+ WHY STRIVE FOR HIGH SURFACE FINISH
The increased use of plastic products has created a higher demand for mirror finish of molding tools,The highest demands for surface finish are for optical lens molds,which must be highly polished.But in general,there are other advantages with high surface finish,including :
Easier ejector of the plastic articles from the molding tool,applies to most plastics.
Reduced risk of local corrosion
Reduce risk of fracture or cracking due to temporary overloading or pure fatigue.
+ HOW TO JUDGE THE SURFACE QUALITY
Two thing are important when judging the surface quality of the mold,First the surface must be have geometrically correct shape,without any long macrhowaves.This macrhoshape derives mostly from the early grinding and stoning steps.
Second,the mirror finish of the mold surface is normally juged by naked eye.The surface must be free from scratches,pores,orange peel,pitting9needle stick,phinholes) etc.There are certain difficulties involved in a visual evaluation.A flat surface can look perfect even though its not geometrically completelly flat.Thus the eye can be fooled.
The finish can be juged in a more sophicated manner with instrumental methods,e g,optical interference methods.
B . WHAT IS SURFACE ROUGHNESS ?
C. FACTORS THAT AFFECT POLISHABILITY
D. PRACTICAL HINTS FOR GRINDING AND STONINNG OF MOLD
E. PRACTICAL HINTS FOR POLISHING OF MOLDS
Diamond paste is the commonest abrasive agent used in polishing.Optimum performance is obtained with right paste on the right polishing tool.Th commonest polishing tools are stricks,pins and plate for manual use and bobs or brushes for machines
Polishing tools are available in materials of different hardnesses,from metals through different type of fibers(wood,synthetic fibers) to soft felt.The hardness of the polishing tool affect the exposure of the diamond grains and the removal rate.
Time consuming,expensive polishing can be avoided by following certain rule.Cleanliness in every step of the polishing opeation is of such great importance that it cannot be overemphasized.
Polishing should be carried out in dust and draft free places.Hard dust particles can easily contaminate the abrasive and ruin an almost finished surface.
Each polishing tool should be used for one and only one pate grade and kept in dust proof containers.
The polishing tool gradually became impregnated and with time.
Hands and workpiece should be cleaned carefully between each change of paste grade,the workpiece with a grease dissolving liquid and the hand with soap.
Paste should be applied to the polishing tool in manual polishing,whereas in machine polishing the paste should be applied to the workpiece.
The finer the grain size,the less the thining liquid.
Polishing preassure should be adjusted to the hardness of the polishing tool and the grade of the paste.For the finnest grain sizes,the pressure should be only te weight of the polishing tool.
Removal of large amounts of material requires hard polishing tools and coarse paste.
Finish polishing of plastic mold should be carried out in the release direction.
Polishing should start in the corner,edge and fillets or in other difficult patr of the mold.
Be careful that sharp corners and edges are not rounded off.Preferably use hard polishing tools.
F.EXAMPLES OF OPERATION SEQUENCES
The following is a simple sequence :
Start with a hard polishing tool and a coarse paste.
Then change to a softer polishing tool with the same paste.
Then use a medium hard polishing tool and medium coarse paste
Change to a soft polishing tool with the same paste.
Finally,use a soft polishing tool and a fine paste.
G. PARTICULAR PROBLEMS
+ DO DIFFERENT SURFACE CONDITIONS PRIOR TO POLISHING METHODE
+ DIFFERENT SURFACE ROUGHNESS AFTER DIFFERENT HEAT TREATMENTS
+ OVERPOLISHING
The predominant problem in polishing is so called ovepolishing.Ovepolishing is the term used when a polished surface gets worse the longer its polished.Basically two phenomena appear when a surface is overpolished : orang peel and pitting(needle stick and pincholes).It should be pointed out that overpolishing often occurs in connection with machine polishing.Manual polishing seldom gives rise to these problem.
+ ORANGE PEEL
The appearance of an irregular ,rough surface,which is normally referred to as orange pell,may depend on a number of different cause.The commonest cause is overheating or or overcarburization from heat treatment in combination with high pressure and excessivelly prolonged polishing.Harder material can better withstand a high polishing pressure :softer steels overpolish easier.Studies have shown that the overpolish efffect occurs at different polishing time for different hardeness.
The normal reaction of some who sees that surface has deteriorated is to increase the polishing pressure and continue polishing.Such a course of action will inevitably result in further surface deterioration.
Either of the following alternatives can be adopted to restore the surface :
Remove the defective surface layer by grinding the surface using the next to last grinding step prior to polishing.Start againt at the final grinding stage.Use a lower pressure than was used before during polishing.
Stress relieve at a temperature about 45 F(25 C) bellow the last tempering temperature.Regrind using the final grinding step prior to polishing until a satifactory surface has been obtained.Start polishing again,but at a lower polishing pressure than before.
If the result is still not good,the hardness must be raised.This can be done in a number of ways :
Increase the surface hardness of the steell by means of nitriding or tufftride treament.
Select a steel of higher hardnedd or heat treat the tool to higher hardness.
+ PITTING
The small pits that can accur in a polhished surface generally result from slag inclusion in the form of hard,brittle oxides torn loose from the surface by the polishing process.Important causal factors of pitting are :Polishing time and pressure
Purity of the steel,especially with regard to hard slag inclusions
The polishing tool
The abrasive
One of the reason pitting can occur is the difference in hardness between the matrix and the slag inclusion.During polishing the matrix will be removed at a more rapid rate than the hard slag particles.Polishing will gradually undermine the slag particle until its torn out of the material by further polishing,leaving a pit.The problem is most oten encountered when a paste grain size of less than 10 micro m and soft polishing tools are used.
Steel with high cleanliness should be selected steel that is ESR remelted and vacum degassed has purity.
If pitting appears despite this precaution,the following measures should be taken:
Regrind the surface carefully using the next to last grinding step prior to polishing.Use a soft,free cutting stone.Then start with the final grinding step and finally polish.
When using grain size 10 micro m and smaller,the softest polishing tools should be avoided.
Polish the shortest possible time and under the least possible pressure.
Avoid machine polishing with aluminum oxide paste.
Design | Injection | Blow Mould | DieCasting| PunchingTool | TechStore | Holiday
No comments:
Post a Comment