Without wishing to give away all
our secrets, this glossary is designed to assist the customer by
unravelling some of the terms used in the Vacuum Forming Industry.
This is the process of heating materials
within an oven to reduce the stresses caused by machining of susceptible
materials such as Acrylic. Stresses caused by the machining of holes or
apertures may start to crack after a short while especially if the router speed
or cutter are incorrect. Annealing reduces the possibility of components failing
in service.
Essentially this involves a co-extrusion
of a layer of a different material onto a substrate of another material, I.E. ABS.
This performs two major functions:-
1. To impart a decorative surface to the material.
2. To increase outdoor weatherability.
Typical examples are the use of Acrylic (PMMA) which does not transmit UV light
this has the effect of protecting the
substrate from the UV generated by sunlight. This increases the outdoor
life of the substrate dramatically which otherwise would deteriorate, losing
impact strength and colour. Another material used to protect material against
ultra-violet is ASA.
Many materials can be manufactured to a
special colour either using a colouring numbering system such as RAL or PANTONE.
In the absence of this information colours can be photographically matched to a
sample provided by the client. Should an exact colour shade not be important, a
massive range of pre-matched colours are available. Minimum quantities apply for
this service and are they usually are around 500kg.
Vacuum formings can easily lose detail no
matter how much work is put into tooling. This is due to Vacuum Forming relying
on atmospheric pressure to force the material over and into cavities on the
tooling. Various methods can be employed to resolve this issue and all of them
involve increasing the air pressure on the material during the forming cycle to
increase definition.
Materials that absorb water are termed
Hygroscopic. These materials must be pre-dried before they can be formed
successfully. Failure to dry these materials correctly will result in the
captured moisture boiling through the surface of the material leaving thousands
of tiny blisters on the material's surface. In clear materials thousands of tiny
bubbles will become visible throughout the material.
A very different moulding process.
Injection moulding uses cavity tooling into which molten liquid plastics are
injected at very high pressures. Detail is superior and piece part costs are far
lower, unfortunately to reap the benefits of this process one has to pay vastly
increased tooling and set-up costs. Injection moulding makes sound financial
sense for high volume ongoing products, however it cannot begin to compete on
overall costs in the short to medium batch market that is catered for by Vacuum
Forming.
Whilst I think all adults amongst us
appreciate this term, it may help to understand the process that determines if
tooling should be male or female. A number of criteria should be considered not
least of which is production dimension requirements.
1. Male tools should be used when
internal product dimensions are important.
2. Female tools should be used when
external dimensions are important.
3. Textures can be applied to female
tools to produce well defined textures on the mouldedfproducts.
4. Tool height x width x depth /
tooling type = moulded material thickness.
5. Female tools are more expensive to
manufacture than male tools.
All the above are relevant to tooling
design and gender selection and should be considered carefully. A number of
other factors also have important parts to play in this area and should be
adequately addressed to ensure that the final tooling selection produces the
best product possible.
All thermoplastics expand when heated. The amount that
they expand is defined by their co-efficient of thermal
expansion which, is documented within each materials technical
specification. It follows therefore that once the heating process is completed
and the material is formed, they will shrink as they cool down. This is known as
'post forming shrinkage' and this must be taken into account during tooling
manufacture. Post forming shrinkage can set up stresses in the products and this
can lead to 'Post forming distortion'
unless adequate precautions are taken.
This phenomenon is the result of uneven or rapid
cooling of the hot moulded product. Essentially a moulding consists of two
sides, the side in contact with the tooling and the side in contact with the
atmosphere. The side in contact with the hot tooling will remain at a greater
temperature than the side that is in contact with the atmosphere and which
additionally is being cooled by the machine's cooling fans. This difference can lead to one side
of the material cooling before the other and therefore shrinking at a
greater rate. The result is distortion. This can be significantly reduced with
the use of post-form clamping but only accurate temperature control of both
tooling and material will cure the problem completely.
The process of printing on the 'inside'
face of a clear material. The image is then viewed 'through' the material and is
protected against scratches etc.
A solvent joint is not to be confused with an adhesive
joint. In an adhesive joint a separate agent acts on the jointing surfaces and
REMAINS within the joint continuing to act on those surfaces. A 'solvent joint'
or 'solvent weld' uses a chemical agent that acts aggressively on the jointing
surfaces 'melting' them and allowing them to fuse together. The solvent agent
evaporates over a short period of time allowing the 'melted' surfaces to 'harden
off' leaving no trace of itself within the joint. The joint fused surfaces
are said to be 'solvent jointed' or 'welded' together.
Another area of Vacuum Forming, thin film
moulding generally uses high speed reel fed machinery to produce packaging
products such as blisters and clam packs.
Chewing gum and string? Not quite, but a much wider
range of tooling materials are available to the Vacuum Former than in any other
moulding industry. Plaster, filler, concrete, resins, softwoods, hardwoods,
plasticene, Steel, Aluminium and many many others. Tooling construction is very
much dependant on the product material selection, batch size, tooling budgets
and the final product quality required. Again, correct selection of a wide range
of parameters will have a direct effect on the final product.