8. lecture . level 3. term 2 investing & casting (1).pptx

One of the best summary of the investment and casting for fixed prosthodontics in dentistry

1. wax pattern Investing & casting Dr. Hagar Fatehi Al-zabadi Lec. 7,8 Level 3 – T.2

2. Content Wax pattern . Definition . Requirements of the wax pattern . Techniques of construction of the wax pattern . The lost wax cast technique .

3. Definition : It is the precursor of the final cast restoration that will be placed on the prepared tooth. Requirements of the wax pattern: 1. It should be clean and smooth. 2. It should duplicate accurately the anatomical features of the original tooth. 3. It should be free from any debris. The finished and accurate cast restoration is duplicated exactly through fabricating a wax pattern . Information needed for duplication of the anatomical features is taken from the adjacent & opposing teeth and from general knowledge of dental anatomy. Wax pattern

4. Types of Wax used to Form a g ood Pattern : Type I : Medium hardness wax : Used in direct technique . Not flow at mouth temperature . Accurate shape . Enough strength to resist any deformation . Support fine details . Type II : Softer wax : Used in indirect technique . Resist flow at room temperature . Maximize fitness between wax and die .

5. Requirements of Casting wax: 1. Have different color than the used die material. 2. Flow readily. 3. Be carved without shipping . 4. Accept addition and carved easily and precisely. 5. Evaporate without leaving any debris to avoid the contamination of cast restoration. 6. Have and preserve the fine and sharp details of the pattern till it investing without any distortion. 7. Have a minimum rate of stress relaxation after carving and before investing to prevent any distortion. 8. Can be burnished and polished . 10. Have acceptable strength and rigid after cooling .

6. Techniques of construction of the wax pattern : 1.Intraoral technique (direct technique): The wax pattern is constructed inside the patient's mouth using type I inlay wax . This technique is mostly used to construct the posterior inlay restoration & anterior post-crown . 2.Extraoral technique (indirect technique): Type II inlay wax is used to construct the wax pattern on the die of the working cast. Advantages of indirect technique over direct technique: 1. It reduces chair time , so it saves the dentist's time, and is more comfortable to the patient. 2. It offers an opportunity for visualization of the restoration and direct access for waxing of the margin. 3. It is probably the easiest mean of fabricating a cast restoration.

7. M ethods of wax pattern construction : 1. Dipping method : - A thin, uniform and adapted layer of wax on the die . - This is done by dipping the die into wax that has been thoroughly melted (using wax dipping pot). - Dipping the die in molten wax pot is easiest and better way to fabricate a coping

8. 2. Addition method: - To obtain a bulk of the wax pattern then carving is carried out. - This is done by melting the Wax and dropping it on the die using a heated carver until complete building of the pattern, and then Wax is carved by sharp carver. - The main drawback of this technique is due to the strains collected from multiply additions of wax which tends to be released with time and subsequently distorts the wax pattern.

9. 3. Molten press method : It is most suitable method to construct an accurate and adapted wax pattern. This method is done by adding molten wax and pressure is applied with fingers at each application , ( This method overcomes the most important drawback of addition method ) . Electric waxing instrument may be preferred for wax addition to control the temperature during wax pattern manipulation and produce burnishing effect by the blunt attached instrument. Wax is then carved using sharp carver.

10. 4. Injection method: This method is used in producing wax pattern for researches only, which require fixed and accurate dimensions. This method is performed by injecting the molten wax through metallic syringe (heated) into enclosed mold which is relieved by a hole opposing to the feeding hole. The wax feeding is continued under pressure until complete hardening of the wax. Steps of Wax Pattern Fabrication : 1. Coping fabrication. 2. Wax pattern removal and evaluation. 3 . Finishing and polishing of the wax pattern.

11. Waxing procedure (extraoral technique): 1. Coping fabrication : a - Marking the preparation margin without any pressure to: - Preserve the preparation without any scratches and to - F acilitate the carving of wax at this margin accurately.

12.   b . Apply die spacer to maintain constant thickness (space) between the side walls of the preparation and the restoration. A band of about 1 mm must be left unpainted with this relief agent at the margin of the die to maintain good marginal adaptation . 2-3 Coats of die spacer are placed . The produced space (20-40 micron) is necessary to: - provide room for the luting agent - permit complete seating of the restoration during cementation. c . Remove excess with die setting retardant & leave it to dry.

13. d . Cover all the preparation in one time with molten wax to make a coping of wax , This can be obtained by immersing the preparation in a dish containing molten wax . (this is the first layer of wax pattern). e . Add wax to the proximal surfaces of the preparation to build the proximal surfaces and contact areas with the adjacent teeth & then build the axial wall (buccal and lingual wall) to the normal contour.

14. f. Build the occlusal surface of the restoration following the curve of Spee and curve of Wilson, then check and adjust the occlusal relation with the opposing teeth in centric and eccentric relation. g . Check the margins of the wax pattern to ensure that the margins of the wax pattern have no over or under extension.

15. 2. Wax pattern removal and evaluation : The wax pattern should be sufficiently cooled. The wax pattern r emoved using the thumb and forefinger pressure with light grip If the pattern is not coming out the excess wax at the margin should be trimmed off. R emove the pattern & evaluate it .

16. The lost wax cast technique : it is the technique that has been used to convert wax pattern to cast metal . The process is achieved by : 1- Surrounding the wax pattern with a mold of heat resistance investment material (Investing ) . 2- Eliminating the wax by heating ( Burnout ). 3- Introducing molten metal into the mold through a channel called sprue (Casting ) . Investing & casting

17. Investing: it is the process of surrounding the wax pattern with an investment material that can accurately duplicate it’s shape and anatomic features. Investment: I t is a ceramic material which form a mold into which alloy can be casted. It consist of : Refractory material : - 60 - 65% which may be : Silicone dioxide . - Silica is a refractory material that withstands the temperatures used during casting . - Silica is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy. Investing

18. b) Binder material : - It may be : Gypsum /Phosphate /Silica - Binds to silica particle . - I ncrease strength, setting & hygroscopic expansion. c) Accelerators and retardors In small amounts to modify physical properties, reducing agent (decrease oxidation), produce smooth surface, regulate setting time & expansion.

19. Requirements of investments for alloy casting procedures : 1 - S hould be capable of reproducing the shape, size and details recorded in the wax pattern . 2 - S hould have a sufficiently high value of compressive strength at the casting temperature . 3 - Should compensate for the casting shrinkage. 4 - Porous to allow air escaping during casting procedure. 5 - Broken easily so that the cast can be retrieved easily. 6 - Easily manipulated and applied. 7 - Cheap.

20. Spruing The process of whereby the wax pattern is attached to a conical base (crucible former) by a connector called sprue former. Sprue Former: - It is that passage leading way from the crucible to the mold cavity. - It is a small diam pin : wax, plastic, metal. Crucible Former: - It is the base to which sprue is attached while wax pattern is being invested in refractory investment . - It may be convex rubber, plastic or metal base that forms concave depression or crucible in the refractory investment.

21. Requirements of sprue former: - Sprue must allow the molten wax to escape from the mold (during burnout). - Sprue must enable the molten metal to flow into mold. - Metal within it must remain molten slightly longer than the alloy that has filled the mold ( provides reservoir to compensate for the shrinkage occurs during solidification of the casting ) . T ypes of sprue : I- Wax : preferred because they melt at the same rate of the wax pattern*** . 2- Plastic : they may be used when casting fixed partial dentures because of the added rigidity . 3- Metallic : - It m ust be made from non-rusting materials to avoid contamination of the interface area between the wax and investment.

22. Sprue Former Diameter : - Diameter available 8, 10, 12 gauge. - Always use the largest possible to : - Improves the flow of molten metal into the mold. - Ensures a reservoir during solidification. a) For centrifugal casting machine: - The sprue should be thicker than the thickest portion of the wax pattern. - Large 2.6 mm (10 gauge) for molar and metal ceramic restorations. - A smaller 2 mm (12 gauge) for premolar and partial coverage. b) For air pressure casting machine: - Narrow sprue at the area of attachment to wax pattern is used for forcing of metal to mold cavity.

23. Sprue former length: • The length should not be more than 6 mm and not less than 2 mm** . • Gypsum distance between ring and pattern is 6-8 mm. • Phosphate bonded distance is 3-4 mm. • The sprue should be placed in the center of the ring (hottest zone) Too close to end of ring cause casting alloy breaking through the end of investment. Too far, gases may not escape rapidly enough to complete filling of mold with alloy.

24. Sprue former location and angulation : • The sprue former is attached to the largest non functional cusp with angle 45 degree. • In anterior teeth it is found mid incisal. • Point of attachment : should be well flared and avoid any constriction to allow good flow of molten metal. Correct sprue placement on the bulkiest nonfunc - tional cusp allows molten alloy to flow to all parts of the mold. Incorrect sprue placement in the central fossa obliterates occlusal anatomy and may result in poor mold filling because the molten metal is not pushed into the cusp tips by centrifugal force.

25. Number and shape of Sprue former : 1- Single may be direct or indirect (zigzag) 2- Double may be direct ( v and y shape) or indirect (stable shape) Accessory sprue (Venting) : • Small auxiliary sprue or vent have been recommended to improve casting of thin patterns. • Their action may help gases escape during casting. Crucible former: Constitute the base of the casting ring ( Rubber , Metal , Plastic )

26. Casting Ring and liners : • The casting ring : serves as a container for the investment while it sets and restricts the setting expansion of the mold. • The casting liners : - A resilient liner placed inside the ring leaving 2 -3 mm from both ends . - Aim of liners: 1 - Venting. 2 - Facilitate removal of investment after finishing of casting. 3 - Share in setting expansion when it is placed wet .

27. Investing techniques : I. single step investing technique . II. Two step investing technique . I. single step investing technique : 1. B rush technique . 2. Vacuum technique 1. B rush technique : Water powder ratio is mixed under vacuum & the wax Pattern is painted with the investment by using a b rush (free from bubbles). Casting ring applied over crucible former & the Ring is filled under vibration until complete fill.

28.  Vacuum technique : W ater powder ratio is mixed manually at first . Then with the crucible former and pattern in place in the ring is attached to the mixing bowel. T he vacuum hose is attached to the assembly ( the mix is mixed at vacuum ). After mixing , the bowel is inverted and the ring is filled under vacuum.

29. II) Two step investing technique : T he wax pattern is painted with a thick mix , after setting it immersed in water for 10 min. Then the casting ring is applied over the crucible former & filled with investment (second coat). It produce - Smooth surface casting. - Decrease the Setting expansion, therefore decrease the wax pattern distortion. After investing, the ring should left for 1 hour with no premature handling to avoid cracks within investment.

30. Burnout : It is the process of r emoving the wax pattern by heating for : 1) Complete elimination of wax to obtain an empty mold. 2) Elimination of carbon vapors from the mold. 3) Obtain adequate thermal expansion. 4) Elevates the temperature of the investment to prepare it for casting . .

31. Dental casting alloys According to noble metal content: * Noble-metal alloys * Semi noble alloys * Base metal alloys * Titanium and its alloys Casting : Introduction of molten alloy into the mold. B. According to hardness: Type I : ( soft ). simple inlays Type II : ( medium ) complex inlays / onlays Type III : ( hard ) crowns and FPDs Type IV : ( x-hard ) long span FPDs, RPD s

32. Thank you