10. Molding and casting¶

This week’s assignment was to design and make a mold, using milling(micro milling or CNC), and using different materials to cast in it.

What is a mold?¶

A mold or mould is a hollowed-out block that is filled with a liquid or pliable material such as plastic, glass, metal, or ceramic raw material.[2] The liquid hardens or sets inside the mold, adopting its shape. A mold is the counterpart to a cast.

Types of molding¶

1. Blow molding - Blow molding (BrE moulding) is a specific manufacturing process by which hollow plastic parts are formed and can be joined together. The blow molding process begins with melting down the plastic and forming it into a parison or, in the case of injection and injection stretch blow molding (ISB), a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
   

2. Comprrssion Molding - Compression Moulding is a method of moulding in which the moulding material, generally preheated, is first placed in an open, heated mould cavity. The mould is closed with a top force or plug member, pressure is applied to force the material into contact with all mould areas, while heat and pressure are maintained until the moulding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.
   

3. Extrusion - Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finish.

4. Injection moulding - Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould. Injection moulding can be performed with a host of materials mainly including metals, (for which the process is called die-casting), glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed (Using a helical shaped screw), and injected (Forced) into a mould cavity, where it cools and hardens to the configuration of the cavity.

5. Rotational molding - Rotational Molding involves a heated hollow mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase.
   

Casting¶

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

The do’s and donts of casting¶

• The first thing you need to take care of while casting is there should be no surfaces having 90 0r lesser angle with the base as shown below. Even if you want a straight surface, give a small draft angle for easy removal of the final cast and not damaging your muld while removing the cast.

• There should be no under cuts as shown below. There should not be material anywhere above a void. Or else it would be impossible to remove the mould without damaging the mould or the cast or both.

Designing the mould in CAD for 3D milling¶

• When starting to design the mould, I had decided to make something geometric.

• Thus, I made different geometric shapes at different heights which had the same circumcircle of radius 30mm. The shapes I chose were triangle, hexagon and a dodecagon. The dodecagon was placed at the centre since it had the largest surface area and thus the shape would not have any undercuts in this case.

• Next I joined the vertices of triange to the vertices of dodecagon as shown below and made the resultant edges into surfaces using surface patch(SURFPATCH) command. Then used Join command to join these surfaces into a solid.

• Did the same process with dodecagon and hexagon The resultant was something like this.

• Next was adding the base and side walls for filling in the mould material. The wall was kept of 6mm.

• Lastly, I added the cavities for pouring the material and air to come out when pouring. Positive-Negative notches were added in both positive moulds to better align the negative moulds when making final cast. The final design looked something like this.

• Put the 2 blocks together and save this file as stl.
• Go to fabmodules.org and in input format, select mesh(.stl) and select the stl file you want to machine.

• Select Roland mill(.rml) in the output format. And wax rough cut(1/8) in process.

• On the right, select the machine MDX-20. Set the x,y and z to zero and click on calculate. You should see the toolpath as shownw here.

• The setting for cut depth, tool diameter, number of offsets, offset overlap are shown below.

• Click on send and the machining process will start.

• Once this is done change the bit to ballnose, and move the drill-bit to same x,y,z co-ordinates. Follow the same process with only difference being selecting wax finish cut(1/8) in process.

• Such will be the finished positive mould.

• Once the positive mould was ready, it was time to make the negative rubber mould. I applied vaseline as lubricant on all the surfaces so it would be easier to remove the rubber mould later.

• I used the Smooth-on Oomoo 30 - Silicon Rubber Compount to make the mould. The proportion to use was 1:1.3.

• Hence, estimating the size of my mould, I used 100g of A and 130g of B.

• Use a stick to mix them properly until it forms a uniform mixture.

• Then pour them in your lubricated mould and fill it until the brim.

• Shake the mould a little to remove any bubbles that are left inside.

• The curing time here is 6 hours. After waiting for that amount of time, care fully remove the negatives from the wax mould.

• This is the result.

Casting¶

• Now we can use this mould to try out different casting materials.

• I started with Smooth-On 300. The proportion of A to B here is 100:90.

• Hence I poured 30g of A in a cup and mixed then added another 27g of B in it, and mixed them properly using a stick.

• Then poured them in the mould using the holes I keft at the top for this very purpose.

• The curing time here was just 120 seconds so I had to be reaaly fast with pouring or the mixture would harden in the container itself. This is exactly what happened during the first attempt.

• So I redid the entire process and this time it was successful.

• I demoulded it after a few minutes and this was the result.

• Next I tried a different material, Smooth-On 305. The proportion here was 100:100.

• I poured 30g of A and 30g of B, mixed the mixture and poured it in the mould.

• This compound expanded a little after it solidified and hence the two moulds came apart a little as seen in the below picture.

• The 3rd material I tried out was drystone

• I mixed the DryStone powder and water in a Cup in 100:20 proportion until it formed a uniform mixture. Here, 100 is water and 20 is drystone Powder.

• I then carefully poured it in the mould.

• The curing time here is 6 hours. After waiting for that amount of time, I demoulded the cast.

• This was the result.

Conclusion¶

Thus, I explored using wax to make moulds from 3d design, the do’s and dont’s for this process, and also explored various materials and their results. We now understand the amount of time this process takes but also, that it can manufacture multiples of one object faster than any other process can.

• All the files for this week are attached here

Group Work¶

The Group Assignment was to review the safety data sheets for each of your molding and casting materials, then make and compare test casts with each of them. The group page can be found here