This material was produced by the Royal Society of New Zealand (RSNZ)
under contract to the Ministry of Education in 2000 and 2001. It was written
to assist teachers and schools in their delivery of the technology/ hangarau
curriculum statements. The project was jointly coordinated by personnel
from the Technology Education New Zealand (TENZ)
and National Association of Māori Mathematicians, Scientists and Technologists
(NAMMSAT) networks. Monitoring and evaluation of the material was carried
out by a national project advisory group.
Thermoforming of plastics is an important forming process in manufacturing.
It begins with an already formed plastic sheet which is warmed until soft.
Then, in one of four forming processes, it can be either:
pressed between matching moulds (matched-mould forming);
pressed over or into a mould by compressed air (pressure forming);
sucked over or into a mould by a vacuum – although it is atmospheric
pressure which does the shaping (vacuum forming); or
pulled over a mould (ring and plug forming)
The last two methods can be done relatively easily and cheaply and are
being incorporated into an increasing number of junior and senior school
technology programmes. A simple
vacuum former
A plan for a simple "home-made" vacuum former can be found in
The BP Technology Challenge File, 3rd edition, page 5.39. This
book can be obtained at no cost by writing to: The BP Challenge Coordinator,
The Royal Society of NZ, P O Box 598, Wellington. A number of these simple
vacuum formers were donated to Teacher support Services around the country
to be available to schools for borrowing.
An updated vacuum-former plan is included in the Teacher Guide of the
PAC-IT packaging resource, page 91. This resource kit (which has a 150
page teacher guide, a 32 page activities booklet, a video, and packaging
samples) may be ordered from www.pac-it.org.nz and costs $35 (which includes
GST, p&p). The plan is part of a section on plastic forming processes.
Both resources are written by the same author, who commented that the difference
between the vacuum former plans provides a good illustration of product
development. The first plan showed a hinged clamp with a single counter-sunk
machine screw. Unfortunately, it was found that after about 50 uses the
hinge stretches the particle board and loosens, the clamp warps, and the
single machine screw bites into the particle board. To overcome this, the
new plan suggests modifications.
For those using this basic device it should be noted that the clamp needs
to hold the plastic sheet firmly. The author suggests the use of "Vivak"
plastic, the brand name for PETG (glycol modified polyethylene terephthalate)
which has a wide moulding temperature range in this situation. He concedes
that it is more expensive than sheet PVC or polystyrene but considers
it easier to mould. 0.5mm thickness should be adequate for shapes up to
2cm deep (shapes with greater depth need thicker sheet, but thicker than
1mm is not recommended for this area of plastic. Hints
for successful forming using the device
A heat gun (1600 watts minimum) is required. A hair drier is not
hot enough. Look for heat guns with the least exposed metal nozzle,
as it is this metal part which can inflict severe burns. Makita make
a suitable models, later Black & Decker models have too much exposed
metal nozzle.
Heat the plastic sheet in the clamp until it sags. It will first
wrinkle, then tighten (which is when it is moulded commercially),
then it begins to sag. Keep heating it evenly, including around the
edges, let it sag about 12mm (just touches whatever is beneath the
clamp), and then quickly place it over the plug mould which is on
the gauze of the vacuum box. The vacuum cleaner must be turned on
before placing the clamp over it, and speed is essential to avoid
it cooling.
Let the vacuum cleaner run until the plastic is cool enough to touch.
Don't let it run too long as you risk overheating it.
Make sure that the heat gun nozzle can not be touched, it remains
hot for some time after use. It is best if the moulding station is against
a wall so that the gun points away from the users.
Making
plug moulds
These can be ready-made by using shapes from fridge magnets (remove the
magnet and paint), or, preferably, make your own. Suitable materials are
play-doh (recipe on the side of a cream of tartar container), clay, commercial
play-doh type of moulding materials, layers of paperboard, balsa wood,
or similar soft materials. Foam polystyrene from produce trays does work,
except that it is compressed so that the edges are rounded and it can
melt to the plastic – prevented by covering in aluminium foil. If
using mouldable materials it is advisable to dry or cook them slowly in
a conventional oven rather than in a microwave oven.
Ensure that all plug moulds have no 'undercuts', where the mould curves
underneath its plan shape. The plastic will 'follow' the mould underneath
and make it difficult or impossible to remove the mould!
The size and depth of the mould will obviously depend on the materials being
moulded. Moulds for chocolate need a minimum depth of 4mm if the moulded
chocolate is to be removed unbroken (and is most easily removed after refrigeration).
Increase the minimum depth if its length is greater than 25mm. The depth
versus size for chocolate is a good opportunity for investigation.
Chocolate is best melted in a container in a hot-water bath (for example,
use a plastic 2 litre ice-cream container – cut a hole in the icecream
container lid to hold the chocolate container in the hot water).
Wendy Turnbull (Christchurch College of Education) used this type of basic
device when working with with Liz Dean's year 1 class at Amberley School.
The children wanted to make simple gifts for their parent helpers and
decided to make chocolates. They made the plug moulds from "ceramic
dough" (salt and flour in equal quantities). The children matched
the shapes to parents, one pair making a shape of a trimming knife for
a parent who helped them with cutting. To minimise safety risks, the teacher
used the heat gun, with the children participating in the moulding process
by turning the vacuum cleaner on and off. The children were then able
to do the chocolate moulding and the wrapping of the chocolates for presentation
themselves. Suggestions
for other moulding applications
Moulding coloured jelly shapes for use on pavlova or other foods
Moulding plaster shapes for painting for Christmas decorations
Making plastic cases for electronic projects
Moulding toys (for example, vehicle shapes to fit a slot-car chassis;
yachts)
Larger
commercially available machines
Sue Shore, a technology teacher at Sacred Heart College in Lower Hutt
is able to make extensive use of a SATRA vacuum forming machine which
the school has had for a number of years. Supplied by the local company
Techsoft, it is used by students in their Year 10 -12 Design Technology
programme to form moulds for such things as chocolate, CD racks and containers
to house electronic circuit boards.
Sue commented that the machine works quickly, is safe to handle and easy
to use. The plastic supplied by Techsoft produces a high quality finished
product. Techsoft is one of a number of companies supplying a comprehensive
range of safe, convenient to operate plastic forming equipment for use
in schools. Teachers wishing to find out more about materials, equipment,
and processes can find their website at http://home.clear.net.nz/pages/techsoft
At Hastings Boys High School the technology team had a good look at the
options available for thermoformin and eventually decided to have a go
at developing their own machine. The school approached local company Classique
Plastics and together they set about developing a machine which was tailored
to suit their needs. The result is the MiniVac66 – marketed as a
"compact, light commercial and instructional, vacuum thermoformer". In
the higher price range, it is nevertheless creating a good deal of interest
– particularly in large secondary schools. Paul Beard, HOD Technology
at Hastings Boys' says that it is used by students all the time. It can
cater for a wide size range in solutions developed to address a specific
technological problem, is able to be quickly and easily operated by students
and has a number of critically important safety features built in.
