Model fabrication - common mistakes

making work easy

Oliver Dreher
Dental Technician
Germany

Model fabrication is the business card and a basic element of the work of a dental technician. Many steps could be significantly simplified by tools such as vibrators or vacuum mixers. Nevertheless, there are many pitfalls that affect the quality of work significantly. Today we take a look at problems that may arise in the process of model creation, showing the cause and give valuable tips on how these errors can be avoided in order to ensure flawless models.

Plaster sets too slowly.

Cause	Remedy
Impression was not cleaned of blood etc.	Clean the impression thoroughly.
With hydrocolloids: impression was not immersed or only briefly immersed in a potassium sulphate solution.	With hydrocolloids: Immerse for a minimum of 10 minutes.

Plaster sets too quickly.

Cause	Remedy
Set plaster residue on the spatula and/or in the mixing bowl.	Always use clean bowls and instruments.
Too little water used for mixing.	Adhere strictly to the manufacturer‘s instuctions, always use distilled water.

Plaster forms lumps.

Cause	Remedy
Plaster was stored unsealed.	Plaster is hydrophilic, so always store airtight.
Set plaster residue in mixing bowl.	Always clean the mixing bowl thoroughly.

Final hardness of plaster is too low.

Cause	Remedy
Salt was added to speed up the setting time.	Never use additives in the mixing water.
Too much water was used for mixing the plaster.	Adhere strictly to the water ratio given by the manufacturer.
Plaster was mixed for too long.	Adhere strictly to the mixing time given by the manufacturer.

Some surfaces of the model are soft.

Cause	Remedy
Mixing bowl was on the vibrator during pouring.	Never place the mixing bowl on the vibrator.
Impression was vibrated too strongly.	Always use the vibrator on the lowest setting.

Surface of the model is porous.

Cause	Remedy
Plaster powder was not sprinkled in properly.	Always sprinkle in the loose plaster powder.
Vacuum pump and mixing paddles were started at the same time..	Build up the vacuum before starting the mixing paddles.

Smears in the plaster.

Cause	Remedy
Mixing bowl was on the vibrator during pouring.	Never place the mixing bowl on the vibrator.
Vibrator setting was too high.	Always pour impressions on a low vibrator setting.
Mixing time in the vacuum mixer was too short.	Allow a minimum mixing time of 60 seconds.

Cracks in the model.

Cause	Remedy
Plaster was mixed with too little water.	Adhere to the manufacturer‘s instructions.
Water removed from the model during setting.	Do not place the model on paper during setting.

Raised bite.

Cause	Remedy
Trimmer water was used instead of distilled water.	Always use distilled water.
Salt was added to speed up the setting time.	Never use additives in the mixing water.

Marginal gap between the dental arch and base.

Cause	Remedy
Base plaster was mixed for too long.	Adhere to the mixing times.
Excess / incorrect separating agent used.	Always use the special plaster separating agent.