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Thread: New to injection moulding - Am I being realistic?

  1. #1
    Join Date
    Oct 2018

    New to injection moulding - Am I being realistic?

    Hi folks, my interested in recycling plastic and injection moulding was recently sparked by the Precious Plastics project.

    Whilst the equipment they have designed coopers to work ok I started looking at more commercial injection moulding equipment.

    I want to be able to produce slightly better quality products with a really good finish.

    I started to look at better machines and found the Travin machines. Initially these appeared to be exactly what I needed but then I found out even the biggest of their machines can only press 100mm x 100mm. I need to be able to closer to 250mm x 200mm at least.

    So, I started the search again. I've not been informed that to do this size of product I need a 50 tonne machine. I got a quote for a Boy machine for 48k which is a little outside my budget ;-). I've seen plenty of older monster sized machines for around 2/3k. I'm not going to pretend I have any experience of these.

    I also found the Morgan Press Model G-100T in the USA which looks very good. Around 20k new so still expensive when you factor import taxes etc.

    The question is, am I being naive in thinking I would be able to produce these products myself on a reasonably priced machine? I would appreciate direction on exactly what machine I would need.

  2. #2
    Join Date
    Aug 2014

    Re: New to injection moulding - Am I being realistic?


    To answer your question directly, no you are not being naive thinking you can make great parts on budget machines. My company buys used machines all the time and we make automotive parts with tight tolerances. You are a little nave when it comes to the equipment I thinks, as you have not mentioned a hopper and dryer, mold temperature controller or the plastic pressure capability of the machine, all of which will play a role in the part dimensions and surface finish.

    Just as an example; the tonnage needed to hold the mold closed is only part of the process. If your mold needs 25,000 Psi plastic pressure and your machine is only capable of 20,000 Psi, then all the clamping tonnage in the world won’t help you as you won’t make full parts.

    Where did you get the 50 tons from? Was a computer analysis done or hand calcs? A lot of people make the mistake of using the area and ignoring the depth. If you have a poker chip and a shot glass that has a bottom that is the same diameter as the poker chip, the area is the same but the force to close it and injection is not. The shot glass is deep and requires more pressure to fill than the chip, which will require more clamp force. Just another thing to think about and verify.

    So, you need to know:
    • The tonnage needed to hold the mold closed during injection
    o Your machine needs to have tonnage above this number.

    • The amount of plastic pressure needed to fill the mold
    o This is not the same as hydraulic pressure
    o Machine must be capable of going higher than this number (at least by 3000 Psi)

    • The dimensions of your mold and the dimensions of your tie bars (can the mold physically fit?)

    • The ejector stroke of your mold (can the mold open wide enough to eject the part safely without crashing into the other side.)

    • The recommended mold temp of your material
    o Used to determine if you need an oil heater or water heater
    o Mold temp dictates part finish in most cases.
    o Correct mold temp usually leads to a good finish and good dimensions

    • The amount of plastic you need for one shot.
    o You should be using around 60% of your barrel capacity per shot.
    o Using too little or too much of your barrel can cause process issues.
    o Use melt density not solid density for this calculation.

    • Drying temp of your material (if it needs a dryer).

    • Is a gate an issue? Do you need a hot runner system?

    • Material data to process correctly.

    These are just some key points of what you will need to consider. I am not listing this out to deter you, I am listing this out, so you don’t get surprised later on and find it too overwhelming after you have already bought equipment. If you have an idea you are passionate about, I encourage you to go for it and learn all about this industry. The recycled plastic industry is ramping up, so you might be onto something with what you are designing. The plastics industry is a helpful community so don’t be afraid to call a molder or mold maker and ask some questions.

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