The polishing described in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. Not only does it have high requirements for polishing itself, but it also has a high standard for surface flatness, smoothness, and geometric accuracy. Surface polishing generally requires only a shiny surface.

1. www.moldclamps.net info@moldclamps.net 0086-17322110281
DGMF Mold Clamps Co., Ltd
mingfeng425@gmail.com
Please browse our website or contact us for more articles, thank you.
How to polish and maintain injection molds?
Translated by Jasmine
The polishing described in plastic mold processing is very different from the surface polishing
required in other industries. Strictly speaking, the polishing of the mold should be called mirror
processing. Not only does it have high requirements for polishing itself, but it also has a high
standard for surface flatness, smoothness, and geometric accuracy. Surface polishing generally
requires only a shiny surface.
The standard for mirror processing is divided into four levels:
AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm, it is difficult to accurately
control the geometric accuracy of parts due to electropolishing, fluid polishing, etc., while
chemical polishing, ultrasonic polishing, magnetic The surface quality of the methods such as
grinding and polishing is not up to standard, so the mirror processing of precision molds is mainly
based on hand polishing.
1. Basic procedures for manual polishing
For high-quality polishing, the most important thing is to have high-quality polishing tools and
accessories such as oilstone, sandpaper and diamond paste. The most important thing is the
polished working environment, which requires a cleanroom. The choice of polishing procedure
depends on the surface condition of the previous processing, such as machining, EDM, grinding
and so on.
The general process of hand polishing is as follows:
(1) Manual oil stone grinding, strip oil stone plus kerosene as lubricant or coolant.
The general order of use is #180~#240~#320~#400~#600~#800~#1000. Many mold makers
choose to start with #400 in order to save time.
(2) Semi-fine polishing
Semi-finishing mainly uses sandpaper and kerosene. The number of sandpaper is
#400~#600~#800~#1000~#1200~#1500. In fact, #1500 sandpaper is only used for hardened die
steel (above 52HRC), not for pre-hardened steel, as this may cause burns on the surface of
pre-hardened steel.
(3) fine polishing
Fine polishing mainly uses diamond abrasive paste. If the polishing powder is mixed with a
diamond abrasive powder or a polishing paste, the usual polishing order is 9 μm (#1800) to 6 μm
(#3000) to 3 μm (#8000). The 9μm diamond paste and polishing cloth wheel can be used to
remove the hair marks left by #1200 and #1500 sandpaper. The polishing is then carried out using
a felt and a diamond paste in the order of 1 μm (#14000) ~ 1/2 μm (#60000) ~ 1/4 μm (#100000).
A polishing process with an accuracy of more than 1 μm (including 1 μm) can be performed in a
clean polishing chamber in the mold shop. For more precise polishing, an absolutely clean space
is required. Dust, smoke, dandruff, and saliva can all leave high-precision polished surfaces that
are obtained after hours of work.
2. Problems to be paid attention to in manual polishing

2. www.moldclamps.net info@moldclamps.net 0086-17322110281
DGMF Mold Clamps Co., Ltd
mingfeng425@gmail.com
Please browse our website or contact us for more articles, thank you.
Polishing with sandpaper should pay attention to the following points:
(1) Polishing with sandpaper requires the use of softwood sticks or bamboo sticks. When
polishing a round or spherical surface, use a cork stick to better match the curvature of the round
and spherical surfaces. The harder wood strips are like cherry wood, which is more suitable for
the polishing of flat surfaces. The end of the trim strip is adjusted to match the shape of the steel
surface so that the acute angle of the strip (or bamboo strip) is prevented from contacting the
surface of the steel and causing deep scratches.
(2) When changing to different types of sandpaper, the polishing direction should be changed
from 45°to 90°, and the streak shadow left after polishing the former type of sandpaper can be
distinguished.
Before changing the different types of sandpaper, the polished surface must be carefully wiped
with 100% pure cotton and a cleaning solution such as alcohol, because small gravel left on the
surface will destroy the entire polishing work. This cleaning process is equally important when
polishing from sandpaper to diamond polishing. All pellets and kerosene must be completely
cleaned before polishing continues.
(3) In order to avoid scratching and burning the surface of the workpiece, special care must be
taken when polishing with #1200 and #1500 sandpaper. It is, therefore, necessary to load a light
load and polish the surface using a two-step polishing method. When polishing with each type of
sandpaper, it should be polished twice in two different directions, with each rotation between 45°
and 90°.
3. Diamond grinding and polishing should pay attention to the following points:
(1) This polishing must be carried out under light pressure as much as possible, especially when
polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with
#8000 abrasive paste, the common load is 100~200g/cm2, but it is difficult to maintain the
accuracy of this load.
To make this easier, you can make a thin and narrow handle on the strip, such as adding a piece
of copper; or cut a part of the strip to make it softer. This helps control the polishing pressure to
ensure that the mold surface pressure is not too high.
(2) When using diamond grinding and polishing, not only the working surface is required to be
clean, but also the hands of the worker must be carefully cleaned.
(3) The polishing time should not be too long, the shorter the time, the better the effect. If the
polishing process is carried out too long, it will cause "orange peel" and "pitting".
(4) In order to obtain a high-quality polishing effect, polishing methods and tools that are prone
to heat should be avoided. For example polishing wheel polishing, the heat generated by the
polishing wheel can easily cause "orange peel".
(5) When the polishing process is stopped, it is very important to ensure that the surface of the
workpiece is clean and carefully remove all abrasives and lubricants. Then, a layer of mold
anti-rust coating should be sprayed on the surface.
(4) Factors affecting the quality of mold polishing
Since manual polishing is mainly done manually, polishing technology is still the main reason for

3. www.moldclamps.net info@moldclamps.net 0086-17322110281
DGMF Mold Clamps Co., Ltd
mingfeng425@gmail.com
Please browse our website or contact us for more articles, thank you.
the quality of polishing. In addition, it is related to the mold material, the surface condition
before polishing, the heat treatment process, and the like. High-quality steel is a prerequisite for
good polishing quality. If the surface hardness of the steel is uneven or the characteristics are
different, it will often cause polishing difficulties. Various inclusions and pores in the steel are not
conducive to polishing.
(5) The effect of different hardness on the polishing process
The increase in hardness increases the difficulty of grinding, but the roughness after polishing
decreases. Due to the increased hardness, the polishing time required to achieve a lower
roughness is correspondingly increased. At the same time, the hardness is increased and the
possibility of excessive polishing is correspondingly reduced.
(6) The influence of the surface condition of the workpiece on the polishing process
During the crushing process of steel machining, the surface layer will be damaged by heat,
internal stress or other factors. Improper cutting parameters will affect the polishing effect.
Therefore, high-speed CNC finishing is required, and the cutting amount is controlled to be
0.05mm~0.07mm.
The surface after EDM is more difficult to grind than the surface after ordinary machining or heat
treatment. Therefore, the fine gauge quasi-electric spark should be used before the end of EDM,
otherwise, the surface will form a hardened thin layer. If the EDM is not properly selected, the
depth of the heat-affected layer can be up to 0.4mm.
The hardness of the hardened layer is higher than the hardness of the substrate and must be
removed. Therefore, it is better to add a rough grinding process to completely remove the
damaged surface layer to form an average rough metal surface, which provides a good basis for
polishing.