The reasons for a high-gloss finish in plastic molding vary. Some examples include extreme transparency of the molded part, reduced localized corrosion or highly crack-sensitive tooling (due to design or operational stresses). Then there is the impact of mold surface finish on injection molding productivity, as the release forces of the plastic part from the mold steel surface depend upon the polymer’s adhesion properties. Remember that improved smoothness may lead to higher release forces, and, eventually, to a sticking phenomenon, which you can partly overcome with the appropriate tool steel choice and preparation strategy. Achieving a surface with a mirror finish requires several grinding, paper and diamond polishing steps, which you must perform in a clean workplace with the proper tools.
Polishing is a critical step when it comes to the manufacturing of high-quality inserts for plastic molding. Some of the most common defects include pitting, raised defects and orange peel. Pitting refers to scatter pin holes dispersed over most of the material’s surface, creating depressions. Raised defects are small, outwardly directed features that are often irregular in shape. Orange peel appears as hills and valleys covering the surface
Let’s break down some key challenges and considerations for achieving the best mold finish.
The Steel Factor
Your steel choice does impact part defects. For example, microcleanliness and microstructure strongly contribute to pitting, as polishing removes the softer matrix around an inclusion or carbide, resulting in peaks. Then, as polishing continues, the peaks are torn from the surface, yielding pits and needle marks
When projects demand high-quality surface finishes, use electro slag refined/remelted (ESR) grades and powder metallurgy (PM) steels, which have high cleanliness and whose carbides are smaller and rounder compared to conventionally produced steel.
When polishing, a low defect content is beneficial.
The hardness of the matrix also influences part defects. The higher the hardness, the lower the risk of encountering issues. For example, steel at a lower hardness (<45 HRC) is more prone to orange peel, so the polishing technique becomes very important. Orange peel can worsen with segregation or small differences in alloying content, leading to softer and harder layers in the matrix.
Surface Preparation and Polishing Techniques
Problems can arise even when you use a high-quality steel grade, so check the polishing sequence. Surface finish before polishing plays an important role. For example, polishing on too rough a surface is one of the most common mistakes. It is recommended to begin stoning on a surface roughness of Ra/Rz = 0.5/5 µm. Then, to avoid pitting and orange peel, spend more time on the rougher stages and less time on the final stages.
Another mistake is over-polishing, which is polishing too long and often in combination with too much pressure. Instead, it would help if you used large polishing tools, since smaller tools tend to enhance variations in the steel that yield orange peel, pitting and peaks. The same goes for the hardness of the carriers. Harder carriers are preferred over softer ones since the latter removes more matrix in softer areas.
Poor choice of polishing consumables like stone, paper and diamond paste can also cause problems. Silicon carbide stones and a high concentration of diamonds in the paste provide shorter polishing time and finer surface finishes.
The environment also plays an important role when polishing, so polish in a separate toolroom to avoid corrosion caused by high humidity, dirt in the air landing in a cavity and scratch marks.
Corrosion and EDM
Pitting is also the result of corrosion, which occurs due to high humidity or using water during cleaning. Pitting may also be the result of electrolysis corrosion after wire EDM. You will typically experience this result directly after the EDM operation. However, it depends on the depth of the corrosion attack.
Furthermore, pay extra attention to how the EDM layer is removed. A typical EDM layer consists of a remelted and rehardened surface layer with many peaks and valleys. If the entire heat-affected zone is not entirely removed, the residues from the layer (usually the valleys) will appear as pitting in the surface.
Arcing during EDM is also common, which causes craters that are deeper than the surrounding valleys. It is difficult to judge the reason for these pits with the naked eye or at low magnification, but a higher magnification reveals a surface that has been remelted . Use special polishing stones to remove the EDM layer properly
Solving the Problems
Immediately cease polishing when a defect appears, or you will worsen the problem. The best plan of action is as follows:
Re-stone the surface, repeat polishing with lower pressure and check the carriers used.
Perform in a cleanroom that controls the temperature, humidity and particles in the air.
Use a face mask to avoid the risk of saliva contaminating the surface.
Clean hands, clothes and tools carefully between each step.
Spend more time on the coarser steps.
Use hard carriers instead of softer ones.
Material Choice and Other Variables
Your mold material choice has its own impact on the final surface finish. For example, a martensitic stainless grade1 was developed to meet the high requirements of a high-gloss surface finish. This material type is considerably easier to polish than Werkstoff Number (W.Nr) 1.2083/AISI 420 steel types. This material’s most important feature is the microstructure, as the carbides are fewer and smaller compared to W.Nr 1.2083/AISI 420.
An additional benefit is that this martensitic stainless grade can reach a hardness level of 58-60 HRC, making the material easier to polish. The alloying composition of the carbides has been adjusted, which results in improved wear resistance compared to W.Nr 1.2083/AISI 420. Optimized carbide distribution and alloying design also improve the mechanical properties, yielding less risk of crack initiation and crack propagation compared to other martensitic stainless grades.
Heat treatment is another critical polishing factor, as it impacts the surface. For example, decarburization and quench speed impact the microstructure. Also, the method used to machine the surface might affect the final result. For instance, if you used EDM, it is essential to remove the recast layer and the affected layer underneath. To avoid corrosion that will cause defects, carefully dry the workpiece between every machining operation and ensure the wire EDM is in good condition.
Achieving a high-gloss surface finish requires the proper tool steel with high cleanliness and a small, even carbide distribution. Then, when polishing, carefully select the environment, starting surface, pressure, time and consumables.
Polishing is done for some of the following reasons:
Whatever be the machining processes used for machining the profile in the core and cavity, there is bound to be some surface roughness which is inevitable. This affects the asthetics of the part surface. So, to get a smoother surface finish for the molding, polishing is needed. (Unless a specific surface roughness is specified for the component, in which case other methods are used such as sand blasting, EDM,etc.)
Other main reason is, the lay of the machined surface of the impression may not be in the ejection direction of the mold. This causes ‘catching’ of the molding in the core or cavity during ejection. To counter this, polishing is done usually in the ejection direction and to get a smooth surface finish.
Polishing also helps in the smooth flow of the molten plastic material during injection.
Lastly, polishing is actually an art that requires a lot of patience and some level of skill. Even though unskilled labour is used for this activity (especially in India), they are skilled enough by their experience and know the importance of the mold part that they handle.
The most basic type of finish is a “stoned” finish which uses stone abrasives to get a matt finish. They range from 100 grit(roughest) to 1200 grit (finest). This is the cheapest & quickest form of polishing. Normally used to remove machining marks for the purpose of easy part ejection during molding.
Also, when a grit blasted finish is required, the surface must be stone finished first in order to get a quality grit blasted surface finish.
Then there are “paper finishes” which is essentially a fine abrasive attached to paper which is used after stoning to get some level of gloss on the mold tool surface. The plastic part will also replicate this same level of gloss on its surface.
Finally there is the mirror polish. There are different levels of mirror polishing the highest level looks just like a glass mirror and is used for lens parts. This is the most expensive and time consuming type of finish to achieve and needs the first 2 stages (stoning and papering) to be completed before mirror polishing can begin. Mirror polishing requires the use of a buffing wheel & is often referred to as buffing.
Know The Minimum Type of Finish Required.
In order save costs and time a mould maker must know the minimum type of finish required in mold polishing. The best way to do this is to find several sample parts of different finishes and get your customer to approve the minimum requirement. This sample can then be passed on to the mould polisher so he can replicate the required finish.
Also, knowing if the polished finish is for functional or cosmetic reasons helps in deciding the type of finish. A stoned finished is usually the minimum requirement for functional reasons (for easy ejection off the mold tool core). Mirror polished mold tool surfaces will more likely give part ejection difficulties especially for deep parts with little or no draft. This will result in longer cycle times and more part quality issues. So avoid mirror polishes if possible.
Mold polishing techniques are very important for manufacturing molding tools for plastics. It does not only affect the mold’s quality but also plays a vital role in the quality of the plastic component.
Generally, mold polishing has two primary purposes. First, it ensures easy and smooth ejection of the plastic material. With the mold having a smooth surface finish, molders can easily remove and demold plastic parts. Thus, you can be sure that the plastic does not stick to the mold. This way, you can prevent the occurrence of injection molding defects.
Furthermore, a smooth mold surface improves the aesthetics of the workpiece. At the same time, it helps to improve the wear and corrosion resistance of the plastic surface. Generally, injection cycle time reduces with effective plastic mold polishing techniques.
Plastic injection mold polishing is a technique that helps to obtain smooth mold surfaces. It does this by removing the convex parts of the mold through plastic deformation and cutting. Generally, mold polishing techniques use wool wheels, whetstone strips, and sandpaper for manual polishing.
the mold polishing process
In other cases, manufacturers use super-precision polishing methods for materials requiring a high-quality surface finish. For this, special mold polishing tools tightly press liquids containing abrasives on the workpiece surface at high speed. Thus, this technology helps to get a surface roughness of Ra 0.008 micrometer.
Polishing surface finish is quite different from surface finishing for other industries. This type of polishing often answers to the moniker “mirror processing.” It has high standards for the polishing process, smoothness, and geometric accuracy.
The important polishing considerations are as follows:
(1) Before you begin to machine a new cavity, first inspect the surface of the workpiece and then clean it with kerosene, so that the cutting feature of the oilstone does not malfunction due to attraction of dirt.
(2) When grinding a rough surface, the job should be carried out from the difficult part to the easy part. Especially for some dead corners that are difficult to grind, the deep bottom must be ground first, and finally the sides and the large flat surface.
(3) Some workpieces may have multiple components assembled together for grinding. It is necessary to separately grind the rough texture or EDM pattern of a single workpiece before grinding and smoothening the assembled workpiece.
(4) For workpieces with a large flat surface or a side surface, use oilstone to grind the rough lines and then use straight steel sheets for light transmission inspection, to check whether there are any unevenness or undercuts. If so, it will be difficult to release the part or the part may be damaged.
(5) To eliminate undercuts caused by mold part grinding, or respond to the situation where some contacting surfaces need to be protected, a saw blade or sandpaper can be attached to the edge to obtain an ideal protective effect.
(6) Pull back and forth to grind the flat surface of a mold, and try to keep the handle of the oilstone as horizontal as possible. Do not exceed 25°, because when the slope is too large, the force is pressed top-down, which may lead to the creation of many rough lines on the workpiece.
(7) When the surface of a workpiece is polished with copper- or bamboo-pressed sandpaper, the sandpaper should not be larger than the tool area, otherwise the parts that are not supposed to be ground will be ground.
(8) Try not to use a sander to repair the parting surface, because the parting surface polished by the wheel head is rough with uneven waves. When necessary, the wheel head must be fixed with concentric balance.
(9) The shape of the grinding tool should be close to the surface shape of the mold, so as to guarantee that the workpiece is not deformed by grinding.
Cavity polishing during mold manufacturing is a very important process step that is closely related to the quality and service life of a mold, while also determining the quality of the product. Understanding how polishing works, mastering the technological process and selecting an appropriate polishing method can improve the quality, as well as the service life of a mold, thus improving the quality of the final product.
The purpose of my idea is to lay out a simple and foolproof method of polishing mould steel. With a few variations, it will work on any kind of steel and can be done by anyone.
The emphasis will be mainly on bringing up an adequate finish in the least possible amount of time.
I will also be concerned about doing this using the least costly and simplest number of tools and supplies.
There is a great deal of equipment available today to assist the mould polisher, but we will keep it as simple as possible. We will be concerned with bringing up an adequate finish in a commercially competitive time frame with basic equipment.
The Mechanical Equipment
The first thing you will need is a flexible shaft power unit such as the KFD or NSK Powered kit with foot control for accurate speed control.
You will need a reciprocating unit such as the Diprofil for lapping slots and stoning areas where only a very short stroke is possible. This can be fitted to the KFD.
You will need handpieces for the KFD. The first is a straight handpiece with collets. This is the most often used.
A telescoping right-angle head (the VTN) is also essential. It reaches places that cannot be reached with the straight heads and is very helpful in reducing operator fatigue.
There is also available a right-angle attachment (VTN-S) which accepts 1/4″ collets.
A Diprofil Mini Filing Handpiece is available for very short stroke work, there are two versions, one has 0.5mm stroke, the other 1.8mm.
There are maximum speed restrictions on all Reciprocating Hand Tools:-
10,000 RPM for Mini Diprofil
15,000 RPM for Standard Diprofil