Circular Machine: Plastic-Shredder

Goal:

We developed a prototype of a shredder for the plastic waste circulating at the HAN, University of Applied Sciences.

The new shredder is used in combination with an existing smaller shredder present at the HAN. The prototype shreds large pieces of plastic which are to fed into the smaller shredder, to decrease the plastic dimensions even further. The shredder consists out of several components, such as: blades, funnel, axle and bearings. The output plastic is to be used for new products.

Material list:

• Steelplate 15mm
• Steelplate 1500x1000 - 6mm
• Steelplate 1000x1000 - 15mm
• 1x Steel tube square 50x50x3
• 1x Steel tube round 25x2 - 2m

Parts:

• Electric motor
• Reducer
• 2 Flanges
• 2 Stub shafts
• 24 Knives

Tools used:

• Waterjet
• Sandblaster
• Welder
• Lathe
• Grinder

Semester 6 project planning

Requirements

The concepts are made according to the following requirements:

1. The machine is able to shred 70cm wide 6mm thick plastic plates.
2. The machine is able to shred 10L jerry-cans.

Concepts

Multiple concepts are made with foamboard to visualize the various ideas. The visualization prevents possible problems with the real design. The three-bladed knife with hooks is chosen for the machine, because:

1. The material gets trapped between the stationary and rotating knives, due to the large cutting radius and hook on the blades. This ensures that the material is pulled into the machine, and does not bounce on top of all the rotating knives.
2. The throughput of the machine stays high, without losing the gap between blades. The latter allows for material to fall into the blades. (Two blades would have too little throughput, four blades would have too small of an angle between the blades).

The calculations of the shredder are divided into two parts. The first parts discusses the deflection and stresses in the axle, and the second the power required for the shredding. The power calculations also contain an overview of a MATLAB script, which helps to quickly see the effects of different shredder designs.

Axle Calculations

1. Linear deflection and stresses.
2. Angular deflection and stresses.

Power Calculations

1. Required power for given material.
2. Shredding ability of machine with given power.
3. MATLAB scripts.

The previous mentioned concept was further developed into a complete CAD model, which includes the frame. The axle is later changed to a cheaper design, to cust costs. The new design uses the stub shafts, as seen in step 5.

SolidWorks Part list:

From automotive we received a steelplate of 15 mm for the production of the knives. 24 Knives were cut out of the steelplate with the waterjet. The time for cutting the blades with the waterjet was around 12 hours.

Knives

The stub shafts for the shredder were first designed in CAD. The shaft itself is going to be a steel square tube of 50x50x3mm. Therefore the endcaps of 50x50 are used from waste material of the knives (square mid-products).

Using the lathe the holes are centered and drilled. The shafts itself are machined according the 2D drawing.

An old metal circular saw was found at automotive and is disassembled for the electromotor. This has a reductor included, the motor is checked and the next step is to test the state of it and count the outgoing RPM.

Also a rubber coupler was found, flanges can be drawn in CAD after the working state of the motor is checked.

Update 19-5-2022:

We tested the electric motor by putting it in the wall outlet to see if the electric motor still worked. The result was that the electric motor still worked, without fall out of any fuses. Because the previous owner said that there could be an earth leakage or short circuit between the coils. The result of the RPM test was 26 rpm.

The bearing that will be used is a SYF 30 TF short base pillow block ball bearing units (2x). The bearings were found and collected at the HAN.

Information about the specific housing of the bearing:

Cast iron housing (from manufacturer)

• Material is suitable for most applications.
• Grease fitting for relubrication is included – enabling maximized service life under severe operating conditions.
• Housings are painted with a blue (RAL 5007) water-based alkyd/acrylic paint that:
• protects the housing in accordance with ISO 12944-2, corrosivity category C2, i.e. exterior atmospheres with low levels of pollution, or interior atmospheres where condensation may occur
• is not affected by most lubricating and engine oils, cutting fluids or alkalescent washing chemicals
• can be repainted with most water- or solvent-based 1- or 2-component paints
• Unpainted surfaces are protected by a solventless rust inhibitor.
• Housings can be ordered as separate products for combination with any SKF insert ball bearing 

Both the stationary knives as the shims are made out of the same 6 mm steel plate. The cutting time for the stationary knives and spacers on the water jet was 5 hours. The spacers guide the axle during use of the machine, to prevent the axle from deflecting to much under heavy loads.

Material

15mm steel is required once again for the shims. The total plate was too heavy to lift onto the waterjet table. The plate was therefore cut into two parts, 120cm and 80cm long each.

The cutting of the plate proved quite troublesome. First, a plasma cutter was used. However, this proved to be insufficient for the plate thickness. The result is seen in the 4th picture. Secondly, an oxy-acetylene torch was used. This worked perfectly, until the gas ran out. Thus, lastly, a grinder was used to cut the last bit.

Shims

The shims were cut over two days, with a total cutting time of 8.5 hours. The holes in the shims were not perfect, and were made correct by hand with files.

The 50x50x3 square steel tube is cut to correct length. We unfortunately found out that these dimensions differ quite a lot. Since the square holes in the knives were already cut, we chose to sand the square tube. This removes the mill scale and a small layer of the steel. The axle stubs are welded to the tube, as seen in the first figure.

The rotating knives will be in direct contact with the stationairy knives. Therefore, we removed the mill scale and rust spots. Sandblasting is the most efficient in a closed cabinet, the blasting sand is constantly collected and reused. Only compressed air (+/- 8 Bar) and some electricity for the LED light in the cabinet is needed.

The main-axle is guided by rings in the stationairy knives, this will become more clear in the assembling phase. The four mounting axles hold the all the stationary knives in their place. The mounting axles will be clamped at the ends to hold the whole shredder block tightly together.

The housing is an exterior case that protects the mechanisms/parts within. The housing prevents that the mechanisms/parts are fouled by debris. One of the important functions of the housing is to provide safety when the machine is in operation.

The flange joins the axle and the electric drivetrain. The plastic middle piece provides some freedom for the axle to move if it is not perfectly aligned. The latter ensures that the machine does not vibrate or damage itself.

Step 1: Putting spacers, rotating knives and stationary knives on the main axle.

Step 2: Putting the shims into the shredder together with the mounting axles.

Step 3: Attaching the outer frame to the mounting axles.

Step 4: Attaching the flange to the main axle.

Step 5: Attaching the motor to the flange, and thus to the shredder.

For a seperate part of the project (Safety), there was made a planning.

6 weeks Research into matching standards and draw up a checklist of this.
5 weeks Search for safety components (purchasing) that make the shredder safe (possibly dummy-proof).
4 weeks Designing safety components in CAD (in which the above components can be processed)
4 weeks Completion of ME+ (Components such as purchasing and production are ready and the shredder is safe)

The outcome of this study, as well as the DXF files, can be found in the attached files.