Injection The necessity for better melt homogeneity

Injection moulding is one of the most
prevalent techniques that is used for the manufacture of plastic products.
Injection moulding has developed to its current state due to a few key reasons.
Due to advances in the fields of technology and science the specifications
required from the moulded parts have become increasingly demanding. Tighter
tolerances and increasingly complex parts have become commonplace within the
industry. With the development of polymer materials for injection moulding,
also came the development of the processing. The necessity for better melt
homogeneity which was established due to the discovery of varied morphologies
in polymers has led to the injection screw being introduced. This has been
further developed with the introduction of material specific screws. Mould
temperature controllers and high temperature ceramic heaters where introduced
to improve the processing of high temperature materials that require high mould
temperatures. Advancements in electronic technologies have allowed for machines
that are increasingly accurate, efficient, and have increased control. All
electric machines are increasing in popularity in particularly with the medical
device industry as they are cleaner, quieter and often more accurate.

The most demanding aspect of injection
moulding is to ensure consistency of quality, from cavity to cavity, shot to
shot and production run to production run, it is for this reason that the
process needs to be documented clearly and thoroughly. The moulding process can
be defined as inputs into the moulding machine, these inputs include the
setting of speeds, temperatures, pressures and times such as holding pressure,
injection speeds, cooling time and melt temperature. The inputs are established
through setting up of the process and are recorded on a process sheet. The
process sheet should also contain any other aspect of the operation that can
impact on the quality of the finished part. Storage of the material, drying of
the material, temperature profile of hot runner systems if applicable, cooling
times and ejector parameters should all be established and recorded.

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The objective of developing a moulding
process is to develop a process that is robust, one that will not require any
modifications once the processing parameters have been set. Process consistency
is directly linked with quality consistency. There are three consistencies
required for injection moulding, these are cavity to cavity, shot to shot and
run to run. Cavity to cavity consistency is necessary in multi cavity moulds in
order to ensure that each cavity has an identical quality level as all the
other cavities, shown in figure 3.

Shot to shot consistency signifies that
each successive shot will be identical with the last, or that any two given
shots will be identical to each other so long as the process parameters are
maintained throughout the entire production run. This is indicated in figure 4.

When process parameters remain the same
over two or more runs they will then produce parts of the same quality, this is
called run to run consistency and is shown in figure 5. This information shows
that a robust process will always produce consistent results of high quality.

Scientific moulding was first introduced
and promoted by John Bozzelli and Rod Groleau, two pioneers in the field of injection
moulding (Fimmtech, 2015). The principles that they introduced
are the basis of the industry standard and are widely implemented to date.
Scientific processing encompasses the actual plastic pellet that enters the mould
throughout the moulding operation at the machine. Scientific processing is the
entire process, from the moment a pellet enters the facility up to when the
finished product leaves the facility. Figure 4 below shows the lifecycle of a
pellet within a moulding facility.

Kulkarni (2010)
further states that scientific processing is the act of accomplishing
consistency in quality of the part through the application of the elemental
scientific principles that regulate the parameters of the moulding process. In
order to attain this consistency, control must be achieved in each task that is
to take place during the process, in order to ensure control there must be
understanding of the underlying scientific principles. A robust process is the
main goal of scientific processing. Attaining robustness at each of the stages
that the material travels through systematically translates to an all-encompassing
robust process.

There are five critical factors that must
be carefully chosen in order to ensure quality and consistency of the final moulded
part, these are shown in figure 7. Distinctive Plastics (2012)
state that these include;

1.     
Part design

2.     
Mould design and construction

3.     
Material selection

4.     
Moulding machine

5.     
Moulding process

Every one of these factors play a vital
role in the manufacture of the moulded part and thus each of them must be
optimised for manufacturing the moulded part. Each of these aspects will be
discussed briefly below.

Part concept begins with the design
engineer. It is important that the part is designed with moulding in mind and
that each of the design laws for plastics are considered. Design laws for
plastics vary greatly from metal part design due to the intrinsic nature of the
plastic. For example, it is essential for all corners to have a radius to
minimise stress concentration as well as premature failure. Additionally, to
avoid sink defects, thick sections must be avoided. Part designers face added
pressure, due to the rising cost of labour as well as the need to maximise
efficiency of the process, to design assembly parts that use numerous materials.
This is commonly called multi-material or multi-component moulding