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MYTH in INJECTION MOULDING OF PLASTICS

From " A Guide to Injection Moulding of Plastics" by Prabodh C. Bolur

ISSUE

MYTH- General belief prevailing in the industry.

TRUTH

Role of the machine.

Machine should solve all the quality-related problems.

Role of the machine is to provide good homogeneous melt and provide consistent reproduction of set parameters all the time during production run. Machine should not break down often. Energy efficiency can be the merit of machine design.

Connected load of the machine

Connected load of the machine depends on Clamp unit specification.

Connected load of the machine is decided by the specification of injection unit. - Injection rate and maximum injection pressure.

Energy efficiency

Lower connected load is the indication of energy efficiency.

Amount of power required to carry out a work can not be different in different machines. It is the reduction or elimination of power wastage in a given machine that makes it energy efficient.

ISSUE

MYTH- General belief prevailing in the industry.

TRUTH

Comparison of POWER in injection unit

Lower connected load will consume less power.

Lower connected load will have lower injection rate for a given screw diameter and L/D ratio.

Higher connected load will have higher injection rate for a given screw diameter and L/D ratio.

Lower connected load will give lower capability of spreading melt (lower flow ratio) in the mould.

Higher connected load will give higher capability of spreading melt (higher flow ratio) in the mould.

It means that thinner and bigger part (high flow ratio) can be moulded with higher connected load machine.

Mouldability.

Any shape / part geometry is mouldable.

Perfection in mouldability depends on balanced melt flow, balanced heat exchange in mould and uniform freezing of moulding in the mould.

What is responsible for the filling of melt in the space between clamped mould?

Pressure

Speed (injection rate) component of Injection power. Injection power is product of injection rate and actual pressure during filling phase. Injection power is product of injection rate and set pressure during follow-up pressure phase.

Maximum shot weight

Any part having lower weight than the maximum shot weight of the machine can be moulded.

To mould the biggest possible part of highest flow ratio requires higher injection rate for a given size of screw.

Therefore bigger moulding (with in the maximum shot weight) is possible only with the machine having larger injection rate.

ISSUE

MYTH- General belief prevailing in the industry.

TRUTH

Quality problems- sink marks, warpage, shrinkage, dimensional inaccuracies.

Quality problems- sink marks, warpage, dimensional inaccuracy, can only be corrected by machine parameters.

Part design and mould design should take care of the moulding process constrains like sink mark, warpage, dimensional inaccuracy etc. Therefore part and mould design are more responsible for quality of moulded parts. Machine parameters should lie in the middle of possible parameter range. So that the influence of any minor swing in the parameter on either side of the set value still remains with in the tolerance band.

Injection speed profile and;mp; pressure profile.

It improves quality.

For uniform surface quality of moulded part, it is necessary that skin development while spreading melt in the mould should be uniform. That is possible only when melt moves with uniform velocity in the complex shape of part. Therefore the object of injection profile setting is to achieve uniform melt front velocity. By trial and error method it is not easy at all. CAE software can establish quickly best possible profile for injection speed and follow-up pressure.

RESIDENCE TIME

Soak time - significance is not known, therefore mistakes while selecting the screw size or machine capacity.

Lack of sufficient residence time increases the cycle time particularly in fast cycling thin walled disposable mouldings. This demands lower utilisation shot capacity (in higher screw size) of machine to increase residence time.

What is responsible for the filling of melt in the space between clamped mould?

Pressure

Speed (injection rate) component of Injection power. Injection power is product of injection rate and actual pressure during filling phase. Injection power is product of injection rate and set pressure during follow-up pressure phase.

To understand PRESSURE, INJECTION RATE and POWER see

Engineering basics for Injection Moulding Machine.

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