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finishing a 4" deep bore .5" dia

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  • finishing a 4" deep bore .5" dia

    I'm finishing up a piston type bench plastic injection molder. I started with the Gingery books design but I have made numerious refinements.

    My current problem is in finishing the cylinder walls. I want a nicely polished surface, the dimensions aren't as critical as finish. I can make the piston to fit but I want the cylinders surface as polished as I can reasonably make it.

    The hole is 4" deep, 0.5" dia and is through the block(not blind). It is in Cold Rolled steel of unknown breed.

    I don't have a toolpost grinder yet. I was thinking of building a simple one to do this. I was also looking at copper/bronze laps or a small rotary hone(if they make one that small). I think I like the rotary hone best as it's self-centering, self guiding. Right now the hole has been reamed only.

    Since the bore is going to be holding 450-500deg plastic under pressure I want the fit to be very close and I will be placing shielding between me and it.


  • #2
    I'd use a barrel lap, which you can buy for not much money. You can also make 'em, but it's hardly worth the trouble. I'd probably start with #240 or #320 lapping compound, finish with #600.

    There is a strong urge to use a lot of lapping compound to speed things up, but that tends to lead to bell-mouthed holes (the lapping compound piles up.) So go easy.

    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
    Don't own anything you have to feed or paint. - Hood River Blackie


    • #3
      If you have Dremel, figure out a way to secure it on the tool post, and use #425 polishing wheel. The wheel feels smooth to the touch and produces a mirror like finish. I have had great results. Try to use one of the newer Dremel (blue/gray colour) since it has a tighter axial play.

      For plastic molding, you may even have to polish it using a small buffing wheel and polishing compound. It's amazing how even finger print left on the mold shows up in the final product.

      Sounds exciting. Please post pictures as I and others would be interested to see the project. Good luck.


      [This message has been edited by Rotate (edited 02-21-2003).]


      • #4
        Have you considered purchasing a section of precision lapped and polished tubing which could be inserted into the block. This would save so much time.

        At the moment I cannot think of a source for this material but surely someone in this gang knows of it.


        • #5

          It's looks like McMaster-Carr has an expandable barrel lap .5" for $7.70 with replacable barrels and adjusting screws. I figure getting a barrel for each grit would be correct as they're made of brass which gets embedded with grit. I can model my own on these if I want to make some.

          I tested the heating block last night and it sure does work. It has a digital, thermocouple control unit for regulating the block heat.

          I was thinking of using a lining. McMaster has some precision ceramic tubing and rods that would work and don't cost alot. The only problem is I have already placed the hole for the thermocouple probe in the block and don't have enough room left to expand much. Maybe 0.01-0.02" wall left.

          I'm taking pictures of the construction and will post them on my site when it's working. Hopefully in the next 2 weeks. I've been studying the technology and have a couple of the expensive books on plastic injection theory. They all cost $100+ sheesh.
          As soon as I get a bigger lathe (12x36) I plan on attempting a hobbyist sized screw injection type. It may not be as efficient, ultra-tuned or 300 tons of clamping force like commercial units but there's no reason it shouldn't work.

          It's just plain amazing what a hobbyist can do nowadays and what they can get in off-the-shelf components.


          • #6
            For a nicely polished cylinder try using a cast off shock from a car. They seem to be about 5/8 dia. I would think a hatch strut from a hatch back would be smaller. If the wall is too thin, use it as a liner in a thicker block and epoxy or solder it in place. Have fun with your project. Buzzer


            • #7
              I've made a a few bench top injection molders. The first was from HSM plans. The last were loosely...veryloosely based on gingery's ideas...which is loosely based on the HSM plans : ) (very good reference).

              The last ones I made with students, we used 3/4" thick pieces of 6061 aluminum for the barrel machined on the cnc mill. Milled two halves mirror images of each other, contained pockets for the ram and the heating element. Bolted them together with gr 5 bolts then reamed the barrel hole with a 1/2 " hand reamer.

              The moral of the story is, if your ram alligns properly, and you have a relatively snug slip fit with the barrel, you will be able to make nice little injection molded parts with just a reamed bore. A reamed hole will work fine in my experience. I'd be glad to e-mail pictures if you are interested.

              I mainly used this method because I'm cheap. I don't have a lot of $ for supplies and tools (did I say we had a CNC mill? opps). We used materials on hand, and I only had to come up with an inexpensive reamer to finish the bore. I even made one last year that was a drilled bore,and used a propane torch for heat. All I can say about that one is it worked...OK not everything works well.



              • #8
                Techtchr- Seeing pictures of what you've done would be real cool, thanks. Did you have any problems with plastic squeezing around the piston? or in withdrawing the piston to reload?

                I'm using his(Gingery's) heating block and basically nothing else after redesigning it a doz times.
                I picked up the temp controller on Ebay for $24. It works just fine in tests. The only problem for me is that it reads in degrees C. I found the chips to build one for $9.00. It's really quite simple to build a thermocoupler controller with them.


                • #9
                  Kevin, I'm sending you an e-mail with some pictures of a student made inj. molder. and an inj. molded project. Unfortunately all I had for pictures on my computer at home was a screen captures of the solid model drawings of the molder. When I get back to school, I'll take some digital pics of the molder and send them from school. Keep in mind that this was a student group design, it works very well, but I'd probably do some different things.

                  As far as plastic running out of the barrel between the ram as you press the lever down, this does not happen with a reamed bore. The viscosity of the plastic when it is heated is such that it is too thick to squeeze between the parts. Remember, you have cooler plastic on top of melted plastic, and the ram pushes on the cooler stuff. Its like very thick maple syrup at the nozzle. Some of the plastic does stick to the side of the ram and the bore, so you will get a film of plastic on that.

                  You will need to play with the temp setting on your machine, depending on the size of your molds and the plastic being used. Too hot in a big mold, and the plastic will shrink as it cools and leave divits in your parts, too cool, and your part won't mold.

                  When I first started doing this plastics stuff, I used discarded 5 gallon plastic buckets as a plastic source. (HDPE) Mostly old drywall mud buckets. I just cut them into strips with my wood working band saw and fed them into my machine. The nice thing about these buckets is that the plasticisers used to mold the buckets works well in little molds, and your parts come out pretty nice. The buckets also take on the smell of what ever was in them. I got a green bucket from my sister inlaw who works in food service. When I used the bucket to make some parts, there was this odd warm smell of garlic as I was molding, yep, it was a green pickle bucket. Parts smelled like garlic pickles.

                  At school we buy plastic from Iasco which is an industrial arts supply company. Polyethelene or polypropolene is a good general molding plastic. I use polystyrene for blow molds (notherstory), and butyl is a good plastic if you want to make flexible plastic stuff like key fobs, or rubber stamps.

                  I purchased my heating elements and thermal switch from a company in Syracuse NY. They were very good to me when I told them what I was building, and that kids were doing it as a class project. The incidental electrical stuff was scronged from a variety of sources. I did not use a thermometer in mine, as it was just an added expense I thought I could do without.

                  As far as Gingery's design, I love his thoroughness, and his step by step process, but the libral use of angle steel is not apealing to me. This is not a criticism of what he has done. I just prefer other materials if I can aford them and find them. I think his books are really to be used as you and I have both done. Build on a good idea. I would enjoy seeing your molder when it is finished.
                  Happy Molding, Matt


                  • #10
                    I was looking at the models at the Iasco website. Mine is styled more like their's. I'm using two .5" steel plates for the bases (donated). One of the agents I work with also works for a local steel supplier so he get's me stuff. If only they stocked aluminum and brass as well.

                    Here's a link to some drawings showing how I'm building mine:
                    It shows a general overall view and some details.


                    • #11
                      Nice drawns. I like the large area under the nozzle for larger molds. The thermocouple idea is very interesting. The toggle mechanism is also very nice.

                      Just some food for thought. How will you attach the nozzle to the barrel? Weld, solder, threads? How will molds be held together when injecting? How will the sprue in various size molds be held against/lined up against the nozzle?
                      Its gonna be great fun when you are done, keep up the good work.



                      • #12
                        The lever was pretty close to the one you sent me. Great minds...

                        Nozzle: I don't like his idea of brazing it in. I'm going to thread it so it can be removed for cleaning or damage. That's how industrial units are made. The threaded part will be larger than .5" so the surfaces don't meet at the threads. Maybe I'll make an anti-backflow version someday.

                        I'm using 1/8" hardened dowel pins for alignment of the mold halves which will be clamped together. The aluminum I have for my first molds is 1x2x2" and .5x2x2" so I can lay them all on edge. If I can or need to do a larger mold I'll try piercing the center of one half in the industrial manner.

                        The controller I used is this one:
                        Same person I bought mine from. The Ad says it uses 220 but the back of mine shows hookups for 110/220 so that shouldn't be a problem.
                        I'm still planning on building my own so I can control it better and add other services like remote monitoring, gradient sensing or controlling multiple heaters/sensors from one unit.

                        I simply love the idea of doing something well once and then making numerious copies for pennies a piece. I have a friend who has a hobby shop who is collecting leftover model sprue's from customers for me to melt down. I also get all the printer cases from work so I have pounds and pounds of grey plastic to use.

                        It would be interesting to make a continous extruder to turn scrap into .5" rod to feed the molder with. An extruder.. hmmm


                        • #13
                          What kind of plastic works best for this type of home-made plastic injection? Do you buy pallets and where?



                          • #14
                            Pretty much, if it melts you can mold it. I'm using scrap plastic before I buy pellets in bulk. Any plastic shell, case or container that has mold lines can probably be put through it.


                            • #15

                              I used to work in an injection molding plant. Dupont sells the clear polyethlene in 55 Lb bags (takes four bags to fill a 55 gallon drum).