Ceramics And Rapid Prototyping 3

Question 1

Why can you use ceramics for medical implants like hip replacements

Answer 1

Chemical/abrasion resistant in the body (aggressive environment)

Question 2

List the basic steps in the traditional ceramics processing sequence

Answer 2

1) prep raw materials
2) shaping
3) drying
4) firing

Question 3

What’s the technical difference between crushing and grinding in preparation of traditional ceramic raw materials

Answer 3

Crushing reduces big lumps from the mine so they can be further reduced. Mineral crushing is done by compression or impact against rigid surfaces. Grinding refers to reducing small bits from crushing into a fine powder. It’s done by abrasion and impact from movement of other hard pebbles rods balls etc

Question 4

Describe 3 main types of comminution via crushing

Answer 4

1) jaw crushers, where large jaw toggles back and forth against a hard rigid surface
2) gyratory crushers, which use a gyrating cone to compress lumps against a rigid surface
3) roll crushers, in which the ceramic lumps are squeezed between rotating rolls
4) hammer mills, which use rotating hammers impacting the material to break up the lumps

Question 5

Describe the 4 main types of comminution via grinding

Answer 5

1) ball milling: hard spheres are mixed with the stock and tumbled inside a rotating cylindrical container. Rotation causes the balls and stick to be carried up the container wall and pulled down by gravity which makes a grinding action.
2) roller mill. Stock is compressed against a flat horizontal grinding table by rollers riding over the surface
3) impact grinding: particles of stock are thrown against a hard flat surface either in a high velocity air stream or a high speed slurry. Impact fractures pieces into smaller particles

Question 6

Name 3 components of ceramic paste used for shaping

Answer 6

1) clay (for consistency and plasticity)
2) no plastic raw materials like alumina and silica (don’t shrink in frying/firing but unfortunately reduce plasticity in mixture during forming)
3) other ingredients like fluxes that melt (vitrify) during firing and promote sintering of the ceramic material, and wetting agents that improve mixing ingredients

Question 7

Why is it good to use ball killing to prepare powder for shaping?

Answer 7

Ball milking grinds the material as well as mixing the various additives in one step. Other methods need another mixing step.

Question 8

Dis/advantages of a low and high water content for shaping of ceramic paste

Answer 8

Low water content results in low amounts of shrinkage during drying and also needs high pressure to form the component and binder materials. High water content makes the paste easy to form but results in more shrinkage which can cause cracks

Question 9

Describe the drain slip casting method

Answer 9

1) slip is poured into mold cavity
2) water is absorbed into plaster mold to form firm layer
3) excess slip is poured out
4) part is removed from mould and trimmed

Question 10

Describe the various forms of manual forming for ceramics

Answer 10

Hand modelling involves the creation of the ceramic product by morphing the plastic clay into shape. Art and patterns for plaster molds in slip casting are made this way. Hand moulding is similar only a mould or form is used to define portions of the geometry. Hand throwing on a potters wheel is another refinement of the handicraft methods.

Question 11

Difference between dry pressing and semi dry pressing of traditional ceramic parts

Answer 11

Moisture content is lower. Binders must be used to hold powder together. Lubricants are added to the die. Die must be harder due to abrasive particles. No flash is formed and parts are typically simpler due to flow ability of powder.

Question 12

Describe what a flash is in the context of ceramic manufacturing and why it occurs in semi dry pressing but not dry

Answer 12

Flash is formed from excess clay being squeezed between die sections. Doesn’t form in dry pressing as the ceramic is much less flowable

Question 13

Describe the change in volume as water is added to a ceramic aggregate. What happens to a ceramic material when it’s sintered?

Answer 13

Initially the aggregate volume is constant as water is added as the water replaces the air in the voids between particles. After all the air voids have been filled the volume increases linearly.

1) bond individual grains into solid mass
2) increase density
3) reduce/eliminate porosity

Question 14

When is it good to have an aggregate water content above 50%?

Answer 14

Above 50% behaves like water so is easy to transport

Question 15

Describe the volume changes in a ceramic body as it dries and relate this to the drying rate and physical processes occurring

Answer 15

Initially there’s a linear decrease in volume with constant drying rate as the water evaporates from the surface and draws water deeper via capillary action. After particles come completely in contact the volume changes stop. Drying rate decreases as the rate of evaporation drops from lower capillary action.

Question 16

Why is it undesirable to rapidly dry a ceramic?

Answer 16

Can lead to uneven evaporation and thus uneven volume changes that can cause cracking

Question 17

What happens in sintering?

Answer 17

Loose powder is in contact with other particles. The mix is heated and point bonds form. These then grow into necks. Growth continues reducing pore size and developing grain boundaries between particles.

Question 18

Comparing an equivalent part which was hand modelled and dry pressed, which would have a higher tensile strength?

Answer 18

Dry pressed part would have higher tensile strength as porosity is lower as powder is lacked more tightly together. Tensile strength of a ceramic is often defined by density of defects in structure and thus hand modelled are more likely to have pores (stress concentrators)

Question 19

Why should large cantilever features be avoided in ceramic components?

Answer 19

When losing a cantilever one side is in compression and the other tension. The tensile side is prone to fracture

Question 20

Describe the various stages for creating an additive my manufactured component from a software perspective

Answer 20

1) create shape in CAD
2) convert to STL file
3) Import/support part as needed
4) slice various layers

Question 21

What’s the issue with converting CAD to STL for a sphere

Answer 21

STL approximates geometry using triangles and polygons. Sphere won’t be completely continuous when STL.

Question 22

If you were 3D printing a hollow sphere using stereolithiography what considerations would you need for the design

Answer 22

Consider how to remove incited resin after printing

Question 23

What would you expect to happen to a liquid monomer if left out under ambient conditions?

Answer 23

The photo polymer would slowly cure due to UV in the normal environment

Question 24

In FDM printing would you expect a part built using 3mm or 1.5mm diameter filament to produce a more accurate part?

Answer 24

Smaller filament has higher dimensional tolerance as there will be less die swell from the polymer. A larger filament will remember it’s previous shape more

Question 25

In FDM printing would you expect a part built faster or slower to have higher dimensional tolerance?

Answer 25

Slower will have higher dimensional tolerance because the polymer melt stays in the heating chamber longer and therefore reduces the effect of die swell

Question 26

If I was trying to make a knife or other sharp object using FDM printing what is one limitation of the manufacturing technique relative to the functionality?

Answer 26

There is a resolution limit of the printing process. A knife requires a sharp edge and the FDM printing process can only be as good as the layer height or nozzle diameter

Question 27

If I was producing a shaft which is torsionally loads using FDM, why m if they it be good to print the shaft horizontally rather than vertically?

Answer 27

When vertical, the layers would sit on the shear planes and therefore chances of delamination of the layers is higher than if horizontally

Question 28

Why is additive manufacturing not zero waste?

Answer 28

Depending on the technique you’ll always have a degree of wastage like metal powder that’s lost when cleaning, or support materials that are later removed

Question 29

Name 3 functions of supports in selective laser sintering using metals

Answer 29

Support for overhanging features. Prevent thermal distortion. Relive support heat.

Question 30

Why does polymer based powder bed fusion not need support material but it’s needed in metal based?

Answer 30

The unsintered powder supports the sintered parts. Thermal stresses are higher in metal powder so needed here.

Question 31

Why is polymer jetting based 3D printing able to create parts with multiple materials compared to SLS DMLS SLA processes?

Answer 31

I’m polymer jetting, the monomer is held in a printer head and deposited into a substrate. For parts with multi materials, multiple print beads can be used. It’s more difficult to do this in SLS etc as the raw material is in distributed powder from or in a vat of resin, and thus hard to selectively find raw material

Question 32

You’re making a 3D printed component which has gears. What considerations do you need to ensure it prints well?

Answer 32

Sufficient spacing between gears so they don’t fuse together.