Cast of bones, e.g. Ossicles

Question 1

What is plastic?

Answer 1

“Plastic “ is an inexact term referring to a class of synthetic manufacturing materials that can be shaped and moulded.

Question 2

Why are certain plastics called resins?

Answer 2

Certain plastics have no definite melting points and are called resins.

Question 3

How are thermoplastic resins softened and what do they include?

Answer 3

Thermoplastic resins are softened by heat and they include epoxys, silicones and other polyesters.

Question 4

Which structures are protected by being embedded in clear resin? Which resin is suitable for this purpose?

Answer 4

Ossicles, small fragile casts eg, casts of the coronary arteries, bronchial tree, cleared alizarin stained foetus are protected by embedding them in clear resin.

Clear cystic resin is most suitable for this purpose.

Question 5

How is the resin activated?

Answer 5

The resin is activated by use of 1% catalyst. It maybe tinted to any desired colour.

Question 6

How is any small specimen protected from breakage, dessication and dust?

Answer 6

Virtually any small specimen can be embedded in a block of clear resin where its protected from breakage, dessication and dust.

Resin embedded mounts are clean,convenient to handle,view and easy to store.

Question 7

What do suitable specimens include?

Answer 7

Suitable specimens include delicate specimens that are opaque and dry eg ossicles and small specimens that have been stained.

Question 8

Epoxy clear casting Resin System

Two resins and three hardeners:

Answer 8

Two resins (LR151 and LR155) and three hardeners (EH39 fast, EH42, and EH45 slow) can be mixed together ratio (2:1) in any combination to adjust setting times.

Question 9

Epoxy clear casting Resin System

What do small castings cast with?

Answer 9

Small castings maybe cast with faster resins.

Question 10

What needs to happen to larger & thicker casts to avoid heating & discolouration?

Answer 10

Larger and thicker casts must be slowed down to avoid heating and discoloration.

Question 10

What needs to happen to larger & thicker casts to avoid heating & discolouration?

Answer 10

Larger and thicker casts must be slowed down to avoid heating and discoloration.

Question 11

What are thermosetting resins softened by?

Answer 11

Thermosetting resins are appreciably softened by heat.

Question 12

Describe the resin casts process.

Answer 12

This is a method of plastic casting where a mould is filled with a liquid synthetic resin which hardens.

Question 13

What are resin casts primarily used for?

Answer 13

Resin casts are primarily used for small specimens eg. ossicles and other prototypes.

Question 14

Describe the synthetic resin used in the Resin casts process.

Answer 14

The synthetic resin for this process is a monomer for making a plastic thermosetting polymer.

Question 15

What happens to the liquid monomer during the setting process?

Answer 15

During the setting process the liquid monomer polymerizes into a polymer thereby hardening into a solid.

Question 16

Method of resin casting, e.g. ossicles

Step 1a)

Answer 16

A plastic container suited to the size of the specimen is chosen.

Question 17

Method of resin casting, e.g. ossicles

Step 1b)

Answer 17

As a base, prepare resin and tint with a colour which is in contrast to the specimen being embedded.

Question 18

Method of resin casting, e.g. ossicles

Step 1c)

Answer 18

Having removed air bubbles the base is poured and the setting time is about 12- 18hrs.

Question 19

Method of resin casting, e.g. ossicles

Step 1d)

Answer 19

It is vitally important that at any time the layer of cast should not be more than 2-3mm thick.

Question 20

Method of resin casting, e.g. ossicles

Step 1e)

Answer 20

Any thicker than that has chances of it cracking due to heat generated during the clearing process.

Question 21

Method of resin casting, e.g. ossicles

Step 2a)

Answer 21

Prepare another quantity of resin in another container (sufficient to submerge the specimen) and place the item in it.

Question 22

Method of resin casting, e.g. ossicles

Step 2b)

Answer 22

Introduce negative pressure to remove bubbles trapped around the specimen

Question 23

Method of resin casting, e.g. ossicles

Step 3a) & 3b)

Answer 23

a) Place the specimen on the prepared base and pour clear mixed resin until the height of 2- 3mm is reached.
b) Allow this process to now cure.

Question 24

Method of resin casting, e.g. ossicles

Step 3c)

Answer 24

After curing continue layering clear resin in stages till the entire specimen is covered to about 4mm above it.

Question 25

Method of resin casting, e.g. ossicles

Step 4a) and 4b)

Answer 25

a) When the resin has hardened , place the resin embedded block in warm water (60 deg C).
b) After 10min remove and immediately place in cold water (10 deg C).

Question 26

Method of resin casting, e.g. ossicles

Step 4c) and 4d)

Answer 26

c) The sudden exposure to difference in temperature will release the resin block from its container.
d) Wipe dry and the formed is now ready for the final stage

Question 27

Method of resin casting, e.g. ossicles

Step 5a)

Answer 27

The block is now ready for sanding and polishing.

Question 28

Method of resin casting, e.g. ossicles

Step 5b)

Answer 28

Commence sanding the upper surface(which maybe wavy due to setting cure) with a sandpaper of 60 grit.

Question 29

Method of resin casting, e.g. ossicles

Step 5b), 5c) & 5d)

Answer 29

b) Gradually increase and bring to 1000 grit and sand all areas needed.
c) With a motorised polishing pad polish till gloss.
d) Specimen is now ready for display and usage.