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Cleaning by heat
Heat cleaning is the sole option for nozzles, hot runners, and filters.
Cleaning by heat requires the entire mould to be heated up uniformly
to above the decomposition temperature of the plastic. As a rule,
a temperature of 420°C will suffice. High-temperature -resistant thermoplastics,
however, require a temperature of at least 450°C.
The fluidized bed technique: The mould is placed in basket and immersed
in the heated sand. Any organic soil adhering to the tool is rapidly
degraded and incinerated in an environmentally friendly manner
The pyrolysis furnace: Heat cleaning in a pyrolysis furnace requires
increased cleaning time because of the heating and cooling phase.
The furnace chamber is heated only after the mould has been placed
in the furnace, and is cooled before the mould is removed. Cleaning
times of 12 hours and more can result for big moulds. Sensitive nozzles
require low-oxygen vacuum furnaces during heat cleaning to avoid damage
through overheating or carbon residues on the surface.
(Q): How can we reduce the acetaldehyde gas in our perform made
form PET and what is a practical way to control it?
(A): The acetaldehyde (AA) gas referred to occurs in PET performs
for the mineral water package. In order to minimize concentrations
for AA, first and foremost, the correct resin has to be selected.
You should use a resin of low IV and the performs produced with it
should given an AA level of around 3 ppm. Generally, resin prepared
from terephthalic acid generates less AA than those prepared using
dimethyl terephthalate.
Process Guide
Inlet temperature of the PET resin at the hopper throat before
entering the barrel should be approximately 160°C. · Barrel temperature
should be in the range of 265° to 280°C. · Backpressure and the screw
speed should be maintained at a minimum. · Maintain the shortest possible
cycle time, as lengthy residence times will lead to excessive degradation
in the material. For best results, try to have the shot close to 60
to 70 percent of the maximum shot capacity. · Using additives as brighteners
can also reduce the AA in performs, but that would add to the appreciable
cost. · Once the performs are produced and before they are finally
packed, they should be left exposed to an ambience of 50°C for a couple
of hours as this will allow the diffusion of AA gas that is volatile
at 20.5°C.
(Q): How to detect sinks and voids?
(A): Sinks and voids cab be the nemesis of an otherwise well designed
part. But how does a designer know beforehand that the part he or
she is creating on a CAD screen will cause major headaches at the
moulding press? The creation of sinks and voids during the moulding
process is a 3D finite phenomenon that can be detected using the element
method. The pressure causing sinks and voids is in the thickness direction.
So to minimize void creation, track temperature across various points
of the part, find out how to keep the flow path and gate unfrozen
for the longest time.
Courtesy: Mr. P.M. Jariwala, Kolsite Industries
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