27.0 FINISHING OF METAL MOULDS
27.1 Introduction:
Mould polishing is a very important step in producing a mould that works properly and a part that is functional and visually attractive. Proper finishing of the mould is important for the release of the part. Polishing insures that the part is not damaged during ejection. For example, scratches from rough surfaces or dents from ejector pin. It also prevents damage to the mould that could occur if the mould closed on a part stuck in the mould. The finish of the moulding largely depends on the finish of the cavity and core.
27.2 Release:
Release is the ability of the mould to eject the component. To accomplish this, all cutter marks must be removed from the moulding surface of the cavities and cores. Further, the finish should be vertical, from bottom of the cavity or core to the parting surface, to aid in the ease of ejection. This is called draw stone. This also will speed up the moulding cycle.
27.3 Appearance:
There are different types of appearance required after finishing. They are
a) Rough finishing:
There are mainly three types of rough finish. The first is called draw stone. This is the most basic of all finishes. It is strictly for release. Te second is a sand blast or glass bead. This produces a matte finish, creating non-slippery, non-reflective surfaces. The third type is a textured finish. This is done to create a specific appearance, such as wood grain, leather or pebbly textures.
b) Mirror finishing:
A mirror finish is called diamond finish. There are three basic diamond finishes. The first, a rough diamond, is primarily used for release or as a preparation for a very fine texture. A diamond, the most often specified, is used to enhance the appearance of opaque parts and the ability to see through clear and translucent parts. A high shine or #1 diamond is necessary for optical or highly reflective parts.
c) Texturing:
To prepare a surface for texturing, you must know what the texture will be. This is necessary to prevent over or under polishing. Basically, the finer the texture, the finer the finish needs to be.
The product designer of a finished plastics product desires some sort of surface finish to the component. As a general rule it is considered that all moulding surfaces of a mould must be provide a mirror polish. A mirror polished moulding surface helps the moulds in many ways as given below
• Easy flow of molten plastics material ( melt) into the moulds
• Easy ejection of finished product from the mould
• Glazing attractive surface for a commercial product
• Good transparency for clear plastics materials
• High brilliance and reflectability for optical and automobile lamp reflectors
• Rough surface of moulds would be liable to retain acids and leading to localized corrosion
27.4 Overview of the process:
The polishing process involves a number of steps that must be performed in the proper order. This will ensure the best finish in the least amount of time. Remember, polishing is replacing one set of scratches with finer set of scratches until the desired finish is reached.
Polishing can be done by following method
• Lap first with rough grade lapping paste (emery paper) to remove machining marks
• After that medium and fine grade of lapping pastes (emery paper) are applied to give good finish
• Final polishing is done by chrome powder
• Diamond pastes of different grades are also used
• Amount of material removal is only a few hundredth
• Care must be taken to remove all machining marks initial stage itself to save time.
27.5 Standard specification of finish:
Currently, there are three standards being used to define a finish. They are the “OLD” SPI which has six finishes, the “NEW” SPI which has 12 finishes and RMS values which is the measurement of the depth of the scratches.
| Old SPI | New SPI | RMS | Description |
| #1 | A – 1 | 3 micron | Grade #3 Diamond buff |
| - | A – 2 | 6 micron | Grade #6 Diamond buff |
| #2 | A – 3 | 15 micron | Grade #15 Diamond buff |
| | B – 1 | 20 micron | 600 Grit emery |
| | B – 2 | 30 micron | 400 Grit emery |
| | B – 3 | 40 micron | 320 Grit emery |
| #3 | C – 1 | 25 micron | 600 Draw stone |
| | C – 2 | 35 micron | 400 Draw stone |
| #4 | C – 3 | 45 micron | 320 draw stone |
| | D – 1 | * | Dry Blast Glass Bead # 11 |
| #5 | D – 2 | * | Dry Blast # 240 Aluminum Oxide |
| #6 | D – 3 | * | Dry Blast #24 Aluminum Oxide |
The mirror polish is achieved by adopting polishing process, followed by a proper procedure. The very first step is to remove the cutter marks left over by the machining operation over the moulding surfaces. This is done by using various type of hand tools like files, needle files and raffle files. After obtained surface finish corresponding to the finer grade of files, abrasive stones of various grades are used, which again followed by various grades of emery papers. It is a laborious and time consuming process which determines the final required final dimensions of mould cavity and core. Therefore the polishing of moulds is to be carried out under the close supervision of skilled mould makers. The various sequences of polishing process is illustrated in the following figures.
To avoid manual strain and to speed up the polishing process there are polishing machines available with lot of accessories packed in a polishing kit which is also explained with figures
27.6 Mechanical Equipment in Mould Polishing:
Mechanical equipment (hand held) is important to the mould polisher for three reasons.
1) Mechanical equipment greatly reduces the amount of time required to reach the desired finish.
2) Mechanical equipment provides flexibility (many solutions to one problem) and versatility (ability to do more tasks) for the mould polisher.
3) Using hand held mechanical equipment enables the polisher to be more consistent in finish and in the time required from piece to piece and from job to job.
a) Heavy Grinder (Die grinder):
Die grinders have a quarter inch collets. They are used to drive large mounted points, carbide burrs, abrasive discs, flap wheels, large brushes, and felt bobs. Die grinders are used to install large radii, remove deep cutter marks, and to diamond polish large surfaces. Heavy grinders, called die grinders, are available in air drive and electric with flexible shafts.
b) Pencil Grinders (light duty):
Pencil grinders have interchangeable 1/8” and 3/32” collets. They are used to drive small mounted points, carbide burs, abrasive discs, diamond brushes and felt bobs. Pencil grinders are used to install small radii, removing deep cutter marks on small pieces or tight areas, and diamond finishing. Light duty grinders or pencil grinders are available in air drive and electric with flexible shafts.
c) Right Angled Head:
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Right angled head s is available in large ¼” and small 1/8” collets. They are used with the same cutter and abrasives as die and pencil grinders. They are useful for getting into tight places like cavity side walls. They also afford the polisher more control when using abrasive discs. Right angled heads are available in air-drive and electric with flexible shafts.
d) 45 Degree Head:
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The 45 degree head is a variation of the right angle head. It is used with the same cutters and abrasives as the right angle head. They are good for getting into tight spots and the bottom surfaces of deep cavities. 45 degree heads are available in air drive and electric with flexible shafts.
e) Profiler:
The profiler is a reciprocating hand piece. It reciprocates 2000 to 10000 times per minute. Stroke length is set on a cam inside the body. Profilers are the most frequently used piece of polishing equipment. They are used to push small files, polishing stones, wood and brass laps, and felt buffs. Profilers are available in air and electric with flexible shafts.
f) Ultrasonic Head Piece:
The true ultrasonic hand piece operates on a wave length. They are used with similar tools and abrasives as a profiler. Because ultrasonic vibrates in a 4 to 30 micron range, their use is limited to small areas and intricate details. They are very efficient when used properly. Tool length is critical for proper operation. It is available in air and electric drive. It should be noted that the air drive is not a true ultrasonic tool.
27.7 Mould Polishing Perishables
Why are perishables important?
Perishables perform the actual cutting, smoothing, and shining of the mould. Mechanical equipment may make the perishables work faster, but it is the perishables that are doing the polishing work. From the coarsest files and discs through stones and emery, and finally to the fine diamond compounds, all perishables work together to refine the moulding surface to conform to the customer’s specifications.
a) Mill bastards
Large files, called mill bastards, are rarely used in polishing but they deserve a mention because on occasion they are used during the benching process. Mill bastards came in four basic shapes. Flat, round, triangle and oval. Typically 8 to 12 inches long, they have coarse teeth and can remove larger amount of steel. Because of this, caution should be used with these tools. They are used for flattening, initial smoothening or large radii and cleaning out corners. They are always pushed by hands.
b) Refflers
Small files, called refflers, are used much more frequently than larger files. Rifflers are used in the benching phase of polishing. They come in many shapes and configurations.
Typically rifflers are six inch long with a small cutting surface on each end. They are used for initial smoothing in hard to reach and detail areas. Rifflers can be used by hand or with a profiler.
c) Diamond files
The name diamond file refers to the cutting medium not the shape. They are used in both benching and smoothening phases. Diamond files are 3” to 6” long with a cutting
surface if 1” to 4”. They come in four shapes flat, round, half round and triangle. Diamond files are made by electroplating diamond particles to a file blank. They are excellent for use during an EDM process. Diamond files can also be used to install all
radii. They can be used by hand removing cutter marks from thin ribs, especially when the ribs have been burned during EDM process. Diamond files can be used by hand but are most effective when driven by a profiler.
d) Grinding wheels
These are wheels mounted at the tip of a shank. Different shapes are available depends on the usage. These wheels are clamped on air grinders and can be used for removing material during benching. Deep cutter marks can be removed by this process.
e) Flap wheels
Flap wheels are strips of emery with one end epoxied to an arbor. Flap wheels are 1” t0 4” in diameter. They are used with the die grinders. Because they are resilient, flap wheels works well on flat and contoured surfaces. Flap wheels are not aggressive enough to remove cutter marks. They are used to take out disc and cartridge roll marks. Flap wheels work best when only light pressure is applied.
f) Stones
Stones are manufactured “sticks”. Aluminum oxide and silicon carbide are the abrasive particles used. They are used in the final finishing phase of polishing. Stones are square, rectangular or round. Sizes range from ¼” x 1/8” to 1 square inch, but all are 6” long. Grits range from 120 very coarse to 1200 very fine. Coarse stones (120 – 400) are for removing cutter marks and benching marks. Finer stones are used to refine the finish and to prepare for diamond finishing. Stones are used with a profiler and by hand for a draw stone finish. Stones should be used with a lubricant such as mineral spirit to prevent “loading up.”
g) Emery
Emery is a cloth with abrasive material glued to one side and is used in the final finishing phase. Emery comes I 1” and 2” widths on rolls of varying lengths. Grits range from 60 to 600. Emery is pushed by hand or by wrapping it over a stick. When wrapped over a stick, a rubber pad should be glued to the contact surface. The emery should be used with lubricant to prevent loading and to produce a finer finish. Emery is used to smooth large surfaces and refine a stone finish prior to diamond finishing.
h) Diamond compound
Diamond compound is a mixture of diamond particles and a grease base. Diamond particles are graded for size by filtering through a mesh. They are used in the final finishing phase. It is available in grades from 90 – very coarse (80 – 100 microns), to 1/10 – the finest (0 – ¼ microns). The coarse grades are used for cutting and the finer ones are used for shining. This can be used with brass, wood, nylon and felt laps pushed by a profiler. Diamond compound is also used with brushes and felt buffs driven by a die or pencil grinder. Diamond polishing removes stone and emery scratches and is the final step in producing a “mirror” finish.
i) Brushes
Brushes are made from animal hair, nylon and brass. Brushes are available from coarse to soft. They come in three shapes- cup, wheel and end. They are available in many sizes. Brushes are the most common way of driving diamond compound. Cup brushes are used to diamond corners and flat surfaces. Wheel brushes are used in inside diameters, side walls, parting lines, radii and round pins. End brushes are used on flat or lightly curved surfaces. Brushes are driven with die or pencil grinders, right angle and 45 degree angle heads. Light pressure and slow speeds of 2000 to 3000RPM are recommended.
j) Buffs
Buffs are available in many different sizes and shapes. Buffs are used to eliminate brush marks and to increase the shine. Buffs come in hard, medium and soft. The harder the buff, the more cut. The softer the buff, the more shine. Buffs are driven with die and pencil grinders, right angle and 45 degree angle head. Light pressure and a speed of 4000 to 6000 RPM are best.
27.8 Finishing process
Following the basics is critical to achieve the best possible finish in the shortest possible time. By completing each step in the process before going on to the next, a mould polisher will avoid doing double work and over-polishing. Understanding and following the proper finishing process will save time, money and avoids removing too much stock. The experienced mould polisher will follow the finishing sequence shown below to achieve the desired finish in the most efficient manner.
a) Bench work
b) Stone Finishing
c) Rough Diamond Finishing
d) Fine Diamond Finishing
e) Buffing
a) Bench work
Benching is the process of preparing the work piece for actual polishing. A benchwork must be complete before you begin to polish. Benching can be as little as installing a radius or as involved as discing, grinding and blending converging surfaces and then smoothing with flap wheels or coarse emery. When extensive benching is required, deep cutter marks and stepped surfaces should be worked first, then the radii should be installed or refined. A polisher will always work from details to open areas. Start benching with coarse perishables, progressing to finer.
b) Stone Finishing
Stone finishing, also called stoning, refines all surfaces of the work piece. When each phase of stoning is completed, all surfaces should have the same uniform surface. Stone finishing is done with a profiler. To get the best possible finish in the least amount of time, stoning should always be done in the following order, starting with the deepest area and working out.
1) Stone all corners and detail such as radii
2) Stone all horizontal surfaces
3) Stone all vertical surfaces
All surfaces must be done before changing to a finer grit.
c) Rough Diamond Finishing
Rough diamonding removes the stone marks and begins the shining process. All brushes and laps used for diamonding should be dedicated to each grade of diamond compound. To begin rough diamonding, all surfaces should have at least #600 stone finishes. A #400 to 600 emery finish is the best. Diamond steps are the same as stoning. A mould polisher will diamond the deepest detail first and work out to larger surfaces. The largest possible brush is used. In all diamonding, the key is ti use a minimum of pressure and a brush speed of 2000 to 3000 RPM.
d) Fine Diamond Finish
It is done in one to three steps. The number is determined by the final finish required. Fine diamonding techniques are the same as for rough diamonding. Start the fine diamond finishing with #6 grade diamond compound. If required, the second step is done with #3 grade diamond compound. For an optical quality finish, a third step is done using #1 grade diamond. Every time the grade of diamond compound is changed, it is critical that all surfaces are free from previous compound.
e) Buffing
Buffing is the final step. It is done both mechanically and by hand. Buffing begins when you have completed your last diamonding step. When buffing, a mould polisher will start with the same grade of diamond as the last brushing steps. Techniques are same as diamonding -working from details to large surfaces using the large buff possible. Light pressure and a sped of 4000 to 6000 RPM will produce the best results. When mechanical buffing is complete, hand rubbing with soft felt ot a cotton ball will remove any swirl marks left by the buffing process.
Chrome Plating
It is another method to for finishing the metal moulds other than polishing. The thichness of the plating should be grater than 0.025mm. The followings are the advantages of chrome plating
1) chrome plating has a low co-efficient of friction and thus facilitates
27.9 problems in mould polishing and solutions
| Problems | Cause | Solution |
| Orange peel | Too much pressure or speed during the diamond finishing steps | Re-stone until orange peel is removed and re-diamonded. |
| Pitting | Defective steel. Too much pressure or speed when diamonding. Did not remove all EDM marks when stoning. If pits have a comet-like tail then too much pressure was applied. If no tail exits, then steel may be defective or marks were not completely removed. | If from diamonding or defective steel, re-stone and diamond carefully. If from EDM, re-stone and diamond. |
| Dull shine | Contamination from stone grinder or diamond from previous operation left on surface. Contaminated brushes or buffs. | Clean all surfaces and re-diamond using new brushes and buffs. |
| Estimating surface condition | Underestimating or overestimating the surface condition will slow the polishing process | Decide beforehand how aggressive your initial abrasives should be. Adjust up or down to reach optimum speed. |
| EDM surface finish | Cutter marks left by an EDM, characterized by a pitted surface. This process leaves a hard scale on the surface | When ever possible, start an abrasive with loosely packed particles such as diamond files or discs. When approximately 50 to 75% of EDM is removed, switched to stones. |
| Lack of cleanliness | Improper care and storage of abrasives. Dirty work area. Not thoroughly cleaning surfaces when changing to a finer abrasive. Slow down job and produces inconsistent finishes. | Store each grade of stone in its own container. Decide all diamonding brushes and buffs to only one grade of diamond. Store in covered containers. Keep work area clean. Never set diamonding supplies on bench. Meticulously clean pieces between each operation. |
| Lack of methodology | Slow down job and produce inconsistent finishes | Always follow proper procedure. |
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