Compared with ordinary flow channel molds, hot runner mould have significant advantages such as saving time and materials, high efficiency, and stable quality. However, its wide application has been affected by their failure in use. With technological advancement of mold industry, hot runner mould has made great progress in flow channel melt temperature control, structural reliability, hot runner component design and manufacture, which makes hot runner technology regain people’s attention and favor.

1. Analysis of common faults in hot runner system
Compared with ordinary flow channel molds, hot runner mould have significant advantages such
as saving time and materials, high efficiency, and stable quality. However, its wide application has
been affected by their failure in use. With technological advancement of mold industry, hot
runner mould has made great progress in flow channel melt temperature control, structural
reliability, hot runner component design and manufacture, which makes hot runner technology
regain people's attention and favor.
 First, residue at the gate is protruding or flowing, and surface
appearance is poor.
 Main reason
Gate structure selection is unreasonable, temperature control is improper, and there is a large
residual pressure in the melt of flow channel after injection.
 Solution
1) Improvement of gate structure. Usually length of gate is too long, which will leave a long gate
material on the surface of plastic part. If gate diameter is too large, it will easily lead to
phenomenon of drooling. When above failure occurs, it is important to consider changing gate
structure. Common gates for hot runners are direct gate, point gate and valve gate .
2) Reasonable control of temperature. If amount of cooling water in gate area is insufficient, it
will cause heat to concentrate, causing runny, dripping and drawing. Therefore, cooling of this
area should be strengthened when above phenomenon occurs.
3) Resin pressure release. Excessive residual pressure in flow path is one of main causes of runny.
In general, injection machine should adopt a buffer circuit or device to prevent runny.

2.  Second, material discoloration, coke or degradation
 Main reason
Improper temperature control; size of runner or gate is too small to cause large shear heat
generation; dead point in runner causes retaining material to be heated for too long.
 Solution
1) Accurate control of temperature. In order to accurately and quickly measure temperature
fluctuation, thermocouple temperature measuring head should be reliably contacted with flow
channel plate or nozzle wall, placed at the center of each independent temperature control zone.
Distance between head temperature sensing point and flow channel wall should be no more than
10mm, and heating elements should be evenly distributed on both sides of flow channel.
(2) Correct gate size. Runner point should be avoided as much as possible, and gate diameter
should be appropriately increased within allowable range to prevent excessive shear heat
generation. Melt of inner hot nozzle has a large temperature difference in radial direction of flow
passage, which is more likely to cause phenomenon of coke and degradation. Therefore, it should
be noted that radial dimension design of flow passage should not be too large.
 Third, injection volume is short or no material is injected
 Main reason
Obstructions or dead spots appear in flow path; gate is blocked; a thicker condensation layer
appears in flow path.
 Solution
1) When designing and processing runner, transition of melt to arc of wall at the corner should be
ensured, so that entire runner is smooth without flow dead angle.
2) Appropriately increase material temperature without affecting quality of plastic parts, and
avoid premature condensation of gate.
3) Appropriately increase hot runner temperature to reduce thickness of condensation layer of
internal heat nozzle and reduce pressure loss, thereby facilitating filling of cavity.
 Fourth, serious leakage

3.  Main reason
Sealing element is damaged; heating element is burnt and causes flow plate to expand unevenly;
nozzle and center of sprue sleeve are misaligned, or projected area of melt insulating layer on
nozzle determined by leakage preventing ring is too large, causing nozzle to retreat.
 Solution
1) Check sealing and heating element for damage. In case of damage, careful inspection before
replacement is a component quality problem, a structural problem, or a result of normal service
life.
2) Choose appropriate leak stop method. Depending on the way nozzle is insulated, leakage can
be prevented by a leak stop ring or a nozzle contact. Care should be taken to maintain a reliable
contact between leak-proof contact areas.
 Fifth, hot runner cannot be heated normally or heating time is too
long.
 Main reason
Wire channel spacing is not enough, causing wire to break; when mold is assembled, wire
intersects with short circuit and leakage.
 Solution
Choose correct processing and installation process, ensure that all wires can be placed, use
high-temperature insulation materials as required, and regularly detect damage of wires.
 Sixth, refueling or poor color change
 Main reason
Improper method of refueling or color change; unreasonable design or processing of runner
leads to more stagnant material inside.
 Solution
1) Improve structural design and processing of runner. When designing flow channel, runner
dead point should be avoided as much as possible, stress at each corner should be circular.

4. Within scope of permit, size of runner is as small as possible, so that less material is retained in
runner and flow rate of new material is larger, which is beneficial for quick cleaning. When
processing flow path, no matter how long flow path is, it must be processed from one end. If it is
processed from both ends at the same time, it is easy to cause center of hole to not coincide,
thus retentate portion is inevitably formed. Generally, external heating nozzle can clean flow
passage relatively easily because heating device does not affect melt flow, inner heating nozzle
easily forms a condensation layer on outer wall of flow passage, which is disadvantageous for
quick refueling.
2) Choose correct refueling method. Hot runner system refueling and color changing process
generally pushes out all retentive materials in flow channel directly from new material, and then
moves retaining material on flow channel wall as a whole. Therefore, cleaning is relatively easy.
On the contrary, if viscosity of new material is low, it is easy to enter center of retentate,
retentate is separated layer by layer, which is troublesome to clean. If viscosity of new and old
materials are similar, fast refueling can be achieved by speeding up injection rate of new material.
If viscosity of retentate is sensitive to temperature, temperature of material can be appropriately
increased to lower viscosity to speed up refueling process.