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  • injection molding questions

    Recently saw a tv show talking about injection molding. What are the types of plastic used,what temperatures and pressures are used, and can these procedures be done in a home workshop? Thanks Paul

  • #2
    Indeed, home injection molding can be done. There was an article in HSM some many years ago about a lever-operated molding machine. In the article, the author used cut-up milk jugs for his plastic stock. A few years back I needed to mold some magnets into a cavity in a detail, so I build a very small lever-operated press. I had virgin glass-filled nylon and used that. It was difficult to use but produced good results. White pill-bottle plastic would also be nice to mold.

    Anybody remember the toy "Moldmaster" molding outfit? It used a wax-type material.
    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
    ~Practitioner of the Electromechanical Arts~

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    • #3
      http://www.lindsaybks.com/dgjp/djgbk/inject/index.html

      There was an article in Model Engineers Workshop a while back where they built a version of this Gingery IMM. He was getting very good results. I believe he dhad to beef up the linkage and piston.

      Many different plastic resins can be molded. The molding process parameters will vary by resin. I worked in a toolroom so have only a basic understanding of the processing. HSM member Liger Zero has worked as a molder so he will be able to help with specific process questions.
      Jon Bohlander
      My PM Blog

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      • #4
        I built the Gingery IMM and it works fine but on a small scale. All the small knobs on my knee mill are were made with it and are a cool blue color, Chevron oil bottles. I also have a mold the produces a dart that will slide on the end of my air blow gun. It was the recourse for the guy that brought the paint pellet gun to work.
        Mark Hockett

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        • #5
          Originally posted by moldmonkey
          . HSM member Liger Zero has worked as a molder so he will be able to help with specific process questions.
          Indeed. As a "process tech" no less... They'd give me a mold that has never been run before and I'd figure out every aspect of the "process" and make it go.

          Ideally injection should take place in three steps. A fill-stage, a pack-stage and a holding-stage.

          In a lever-type press... make sure your material is fully liquid, stir it often with a heated rod.

          You will need more than you calculate for the size of the part, this will leave a "cushion" in the barrel for transmitting pressure.

          Once you have your melt at the right temp, pull the lever with fast steady speed. When you get near the end of the stroke slow down and PULL HARD FOR MAXIMUM PRESSURE until she won't go no more. Now hold that pressure.

          Back off, let your part cool, remove your part. It'll take awhile to get the hang of it but this will yield good parts with practice... or you can buy a molding machine and pay me $4,000(*) an hour to teach you what I learned in university + 7 years experience.


          (*) or teach me to weld.
          This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
          Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
          Plastic Operators Dot Com

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          • #6
            I'm building one now

            I'm presently making the Vince Gingery drill-press version of the plastics injection molder. The book is available from Lindsay Publications. Try this link: http://www.lindsaybks.com/dgjp/djgbk/injatt/index.html

            This version uses a drill press for support and pressure, so you don't need to build the frame.

            Yesterday I ruined the three-step piston after working on it for two days. That's typical for me.

            If you use the plans in the book, check the dimensions. For example, I think the note for the depth for the main cylinder (where the plastic goes) may refer to the point of the drill bit and not the bottom of the drilled cylindrical hole.

            That said, there is considerable leeway in the dimensions and you can modify them to suit your needs (and the materials in your scrap bin).

            James (seatlanta)

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            • #7
              The Gingery design has about a one ounce shot. That's not a lot. I wonder how large a run of items you need before it's more practical to use an injection process versus machining?

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              • #8
                ...I must build one of those.
                This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
                Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
                Plastic Operators Dot Com

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                • #9
                  I have a couple of questions. Would it be benificial to pull a vacuum in the mold? And what is the correct temperature for melting Chevron oil bottles?

                  Kerry

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                  • #10
                    Gingery book

                    Thanks for the thread. I just ordered the Gingery book you listed on the Drill Press Injection device fro Amazon. Sounds like a neat type of project.

                    I'm putting the finishing touches on a flower pot aluminum melting furnace now. I've been collecting stray pop cans to melt. I've been looking for another recycle oriented machine/project.

                    Thanks again.

                    HerbD

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                    • #11
                      Originally posted by kmccubbin
                      I have a couple of questions. Would it be benificial to pull a vacuum in the mold? And what is the correct temperature for melting Chevron oil bottles?

                      Kerry

                      If you are melting oil bottles yes. It'll help degas the mold.

                      See if your bottles are HDPE or PP and look up "blowmolding grade" resins online, that'll get you close, your actual settings will depend on many many factors.
                      This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
                      Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
                      Plastic Operators Dot Com

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                      • #12
                        A device to reduce plastic recyclables to near-dust would help too.

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                        • #13
                          One thing to keep in mind if you guys get to researching molding, the jargon varies greatly even within different plants of the same company. One moldmaker called a part one thing and another something totally different. What Liger calls a process technician we called a molder. (Actually the toolroom called them lots of things. )
                          Jon Bohlander
                          My PM Blog

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                          • #14
                            Originally posted by moldmonkey
                            One thing to keep in mind if you guys get to researching molding, the jargon varies greatly even within different plants of the same company. One moldmaker called a part one thing and another something totally different. What Liger calls a process technician we called a molder. (Actually the toolroom called them lots of things. )

                            If you assholes would build the mold right we wouldn't complain so much.


                            Process Tech is a senior molder.

                            Typically it goes:

                            Press tender/operator
                            Auto press tender/runner-puller
                            Resin-handler
                            Steel-hanger/mold-setup
                            Setup-programmer/technician
                            Process-tech
                            Process engineer


                            ...at least around here.

                            Plastic ain't hard to grasp, but it sure isn't machining. Keep in mind some materials can and will bite you with gas and sudden violent eruptions... others flow like toothpaste, some are sticky, others flow like water.

                            Even with a degree and experience it's alot of trial and error... what worked for one mold sometimes won't work for an identical "backup" mold for the same part. (That's why we curse the mold-makers. )
                            This product has been determined by the state of California to cause permanent irreversible death. This statement may or may not be recognized as valid by all states.
                            Heirs of an old war/that's what we've become Inheriting troubles I'm mentally numb
                            Plastic Operators Dot Com

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                            • #15
                              A coupla ideas for you guys wanting to build a injection press.

                              There's a coupla companies making small machines for schools(at obscene prices). They're simple designs using air cylinders for pressure. When one of my friends had one up for sale I took some photos, and it may give you guys a few ideas:






                              EGO partum , proinde EGO sum

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