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Opportunities to Optimize Cycle Times in Plastic Injection

Establishing an optimized molding cycle is imperative to ensure
a plastics company's profitability. It is important to remember that
running efficiently improves overall part quality and helps
a company increase efficiencies, as well as creates opportunities
to accept more jobs created by more press time availability.
The factors listed below are meant to define what areas strongly affect cycle times
and these areas should be focused on to develop strong cycles and increased productivity.
The following tutorial explains the procedure for developing
an optimized process:

Optimizing Process

Hold time:

A hold time study should be performed to establish the actual point of gate seal,
and this time should generally be set 1 second higher than the actual established gate seal time.See the tutorial
On this site for the procedure for doing this.

Cooling time:

Cooling/cure time should be set as low as possible to maximize cycle
time. When determining this setpoint, great care should be taken to inspect part aesthetics as well
as measuring key dimensional part values. The screw should also recover to full shot position 1 second prior
to cure time setpoint.

Clamp speeds:

One of the most common issues I see in regards to poor cycle times is improper
clamp set up. The following describes general clamp settings and their relationship to each other:

1. Mold Open Position: Mold Open position should be set to minimum daylight allowing just enough room
for part ejection and/ or robot extraction.

2.Mold Open Slow:

Open slow position should be set 1.00" prior to the mold full open position. Pressure and speed should
be maximized while still allowing full open to stay consistent.

3.Open Breakaway:

Open Break should be set at maximum speeds and pressure based on the complexity of mold configuration,and
should also phase into mold open fast at 2/3 of the full guide pin length.

4. Ejection pressure/ speed:,

Ejectors should be maximized taking great care to inspect for pinpush condition
as well as its relationship to robot extraction.

5. Low Pressure Close:

Low pressure close is one of the most important setpoints of any molding operation.
Speed and pressure should be maximized based on mold complexity (configuration, cores, slides, etc.) with great
care being taken to assure that mold will not lock up on high pressure if some component is caught between the
mold halves.Your clamp speed should phase into low pressure close (close slow) when the moveable side bushings
are positioned on the guide pin 1/3 of the overall length (most cases: again..dependent upon the mold
configuration in relationship to cores/slides/details etc.)

Fill Time:

When establishing process, it is important to realize the impact of fill time on your cycle..there are times you
will need to profile your velocities to accomplish good visual/ dimensional aesthetics... but whenever possible
velocity should be maximized to help maximize cycle efficiencies.

Screw Rotate/ Back Pressure:

Screw rotate time is important to the integrity of your productivity. There is a direct relationship to back
pressure. It is important to remember that back pressure supplies 70-90% of your total heat generation. Your
material specs often give values to proper back pressure setpoints, but these should be considered as general data
based on the material manufacturer's machinery..not your own. Back pressure validity can be measured by
performing a melt temperature study at different levels of back pressure. As back pressure is increased, so should
melt temp. Once back pressure increases no longer result in higher melt temps, max back has been established and no
further increases should be performed.

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Garrett MacKenzie: plastic411.com Editor & Technical Writer

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Opportunities to Optimize Cycle Times in Plastic Injection

Optimizing Process

Hold time:

Cooling time:

Clamp speeds:

Fill Time:

Screw Rotate/ Back Pressure:

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