Method of blow-moulding

A parison 4 of hot thermoplastics material is delivered into a mould 1 comprising two mould halves 2, 3 which are closed towards one another. A stress force is applied to the wall of the parison 4 either by a projection 5 extending from one of the mould halves 3 and/or by blowing air at a low pressure against the parison 4 through a blow pin 10. The parison 4 is then penetrated by the blow pin and high pressure air is blown through the blow pin 10 to inflate the parison 4 against the mould halves 2, 3. The blow pin 10 is withdrawn and a formed article such as a case is released from the mould after a preset time.

The invention relates to a method of blow-moulding thermoplastics material 
to form a blow moulded article such as a case, for example, a tool case 
comprising two case halves having an integrally moulded hinge 
therebetween. 
It is known to form such blow-moulded articles by firstly extruding a tube 
or parison of hot thermoplastics material, bringing two halves of a mould 
together on opposite sides of the parison and inserting a blow pin through 
the mould into the parison. Air is then blown into the interior of the 
parison through the blow pin at high pressure to inflate the 
thermoplastics material against the walls of the mould. After a preset 
time when the moulding has set, the two halves of the mould are separated 
and the moulded article is released. 
In conventional arrangements, the blow pin which penetrates the wall of the 
mould comprises a sharpened tip which is normally chamfered. On 
penetration, the sharpened tip essentially rips through the wall of the 
parison resulting in a ragged edge through which air may back-leak out 
from the mould. Because of this, a relatively low moulding pressure of 
between 3 and 5 bar is generally only achievable using conventional 
technology. This results in an unsatisfactory surface finish as the air 
pressure is insufficient to avoid flow wrinkles being visible on the 
surface of the moulding where distribution of thermoplastics material has 
been uneven. Further, the small apertures left in the moulding after 
withdrawal of the blow pins are frequently poorly formed which again 
detracts from the surface finish of the finished product. 
It is also known from UK-A-2275441A to blow air at a higher pressure of 10 
bar through a blow pin to deform an outer skin of the thermoplastics 
material as the mould parts are closing. The blow pin is then inserted 
through the skin and an article such as a case is blow moulded. In this 
known arrangement the pin has a blunt tip and a tubular sleeve forms 
around the pin to sleeve the pin as it is operated. This assists in 
preventing leak-back of air and facilitates higher cycle times with 
improved surface finish. 
However, the control of the blow pin during the operation of this known 
system is sometimes difficult and consequently the blow moulding cycle and 
surface finish are not fully optimised. 
This invention is therefore directed towards providing an improved method 
of blow moulding which will overcome at least some of these difficulties. 
STATEMENTS OF INVENTION 
According to the invention there is provided a method of blow-moulding 
thermoplastics material comprising the steps of: 
extruding a parison of hot thermoplastics material; 
slowly closing a pair of mould halves around the extruded parison; 
applying a stress force to the wall of the parison as the mould halves are 
being closed; 
penetrating the parison by passing a blow pin through the deformation 
formed in the wall of the parison; 
blowing high pressure air through the blow pin to inflate the parison 
against the walls of the mould halves; 
withdrawing the blow pin; and 
releasing a formed article from the mould after a preset time. 
In one embodiment of the invention, a stress force is applied to the wall 
of the parison by blowing air at a low pressure against the parison 
through the blow pin. 
Alternatively or additionally, a stress force is applied to the wall of the 
parison by providing a projection from the mould surface over which the 
parison passes. In this case, preferably the projection is provided in the 
region of the blow pin. Most preferably, the projection is defined by a 
sleeve in which the blow pin is mounted for movement between a retracted 
and an extended position. 
In a particularly preferred embodiment of the invention the mould halves 
are closed in timed relation to the blowing of the low pressure air. 
Preferably the wall of the parison is deformed by blowing low pressure air 
at a pressure of from 3 to 5 bar against the parison. 
Preferably the high pressure air at a pressure of from 8 to 10 bar is blown 
through the blow pin to inflate the parison. 
In one embodiment of the invention the low pressure air is blown through a 
parison deforming outlet hole at the end of a blowing pin to provide a 
stream of low pressure air directed against the parison to deform the 
parison at the point of entry of the blowing pin into the parison. 
Preferably, after insertion of the blow pin into the parison, the high 
pressure air is blown through blow pin inflation outlet means in a side 
wall of the blow pin to inflate the parison against the walls of the 
mould. Typically the blow pin inflation means comprises at least two 
outlets in the side wall of the blow pin. 
There may be two blow pin inflation outlets which are substantially 
diametrically opposed in the side wall of the blow pin. Alternatively the 
blow pin inflation means comprises four outlets spaced-apart around the 
side wall of the blow pin. 
In one embodiment of the invention the parison deforming outlet also 
provides a blow pin inflation outlet. 
In a preferred arrangement the mould halves are closed to a distance of 
less than 100 mm apart before low pressure air is blown against the 
parison. Preferably the mould halves are closed to a distance of less than 
25 mm apart before low pressure air is blown against the parison. 
In a particularly preferred embodiment the method includes the step of 
initiating a timer to control the blowing of the low pressure air on 
partial closing of the mould halves. 
The invention also provides a blow moulded article, especially a blow 
moulded case whenever produced by the method of the invention.

Referring to the drawings, there is illustrated a method of blow-moulding 
thermoplastics material according to the invention. A blend of 
thermoplastics material is delivered into a mould 1 comprising two mould 
halves 2,3. A parison 4 of hot thermoplastic material is extruded as the 
mould halves 2,3 are closed towards one another. In this case, a stress 
force is applied to the wall of the parison 4 by a projection 5 extending 
from one of the mould halves 3 over which the parison 4 is passed. The 
projection 5 defines a sleeve for a blow pin 10 which is movable by a 
piston 11 from a retracted position as illustrated in FIGS. 2(a), 2(b) and 
2(c) to extended positions illustrated in FIGS. 2(d), 2(e) and 2(f). As 
the parison 4 flows over the mould half 3, it is pretensioned by the 
projection or sleeve 5 as illustrated in FIG. 2(b). In this arrangement, 
the pre-close distance d between the mould halves 2, 3 is at most 100 mm 
and is typically 25 mm. 
In the method of the invention, after partial closing of the mould halves 
2,3, low pressure air at a pressure of from 3 to 5 bar is blown through 
the blow pin against the parison 4 as illustrated in FIG. 2(c). The stream 
of low pressure air deflects the wall of the parison 4. The mould halves 
2,3 continue to close in timed relation to the blowing of the low pressure 
air and the parison 4 is further pre-shaped as illustrated in FIG. 2(d). 
The blowing of low pressure air is continued and the blow pin 10 is 
further extended during further closing of the mould halves 2,3 until the 
air and blow pin 10 breaks through the parison 4 as illustrated in FIG. 
2(e). Finally, an air seal is formed by blowing high pressure air at a 
pressure of from 7 to 10 bar through the blow pin 10 as illustrated in 
FIG. 2(f). In this configuration, the mould halves 2,3 are fully closed. 
After a preset time allowed for the moulded article thus formed to set, the 
blow moulded article is released from the mould. The article may be a blow 
moulded case for speciality tools and the like. 
The blow pin 10 is operated under the control of a control system 
illustrated in FIG. 3 and comprising a proximity switch 11 to monitor when 
the mould halves 2,3 are in the pre-close position illustrated in FIG. 3, 
a delay timer 12 and a low pressure solenoid valve 13. A similar control 
system is provided for high pressure air. The blow pin 10 is mounted in a 
mounting block 15 having an inlet port 16 for high pressure air at a 
pressure of typically 8 to 10 bar and for low pressure air at a pressure 
of typically 3 to 5 bar. 
In more detail, and referring particularly to FIG. 1 the blow pin 10 has a 
rounded tip 21 to prevent damage to a parison 4 on insertion. The tip 21 
in this case has a single parison deforming outlet hole 22. Blow pin 
inflation outlet means is in this case provided by two substantially 
diametrically opposed blow pin inflation outlet holes 23 in the side wall 
of the blow pin 10 stepped back from the tip 21 of the pin 10. 
It will be appreciated that a stress force may be applied to the wall of 
the parison either by providing a projection from a mould over which the 
parison wall passes and/or by blowing air at a low pressure against the 
parison through a blow pin. 
The invention offers very considerable advantages. Because of the lower 
pressure used, the delayed mould closing, the controlled timing and the 
modified blow pin it is possible to form blow holes more reliably than by 
using standard techniques. Lighter walled parisons may be used resulting 
in materials savings. Different materials which have lower melt strengths 
may also be used. In addition, production efficiency is optimised as there 
is no risk of blockages in the blow pin. 
A substantially improved surface finish is also achieved. In addition, 
there is improved contact of the thermoplastics with the mould surface 
with consequent improved heat transfer and cooling efficiencies which in 
turn leads to lower cycle times and improved rates of production. 
As the quality of the moulded article as produced using the method of the 
invention is improved there is a lower reject rate on testing, with 
consequent increase in production. 
The invention is not limited to the embodiments hereinbefore described, but 
may be varied in both construction and detail.