GB2114502A

GB2114502A - Method for forming hollow thermoplastic bodies

Info

Publication number: GB2114502A
Application number: GB08301334A
Authority: GB
Inventors: Giannunzio Guzzioni
Original assignee: Iguzzini Illuminazione SpA
Current assignee: Iguzzini Illuminazione SpA
Priority date: 1982-02-04
Filing date: 1983-01-18
Publication date: 1983-08-24
Also published as: GB2114502B; NL8300210A; AU1110683A; IT8219446A0; AU557576B2; FR2520658A1; IT1149745B; GB8301334D0; DE3301975A1; JPS58155931A

Abstract

The invention relates to a method for forming hollow thermoplastic bodies, such as lamp globes, provided with an aperture which is relatively small compared with the body dimensions, characterised by comprising at least the following stages: moulding a cup-shaped preform 1 having its open end provided with a flange 2; connecting said preform in a sealed manner to a blowing head 4 which is able to abut against said flange and to lock it; inserting the preform connected to the blowing head into a forming mould 5, 6 until it reaches the operational position in which said flange is in proximity to, but not in contact with the outer surface 7 of the mould; and subjecting said preform to a forming operation to produce the finished hollow body by blowing air at a pressure which increases progressively from an initial value close to zero, and at a predetermined temperature. The invention also relates to an apparatus for carrying out said method. <IMAGE>

Description

SPECIFICATION Method for forming hollow thermoplastic bodies It is known to use mainly rotational moulding or injection and blow moulding for manufacturing hollow thermoplastic bodies provided with an aperture of small dimensions relative to the dimensions of the body.

Rotational moulding can be used for manufacturing spherical bodies, such as globes to form the outer covering of a lamp. The thermoplastic material is fed into a steel or aluminium mould composed of two hemispherical shells, the mould is then placed in an oven and rotated about its own normal axes so as to uniformly distribute the material which, on setting, adheres to the inner walls of the mould. After cooling, a hollow globe is extracted from the mould, and has a uniform thickness at all points, this characteristic being the major attribute of the article thus produced. However, the long processing time, the cost of the process and moulds, and the need to continuously maintain these latter represent drawbacks such as to make rotational moulding hardly convenient industrially, especially for producing lamp globes.

On the other hand, injection and blow moulding is widely used for manufacturing hollow thermoplastic bodies such as containers, particularly bottles.

This method uses two successive steps. A preform or parison is moulded by injection, and is blown at a predetermined temperature, with simultaneous biaxial mechanical stretching to enable the desired final shape to be obtained. Biaxial orientation is a very delicate operation. Determined physical characteristics of the finished article, such as mechanical strength, modulus of elasticity etc. depend on its correct execution.

Injection and blow moulding can be carried out either in two separate stages or in a single stage according to the required mechanical characteristics and the material used. More specifically, in the two stage process the finished preform is produced separately in a first machine, after which in a second apparatus the preform is heated to a predetermined temperature, at which blowing and biaxial stretching are carried out. For example, if polyethyleneterephthalate is used, biaxial stretching is carried out at the glass transition temperature of the polymer, which crystallises under stretch conditions, so that the finished hollow body acquires excellent mechanical characteristics.

In the case of the single stage process, the preform is moulded, and the finished hollow body is ready when the predetermined temperature has been reached during the cooling of the preform.

In either case, the complications of this type of technology are apparent, due mainly to the need to carry out biaxial mechanical stretching, with the consequent complexity of the required apparatus.

Such a method is also unsuitable for producing spherical hollow bodies such as lamp globes with satisfactory characteristics, particularly with regard to thickness uniformity of the finished article.

The main object of the present invention is to produce hollow bodies of thermoplastic material in a substantially rapid, simple and direct manner, and thus avoiding the complications oof the aforesaid known method, by a method which comprises operations not requiring costly or complicated apparatus, so asto make small series production of articles industrially convenient.

A further object of the invention is to allow precision production in particular of spherical bodies suitable for forming lamp globes, which have excellent physical characteristics, particularly with regard to the thickness uniformity of the finished globe.

These objects are attained according to the present invention bya methodforforming hollow thermoplastic bodies, such as lamp globes, provided with an aperture which is relatively small compared with the body dimensions, characterised by comprising at least the following stages: moulding a cup-shaped preform having its open end provided with a flange; connecting said preform in a sealed manner to a blowing head which is able to abut against said flange and to lock it; inserting the preform connected to the blowing head into a forming mould until it reaches the operational position in which said flange is in proximity to, but not in contact with the outer surface of the mould; and subjecting said preform to a forming operation to produce the finished hollow body by blowing air at a pressure which increases progressively from an initial value close to zero, and at a predetermined temperature.

The invention also relates to an apparatus for carrying out the aforesaid method.

The characteristics and advantages of the aforesaid method will be more apparent from the description given hereinafter of a non-limiting embodiment with reference to the Figures of the accompanying drawings.

Said Figures show three different operational stages of the method viewed in axial section, wherein: Figure 1 shows a preform and blowing head at the moment of their connection; Figure 2 shows the preform and blowing head connected together in a sealed manner and inserted into a mould in the operating position before blowing; Figure 3 shows the final forming stage, with blowing terminated.

With reference to said Figures, a cup-shapd preform 1 of thermoplastic material such as polycarbonate, methacrylate or the like is initially prepared, for example by injection moulding.

Said preform is provided at its open end with a perimetral flange 2, and at the opposite end 3 is shaped as a spherical cap, of which the thickness is slightly less than the thickness of the remaining part of the preform. In this latter part, the thickness can however be suitably varied according to the shape and characteristics of the final article to be obtained.

After the moulded preform has been cooled to ambient temperature, it is positioned by being coupled to a blowing head 4 according to the arrangement shown in Figure 1.

Said blowing head 4 comprises a shaft 9, on the lower zone of which there is mounted a jacket 10.

The shaft 9, connected to a motor, terminates lowerly in a flanged rim 30 able to abut against that of the preform, and can be rotated independently of the jacket 10, which remains fixed. For this purpose, ball bearings 11 are provided between the shaft and jacket. An annular header 12 fitted with gaskets 13 is provided in the upper part of the jacket 10, and into it there opens a conduit 14forfeeding the blowing air.

A conduit 15 is provided for the discharge of the blown air. Grippers 16 for locking the preform are mounted on the lower end of the shaft 9. These grippers are swivel-pivoted by pins 17 on the blowing head, to which they are also connected by springs 18. Bearings 26 are mounted on the head of the grippers to enable it to rotate about the fixed jacket 10. To enable the grippers to be opened and closed, the jacket 10 is provided with a double acting cylinder 19, of which the lower end 20 is wedge shaped. This cylinder can move along the jacket 10, and is provided internally with a piston 21 fixed rigidly to said jacket and fitted with an O-ring 22 for pneumatic sealing.

The walls of the cylinder 19 are provided with a lower aperture 23 and upper aperture 24 for entry of airfrom the pneumatic circuit for moving the cylinder.

Each gripper 16 terminates lowerly in a hook 25 intended for coupling to the flange 2 of the preform 1.

In order to lock this latter to the blowing head, the parts are arranged as shown in Figure 1, with the grippers 16 in their open position. Air is then fed through the aperture 23 into the cylinder 19, so causing it to descend until the wedge-shaped end 20 engages with the head of the grippers 16, which then close about the flange of the preform to assume the operating position shown in Figure 2.

In contrast, to open the grippers, air is fed through 24, causing the cylinder 19 to rise, and to disengage from the grippers.

In the closed position shown in Figue 2, the pneumatic seal between the blowing head and preform is provided by a gasket of the O-ring type 27, inserted in a seat formed from an annular groove 28 provided on the shaft 9 of the blowing head 4.

When the grippers are locked about the preform flange, the preform can be heated in orderto attain the suitable predetermined temperature for carrying out the subsequent final forming by blowing. For this purpose, the forming apparatus can be provided with a cylindrical infrared chamber of known type, which by means of an ascending movement rises until it completely contains the preform locked to the blowing head. This latter is then rotated at constant speed so that the heat emitted by the heating chamber is distributed uniformly over the entire surface ofthe preform.

Having attained the required temperature, rotation of the preform is interrupted, and the heating chamber is withdrawn therefrom by a descending movement.

At this point, the final forming mould is immedi atelyclosed aboutthe preform, to assume the operative position shown in Figure 2.

This mould consists of two half shells 5 and 6 which, when coupled together, define a spherical cavity for forming the final article. Having closed the mould by coupling the two half shells together by a converging translatory movement along their horizontal axis, the blowing head is lowered with a rapid movement until the preform becomes located inside the mould in the operating position shown in Figure 2. In this respect, it is important that, on termination of this operation, the peform flange locked by the blowing head grippers is in the immediate vicinity of, but not in contact with, the upper outer surface 7 of the mould. In this manner, a small gap is provided between the mould mouth and the preform neck in proximity to the flange.

The preform, which is at the predetermined temperature suitable for blowing, is now blown in the position shown in Figure 2.

Compressed air is thus fed into the preform through the conduit, with a throughput which increases progressively from a very low initial value close to zero, in accordance with an exponential pressure-time function. For this purpose, the blowing head can be suitably connected to a pressure timer automatically controlled by a flow controller connected into a secondary pneumatic circuit downstream of a pressure reducer.

The compressed air feed causes progressive deformation of the softened preform until it rests uniformly against the inner surface of the mould in order, when blowing is completed, to form a sphere 8 as shown in Figure 3, of uniform thickness and free from any defect.

On termination of blowing, the two half shells forming the mould are withdrawn bytranslatory motion along the horizontal axis. At this point, the spherical body 8, which has now hardened into its final configuration by cooling, is still retained by the blowing head by means of the grippers which act on the flange.

The grippers are then opened by the pneumatic movement heretofore described, until they assume the release position already shown in Figure 1.

The finished body is then recovered and sent to storage.

A second possible embodiment of the method according to the invention is as follows. Having prepared the preform by injection moulding, it is connected as quickly as possible to the blowing head, and its cooling in the operating position shown in Figure 2 is monitored.

Having reached the predetermined temperature suitable for its final thermoforming, blowing is carried out as heretofore described and iliustrnted in Figure 3.

This second embodiment is therefore more rapid and simple, but the physical-mechanical characteristics of the finished article may not be as satisfactory, in that the stress relieving of the material, which in the first embodiment is complete, is interrupted in this case before attaining ambient temperature when blowing is carried out, this leading to a slight tensile stress in the finished body. It is also apparent that there is the further advantage of saving of thermal energy in that the blowing temperature is attained by simply retaining the preform moulding heat.

In each case, from the aforegoing description it is apparent that the invention completely attains the initially proposed objects of providing hollowthermoplastic bodies of small aperture having excellent physical-mechanical characteristics such as strength, thickness uniformity etc., in an extremely rapid, simple and direct manner with an apparatus which is simplified and of low cost in comparison with the known art.

In particular, the concept of connecting the blowing head and preform together in a mechanically and pneumatically tight manner, and to blow the preform at a progressively increasing pressure while it is retained securely but independently by the mould, avoids the need to mechanially stretch the article during blowing, while equally obtaining excellent thickness uniformity characteristics.

Because of the simplicity and speed of the required operations, it is also apparent that the method of the invention is suitable for small series production of articles having excellent physical and mechanical characteristics.

Numerous modifications can be made to the invention as described.

Suitable materials for use in the method of the invention theoretically comprise all thermoplastic materials, and in particular polycarbonate, methacrylate and polypropylene.

The preform can also be prepared by moulding methods other than injection moulding, and using any type of flange, which does not necessarily have to be of flat cylindrical type.

Claims

1. A method for forming hollow thermoplastic bodies, such as lamp globes, provided with an aperture which is relatively small compared with the body dimension, characterised by comprising at least the following stages: moulding a cup-shaped preform having its open end provided with a flange; connecting said preform in a sealed manner to a blowing head which is able to abut against said flange and to lock it; inserting the preform connected to the blowing head into a forming mould until it reaches the operational position in which said flange is in proximity to, but not in contact with the outer surface of the mould; and subjecting said preform to a forming operation to produce the finished hollow body by blowing air at a pressure which increases progressively from an initial value close to zero, and at a predetermined temperature.

2. A method as claimed in claim 1, characterised by comprising the following stages: moulding said preform, cooling it to ambient temperature, heating it to said predetermined temperature for effecting the blowing, and carrying out said forming by blowing.

3. A method as claimed in claim 1, characterised by comprising the following stages: moulding said preform, waiting for it to cool to said predetermined temperature for effecting the blowing, then carrying out said forming by blowing.

4. An apparatus for carrying out the method of one or more of the preceding claims, characterised by comprising at least one blowing head terminating in a flanged rim arranged to abut against the flange of said preform, and a forming mould.

5. An apparatus as claimed in claim 4, characterised in that said flanged rim of the blowing head is provided with an annular seat arranged to receive a gasket for pneumatic sealing between the head and the preform.

6. An apparatus as claimed in claim 4, characterised in that said blowing head comprises a shaft, on the lower end of which there are mounted grippers for locking the preform flange.

7. An apparatus as claimed in claim 6, characterised in that the locking movement and release movement of said locking grippers are controlled by a double acting cylinder.

8. An apparatus as claimed in claim 7, characterised in that said double acting cylinder terminates in a wedge-shaped and arranged to effect the movement of said grippers.

9. An apparatus as claimed in claim 6, characterised in that said blowing head shaft can be rotated.

10. An apparatus as claimed in claim 9, characterised by comprising a heating chamber arranged to contain said preform when connected to the blowing head.