Causes & Prevention Methods of Common Investment Casting Defects

During theproduction of investment castings, it isavoidless that some investment casting defects are occurred due to some wrongoperations. Below are some of the common defects:

1. Gas hole

Gas hole is thehole that exists in the casting surface or internal area. It can be round,oval, or irregular shape, sometimes is an air mass made up of multiple gasholes. Internal gas holes is usually pear-shaped. Shape of choking hole isirregular, and the surface is rough. Gas nest is casting dents on the surface,the surface is much more smooth. Surface gas holes can be found by appearanceinspection, however, subsurface gas holes can ony be tested after machining.

Causes:

·        Preheating temperature of casting mouldis too low, and cooling of liquid metal is too fast when it goes throught thepouring system.

·        The exhaust design of mould is not good,so that gas can not flow out.

·        Bad coating material, poor exhaust, evenvolatile or break down the gas itself.

·        Holes and pits are exsited in thesurface of mold cavity, when pouring liquid metal, the gas in the holes andpits are rapidly expanding, and compress gas liquid metal, thus to form chokinghole.

·        Surface of mold cavity is rust, and didnot clean up.

·        Raw material (sand core) is improperlystored, without preheating before use.

·        Poor deoxidizer, improper dosage orimproper operation, etc.

PreventionMethods:

·        Mould should be fully preheat, thegranularity of coatings (graphite) shoulds not be too thin, permeability needsto be good.

·        Using tilt pouring way when casting.

·        Raw materials should be stored in ventilated,dry place, and should be preheated when used.

·        Choose deoxidizer (magnesium) withbetter deoxidization effect.

·        Pouring temperature should not beexorbitant.

2. Shrinkage Cavity (Shrinkage)

Causes:

·        Control of mold working temperature didnot meet the requirements of directional solidification.Improper coating

·        selection, the control of the coatinglayer thickness control in different areas is bad.

·        The position of the investment castingpart in the mold is not proper designed.

·        Poured riser design did not meet fullythe effect of feeding.

PreventionMethods:

·        Improve mold temperature.

·        Adjust the thickness of coating layer,even paint spraying, and avoid local coating accumulation phenomenon whencoating paint falls off or filling.

·        Local heating the mold, or local heatpreservation using thermal insulation material.

·        Design the heat sink in the mold , orthe cooling speed accelerate the cooling speed in local area through water, or elsespray water outside the mold.

·        Place detachable handling chill blocksin turn inside the cavity , to avoid sufficient cooling of blocks whencontinuous production.

·        Design pressure device on the riser ofmold.

·        Accurately design the gating system, andchoose suitable pouring temperature.

3. Slag hole (flux slag or metal oxide slag)

Slag hole isclear or unclear hole in the casting. It is fully or partially filled by slag,and the shape is irregular. Small shape flux slag is not easy to be found,after removing slag, it will present smooth hole, general is distributed at thebottom area of the pouring position, around the runner section, or near thecorner. Oxide slag is distributed like meshed shape in casting surface near thesprue, sometimes thin sheet, or irregular cloud shape with wrinkles, orform flake interlayer, or exist as flocculent shape in the inside area ofcastings.

Causes:

Slag hole ismainly caused by alloy smelting process and pouring process (including improperdesign of gating system), the mold itself will not cause slag hole, and themetal mould is one of the effective methods to prevent slag inclusion.

PreventionMethods:

·        Correct gating system design, or usecast fiber filter.

·        Using tilt pouring way.

·        Choose the flux, and strictly controlthe quality.

4. Crack (thermal crack, cold crack)

Causes:

Metal mouldcasting is easy to produce crack defects, because the metal mould does not havedeformability itself. High cooling speed will be easy to increase the castinginternal stress. Too large or small pouring angle, too thin coating thickness,ect. may all cause casting crack. Beside when there is a crack in the molditself, it will also be easy to cause crack.

PreventionMethods:

·        Attention should be paid on the castingstructure, make uniform transition of non-uniform wall thickness. And adopt theappropriate fillet size.

·        Adjust the thickness of the coating, tryto achieve the required cooling rate of each casting area, and avoid to formtoo large internal stress.

·        The working temperature of the metalmould should be paid attention to and adjust the angle of mold.

5. Cold Shut

Cold shut is akind of slot or surface crack with round edge, the middle is separated by ascale, so it is not fully integrated. When the cold shot is serious, it willbecome “owe cast”. Cold shut often appears in the top wall of the casting, thinhorizontal or vertical plane, or in the joint area of thin & thick wallthickness.

Causes:

·        The venting design of metal mold is notreasonable.

·        Too low working temperature.

·        Bad quality or improper operation ofcoating.

·        Improper riser postion design

·        Pouring speed is too slow.

PreventionMethods:

·        Correct design the runner and exhaustsystem.

·        Proper thicken the coating layer forthin wall thickness investment castings.

·        Improve mold working temperature.

·        Using tilt pouring way.

·        Using mechanical vibration metal mouldfor casting.

6. Sand hole

Causes:

·        Surface strength of sand core is notgood, burning or not completely cured.

·        The size of the sand core is notconsistent with outer mold, or is crushed when clamping mold.

·        The mould is polluted by graphite waterand sand.
The sand is rushed into the cavity.

PreventionMethods:

·        Making sand core according to theprocess districtly.

·        Keep the size of sand core consistentwith outer mold.

·        Timely clean graphite water.

·        Clean the sand in mold cavity whenputting sand core.