Making Dimensional Recycled HDPE Stock for Projects





Introduction: Making Dimensional Recycled HDPE Stock for Projects

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The Need--- Restore an 1871 Apple Cider Press with Easily Cleaned, Non Deteriorating Parts

A friend acquired an 1871 apple cider press, the unit was complete, but all the wooden parts were rotten or termite infested and hollowed out, the metal parts were rusted and most were frozen and immoveable.

The metal parts were soaked in an electrolysis bath (electrolytic rust removal instructables) and were freed up to be disassembled, wire brushed, painted or seasoned like a cast iron frying pan and reassembled to a functional state.

The wooden frame work of the press was replaced with new wood.

Since this was to be a functioning press my friend wanted the bottom tray, fruit buckets, bucket sleds, and press plates made of food safe high density polyethene (hdpe) for easy cleanup and rot resistance.

Food grade hdpe is much more expensive than the wood it replaced. So we started trying to see if were could recycle hdpe to make the parts for the press. The following is a 6 month research project that is still ongoing, we have successfully made consistent dimensional stock, 3/4"x6"x20" (that is the longest mold we can get into the oven), 1-1/2" x 6"x 20" pieces (currently only have 6" sides, oven would allow 10" if I had the side material), 12" diameter x 1-1/2" disks (can make thinner), wheels 2" diameter to 6" diameter up to 6" wide. rod material 3/4" diameter to 3" up to 8" long (again oven size in limiting factor)

Since the initial project additional projects have added. A low cost strip sander needed a replacement idler wheel with bearing, knobs for shop made jigs, runners for shop made jigs, bracket for a shop made mouse trap, a future project of beehive boxes, supers, and half supers, and wheels for a shop made 2"x72" belt grinder 2", 5", and 12".

Step 1: Getting Started

Several instructables and on line tutorials were viewed. All made small irregular shaped blocks that were carved into mallet, hammer, slingshots, jewelry, or a small pulley. The piece needed for the fruit buckets was 3/4x1-1/4-x12 inches considerably longer than any of the previous instructions.

One method was to melt the hdpe in hot oil and force the melted hdpe into a wooden mold to make blocks. This method did not seem to be scalable to make larger pieces.

Another was to heat the hdpe in a toaster oven and again hand form the plastic into a wooden mold, again it was felt this method would not produce larger uniform pieces.

The method that was tried first was using the toaster oven to heat the hdpe and use a metal baking pan as the mold. A 9x13 pan was used and the results were discouraging. The hdpe was in chunks just as it was placed in the pan. On another try some of the hdpe flowed out into the pan and other stayed chunky. This led to another mystery, what was the difference in hdpe #2 plastic?

Step 2: Testing the Hdpe for Flow

Hdpe from different items was tested, it was determined that lids and plastic bucket hdpe flowed when heated to 375 degrees for 90 minutes (max time on toaster oven timer). It was also determined that bottles, barrels, jugs, and large items like portable basketball hoop bases and children playhouses did not flow.

Material from a drum (material thickness varies from less than 1/4" to over 3/8") was heated on a cookie sheet and when pliable (60 minutes at 375 degrees in toaster oven) was pressed between wooden plates, 5/16 hex nuts (17/64 thick) were place on the sheet for a consistent thickness. The nuts are 17/64 thick and the material nominal cooling shrink rate is 3 per cent. Theoretical finished size should be 0.258 inches. or nominally 1/4" material. Actual measurement on the 4x6 inch piece varied from .254 to .295 inch. The clamps were removed after pressing and lead weights were place on the plate eliminating the need to retighten the clamps. This method worked well for producing 1/4" stock for small parts. I since have gotten a 16x20 inch tray to make 1/4" material. Also the toaster oven is supplemented with a regular electric range. Oven dimensions, 13x17x23. This limits my material size to 10x14x22 inches..

Shopping bags were also tested, they do not flow. A small melt of 250 bags yielded less than 1" of material in a soup can mold about 3" in diameter. It was decided that it took too much effort for the amount of material achieved. These might be revisited after a solution for larger dimensional stock is achieved.

Step 3: Trying to Make Thicker Stock From Thinner Bottle Material (non-flowing)

The next need was to make 3/4" stock for the fruit buckets on the cider press.

Milk jugs were chosen to make the material. The jugs were cut up into small piece 1/4" to 1/2" squares, Some strips1/4 x 2 to 3 inches. They were placed in a 9x13 pan, parchment paper (available at the grocery store usually by the aluminum foil) was placed on the bottom of the pan and the sides were sprayed with silicone lubricant (available at hardware store or Walmart in automotive chemicals). The hdpe was heated to 375 degrees for 90 minutes. Then was pressed like in the previous step, but no hex nuts were used as spacers. After pressing the clamps were removed and the weights were placed on the plates to keep the hdpe from curling and distorting during cooling. When cool the stock was sawed into strips to see the consistency. In the photos you will see brown lines, this was areas that had milk residue on them and the milk scorched. These areas also did not bond well to each other causing lamination of material that did not adhere. Also note the irregular surface and thickness.

The major failure in this process was the lack of consistent thickness. Also better cleanliness is needed to avoid the adhesion and lamination problem.

Step 4: Trying to Make Thicker Stock From Bucket and Lid Material (flowing)

The next try was using an electrical frying pan as a heat source, it worked reasonably well, but heat was not even as evident from the wrinkly area on the top right side. Cutting the stock it was revealed that there was lack of adhesion and laminations. This was caused by uneven heating and lack of cleanliness.

Step 5: Second Try Using Bucket and Lid HDPE (flowing )

The next attempt was using the toaster oven and a 9x13 pan. The material was the flow type HDPE from plastic buckets and lids. The results were encouraging, but it was difficult to control the thickness. To get uniform thickness the material was weighed so the volume would give a piece over 3/4 of an inch. It was found that the thicker the pieces the more distortion and addition milling was needed to make a repeatable stock. Several attempts were made to get consistency and repeatability, but common factors were not found that gave those results. A different approach was tried.

Step 6: Thinking Outside the Box

Since it was so difficult to fight gravity, it was decided to make a mold that would make the removal of the top and bottom easier.

Mold material

2- metal 2x6 15 inches long (toaster oven is 16" wide)

2- 6" 1x1 aluminum square tubing

1- 1/8x4x16 steel plate

The mold was clamped together with c-clamps for easy assembly, disassembly, and cleaning.

Bucket and lid HDPE was melted at 375 degrees for 180 minutes (2- 90 minute cycles) the resulting piece had a very consistent thickness of 7/8". length was consistent 12-1/4 to 12-1/2 inches. The width was all over, but it was easy to get a finished piece with a 5 inch width. A table saw was used to square the length and width and a jointer/planer reduced the thickness to 3/4 inch. This method allowed for making stock to make 2 slatted fruit buckets for the apple cider press restoration. 15 planks were produced to make 52 3/4x1-1/4x12 inch slats.

Step 7: Cleaning HDPE Off the Molds (cake Pans)

After several batches of HDPE the molds had a build up of charred material.

Scraping would not remove the residue, washing in soap and water did not remove the char.

The method to remove the char is place about 2 tablespoons of baking soda in the pan, add water to fill the pan, place on a burner and heat to a simmer for about an hour. Then allow to cool, empty the water and then using a scouring pad remove the build up. (additional scraping may be necessary)

Step 8: The Making of 12" Diameter Press Plates

A 13" round cake pan was acquired to make the 12" press plates. A disk was made in the toaster oven, but it did not have the heat output to produce a good part. Three attempts were made to make the disks. All had laminations from lack of heating, also cooling was inconsistent causing bottom and top deforming and severe texturing on the top.

A full size electric range was acquired to solve the heat output problem, and it did. A total of 7 disks were produced. The thickness vary from 1 inch to 1-7/8 inch. The distortion was too great to get any parts over 2 inches.

A 13" cast iron skillet was tried for a mold and it produced disk with very flat bottoms and less distortion on the top, but it pulled the cooking residue out of the pores of the cast iron making for a lot of cleanup of the part.

Additional plates have been made and have gotten a 12"x2-1/4". The "secret" is using the 13" cake pan, the HDPE is melted at 375 degrees for it takes about 6 hours to get that volume of HDPE melted, then it is heated at 375 degrees for 2 hours or longer until all the air bubbles stop making domes on the top. Then reset the oven to 250 degrees for 6 hours, then turn off the oven and let the disk cool. The disk produced had a 12-3/4" diameter, bottom distortion was 1/16 inch and the top was smooth with 3/16" distortion. That was the most uniform disk produced to date.

After molding the disks were surfaced on both sides in a shop built planer fixture to be described in a later step. The center of the disk was found and drilled through using a 1/4 inch bit. The disk was then taken to the bandsaw and using shop built fixture was cut round.

Step 9: Latest Techniques for Making Dimensional Stock (flowing)

The material is cut up into pieces to fit in a 9x13 pan

Labels and printing are removed from the HDPE

The pieces are washed in water and dishwashing detergent

The pieces are melted in a 9x13 pan 90 minutes at 375 degrees, if a lot of air bubble domes are visible another 90 minute cycle is added. A piece of parchment paper is placed on the bottom of the pan and the sides are sprayed with silicone.

The mold is assembled. The ends are attached to the sides with the clamps, be sure to get the bottom of the ends pushed down to the table. Once clamped turn mold over, place a piece of parchment paper over the bottom then a piece of aluminum foil (heavy duty) and crimp over the edges so if there is a leak it is contained in foil and not running all over the oven, place the steel plate on the bottom and secure with midget panel clamps. Turn the mold upright and spray inside with silicone spray.

Take the 9x13 piece and cut into smaller pieces 1x4-1/2, Cut down the center the long way then cut into strips approximately one inch wide

Load the mold setting the pieces on edge, by rotating the pieces from horizontal to vertical additional air pockets are reduce and lamination flow out.

Place in oven 375 degrees after an hour check and add more material, keep checking hourly and add material until full, when full bake for another 2 hours, turn off and let cool in oven over night.

Remove from mold, clean up mold (a sharp wood chisel removes most of the material stuck to the mold), trim any flash on the bottom and run through a jointer to true up the bottom edge. Plane the faces to remove distortion and make a consistent size ( 3/4 inch). A piece of 3/4" square tubing is used as a setup gauge for the router bit, place a piece of paper between the end of the bit and the tubing, move the bit down until you can no longer slide the paper between the bit and the tubing, lock the depth setting and the bit should be set correctly. The part can then be sized for a project on a table saw.

Step 10: Making Rod Stock for Knobs and Wheel

Making round stock is similar to flat stock. The difference is the first melt is cut into smaller pieces 1/2x1/2x6 or smaller. The molds are piece of pipe (galvanized is easier to remove the finished piece) 3/4" pipe, 1-1.4", and 2" are used for smaller knobs. Aluminum foil is crimped to the bottom of the pipe, a small piece of parchment paper will prevent the HDPE from adhering to the foil. Place the pipe in an empty can for stability in the oven. Melt at 375 degrees for an hour, add material hourly until filled then heat another 2 hours until all air bubble domes are gone, allow to cool in the oven over night. Using a large drift, punch the HDPE, while holding the pipe in a vise.

To make a knob chuck in the lathe, skin cut the outer diameter for a uniform finish, shape as desired. For a 1/4-20 bolt drill a 1/4" hole through the knob, from the top end drill a 15/32" hole approximately 1/2 way through the knob. Using a vise press the nut into the knob, then place a fender washer on the bottom and insert a bolt into the hole and thread it into the nut, tighten the bolt until the nut is at the bottom of the 15/32 hole. remove the bolt and the knob is complete.

A "tin" can is used as the mold, use a can larger than the finished size of the wheel you want. Follow the steps for making rod stock. When complete most of the time the cool part will fall out of the mold, it not cut the can away (cans are disposable) The piece is chucked in the lathe and again the outside diameter is turned to size, If the wheel is for a belt grinder a crown can be applied. Drill a hole through the wheel if bearings are to be installed drill the center hole 1/32 larger than the shaft size. Next drill or bore the holes for the bearings. Press the bearing into the wheel

Step 11: Shop Built Fixture for Planing Large Pieces


MDF or Plywood 3/4" x 24 x 30

2 - 2x4 30"

4- 2x4 blocks 4"

4 -1/4-20 carriage bolts 5"

2- 1/4-20 threaded rods 24"

8- wingnuts or knobs 1/4-20

8- 1/4 fender washers

The surface planer has a capacity of 10", so for the 12 inch disks an alternate method of planing was needed. A simple shop built fixture was made using a 24"x30" piece of MDF (cutoff from a cabinet shop) 2x4 blocks were screwed on the bottom for legs. A 1/2" groove was cut on both ends approximately 3" from the edge and 1/2 the material thickness. This is head clearance for a 1/4" carriage bolt. A 1/4" groove was cut through the MDF centered on the 1/2 groove. Clearance for the carriage bolts. Two 30" long 2x4 were ripped to the same width. Then 1/4" holes were drilled through the 2x4 to match the 1/4" carriage bolt grooves in the table. Finally four 1/4" holes are drilled approximately 2" from the end in the center of the 2x4's (clamp them together so the holes line up) Cut 2- 1/4"-20 threaded rods 24" long for the clamping. Assemble 2x4's to MDF with carriage bolts and threaded rods using wingnuts or knobs for ease of adjusting.

2- 1-1/2 x1-1/2x 1/16 angle aluminum was cut 24"

2- 3/4x1-1/2x8 hardwood blocks

Place your router between the two angles, clamp the wood block to the angle, do this at both ends then take the assembly to the drill press and drill 8 1/4" holes to mount the blocks to the angles. This completes the router cradle.

Place the part to be planed between the 2x4's. tighten threaded rod until part is clamped firmly, then tighten carriage bolts. Before clamping you may need to raise the part with shims so the router bit will reach the surface also by placing some strips of wood between the part and the 2x4 will save routing the 2x4. A 5/8 diameter bit was used to surface the parts. (it was the largest I had at the time)


Step 12: Circle Cutting Jig


1 MDF 24x30 inches (cutoff from cabinet shop)

1 HDPE strip 3/8x3/4x24 to fit miter slot on bandsaw

1 MDF 4x6 stop block

2 2 inch 1/4-20 bolts

1 wing nut 1/4-20

1 jam nut 1/4-20

1 nut 1/4-20

1 Aluminum t-track for 1/4 bolt 18"

Size the HDPE runner to the miter slot on the saw, place the HDPE in the miter slot.

Place the MDF table on and mark the location of the HDPE with the table clearing the saw arm. (approx. 13 inches on my saw)

Remove the table and using a square mark the miter runner location across the bottom of the table.

Locate the HDPE to the marked line and attach with screws.

Place the stop block on the left side of the table approximately centered between the arm and the blade (on my saw approximately 4 inches from the left side) on the rear edge of the table.

Place the table in the miter slot and saw a kerf until the stop block contacts the table.

Setup a router with a straight guide and route a 3/4" slot centered on the end of the saw kerf 1/2" deep in the table.

The aluminum t-track is drilled 1" from the end with a 1/4" hole centered on the bottom of the track.

The table is drilled 1" from the right edge with a 1/4" hole centered on the routed slot.

Take one 1/4" bolt and cut the head off and round the edges of the stud.

Taking the stud run the jam nut to the end of the thread, insert the 1/4-20 nut into the t-track and align nut and hole, screw stud into nut, when stud bottoms out, tighten jam nut.

Insert head of 1/4-20 bolt into t-track, place track in routed slot and align through hole in slot, tighten wing nut from bottom.

To cut a circle, measure the radius distance between the blade and the center of the stud, mark the center of material to be cut, drill a 1/4" hole through the material, on the bottom side drill a 1/2 clearance hole 1/4 deep for the jam nut. Retract table from the blade so the part can be placed on the pivot stud, turn on the saw and advance the table until the stop is reached, then rotate the part 360 degrees to cut the circle, turn off the saw and remove the part from the table, the scrap will have to be backed out through the first cut kerf

Step 13: Shop Built Clamps

To save space in the oven and allow for making a longer piece, shop made clamps were assembled.

Two types were built, the first is a 1/8x1x5 inch flat stock, 90 degree bends were placed 1 inch from each end. Holes were drilled and tapped for 1/4-20 bolts.

The second clamp is made from a piece of channel iron 3x1.5, one piece was cut 2" and 3". The channel was drilled and tapped for 1/4-20 bolts.

The last clamps are called midget panel clamps and are available on line at several vendors.

Step 14: Tips, Methods, Observations

Tools to cut buckets

I have tried several tools and find the vibrating saw works well to remove the bottom of the buckets and split in 1/2 (usually along the line through the handle mount, this area holds the most dirt). After the bucket is sectioned the bandsaw is used to make pieces that will fit a 9x13 pan.

Sources of Buckets and Lids

My current source for buckets is a local bakery, the buckets contain frosting and icings. Buckets are also available from restaurants pickles, olives and other prepared produce items. Ice Cream is packaged in small HDPE bucket and plants have a lot of scrap that is missed marked or damaged in packaging, (some have a lot of printing that may make them marginal from the time to remove the printing) Beauty Schools get supplies in HDPE buckets, Construction sites get dry wall compound in buckets, Hydraulic oil and some grease come in buckets, Chemicals, insecticides, herbicides, bulk soaps and cleaners all come in buckets. (I do not use buckets that contained poisons) Do not uses buckets that take a lot of time to clean out the residue, it is not worth the time for the amount of material.

Label, decals, printing

When cutting the buckets I try to leave the label or printing a single piece. Labels are soaked in water to remove the paper. If there is a sticky residue paint thinner (mineral spirits) in a spray bottle and most of the residue will wipe off. Printing is scraped off using a card scraper available at a woodworking supply. Some labels are printed on plastic and are very difficult to remove. I just throw that piece away again there is not enough material to warrant spending a lot of time removing it.

Storage and inventory

Containers take up a lot of room to store. I have taken to reducing the volume by melting into concentrated pieces. With colored lids I melt in bread pans to make some sheets of material of a given color. Buckets are reduced to 9x13 sheets 3/8 to 5/8 inch thick. This is about a bucket worth of material. Sharpies permanent marker will write on the HDPE but it will show up on the finished parts sometimes it is still readable (denatured alcohol will remove some of the mark). Some buckets are #5, PP, Polypropylene currently I am melting these in 9x13 sheets and marking them PP and will work with them after I made the parts needed to make a beehive box.

Machining HDPE

Woodworking tools work well in machining HDPE both hand and power. The cuts need to be reduced from wood. Wood can be planed or jointed at a 1/16 to 1/8 while HDPE needs to be 1/32 to 1/16. The feed rate for sawing needs to be reduce about 1/2 of the wood rate. Drilling the chips need to be cleared from the bit if not the HDPE will melt and jam the bit. Routing is also at 1/2 the wood rate and again chip removal is necessary to prevent melting. Turning I have not noticed any major difference from wood as long as the chips do not jam the tool. Sanding works, but heavy cuts melt material into the sandpaper. With hand tool again material removal need to be a half of the wood rate. When using power tools use feather boards and push stick remembering that the HDPE is very slippery and holding devices need to be adjusted properly or the material will slip and the tool may try to throw it at you. The scrap shavings are not worth re-melt, they are too dirty and do not yield much material.

Mixing flowing and non-flowing HDPE

Non-flowing HDPE has been added to flowing to create colored patterns. The materials adhere to each other well, but air will not travel through the non-flowing so only add vertically so that the air pockets can flow around the non-flowing. Additional work will be done in this area, motor oil and detergent bottles have such a good selection of colors.

When filling the mold check the pieces for dirt, laminations, and contamination

Check the pieces for dirt, contamination, and laminations. If sever discard the piece. For laminations re-melt in the 9x13 pan, resaw and if lamination are still present discard. With dirt try scraping to remove. Remember it is not worth ruining a whole piece just to conserve free material. Throw it away and do not waste your time.

Cleanliness and Uniform Heating are the Two Major Elements for Consistent Results

Cleanliness is the major cause of laminations and separation of the melted material. Even after washing the raw material, if it is allowed to set around before melting it will accumulate dust from the shop (I am working in a shop of woodworking machines) I rinse all pieces before I place in pan for melting or the mold for the final stock.

Uneven heating allows areas to solidify at a different rate since the material shrinks when solidifying it pulls the liquid material causing amplified distortion. Ideally the complete form would be cooled and transform from liquid to solid at the same time, if that happened the part would be uniformly 3% smaller, but if it does not the parts are distorted and area shrinks can be as high as 20% change in dimensions.

Glue Does Not Stick to HDPE

Since glue does not stick to HDPE all joints have to be mechanically reinforced. Currently tongue and groove joints are cut for joining parts. These are reinforced with screws or pocket screws. On parts where metal screws are not wanted a hole is drill through the tongue and groove !/4", then the joint is disassembled and the hole through the tongue in enlarged to 5/16". The joint is reassembled and a filling epoxy (JB Weld) is forced in hole, the larger hole in the tongue creates a collar that hold the plug in place.

How to work non-flowing HDPE

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Wow! So glad you posted this instructable. It'll save me hours of experimenting and the like. I've been cleaning and cutting up milk bottles for a few months now, waiting until I have a chance to melt them into something useful. This intructable is so helpful; from where to the plastic to the final stages of the melting process. Thank you!!

Check out my other instructables on hdpe and polypropylene

Also milk jugs are non flowing and you will need to press (clamp) in into shape, also if heat it to 375 degrees you will have more open time for forming and pressing. You will have more degrading of the plastic(brown discoloring ) by heating it over 350 degree. Also if you press in layers you will get better molding detail and forcing into shape with a small punch or an awl, then clamp and press. You have to work fast and preheating tools keep the material from setting up before you finished shaping.

Thank you! I did read through your other instructables too! I found all your instructables by the plastic lounge chairs one (Recycled Polypropylene Bee Hive Box.) And speaking of... someone, right now ironically, is giving away some identical lounge chairs on Craigslist-Reno/Tahoe, in their "Free" stuff.

Anyways, all of your work is so very helpful! I am very grateful for your work and for you sharing it. Thanks again!

Good Job. It reminds me of a dystopian sci-fi I read once where future people are mining landfills to retrieve plastics to sell to the plastic smiths.

I have had a go with various plastic bottles from around the house and cut them into little pieces smaller than baby finger nail sizes, then I put it into a non stick frying pan and slowly melted the bits, on the hob, I took great time to melt it very slowly then kept it on low heat till I could see the shine, I then folded the melted plastic a few times and carried on with the low heat, after a few hours I placed the mixture onto parchment paper and squashed it into a rough square as I was trying to make a tile. The mixture resembled colourful marble all swirled together, I find the more you mix it the stronger it becomes. I never used any heavy weight and the tile is super strong. I am collecting more bottles to make more tiles because I want to use this amazing plastic stuff to cover the cats outdoor house. It's great that I can make these tiles, bit by bit and slowly build up enough for the project. All in all, try slowly and longer with some mixing and hopefully good luck for you.

awesome job this is a great and well written instructable. i have melted hdpe for a while and learned some things about the material that may help you. when i melted and compressed non flowing hdpe into a disc for example it looked great and couldnt find any voids however i tryed to snap it in half to strength test it and unfortunately it broke in a weird fracture pattern almost as if the chips of plastic didnt fully fuse even though it looked like a solid piece. fortunately i found a solution to this problem by taking silicon gloves and kneading the molten piece of hdpe while melted like dough then put it back in the oven until remelted and then finally clamping it in the mold this fully fuses the chips and when i tryed to snap it in half it was completely indestructable i used all of my strength and couldnt even bend a 1/4 disc of this hdpe. a more long term solution for this is by building a mini filament extruder there are lots of ways to make this easily at home and then chop the plastic filament into pellets then the pellets can be compression molded without kneading it because the pellets wont form areas of stress in the plastic unlike plastic flakes.

Thank you for the kind comments. I also appreciate the hint on kneading and reheating the mass. The parts I have made have not needed mechanical strength, they have been used instead of wood in areas where moisture and rotting were problems, or replaced clay parts.

This last week a friend wanted a wedge to replace a commercial wedge that had broken in sub zero temperatures. I made a wedge out of some of the flowing material I had. It was then tested in a vise and hit with a hammer. It failed miserably. It did not break along flat planes like the feed stock, it looked more like a drift wood with gentile curves and cone shaped spikes like stalagmite. Like you said it broke in weird fracture patterns.

I have been experimenting with 1" black hdpe pipe, when heated it shrinks in length over 50% but gets wider. I melt several pieces, then clamp in vise to compress together, then reheat and then compress into "tin" can for making a wheel for an idler on a grinder, then turn the wheels and install bearings. Have not been able to break the wheels.

I will get some silicone gloves and build a wooden form to make a wedge. Will heat pieces, knead and compress into a single mass to fit form (may take several cycles) Then finally form into wedge after last heat and let cool in wood form under pressure.

Will let you know how well this works. Again thank you for your insight and suggestions.

cool cant wait to see the results hdpe is definetely an awesome material to work with

I worked in the recycling industry and funded plastics reprocessing research for some years: "HDPE" is a category of about 2,000 resins with more specific properties and when you blend them together it creates a loss of properties which equals good to use one specific product to recycle if you like the resin properties that went into that product in original manufacture. Many plastic bags are not actually HDPE, but LDPE or LLDPE and there are many versions of each of those, too. For grinding HDPE, I recommend trying low speed shear grinding and then regrind finer if you have to. It's also safer and creates less heat than something high speed. Do not try to compress HDPE in a grinding process or let it "bounce" as it would off a high speed food processor, think tearing / shearing and you'll be in the right direction. Enjoy!