How an Injection Molder Works
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- Опубліковано 4 жов 2024
- How an injection molder works is a question I got recently from a viewer. Here is his full set of questions:
"I’m trying to understand how the injection molding machine works. It looks like a pneumatic piston presses down into a cylinder that is on a sliding axis. Is the force of the piston acting through the plastic the only force that makes the injector press against the mold? Is it a ratio of the piston diameter to the diameter of the injector orifice that causes a tight seal? The force pressing the injector against the mold doesn’t seem like it would be anywhere near the force that is holding the mold halves together presumably it does not leak."
"What is the spring function? Is it to ensure the injector lifts off from the mold when the pneumatic piston is removed?"
That was illustrative taking the cover off and exposing the spring mechanism. I imagined all kinds of worrying complications.
These explanations are really appreciated.
I'm glad I was able to answer your questions.
The idea in this machine and what you normally would call a injection molding machine is the same, but the difference is huge, like comparing a typewriter to a computer. Injection molding is interesting and could be a very precise process.
You're explanation of what goes on inside was very clear and easy to follow. To me, this may be the most important thing I have learned before delving into injection molding. It reminds me of when I learned how my FDM printer works; it's essential information. Thanks.
I was listening to Within Tolerance today and Breaking Taps was talking about his OM and I thought of you and tada! A new video arrives
Yea, we've had a few conversations about OMs. His machine is a little older, with the previous version of the controller.
There are great plastics engineering handbooks available. I highly recommend securing one. They contain a wealth of knowledge. Plus, The formulas needed to accurately calculate everything plastic.
Awesome video. Thank you.
Glad you liked it!
Enjoyed the video, and as I see it... if you have 6000 lbs of pressure in the mold, and you have 2 sq/inches... then indeed you have a force that can be used to lift or move 12,000 lbs... like a jack for instance. However it is wrong to say you have 12,000lbs of pressure in the mold... you don't, you have only 6000, but you could move 12,000. I don't know if this makes sense, but I ran into this with another engineer, and he had a claim of 100,000 pounds in his machine... but he only actually had a 1000 lbs per sq/inch. Sure multiplied he could do the work of the larger amount, but he only had 1000 lb's.
In the video I said it was 12,000 lbs of FORCE, not pressure (ua-cam.com/video/sDNzFKAk79g/v-deo.html) for a part that is 2 sq. in. Unless I misspoke somewhere else in the video, which is entirely possible. FYI, the above is only true as long as the mold halves don't move. If I recall correctly from my mechanics class I took years ago in college, the above is static pressure. But when the mold moves, it become dynamic pressure and you have to use completely different equations.
Great video and enjoying your channel. If you were just getting started in injections moulding which is the easiest plastic to start with? Thanks
Thanks. Polypropylene is probably one of the easiest plastics to start with. It flows really well.
@@JohnSL thanks I will try that first
@@JohnSL thanks I will try that first
how does the plastic get in the cyclinder?
Please John, adding screws to aluminium mold - like you did in the Travin demo - will increase the clamp force to how much ? 15 tons ? 20 tons ?
Thank you for your videos. Very informative !
It depends on the pull-out force of the screws that you use multiplied by the number of screws.
Hello John. I've been watching your videos for a while and find them very interesting. But I don't think it's correct to say that you can increase the pressure by reducing the diameter from 4 to 0.5. The pressure in a (static) liquid is the same everywhere. The pressure is determined by the force on the piston and its cross section. The flow rate increases in the smaller pipe, not the pressure.
Here's how to think about it. The pressure applied to the 4" air cylinder produces a force that is P x A. So the larger the air cylinder, the more force. That force is now applied to the plunger, which is only 0.5" in diameter. So now we have that same force pushing on the plastic in a 0.5" diameter cylinder. When you take that force and divide by the area of the 0.5" heated cylinder you get a much higher pressure/sq. in. (P = F/A) applied to the plastic than applied to the air cylinder. With the numbers I provided, that's a 64:1 (roughly) multiplier.
@@JohnSL OK. You're right. I just watched the video again. Now I understand your drawing. Thank you for your explanation.
Hi John, very nice and instructive videos, I saw every one! congrats! Let me ask, If I have a fixed clamping force, you said I can change injection pressure... Can you explain whats determine if a simple mold/part will going to need 6400psi, or 3000psi or even 1000psi to properly inject ?
There are several factors. First is the material. Some materials, like TPUs, are less viscous than other materials like ABS. That means for the same part, you would likely need more injection pressure with ABS than TPU. Next is the part or wall thickness. Thinner walls require more pressure to fill the cavities. Finally is how far the plastic has to flow from the gate. The longer the flow, the higher the pressure required to fill the mold. There is software to calculate this, but it's generally really expensive, so I just use experience and hope it works.
@@JohnSL Nice! Thanks, Understood! Do you know these small screw based injection machines ? (desktops ones) How they compare with piston based ? Whats your point of view about them ?
I found your channel from MarkP's videos.
Love his channel. Thanks for checking me out.
The injection cylinder is not adjustable for larger mold spacing on that machine. It is only adjusted for alignment with the injection cylinder. If adjusted it you would risk scoring and destroying the injection cylinder. Also it uses cartridge heaters for the heat not coils inside the casting. The thermometer on front is just for visual adjustments to the slow make slow break thermostat that controls the cartridge heaters.
Ah, good point about the front-to-back adjustment.
yay
When the piston enters the cylinder there is a big bite of air included. Does the air ever cause trouble?
I bore 3/8” holes in delrin spheres from McMaster-Carr as part of an assembly and they always have random air bubbles in the interior that intersect with the bore. Sometimes quite large, like a quarter inch bubble.
It's not something I worry about. But for Delrin spheres, I would guess that they're cast rather than injection molded. Material that is cast is far more likely to have bubbles in the resulting parts.
@@JohnSL Sir, Delrin is safely processed on injection machines equipt with reciprocating screws. I wouldn't recommend using it on a plunger-type machine shown in this video.The formalyhide gas could cause sever resperatory problems and fainting. Also delrin must be dried before processing.
Air in the shot will retard flow and cause vent burn ( bubbles)That's one of many limitations of plunger barrels, varying fill rates aren't possible with this machine.
Hi john, do you have an email or website to contact about some work?
Yes. If you click on the About tab on my channel, there is a VIEW EMAIL ADDRESS button.
what was the eink drawing tablet?
Yes, it's a SuperNote A5X I got recently (after debating about buying one for several months because of the cost). I realized after doing this video that I should have rotated it 180 degrees so I wouldn't keep hitting the controls with my palm. I've had it for about a week now.
Great video, and I confess the tablet was also what got me curious
The calculation for a pneumatic cylinders force is F=PA . F = force P = pressure A= area. The correct formula for area is radius squared x pi. When calculating the total pressure you need to subtract atmospheric the pressure of 14.7 psi. Because the pressure is only on the large side of the piston you do not subtract the area of the injection piston. Subtracting the smaller piston or piston end only comes into play if you are calculating the force on the retraction side.
How is pi * d^2 / 4 not correct? Regarding the other things, true, my calculation wasn't exact. And I didn't mean for it to be exact, but rather illustrative. The same is true about clamping force. The actual force pushing the mold halves apart is a lot more complicated and will almost certainly be less than the simple calculation I used.
@@JohnSL It accomplishes the same. However if you look up the proper formula to calculate the area of a circle it is radius squared x pi. Keep up the great videos and content I much appreciate them as well as many others.