Temperature Control in Plastic Injection Molding
Temperature in the molding of Plastic and Rubber
Products need a Digital Proportional Controller.
Once set and Tuned it has good repeatability and
ensures product quality.
Terms in Process Control and Explanation
There are three Controls to be Adjusted to make a
Proportional temperature Controller Perform
Properly. This method has to be practiced and
experience gained from it can be used to get very
good and stable Control of the temperature or
other process parameters.
PID control using OpAmps
1. Set Point. (SP) -
This is the Temperature at which you require the
Heated area to be. Here we have to remember it is
better to control the temperature of the metallic
area closest to heater to avoid thermal Cushions.
In Rubber and Plastic Molding if you are measuring
the plastic temperature directly it may give rise
to oscillations and proper control may not be
In Controlling the Temperature of Air or Water
(Bad Conductors of Heat) Forced Convection with
Fans for Air or Stirrers for Liquids can be used
when Sensing temperature of the Liquid or air
directly. But in Plastic such things cant be done
as it is a semi-solid when heated hence. Control
of Temperature of the Metal Discharging Heat to
the Plastic is most practical. Oscillations are
inevitable if the sensor is away from heater or is
in contact with a non-conductor of heat.
2. Process Value. (PV) -
This is the Temperature at the Tip of the sensor
or the material touching the tip of the sensor. In
Non-Conductors of Heat like plastic if we are
monitoring plastic at a certain point the
temperature of the plastic will be very different
at various points depending on the Distance of the
Heater from that point due to thermal gradients.
3. Proportional Band or Dead band. (PB) -
Dead band or H % or Hysteresis are terms used in
on / off Controllers in proportional controller we
use the term proportional band.
to Use Your PC to Measure Temperature
The Temperature zone in which the Controller turns
on or off The heaters in a time proportional
manner is the proportional Band. Set Point 200 deg
C It is Given in % e.g. 10% PB of 200 deg SP is 20
deg. the Heaters are on till 190 deg C and off
above 210 deg. C. Between 190 to 210 is the PB. A
little above 190 the Heaters are on for 90% time.
A little below 210 Deg C the Heaters are on for
10% of the time. When SP=PV Process Value the
Heaters are on for 50% of the time i.e. 50% Duty
4. Cycle Time -
This is the repetitive rate at which the heaters
are Turned on or off Room Temperature 26 deg C For
a Cycle time of 12 Seconds, when PV=SP heaters are
on for 5 seconds and off for 5 seconds and this
goes on as long as PV=SP.
Tuning or Adjusting a Proportional Temperature
Step # 1 -
Ensure Sensor is properly connected to the
Temperature Controller TC polarity reversal will
show reducing reading in the Display as heat
builds up. The Heaters used and wattage selected
must be able to bring the temperature more than
the maximum required control temperature with TC.
If Supply Voltage is down or heaters are blown or
not in contact TC can not solve the problem.
So when in doubt connect heaters directly to
supply (without TC) and see observe maximum
temperature e. g. if max. temp. is 500 deg C the
TC can control temperature upto 480 deg C.
Step # 2 -
Keep PB in minimum position and power on system
e. g. set temperature is 300 deg C. Now Observe
maximum overshoot. and adjust proportional band as
in table below.
SP 300 deg C
PV (Process Value or Measured Temperature)
|10 % 330 deg C or more
||Near Maximum fully clockwise till end.
|5 % 315 deg C to 360 deg C
||Middle of the PB Control or towards max.
|2% 306 deg C
||Little above present setting.
|Less than 300 deg Droop e. g. 290
||PB is Critically set Do not Change.
After each change turn on system again to see
response till 2 % or less variation or overshoot
or oscillations are obtained.
Thumb Rule ! -
- Increment PB to Decrease Overshoot.
- Increment PB to Decrease Oscillations.
- Stop adjustment when PV droops < SP
- Adjust EC to match SP = PV after PV is stable
at a point less than SP.
4-20mA Loop-Powered System
Step # 3 -
There is an additional control called Error Cal
EC ( manual reset or Integral) which is factory
set for SP=PV 50% duty cycle. In certain cases
after stable reading is obtained after adjusting
or tuning PB the temperature may stabilize say at
290 deg for a set point of 300 deg the process is
stable but a ten degrees process error is present.
this can be compensated in two ways.
Increase setpoint to 310 deg the process settles
at 300 deg but this may not satisfactory even if
it is practical.
Adjust Error Cal provided in the back panel to
increase temperature to 300 deg from 290 deg.When
this is done give some time for system to respond
after every 1/2 a turn 180 deg of the control. the
EC control is a Ten turn potentiometer like the SP
potentiometer after 10 turns the direction of
turning must change. Clockwise Increase
temperature Anticlockwise decrease temperature.
(at min. PB setting EC pot sets the On/Off
Temperature Control using SSR and STC1000PK
Mains Circuit -
Always Connect Phase to Live "L ", This Can Be
"R" in a RYB System 3 Phase . "L" Live Can Be
Verified by a "Neon Tester" . and it is the Energy
Line (Tester Glows). "N" Neutral is the Energy
return line and will be close to Earth Potential
in a Neon tester it will not show a Glow. Earth
"E" is the Local Earth at the site of the
installation. ( "N" to "E" AC Voltage should be
less than 5 Volts ideally)
SSR or Solid State Relay or Electronic Relay -
Generally this is a Thyristor Based Normally
Open 230V Switch that can be turned on/off at a
- No moving parts hence no wear and tear.
- Dissipates Heat when in On Condition.
- Use adequate Heat Sink or SSR will fail.
- Input to Output is optically isolated. very
tight and crimped.
- The one used here is DC Control AC 230V 15A
Components and Points -
Fuse Rating of a HRC Fuse -
High Rupture Capacity (HRC) Fuse is Safe and
Reliable. 10KW Heater at 230V is 10,000 / 230 in
Amps of Fuse Rating. i.e. Watts = Volts x Amps
50 Amps Fuse.
Relay Terminology -
- C Common is connected to NC when Relay is off.
- NO Normally Open is Disconnected when Relay
off. (connected to C when Relay on).
- NC Normally Closed is Connected to C when
Relay off. (disconnected when relay on).
Relay outputs are Potential Free or Floating or
at High Impedance.
Note - The Terminations of High Current
Lines going to Heater must be very tight and
crimped. Loose contacts will Spark and cause Fire.