Make Injection Molder: DIY Guide to Automated Processes

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In summary, an injection molder can produce parts up to 8" in cross sectional area with clamps of 30-50 tons and a pressure of up to 1,000 bar. The extruder uses a different type of injection screw that purges air without causing flash clogging. The mold can purge air without creating flash clogging the vent. The screw and check valve work like a plunger.
  • #1
Serj
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Hello,

I am making an injection molding machine for fun. There is a lack of available information regarding the details of how the process works.

There is something call the Lyman extruder that uses a wood auger bit as the injection screw. This auger is shaped much differently than the injection screws selling on alibaba.

1. How does the screw purge air?
2. How does the mold purge air without creating flash clogging the vent?
3. How do I calculate the needed pressure to fill a mold? (using melt flow index, gate size, and temperature)
4. How do I calculate the pressure a screw can generate on a fluid? (given X torque)

Thank you for your help!

*Disclaimer: this project is for fun, I know there's easier ways to do it like a plunger or 3d printer or resin casting. I currently do resin casting and 3d printing and such but I am obsessed with having an automated injection molder even if it can only make tiny parts and doesn't work very well.
 
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  • #2
I can see that you missed a question.

1. How do I make it safe?

An injection moulder making a part with 8" of cross sectional area will present you with the following major risks.
Clamp force of around 30 - 50t.
Injection pressure of up to 1,000 bar.
Barrel temperatures of over 200°C (~400°F).
Liquid plastic that sticks to your skin and takes a long time to cool.
Even a plasticising pressure (back pressure) of 5 bar is sufficient to splatter the material when you purge the barrel.
As your part gets smaller the clamp force requirement reduces. The other risks remain broadly the same.

Please think about these risks as you proceed.

As for you other questions
1. I don't actually know this, but I assume back up the feed throat.
2. Vents are sized small enough that the meniscus of the molten material will not enter it. 0.010 mm is a starting point. Most do not exceed 0.020 mm.
3. Pack pressure is typically optimised iteratively. It can be predicted using software (e.g. Moldfow) but further optimisation is required. Figure a maximum of 1,000 bar for design purposes.
4. You do not give sufficient information to answer accurately. However, you need to be able to exert a pressure of up to 1,000 bar.

Please also note that the screw and check valve work like a plunger during injection. An extruder does not.
 
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  • #3
Thank you for reply.Safety is #1. I have lexan to enclose the machine.

I do not understand why I need 1000 bar. If I inject with 10 psi what will happen?
Manual piston type injection molders do not use 1000 bar. The piston is usually 3/4" diameter and the operator pulls down without doing a pullup so it must be less than 140 lbs. The mechanical advantage of the lever looks 3:1. So about 741 psi.

I cannot find information on the purpose of injecting under high pressure. I know more pressure means smaller gates, so less fiishing touches. Or more pressure means lower temps and faster cycle times. But I'm looking to do the bare minimum proof of concept. If I had a pump-assisted die cast, is that injection molding?
 
  • #4
My background is industrial and not hobby. In industry, high pressures are needed to maintain part size and compliance with the cavity walls as the part shrinks. Even a slow process with large gates is likely to need 200 bar to properly and consistently pack the part out.

You may find ~750 psi (50 bar) gives you an acceptable part. If you try it then let us know how you get on.
 
  • #5
*Hello,

Thank you for sharing your project with me. I am always excited to see individuals exploring and experimenting with different processes and technologies.

To answer your questions:

1. The screw purges air by rotating and pushing the air out of the barrel as the molten plastic enters. This is aided by the design of the screw, which creates a tight seal against the barrel walls.

2. The mold purges air through the use of vents, which are small openings in the mold that allow air to escape as the plastic fills the cavity. It is important to design the vents appropriately to avoid flash or air pockets in the finished product.

3. The needed pressure to fill a mold can be calculated using the following formula: P = (Q x MFI x L) / (A x T), where P is the required pressure, Q is the flow rate, MFI is the melt flow index, L is the length of the flow path, A is the cross-sectional area of the gate, and T is the temperature.

4. The pressure a screw can generate on a fluid can be calculated using the formula: P = T / (L x A), where P is the pressure, T is the torque, L is the length of the screw, and A is the cross-sectional area of the screw.

I hope this helps with your project. However, I would like to remind you that injection molding is a complex process that requires precision and expertise. It is important to ensure safety measures are in place and to thoroughly research and understand the process before attempting to automate it. As you mentioned, there are easier and safer ways to create small parts such as 3D printing or resin casting.

Best of luck with your project.

Sincerely,
 

1. What is an injection molder?

An injection molder is a machine used in the manufacturing process to produce plastic parts by injecting molten plastic into a mold. This process allows for high volume production of complex and precise plastic parts.

2. Can I make an injection molder at home?

Yes, with the right materials and tools, it is possible to make an injection molder at home. However, it requires advanced knowledge and skills in engineering and mechanics.

3. What are the benefits of using an automated injection molder?

Automated injection molders offer several advantages, including increased efficiency, reduced labor costs, and improved accuracy and consistency in the production process. They also allow for a higher volume of production.

4. What are the main components of an injection molder?

The main components of an injection molder include a hopper for holding and heating the plastic material, an injection unit for melting and injecting the plastic into the mold, a mold clamping unit, and an ejector system to remove the finished part from the mold.

5. Are there any safety precautions I should take when using an injection molder?

Yes, it is essential to follow safety protocols when using an injection molder, such as wearing protective gear, ensuring proper ventilation, and properly maintaining the machine. It is also crucial to follow the manufacturer's instructions and receive proper training before operating the machine.

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