30 Questions
What is the purpose of packing acrylic resin into the mold?
To prevent shrinkage and porosity after polymerization
Why should pressure be applied gradually during the packing of a denture?
To allow the resin to flow within the mold
What are the consequences of adding acrylic resin too early into the mold?
Causes porosity in the final prosthesis
When is it considered too late to pack acrylic resin into the mold?
Stage IV
What method is used to press the flask when packing a denture?
Using a pneumatic press
Why should acrylic flash be trimmed away during the packing process?
To ensure even pressure distribution
What should be done to the teeth in the mold when applying cold mold seal?
Ensure they are not separated and not covered by the cold mold seal
Which component is used to characterize denture bases and create a lifelike appearance?
Colored acrylic placed over the gums
In what ratio are the powder and liquid mixed for an average-sized denture when using acrylic resin?
3 parts powder to 1 part liquid
During which stage of acrylic mixing does the mix become tacky, sticky, and adhere to the sides of the jar?
Stringy or fibrous stage
Why is it important for the cold mold seal to be thin and even on all parts of the mold?
To prevent adherence of the denture base to the mold
Which component acts as a cross-linking agent in acrylic resin?
Cross-linking agent - ethylene glycol dimethacrylate
What is the primary cause of inadequate softening of the powder by the monomer in denture processing?
Too much polymer
What is the main consequence of too much monomer in denture processing?
Excessive polymerization shrinkage
What is the primary reason for contraction porosity in dentures?
Linear shrinkage
Which factor is crucial in minimizing contraction porosity during denture processing?
Powder/liquid system with 5-8% ratio
What is the main reason for the linear contraction observed during denture processing?
Thermal factors
How does excessive monomer affect the potential outcomes in denture processing?
Increased polymerization shrinkage
What is the purpose of slowly cooling the flask on the bench?
To relieve internal stresses caused by contraction differences
Which step is NOT part of the de-flasking procedure?
Pressing the acrylic smooth and dense
What is the potential issue with heating the flask in an ordinary water bath as described?
Higher likelihood of distortion during deflasking
Why should temperature and time be controlled during the curing of acrylic?
To prevent overheating and ensure proper polymerization
In which heating technique does the temperature rise to boiling after reaching 72°C?
The special bath heating technique
What should be done to prevent breakage of the denture during de-flasking?
Cool the flask slowly on the bench in its water bath
What is the term used to describe the volatilization of the monomer due to temperature rises above the boiling point?
Gaseous porosity
How can the potential problem of gaseous porosity be eliminated during denture processing?
Utilizing a long curing period with a hold period at 70°C
What is the consequence of restricting dimensional change during cooling in denture processing?
Internal strains
Why is it important to cool the flask slowly during denture processing?
To prevent internal strains
What can cause gaseous porosity in denture material?
Exceeding the boiling point of the monomer during processing
How do modern heat-cured acrylics behave if the correct powder/liquid ratio is achieved in denture processing?
Their flexibility remains consistent despite processing times
Study Notes
Curing of Acrylic
- The packed mold is heated (cured) in an oven or in water bath to control temperature and time.
- Two water bath heating techniques can be used: heating the flask in a special bath of water or in an ordinary water bath.
- The first technique takes 16 hours, while the second technique takes 2 hours to reach 72°C, then 1 hour at boiling temperature.
- The second technique has a shorter time but may cause distortion during deflasking and increase the amount of free monomer.
Cooling of the Flask
- After curing, the flask is cooled slowly on the bench in its water bath to relieve internal stresses.
- Slow cooling prevents the breakage of the denture caused by the difference in contraction between the acrylic and the mold material.
De-flasking Procedure
- The flask is removed from the clamp, and the upper and lower lids are removed.
- The two parts of the flask are separated using a plaster knife, and the denture is liberated with its model.
- The model is removed carefully, and if there is an undercut, it is split into two or three parts to remove them easily.
- The excess of acrylic is removed using an acrylic bur.
Processing Problems
- Powder/liquid ratio: too much polymer can result in a weak material, while too much monomer can result in excessive polymerization shrinkage, loss of fit, and gaseous porosity.
- Contraction porosity: occurs due to polymerization shrinkage, which can be minimized by using a powder/liquid system between 5 and 8%.
- Gaseous porosity: caused by the volatilization of the monomer, which can be eliminated by having a long curing period with a hold period at about 70°C.
- Processing strains: occur due to the dimensional change during cooling, which can be minimized by cooling the flask slowly.
Stages of Acrylic Resin
- Smooth dough-like stage: where the monomer diffuses into the polymer.
- Rubber-like stage: where the monomer penetrates further into the polymer, making it too stiff to be packed or molded.
- Stiff stage: where the acrylic resin becomes hard.
Packing of Acrylic
- It is the procedure of applying acrylic resin into the mold and pressing the flask to compensate for the contraction of the acrylic after polymerization.
- The acrylic resin should be placed in the mold and pressed in a clamp immediately.
- Too early or too late application of the acrylic resin can result in porosity, loss of detail, or movement of teeth.
Mixing Acrylic Resin
- Acrylic resin is a resinous plastic material of various esters of acrylic acid used as a denture base material.
- It is formed of a powder and liquid, with a ratio of 3 to 1 by volume for an average-sized denture.
- Stages of acrylic mixing: sandy stage, stringy or fibrous stage, and a smooth, stiff stage.
Test your knowledge on the process of sealing the cold mold seal on a denture base, and characterizing denture bases to make them look more lifelike. Questions may cover techniques such as spreading the seal, avoiding covering the teeth, and adding colored acrylic for a natural appearance.
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