There are 2 types of heating hot sprue bushes:
Interior
Exterior

Internally Heated up Hot Sprue Bushes
The building of this sort of hot bush is made up of 3 fundamental models:
1.An exterior body
2.An interior body
3.A torpedo
A typical internally heated hot nozzle is demonstrated in Figure followed.

hot nozzle

The intention of the torpedo will be to drive the melt towards the outside walls of the melt channel. This prevents too big a
large of material amassing , which usually would bring about an irregular melt temperature as well as unequal plasticizing because of a thermal gradient through the melt.
The torpedo includes a number of fins, which expand throughout the melt to touch the outside
walls of the nozzle. This gives an area to the torpedo along with a route for the melt to go through.
With this method there may still be significant heat variances and also the pressure fall could be substantial.

Because of this, internally heated nozzles will not be appropriate for heat sensitive products.
The main advantage of an internally heated nozzle over an externally heated one is that a superior gate observe mark could be attained. Color altering could possibly be more challenging in comparison with(blank) external type, nevertheless .

Externally Heated Hot Sprue Bushes

On the other hand, the construction of an externally heated nozzle is extremely simple. It includes a nozzle which has a central tubular hole by which the melt goes by. Theoutside of the bush can be heated up by way of exterior heating unit , generally employing a coil type heater .Direct gating can be done only using tiny tip diameters up to 2 mm. As there is a substantial amount of heat exchange through the bush to the cavity by using direct gating, it could possibly be difficult to acquire a excellent surface finish within these locations. To avoid this trouble, a short sprue tip (ten millimeters) is generally employed to reduce gate pip height and stringing ; a short torpedo tip could also be used .
The favorable  advantage regarding extern ally heated systems is that they can be utilized with many thermoplastics. Also, they are quicker to preserve and heater replacement is not hard.Color alterations are also simpler.