Rough? Not at all! The surface roughness is more detailed than you can imagine.
Surface Roughness means on the processed parts surface, there is microscopic geometry drop from small peak and
valley, simple way to say, use microscope to see processed parts, there must be some tinny uneven situation, caused
by cutting, steel, mechanical shock etc. even though we see the surface are nice and flat, and we can get the
measure value via instrument, and these are called “Surface Roughness”.
the processed parts surface, there is microscopic geometry drop from small peak
and valley, called “surface roughness”
Surface Roughness is the one of the important indexes to be used for checking whether the metal processing quality
is good or not. Roughness is relative with friction, wear resistance, fatigue strength, stable and tightness etc.
For extrusion dies, the roughness of channel is the essential judgment of die quality, extrusion die chamber is
built by at least two processed steels, when the polymer is flowing in it, the friction is very important, more
friction more abrasion, and if the formula contains corrosive additive, better surface roughness can extend the
lifetime, postponed the corrosion situation.
Surface roughness can be measured by roughness meter, the diamond probe moves very slowly on the surface to
measuring the roughness value of the sampling area, the measuring range is around 0.025 to 6.3 um,
diamond probe of roughness meter moves on the surface for measuring the
there are three roughness parameters, symbol is as Rz, Ry and Ra.
Rz : in the sampling area, the summary is from of max. five peaks average value and min. five valleys average value.
Ry: in the sampling area, the distance of the highest peak and the lowest valley.
Ra: in the sampling area, the average distance between the measuring point and baseline Ra value is the mainly
reference for most people.
Surface roughness quality is greatly affected by processing, for getting fine surface roughness, it depends on
excellent processing, including lathe, milling, grinding and polish etc. different processed parts equip with
different grade surface roughness, and different processing skill. General and basic grinding and polish can reach
roughness Ra 3um, the surface appears light processing knife marks, and if plus fine grinding, the roughness will be
around Ra0.4um, the processing marks won’t be seen, but the cutting direction can be seen. If the roughness below
Ra0.8mm, fine polish process is needed, the surface looks like glossy with highly brightness.
For extrusion die, taking the highest grade of polish processing is very important and necessary.
There are some prerequisites for achieving fine polish process, first, good machines, and operator with nice skill
and lot experience, on the other hand, many mediums will be used in polish process, it means the site has to equips
with quailed waste disposal equipment.
For extrusion die, compared with general polish process, the polish is very important process skill, and require the
best grade quality of polish processing, general polish can make glossy surface, but for extrusion die, mirror
surface is necessary. Mirror polish can make better glossy surface and to avoid from the polymer to stick on the
Good mirror polish needs rich experience operator, the skill will decide the polish quality, meanwhile, the mold
steel is the factor to mirror polish, the steel should be good, stable rigidity and even hardness without flow, it
is more difficulty to reach fine mirror polish quality for bigger die, especial mirror polish take a lot of
There are four grade of mirror polish, A0 means Ra0.008um, A1 means Ra0.0016um, A2 mean Ra0.032um and A3 means
Ra0.063um. for extrusion die, the basic standard surface roughness is Ra0.05um, it means the A2 grade mirror polish
For optical product application die, the standard surface roughness is Ra0.01um, and currently precision processing
can reach to Ra0.005um, but higher processing cost and longer processing time, so the market scale is smaller, only
for few customers who require the extremely fine surface roughness.
precision processing (mirror polish) can reach surface roughness Ra0.005um
In summary, an excellent die has to has good design for fit polymer property, nice processing and fine mirror
polish, ensure even and qualified surface roughness, this is the way to a crafted extrusion die, so it is not only
processing, it is an art of processing.