Randcastle’s Microtruder Technology
Processing Benefits
Compounding
Randcastle compounders use our patented SFEM and AFEM (Elongator and Recirculator) compounding elements for state of the art mixing. Within each SFEM and AFEM compounding element, multiple elongational flow fields are created. As material move to successive elements, the mixing increases exponentially. Up to four elements along the screw create nano scale distributive compounding. Our blending of immiscible polymer blends is much better than twin screw compounders.
Temperature
Distributive mixing is critically important for temperature uniformity. It is well known that the more uniform the temperature through the die, the better the product. The gentle, exponential distributive mixing of the SFEM and AFEM mixers means there’s no need to add expensive static mixersBesides distribution of polymer mixtures (even immiscible polymer blends) temperatures are also blended. No need to add static mixers.
Pressure
The SFEM and AFEM mixers have an interesting property. It is well known that accumulators make pressure more uniform. Randcastle Microtruders have used our patented low volume accumulators for surge suppression for many years. The SFEM and AFEM elements are large volume accumulators. As material moves through each successive element, the pressure becomes more uniform. In conjunction with our patented automatic screw speed control, pressure can be so steady that gear pumps are not required.
Melting Quality
Historically, small screw have not melted as well as large screws. Small screws meant low shear and inferior melting (and poor scale) comparted to large screws. However, the SFEM and AFEM elements add elongational energy just a stretched rubber band warms. This promotes melting even within the first element. The melt is then drained away as in a barrier screw to expose remaining solids to the barrel for more efficient melting. . just as in barrier screws mixers have an interesting property. It is well known that accumulators make pressure more uniform. Randcastle Microtruders have used our patented low volume accumulators for surge suppression for many years. The SFEM and AFEM elements are large volume accumulators. As material moves through each successive element, the pressure becomes more uniform. In conjunction with our patented automatic screw speed control, pressure can be so steady that gear pumps are not required.
Surge Suppression
A theory was presented for a surge suppressor that did not have stagnant areas. Obvious degradation, that might have been seen as discoloration or carbonized material, did not evidence itself in the extrudate. This is encouraging given a somewhat thermally sensitive material processed at very slow screw speeds. A theory was presented that may lead to a simple method of controlling extruder pressures by linking surge suppressors and a pressure control of screw speed. Pressures less than 1% were held in the transfer pipe. It is usually assumed that such pressure stability can only be achieved through the use of metering pumps.
Solids Additional to powder: Click on the Photos for details
Nanopowder 5,000
Nanopowder 10,000
Nanopowder 50,000
Melt Blends
Three SFEM Elements on a Horizonal Screw
Engineering Benefits
Maximum Screw Torque Equation for screw survival Engineering Assumptions
For screw root diameters assuming a typical 1 inch screw with 3:1 ACR where the feed channel depth is 0.180 for 8 L/D’s, straight compression section 8 L/D’s long (where the next to last flight will then measure 0.091 channel depth, and the meter 8 L/D’s long at 0.060 inches then, as given by Machinery’s Handbook, page 263,
T = Ss x Zp
Where, T is the torsional or twisting moment in inch-pounds.
Ss is the allowable shearing stress in pounds per square inch.
Zp is the polar section modulus.
Zp for a circular cross-section, page 266, is given as: 0.196 D3.
There is little published data on the torsional yield stress. The industry rule of thumb is that the yield stress is about two thirds the tensile yield stress. Spirex has published their use of 60% rule. Our standard screws are made from 135M from Crucible Materials Corporation with a published tensile strength at 145,000 psi and yield strength at 125,000 psi for a hardness of 28-32 HRC.
So, using the conservative 0.60 figure,
0.60 x 125,000 x 0.196 D3 = T
For the following screw root diameters, T =
0.640 = 3,851 inch-pounds
0.810 = 7,805 inch-pounds
0.880 = 9,996 inch-pounds
Assumptions For Engineering Load Location:
In a feed driven screw, the entire load of the mixing, metering, melting, and feeding is passes through the root of the feed portion of the screw. This root is the smallest root of the screw. A typical one inch screw has a 0.640 root diameter for a 0.180 channel depth and a resultant maximum torque of about 3,850 inch-pounds.
The maximum load exerted on an extruder screw typically comes from the melting zone. The load from a smooth bore extruder is typically given as zero, and the metering section between 10 and 20%. Thus the melting zone generates most of the load. The greatest load possible in the melting zone is at the end of the melting zone where compression is greatest.
For a feed driven screw, all the load still goes through the smallest root. But for a discharge driven screw, the load is not transmitted by the smallest root but by the largest root. It then matters where the maximum load is to be expected. In a discharge driven screw, the maximum load is at the end of the compression section. So, in the discharge driven one inch screw example (from the calculations above 9,996/3,851) is 2.6 times stronger than the one inch feed driven screw. This can be used as a safety margin, for additional torque, or for deeper screws.
Finally, it is important to understand the implication of the Zp term for smaller screws. As a screw decreases in diameter, the screw's strength decreases with the cube of the diameter (while the output decreases with the square of the diameter). Thus, a 3/4 inch screw, for example, is much weaker than a one inch screw, in terms of survivability, when driven through the smallest root. Long practice shows that 1 inch screws usually survive when driven from either end but screws smaller than one inch do not.
Microtruders Save Space
These are the dimensions for the standard Microtruder. As we are always improving our product, these dimensions should be used only as a guide. Certified dimensional drawings and detailed die mounting drawings are available.
From the President
When I started Randcastle in 1987, I didn’t know that we would become a company that could seriously claim to have advanced the state-of-the-art of extrusion. I thought that we would innovate in small extrusion lines. That worked out and we pioneered everything from proprietary feed sections to patented surge suppression to make them work better.
Looking back, I had no idea that we would supply patented after market compounding screws for over 1,000 pounds per hour that rival, sometimes exceed, what twins can do. That might be hard for you to believe but, fortunately, we have evidence. We can show you nano materials, at 100,000 times magnification, that are well distributed. We can show you sub-micron PS globules, at 5,000 times magnification (whereas some, so called, state-of-the-art single screws are still researching with optical microscopes!). We can show you compounded wood, TPO’s, and solve problems that you may not even know exist (such as reagglomeration).
Now, we’re bringing new technology on-line including:
•Micro-Batch mixers: We’re developing machinery to replace the cumbersome lab batch mixers that you’re used to. You won’t have to scrape hot material off rotors into a messy pile, then grind that pile and then try to mold the grindings. Instead, you’ll take 100 grams of ingredients (your batch size), melt, mix and then extrude directly into a strand. Very shortly, we plan on equipment that will go one step further: In 15 minutes, you’ll go from your 100 grams batch of raw ingredients to a tensile bar.
•Injection Molding Screws: We’re installing our first molding screw. We expect to show serious property improvement.
•Direct Extrusion: We’re already compounding directly into product at the lab scale but as this technology blossoms, the advantages of single screw compounding will let you compound while you extrude at thousands of pound per hour—saving money while you make a better product.
•Venting: We’ve invented a new venting mechanism in the single screw extruder. It’s shorter than your typical two stage screw so we can fit three vents into a 36/1 single. And, since it mixes while it vents, we pretty sure it will vent better too. One of the nice things about this business, is that I don’t know where we’re going. But, everyone at Randcastle looks forward—because that’s our job.
Engineering Benefits
Before we built the vertical extruder, screws smaller than 1 inch broke easily and fed poorly. The solution is the “discharge driven” Microtruder. These are some of the benefits of our state of the art technology. Strong Screw*: An extruder screw must withstand the stress of feeding, melting, mixing, and pumping so Randcastle drives the screw through the metering section of the screw. The metering section root diameter is about 2 to 4 times as strong as the weak feed section root diameter allowing much smaller extruders.
Excellent Feeding:
A functional extruder gets the feed stock to the screw. In a typical extruder, there is a hole in the barrel, proportional to the screw diameter, that allows t he feed stock to enter the screw from the hopper. The hole becomes so small in these extruders under one inch that the feed stock arches (also called “bridging” and “piping”) over the hole. Once the feed becomes erratic, the extruder cannot pump uniformly. Randcastle’s vertical design extends the screw into the feed stock to solve the problem. Attachments to the rotating screw can extend into the hopper to aid in feeding particularly difficult powders and cohesive (non-free flowing) feed stocks.
Superior Compounding:
Mixing is important in most extrusion applications but critical in some. Pending patents for our Recirculator mixing elements demonstrate, in published peer reviewed papers, success in applications previously only possible with twin screw extruders including such high filler loadings as 40% wood flour and pellets directly into sheet. Improved
Melting:
Our patent pending melting technology makes better melted product at lower temperatures and higher extrusion rates.
Feeding Further Enhanced:
Randcastle pioneered unique smooth bore feed sections that can make for significantly better feeding and consequently more uniform pressures1 . These feed sections can be changed in under one minute with just four clamping screws. So often, if you select the wrong screw design for an experiment, you can simply change the feed section to solve the problem without removing the screw and cleaning it.
Uniform Pumping:
Randcastle discovered a novel method of making extruders pump better. We named this method, “Surge Suppression” and we incorporate it into every screw we make. Essentially, if you view a surge as a wave, the Surge Suppressor stores the top part of the wave and inserts the stored material into the trough of the wave. The Surge Suppressor operates automatically giving every Randcastle screw more uniform operation. Batch Size: Until the Microtruder, researchers were forced to make multiple expensive batches to make a single trial. Using the Microtruder, complete trials with as little as 10 grams of polymer are possible. Production applications, such as catheters and coextrusion, also benefit from the Microtruder. Extruders that are too large for the application degrade the polymer because the screw turns too slowly. This makes the residence time excessive and causes a loss of properties in the extrudate. The Microtruders permits normal screw speeds to prevent burning. Space Savings: The vertical design has an extremely small footprint so it is readily engineered into small spaces.
Microtruder Features & Specifications:
All standard Microtruders include the extruder and a control pane .l The standard extruder includes a 24:1 working L/D ratio 4140 hardened screw, removable water cooled feed sections made from hardened stainless steel (s.s.), a 1750 RPM DC motor, 15:1 gear reducer with integral thrust bearings and keyed drive quill, nitrided s.s. barrel with mica band heaters, s.s. barrel cover, rupture disc 1 , and a s.s. breaker plate. The standard control panel includes (3) auto tuning temperature controllers for the barrel and (1) for the die, a variable speed AC motor control with 1000:1 speed range and overload amperage protection, digital motor amme ter, and a digital screw speed readout. Bench mounting is standard and includes height and leveling adjustment (shown on the RCP -1000 below). Typical dimensions are given on page 7. Detailed assembly drawings available. Outputs in the following chart1 can be reduced by about 60% with low output screws and increased about 300% with Recirculator Technology.