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ISSUE 02/2000 |
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| Preform Production |
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Optimum plant layout for PET preform production
In our last issue we drew attention to the importance of making the right factory planning decisions
at an early stage - especially in view of the fact that only 20 % of the costs of preform
production can be influenced by the preform manufacturer. In the concluding part of this
article we examine the factors involved in deciding on production machinery, plant layout
and logistics, and automation.
The detailed factory plan must be firmly based around the product and the way it is to be made, and not be dictated by an existing layout or a piece of space which just happens to be available - this is one of the most commonly encountered mistakes in factory planning. Experience has shown that with new investment projects the company management are calling within a very short time for a factory layout, which the factory manager may even sketch out himself. However, the first questions to be answered are about mould tools, machinery and the production cell - in that order!
Mould tools - type and size
Realistically tool sizes can be from 8 to 96 cavities.
Anything bigger is not necessarily better. Depending on the market size, variations
in demand, flexibility requirements, and
risk management (back-up tools) it will be possible to define the optimum mould
tool requirements. It has been known for companies to opt for a 16 or 48 cavity
tool and to continue using it, purely out of habit, when a move to a 96 cavity tool
had long since become the best economic option.
This is also the point at which questions on general processes have to be asked:
will there be a need for small batches, or unusual wide-mouth bottles, and would
production be better on a two-stage machine or separate injection and blowing units.
Injection moulding machines - standardisation
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Table 1: efficency improvments through machine standardisation
Although the machines have to be appropriate to the type of
preforms required, and of course have to be suitable for the chosen mould tools, it is
still important to standardise as far as possible. It is clear from Table 1 that a strategy
of standardisation can not only reduce the number of machines needed but also improve
machine loading efficiency. Manufacturing cells When setting up a manufacturing cell the level of
automation and any associated processes (e.g. quality checks) must be considered.
If bottles are going to be blown in the same place as the preform moulding there
must be a clear plan regarding the physical link between injection moulding and blowing.
Mould shop service
The mould shop must be fitted out with future generations
of machines in mind, not simply to accommodate existing equipment. Machine banks
containing up to about ten injection moulding machines has been shown to be the
best solution for the production of PET containers. Where more than ten machines
are in operation a second, mirror image, bank of machines is usually set up. The
construction of a mezzanine floor over the injection moulding machines has proved
to be an enormous advantage:
Logistics, automation and factory layout
The whole production logistic has to be tailored
to suit the site and is often influenced by external environmental factors such
as existing buildings, limited ground space for building etc. In the handling,
transport and storage of PET preforms all conceivable methods of transport are
in use. Small plants can operate perfectly well with a fork-lift truck based
system. In the other extreme large plants with really high outputs of the same
preform often decide to automate. Figure 2 shows a factory layout in the USA
where preforms are transported on automatic cranes. Other automation concepts
also rely mainly on driverless transport systems.
Planning the move, and economic considerations
The transfer to the new plant calls for the greatest
care if production is not to be seriously disrupted. The overall plan should have
taken into account both technical and financial considerations. It is particularly
helpful, for example, if the planners, right from the beginning, have allowed for
later expansion. This means that pipework for coolants can be laid with the right
diameters at almost no extra cost and only have to be extended rather than relaid
for the new phase. This principle is not only much cheaper but it also allows the
existing plant to continue running smoothly during the transfer. The same principle
applies to the arrangement of silos, cooling towers etc.
Standardisation in an existing installation does not necessarily mean new machinery.
The process can be carried out over time as long as the strategy is laid down and
taken into consideration when refitting older machines or making new purchases.
The aim is simply to group machines which are capable of accepting as many different
tools as possible on more than one press. Some of the advantages this offers are:
ability to adapt quickly to market changes
better machine loading
reduced risk factor (availability of back-up machines)
lower spare parts holding
simplified operator training
Experience has shown that a direct link from injection moulding to SBM still has a
number of disadvantages.
Inconsistencies in the temperature of the preforms can bring quality problems
Linking individual machines leads to reduced overall machine avail ability or loading
Unless the linked machines have identical capacities loading factors will never be optimised.
The quality question alone nor mally leads to a decision not to directly link the machines.
Fig.1:
Typical plant layout
with mezzanaine floor
a mezzanine floor reduces space requirements in the mould shop,
it reduces the required span of the overhead crane,
it simplifies the provision of serv ice lines and cables,
heat and noise from the PET dryer do not disrupt the machine shop,
the factory has a first class appear ance, Figure 1 shows an example of such a layout.
The supply of PET pellets does not usually present technical problems. For preform
production the operation of a largely automated central supply is the norm. Only in
South America and Asia is the practice of supplying the mould machines with PET
from buckets or tubs still seen. In can nevertheless be a practical solution to
supply very small installations (e.g. only two injection mould machines) direct
from Octabins or similar, and to install a central supply silo as a second phase
of the operation.
In any case the new plant must be laid out from the beginning in a way that
can accommodate central supply, which means leaving space for silos and ducting.
In contrast to plastic granulate supply, in most factories little attention
is paid to the question of coolant supply. Providing the necessary coolant is
not normally a problem, and the system is generally over-specified. What is
sometimes forgotten is that it is of the greatest importance in a high output,
continuous process to ensure that the supply of coolant is kept constant. If
switching off one injection moulding machine results in a sudden improved coolant
supply to the other machines this will immediately result in rejects being produced
and in production losses. The aim, when designing the cooling system, is to build
in as much control of the coolant flow as possible.
The use of relatively inexpensive adjustable valves which will ensure a constant
coolant flow, can work wonders here. A setting for the coolant control valves should
be documented in the set-up details for each mould tool and applied each time the
tool is used. This ensures that the tool is always operating under the same conditions.
The project economic considerations will pull together all of the above stages
and will almost always call for a project review. This means that various alternatives
should be considered at the planning stage. Investment levels and operating costs
should be estimated for each of the alternatives. Personnel requirements should
be examined, and used to decide on the level of automation.
When considering the soundness of the investment, and the potential risks, the
sensitivity of the new plant to changes in economic conditions should be identified.
Examples of such changes may be:
10 % increase in labour costs
25 % less output
Changes in the product mix, with more or less variantsIt is basic fact that, because
of the limited ability to influence costs, a factory which has been set up for high
volume manufacture of a small number of variants cannot be competitive in the production
of small batches of numerous different variants. The reverse is also true. This makes
it essential to analyse caref ully the scenarios resulting from possible changes in demand.
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