Chapter 8 Overview

1. Chapter 8 Overview • The quality of a die casting is more than skin deep • A quality casting is free of defects • In order to determine the quality of a casting, you must be able to identify the defects • There are three common types of defects

2. Chapter 8 Objectives • Correctly identify the common surface defects • Correctly identify the common internal defects • Correctly identify the common types of dimensional defects

3. New Terms • Inclusions • Materials that have been included in the alloy that should not be there, such as aluminum oxide, silicon carbide, fluxes and sludge • Polymorphic • The ability of, in certain environments, the properties of the alumina crystals to change drastically

4. New Terms cont. • Porosity • A void in the casting, caused by trapped gas or shrinkage • Viscous • The state of being semi-fluid; not flowing freely

5. Cold flow Cold shut Flow marks Cold Chill Severe chill Non-fill Poor-fill Laps Flow lines Swirls Knit lines Mis-run Surface Defects Flow Defects Other Defects • Blisters • Cracks • Soldering

6. Flow Defects • Result from how metal flows to and within the die • Adjusting process variables can sometimes impact their occurrence • The alloy begins to freeze before the casting is completely filled out • Several alloy flows converge but do not weld or fuse completely together

7. Fill time Wall thickness Die temperature Alloy temperature Flow distance Gate velocity Alloy type Venting Flow Defects cont. 8 Factors Affecting Flow Defects

8. 8 Factors: Fill Time • The maximum allowable time to fill the die cavity that results in an acceptable casting • If exceeded, the casting will have some defect • Fill time calculation based on several factors • Die temperature • Alloy temperature • Casting geometry • Alloy being cast

9. 8 Factors: Wall Thickness • Part of the casting’s geometry • Heavy wall sections equate to a lot of heat and high cooling requirements • Thin walls equate to very little heat and minimal cooling requirements

10. Flow Defects: Die Temperature • Time-averaged temperature of the die during sustained production • Cannot be measured any time at any place in the die • Ideally, it will: • Be as high as possible • Still permit making the casting • Vary as little as possible over the entire cycle

11. Flow Defects: Alloy Temperature • Temperature of the alloy as it begins to fill the die cavity, as it passes through the gate • Hard to measure in real time as the casting is being made • Estimated to determine fill time calculations • Avoiding delays in alloy transfer can minimize temperature losses

12. 8 Factors: Flow Distance • The distance that the metal must flow once it passes through the gate • Alloy should flow to its terminal location without freezing • If the flow distance is too long and if the alloy speed is too slow, it’s difficult for the metal to fill the cavity without beginning to freeze

13. 8 Factors: Gate Velocity • The speed the alloy travels as it passes through the gate • If not controlled, can be detrimental to the tooling causing washout and erosion • If too low, the alloy may not atomize and not have enough energy to reach the ends of the casting or to properly weld together

14. 8 Factors: Alloy Type • Can make difference in the surface finish • Zinc, Zamak 7 was designed to have the best fluidity and surface finish • Silicon content in aluminum aids fluidity • Alloys closer to the eutectic will be more fluid • Eutectic alloys are regarded as harder to cast

15. 8 Factors: Venting and Vacuum • Trapped air causes blisters and gas porosity and backpressure in the cavity • Back pressure can change the flow enough to cause surface defects • Most noticeable in blind features • May be necessary to add vacuum to remove gasses

16. Other Defects: Blisters • Bubble-like bumps on the casting • Gases trapped in the casting near the casting surface cause them • When casting is ejected and the casting surface is not strong enough to withstand the gas pressure, the surface yields and the blister forms

17. Other Defects: Cracks • Two major causes for cracks are: • Heat • Insufficient • Excessive • Externally applied stresses

18. Other Defects: Soldering • The fusion of aluminum in the alloy with iron from the steel surface of the die cavity • When soldering occurs, the casting sticks to the cavity; casting must be torn away • Aggravated by higher than usual die temperatures, high gate velocities and high metal pressures • Enhanced if the iron content in alloy is low • Can be caused by insufficient draft angles

19. Impact of Internal Defects • Mechanical properties include: • Tensile strength, elongation, hardness, impact strength and others • Measured on samples; results are published to help designers pick best suited material • Internal defects reduce mechanical properties

20. Impact of Internal Defects • Pressure tightness • An important property for some applications • The process has to be controlled while making solid, low porosity castings • Internal defects can cause loss of pressure tightness/leaks • Machineability • Affected by porosity and inclusion defects, the two types of internal defects

21. Internal Defects:Inclusions • Most inclusions are non-metallic aluminum oxide (corundum) • Oxides get into the bath • Most is removed, but some remains and ends up in castings • Size and shape of the individual corundum particles varies widely

22. Internal Defects:Inclusions-Oxide Films and Dross • Inclusions of oxide films and dross are major cause for leakers and excessive tool wear • This is generally gamma aluminum oxide • Oxide films prevent divergent alloy steams knitting together properly as the cavity fills

23. Internal Defects:Inclusions-SiC • Silicon carbide refractories get into castings if furnace-cleaning practices not maintained • As damaging as corundum • Encountered infrequently compared to corundum • Distinguished by its very black, glass-like coloring

24. Internal Defects:Inclusions-Flux • Not usually recognized during a cursory visual inspection • Casting must be submerged in city water overnight • If flux inclusions are present, they will grow crystals on the casting surface • Appears as light mottling on all surfaces

25. Internal Defects:Inclusions-Sludge • Composed of complex inter-metallic compounds of Al-Si-Fe-Mn-Cr • Is quite hard and will damage cutter tooling • Under high magnification sludge is easily recognized by the extremely fine primary crystals and their pentagonal shape

26. Internal Defects:Porosity • A void in the casting • Has two root causes: • Trapped gas • Shrinkage

27. Internal Defects:Porosity-Trapped Gas To solve a gas porosity problem, look at all sources of gas generation • Trapped air • Always present because of the turbulent method used to fill the die cavity • Air in cold chamber • minimized by filling the cold chamber with alloy

28. Internal Defects:Porosity-Trapped Gas • Turbulence: when alloy is subjected to turbulence in the presence of air • Minimize when picking up and transporting alloy to the cold chamber through ladling practices • Slow portion of the shot cycle must be controlled • Optimize timing of plunger • Accelerate plunger tip when past pour hole • When sleeve is filled, follow with a smooth acceleration to the fast shot speed

29. Internal Defects:Porosity-Trapped Gas • Improper venting: another cause for trapped air • Vents must be open to allow air trapped above the alloy in cold chamber to escape • If vent is working, a puff of air coming out can be seen

30. Internal Defects:Porosity-Trapped Gas • Excessive lubricants: can result in gas from two sources • Release of combustion products when some of the die lube burns when the alloy hits • Most releases are diluted with water • Water in lube will turn to steam and produce a great volume of gas • Gas forms when alloy runs over puddled plunger tip lube

31. Internal Defects:Porosity-Trapped Gas • Other sources of trapped gas • If die cavity cracked, it might allow fluid from the cooling line to leak into die cavity • Water or oil in the cavity, when hit by the alloy, will form gas

32. Internal Defects:Porosity-Shrinkage • Shrinkage: porosity that occurs if the alloy solidifies without pressure on it • All alloys shrink a certain percentage • High pressure die casting • Uses intensifiers/other methods to increase alloy pressure once cavity has been filled with alloy • Alloy pressure must be transmitted from the biscuit through the runner to the gate

33. Internal Defects:Porosity-Shrinkage • Shrink defects: occur at the last place in the casting to freeze • characterized by a rough and jagged appearance • tends to be continuous by nature

34. Dimensional Defects • Dimensional variations covered: • Linear variation, across parting line variation, shift and mismatch, warpage • Most dimensional defects related to: • Die temperatures • Condition of the die • Force of injection

35. Dimensional Defects:Die Temperature • Thermal expansion/contraction: objects lengthen when heated, get smaller when cooled • Castings get smaller when cooled • Dimensional problem can occur when one half of die is much hotter than other half • Can be a problem for the die and the casting

36. Dimensional Defects:Die Condition • Flash Buildup at parting line • Prevents the die from closing properly • May cause an oversize dimension • Prevents wedgelock from holding slide in place • Flash buildup at front of slide • Prevents slide from going to “ready to cast” position

37. Dimensional Defects:Die Condition • Soldering • Small core pins can be very susceptible; solder buildup can cause an oversize out-of-tolerance condition • May occur in walls; could cause an undersize or thin wall

38. Dimensional Defects:Force of Injection • Force of injection • Overcomes locking capability, causing tie bars stretch and allowing die to flash • Flashing adds to size, cause slides to backout • Normal injection force, impact, and intensification • Can: • Reduce the mass and speed • Minimize impact • Apply intensification before gates freeze

39. Dimensional Defects • Statistical dimensional control • Product gets larger or smaller over time • Process variables that contribute to the dimensional variation need to be identified • A control technique, such as the average and range chart, needs to be applied

40. Summary • 3 categories of defects: surface, internal, and dimensional, and many defects in each • The operator may or may not have control over them • 2 subcategories of surface defects: flow and other • There are many types of flow defects

41. Summary cont. • 2 subcategories of internal defects: inclusions and porosity • Dimensional defects are related to die temperatures, die condition, and the injection force