Questions and answers relating to etching technology

Which materials are suitable for graining?

Material selection and heat treatment play a key role with regard to the later graining
result. Even at the material procurement stage it is important to ensure that only
steels are used that are suitable for graining. All relevant toolcomponents (sliders,
inserts, etc.) should be produced from the same material to avoid possible risks to
the grain quality and gloss level. 

Suitable materials include: 

Short name Material no. Eschmann name
40 CrMnNiMo 8-6-4 1.2738 EST ES Aktuell 1000
--- Special alloy ES Aktuell 1200 *) Standard for master pattern
40 CrMnMo 7 1.2311 EST ES Aktuell
--- Special alloy ES Multiform SL
X37 CrMoV 5-1 1.2343 ESU *) Limited as shown below ES Maximum 500
---- Special alloy *) Limited as shown below ES Primus SL
     

Use ESR (electroslag remelting) quality material in hot work steels. There may be a difference
in the grain quality and/or gloss level (depending on the thermal conductivity, amongst other
things) in comparison to conventionally hardened and tempered plastic mould steels due to
case hardening and the alloy!

Can non-ferrous metals be etched?

Apart from alloyed steels, the most common non-ferrous metals (e.g. copper, aluminium,
brass or zinc) and non-alloyed steel can also be etched. However, it is generally recommended
to test the desired etching result in advance on a trial plate. For some of these materials a
rougher etching base and/or a completely deviating grain quality, amongst other things, may
be produced as a result of the chemical reaction of the acid with the base material.
 

Non-ferrous metals have good graining capability!

What are segregation lines or stripes?
Segregation lines due to strain hardening!

Surface compressions, streakiness and similar Surface compressions in the material
to be textured often occurs when machining the workpiece. The steel microstructure
can be substantially compressed through mechanical processing, which results in a
tendency to higher surface hardness. When graining the workpiece, this then leads
to an uneven block diagram in the microstructure (e.g. grain depthdeviations, blotchiness,
streakiness, etc.).

Possible causes

  • Blunt milling cutters
  • Incorrectly ground sharpened turning tools
  • Mechanical damage
    (e.g. careless impact on the mould surface when changing inserts)
  • Damage caused by improper handling of clamping equipment or other tools


Similar irregularities may arise with improper heat treatment or due to physically-
induced segregation in the steel production.

Are special polishes required?
reference picture

milling cutters

hardened edge, grooves

Polishing
Areas to be provided with a grain structure must have acertain surface quality (polish)
depending on the design and grain depth.  

Requirements

  • General, 320 grain, groove-free  
  • Fine structures, 400 grain, groove-free 
  • Completely remove spark eroded residues, milling marks, etc.
  • Split lines must be aligned and free of burr
  • It is important to ensure that design and light edges are maintained
  • Provide a test area sized 50x50 ready for graining outside the split plane
     

Graining and high gloss? Highly polished surfaces already suffer minor damage
(fine scratches) through cleaning, paint application and processing of the grain structure. 

Highly polished surfaces should usually be polished up by the customer after graining.

Can structures be welded?
hardened edge

porous

edge

Welding should generally be carried out by a specialist company and the graining capability
of the welded areas needs to be guaranteed. A key factor in determining a structure's
suitability for welding is that the weld metal has the samestructure as the base material
in terms of composition, hardness and strength. Accordingly, the welding method, material
and necessary heat treatment must be selected on the basis of their later graining capability. 


Welding requirements:

  • Sufficient preliminary work (including sanding cracks in U-shaped moulds, etc.)
  • Correct choice of welding electrode 
  • Suitable heat treatment to avoid increased hardness and stress cracks 
  • Please also refer to page 38 of the technical catalogue of Eschmann Stahl GmbH & Co. KG 

ATTENTION! Deviations in the gloss level and grain depth on the finished product cannot
be excluded even if welding is carried out by professionals!

What should be taken into consideration for draft angls?

The minimum draft angles for the ejection of the full grain depth areindicated for
each structure.

The following rule of thumb applies:
A maximum grain depth of 0.02 mm can be ejected for each 1° of conicity.

Plastic properties (e.g. flame-retardant design, glass-fibre reinforced, etc.);material
shrinkage, wall thicknesses and tool structure (e.g.cooling,sprue, sliders, etc.) and
production parameters in injection moulding operations caninfluence this value.

For example, in areas with expected core shrinkage, three times the standard
conicityvalue for the ejection of the full grain depth may be necessary.

 

The following should be taken into consideration for a tool design compatible
with a moulded part:

  • The minimum draft angle for the structure (according to information provided by
    the graining company)
  • Material-related aspects (e.g. for PC-ABS, ABS, PP-compounds, etc.)
  • Tool-related aspects (e.g. core shrinkage, wall thickness changes, gas channels, etc.)

 

A reduction in the grain depth is possible and must be agreedin individual cases with the
end user!!  

What is meant by scuffing?

Scuffing is deformations of the plastic in the direction the item is ejected and is frequently
referred to as "stress whitening". The causes for this may be wall thicknesschanges, high
graining pressure, insufficient shrinkage of the item, material properties of the plastic used,
etc. Although it is possible to optimise the scuffing over the grained surface by reducing the
grain depth,   this must be agreed in detail with the end user as this will change the grain
impression. 

Note:
The occurrence of scuffing cannot generally be excluded, even with adequate draft angles!

What does "texturing over toolsplit" mean?

Witnesses on the sliders and/or insert splitlines are often not permitted for items in which
the graining is in the main visual area. Loose tool parts are grained in the assembly in such
cases, i.e. over partitions.


It is important to ensure the following:

  • Partitions must be aligned and free of burr before etching
  • Inserts/sliders must be aligned when fixed on the cavity (installation aids
    are to be provided by the customer or the (slider) in build, e.g. with large
    tools, is to be supported by a tool maker)
  • All graining areas remain visible and accessible even when fixing loose parts
  • Sharp-edge partitions ("cutter edges/top") are avoided to prevent damage
    caused by the removal of material during etching, otherwise this will affect
    the running time. 

The position and distribution of partitions should generally be agreed with the
end user. 

If loose parts are not suitable for graining in the assembly (e.g. due to restricted access,
tendency towards burring, etc.), witnesses can be agreed. 

ATTENTION! Witnesses should also be completely etched to avoid ejection problems and
are not intended as split line protection. Partial etching is possible and should be agreed
where necessary.

What happens with shut off areas?

Mechanical scribed line (CNC)

tool with scribed line

up to 1 mm around the witness
protecting the shut off area with acid-resistant paint

undercut
etched tool with an optional shut off area

All non-visible shut off areas must be scribed by the customer before delivering the item
for graining. Scribed lines must be made using a CNC or a die-spotting press. Scribed lines
based on the patterned part are inaccurate due to material shrinkage, manual handling,
amongst other things, and can lead to burr formation and/or unacceptable clear areas in
the "worst case scenario". 


Shut off areas (not split lines) are generally provided with up to 1 mm unetched clear areas.
clear areas / witnesses must be dealt with by the customer depending on the position and
draft angles to prevent the grain shearing off when ejecting the item.


Examples of poorly scribed lines:

Shut off completely over-grained,
reposition AS/slider

shut off area over-grained and overmadded
scribed line based on a pattern:
witness uneven
witness too large
What is meant by gloss level?
Photo source: BYK Gardner

"Gloss is a visual perception that occurs when viewing surfaces. The gloss perception is
more pronounced, the more directionally the light is reflected". The light is not only
reflected in the main direction of reflection but also reflected diffusely in other directions
with rough – and thus etched – surfaces. This reduces the reproduction accuracy of the
surface: a reflected object is no longer depicted as a brilliant image, but as a blurred image.
The more uniform the diffusion of light into the room, the lower the intensity of the
directional component and the more matt the surface. 

The gloss level is determined by: 

  • Surface roughness
  • Plastic reproduction accuracy  
  • Tool wall temperature  
  • Plastic and processing parameters
  • Wall thicknesses
  • Measuring angle (medium/matt gloss at a 60° - 85° angle of reflection)
  • Thermal conductivity of the steel used, etc. 

The gloss level can be measured reliably on flat surfaces. A specific gloss level for the
item cannot be guaranteed.

How do tools need to be delivered?





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