downbelowu1928]

@@rjg thanks for the reply, I was doing some day drinking on a rainy day off and just trolling a little. Wouldn’t a simpler test just to say make 100 shots without bp and use an infrared thermometer while purging then run 100 shots with bp to see the difference in temp? Unfortunately you are correct, machines all have the devil in them in one way or another. This is where real world comes into play. No machine is perfect in a manufacturing facility, you can do these test on a perfectly tuned perfect machine but that’s not what we have. Maintenance does their best but money and downtime always an object in the way. So tell me how concentrate color will mix better with bp? You have to be heating the molecules up to mix better, like I said more rotations/friction equals more heat.

[@rjg]

@@downbelowu1928 The increased BP creates more revolutions of the screw. The improved mixing has more to do with allowing the metering zone of the screw to do its job, mixing. Great idea to do 100 shots at one BP measure then repeat at a higher BP. Let us know your results.

[@dp6824]

you might want to check out / The check ring or what ever it has / and what is decomp ( thats what I call it )

[@dp6824]

4 30 guy is right More and viscosity will change rate of flow with will cahange Decoupled parts watch out for shorts and flash

[@BallZachMcgee26]

Dry your material!