| HotFlo! | Warm-up Procedure | |
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The hot sprue bush is fitted with a very powerful and rugged electric element. When starting up from a cold state, it must be brought up to operating temperature at a controlled rate to prevent damage to the sprue bush. There are three ways to achieve this : |
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These type of controllers have a 'Slow Start' option in their program which detects if the starting temperature is below a pre-set value (say 100 o C. If so, it brings it up to a medium temperature (typically about 150o C) and holds it there for a period of time, before increasing it to the set-point temperature.
An equivalent method is 'Ramp' control, whereby the rate of temperature increase is controlled. A rate of 30 degrees Centigrade per minute is usually set.
The temperature controllers supplied by HotFlo! have this feature built-in.
Fit a time-clock start to the hot sprue bush and set the temperature to a low level, say 150o C.
By this means you can set the timer to come on 15 to 30 minutes before you want to start production which will give it time to heat through.
When you want to start production, change the set-point temperature up to the operating point around 450o C, wait five minutes for it to stabilise and off you go.
Change the set-point on your temperature controller to 150o C and switch on the power. When the actual sprue bush temperature reaches 150o C, let it soak there for about five minutes. Then increase it to 300o C and leave for five minutes. Then move it to the operating point, 450o C, wait five minutes and then start production.
The recommended option is the first; using a 'Slow Start Temperature Controller', for the simple reason that it is fully automatic.
With options 2 and 3, someone has to remember to to change the setpoint down to a lower value before switching it on, and then to increase it later on. Whilst this may be easy to do in a trialling situation, it is not recommended for a regular production environment.
It is worthwhile to consider what might happen if the set-point is set to 450o C when starting up from room temperature and a 'slow-start' controller is not being used.
When power is applied, the electric element immediately begins to heat up the outside surface of the sprue bush. However the wall section is quite thick so it takes some time for the heat to transfer to the inside diameter.
Within a short time, the outside surface temperature could be 450o C while the inside temperature may be less than 100o C. This creates an eneven expansion of the metal and induces stresses.
If this happens repeatedly, it can cause cracks to start in the wall of the sprue bush. This will shorten the life of the sprue bush and, in the worst case, could cause it to split open.
| Document Control | Issue 3 | 25th February 2003 | www.hotflo.com |
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