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Author Topic: Educational size rotational molding machine  (Read 49774 times)

johnm

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Re: Educational size rotational molding machine
« Reply #75 on: December 27, 2014, 08:16:43 AM »

Those sound like great updates to the machine.

I wonder if by closing up the venting hole there would be a partial vacuum inside the top when it cools down.

I weighed one of Don's tops without the tip and it measured exactly 130 grams.
Do you think he used less material or lighter plastic? On the one he cut the walls look pretty thick.

If the plastic envelope is complete, in principle there should be a slight vacuum, but I suspect along the screw there may be some openings and air could leak in around the threads. 

I don't know the density of the material Don used (plus different dyes may add different weights).  The HDPE varieties I'm using certainly have different molecular densities.  The sample that Don cut in half has a thick wall because a significant amount of the powder did not melt together completely, it just lightly fused together suggesting longer time in the oven was needed.  The cross-section is quite open except for the outer shell of about 0.09 inch thick.  It doesn't show well in the picture but the inner surface displays lots of little evenly spaced air bubbles just below the surface (the surface is actually quite smooth).



does is smell a lot ? do you wear a protection during the cooking time ?

Since the cleaning, the smell is minimal and really only noticeable for me right next to the machine.  I think this is mostly from various parts of the machine getting warm and not from the material being melted.  Just in case, I have the oven on a cart in front of a chemical exhaust hood and don't spend much time right next to it.

I do have a top like that without a tip.  I do not have a scale nearby, though.  I will have to look around.

So... I've got a scale and the University is officially closed until Jan. 5 so I'm really on vacation all next week.  Just sayin'.   ;) ;D



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Kirk

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Re: Educational size rotational molding machine
« Reply #76 on: December 28, 2014, 01:48:07 AM »

  The plug also has a 1/4-20 screw extending into the top body instead of the venting tube thus eliminating the venting feature which I think is not necessary.  The threaded hole left behind in the plastic may or may not be used for mounting the tip.
Rotomolds that I have seen (only a few) have vents that extend into the center of the opening. Thus no powder spills out.

  There was still a bit of a void where the tip installs but I'm not sure there will be a fix for that, so I'll try again with these parameters to check the reproducibility of the result. 
The pros have tricks to locally thicken or thin the walls.  The rate that a spot of the mold heats up affects the fraction of the plastic that melts and sticks to the mold.  Small insulated sections have thin walls. Pin fin heatsinks will locally heat that area faster and add to the wall thickness.

I will try to beg some powder from a supplier.
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johnm

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Re: Educational size rotational molding machine
« Reply #77 on: December 28, 2014, 09:13:39 PM »

Thanks Kirk for the insights.

Don's original vent tube did go into about the center of the mold but was more or less completely blocked with plastic inside.  I drilled it out before my first run and after the first run it was mostly blocked again so I'm not real confident that no powder would spill out although I didn't notice any of my "flakes" in the oven.  There doesn't seem to be any problem with this particular mold not having a vent.

As far as the tricks go, the mold seems rather well designed with the heat-sink like assembly flanges hopefully causing the walls at this largest diameter to be thicker than the rest and the mass of the plug and clamping plate hopefully adding material to the tip section.  So far the tops have been nice enough that I'm reluctant to saw one open to inspect the cross sectional thicknesses of various regions.  If they continue to be of good quality, I'll probably not sacrifice one until I have at least one for each of the contributing members unless the balance is particularly bad in one of them.  I think my void problem is not a material collection issue due to poor heat distribution but is rather due to the material being completely packed into the mold and initially not being able to roll around and tumble as powder would in a partially filled mold.  The lathe swarf chips or flakes kind of bind together and if there is an unfilled spot while it heats up, that spot could remain empty throughout the process because none of the loose material has an opportunity to fall into it.

Here are some pictures of the materials I'm using.  The swarf in the bags is just the waste from shaping and hollowing the cast HDPE tops.  In the bin is material from the bags selected to fill the mold and then cut up into small bits or flakes to make loading and packing easier.



The bits and flakes are loaded into the mold through a funnel via the opening in the body of the mold where the tip will mount.  I use a 1/2 inch dowel to ram in the plastic to maximize the material charge.  The mold is filled and packed to the brim and the end plug with screw then gets pressed in.



Here are a couple of the tip mounts where the voids generally appear.  The blue one (third from the left) is nice and complete while the others are not unreasonable and should be reparable with a little hot air filler welding.

« Last Edit: December 28, 2014, 09:16:17 PM by johnm »
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the Earl of Whirl

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Re: Educational size rotational molding machine
« Reply #78 on: December 29, 2014, 09:34:26 AM »

I am becoming more and more familiar with lathe swarf from my HDPE efforts.  It is pretty special stuff.

What I do not have any experience with is hot air filler molding.  I have seen some pictures of it but have yet to move into that area of HDPE work.  Are you using a hot air blow dryer?

By the way, thanks for your recap and all your pictures.  They look fascinating!!!
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ta0

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Re: Educational size rotational molding machine
« Reply #79 on: December 29, 2014, 11:24:58 AM »

Yes, it is very interesting to see the details of your work. Thanks a lot!

I imagine the photograph showing how you fill just half of the mold was only for photographic expediency ???
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jim in paris

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Re: Educational size rotational molding machine
« Reply #80 on: December 29, 2014, 12:07:29 PM »

oh oh John!
if i understand well , you have already cooked 4 tops ?!!
the mix of colours is incredible
 8) 8)
jim
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johnm

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Re: Educational size rotational molding machine
« Reply #81 on: December 29, 2014, 01:05:57 PM »

What I do not have any experience with is hot air filler molding.  I have seen some pictures of it but have yet to move into that area of HDPE work.  Are you using a hot air blow dryer?
Since I'm just using hot air welding to fill voids, I want good localized control of the heat and am using a hot air soldering station we bought a long time ago when I had to do a lot of surface mounting of electronic components for one of the experiments.  I'm not sure if a hair dryer gets hot enough for melting HDPE but a heat gun like plumbers etc. might use would certainly have enough power.

I imagine the photograph showing how you fill just half of the mold was only for photographic expediency ???

Sorry for my lack of photography skills and the selection of an ambiguous angle, twice. :-[ ::)  The mold halves are assembled before the loading process, the cap half is just obscured in the picture but you can see the securing nuts, otherwise I'd be filling from the larger opening. ;D  There really is not a practical way to fill the halves and then assemble them since the packed HDPE would just spring back out and go everywhere while trying to fill an open vessel and it certainly would not tolerate tipping up side down to make the assembly.  That was an actual load and became the last top in the row of four.


if i understand well , you have already cooked 4 tops ?!!

The log book says run number 9 is cooling right now. :)  Three tops in one day is possible but it makes a loooong day.  I need to get back to work to recover from my vacation. :P
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ta0

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Re: Educational size rotational molding machine
« Reply #82 on: December 29, 2014, 02:20:43 PM »

Quote
. . . otherwise I'd be filling from the larger opening. ;D
I hope so!  ;D
The nuts should have been a giveaway, but the optical illusion that the cone was sitting on the table was very strong. By the way, your photographs look great. I specially like the one you took of the cross section showing the solid external layer and the honeycomb below.
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JNeff

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Re: Educational size rotational molding machine
« Reply #83 on: December 29, 2014, 08:05:05 PM »

Amazing John. The pictures look great. The technical aspects involved with this are way beyond me. What size are these tops? Are you able to make different sized tops with the molding machine?
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johnm

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Re: Educational size rotational molding machine
« Reply #84 on: December 29, 2014, 09:24:33 PM »

Amazing John. The pictures look great. The technical aspects involved with this are way beyond me. What size are these tops? Are you able to make different sized tops with the molding machine?

I haven't cleaned up any of the surfaces of the raw tops yet to get any final size measurements.  The cap portion where the string gets anchored is normally where I would hold a top like this in the lathe but this region is sloped a bit and doesn't allow easy chucking so I'll need to think a bit about holding these properly.  I'll probably make a little mount that registers the tip side where the screw was molded in and square up the cap side first so that it can then be turned around to true the tip side.

Anyway the approximate size without tip should be around 5.25 inches tall and about 4.75 inches at the largest diameter.  An appropriate tip will add at least another inch to the overall height giving a final overall height above 6 inches.

Here is one pictured next to a standard Spintastics Trompo Grande.



The rotomolder could function with any size mold that could be mounted into the inner rotating frame but currently there is just the original mold that Don had made.
« Last Edit: December 29, 2014, 09:29:20 PM by johnm »
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johnm

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Re: Educational size rotational molding machine
« Reply #85 on: December 30, 2014, 09:33:36 PM »

I made a little mandrel to mount the tops to the lathe using the threaded hole left behind by the mold plug and register the top against the 1/2 inch diameter flat inside the tip pocket.  The mandrel has a little relief to allow tool clearance so that the tip end can be machined square and ready to seat the tip.  The cap end of the top is clamped against a flat disk with pressure from a live center in the tailstock.  The cap is lightly clamped and the outer diameter of the cap is indicated to get that diameter running as true as I can for something that is not completely round and slightly wavy.  For this operation, the dial on the indicator moves around like a hand on a clock when the plunger contacting the top moves in and out while the top rotates.  The "high" spot is located and the top is tapped over and the procedure is repeated until the dial moves very little meaning the test surface is centered with respect to the lathe rotation axis.  Once the cap is running true, the tip end is machined so that these two reference surfaces are square.  Then the excess material at the seam between the cap and body of the mold is removed using a file while it spins in the lathe.  This does not ensure that this diameter runs true with the reference surfaces but it is the easiest way to remove that waste material and profile the top to a somewhat pleasing shape without removing a lot of material.  Finally the whole body is rubbed with steel wool while spinning to get rid of most tooling marks and polish it a bit.



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Daveid

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Re: Educational size rotational molding machine
« Reply #86 on: December 30, 2014, 09:46:08 PM »

those are unbelievably gorgeous...
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ta0

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Re: Educational size rotational molding machine
« Reply #87 on: December 30, 2014, 11:59:50 PM »

Sooo nice!  :)

We should all be proud for helping the machine end up in the right hands.
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the Earl of Whirl

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Re: Educational size rotational molding machine
« Reply #88 on: December 31, 2014, 02:22:59 AM »

wow...Wow...WOW!!!

Merry Christmas and a Happy New Year to us.

Many thanks for your inspiring posts, johnm.  I feel like I am making some good progress up here in Miamisburg with HDPE but then I read what you are doing and cannot help but gasp.

Great work.  Just simply, great great work!!!
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lincolnrick

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Re: Educational size rotational molding machine
« Reply #89 on: December 31, 2014, 11:22:54 AM »

wow...Wow...WOW!!!

Merry Christmas and a Happy New Year to us.

Many thanks for your inspiring posts, johnm.  I feel like I am making some good progress up here in Miamisburg with HDPE but then I read what you are doing and cannot help but gasp.

Great work.  Just simply, great great work!!!

I cannot add anything to Mikes message as he's said exactly what I would have said. Thank you John for your work.
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