Manufacture of plastic containers

The present invention relates to an improvement in a method for thermoforming of containers from thermoplastic materials in which method the rim of the container is formed as an integral part of the pre-form from which the container is thermoformed. The pre-form is obtained by introducing thermoplastic material into a closed space and then mechanically compressing the material. The improvement of the present invention relates to improved uniformity of flow of the thermoplastic material into the closed space, the use of a closed space defined by surfaces having a surface roughness in the range of 50-125 .mu.m rms and to a modified sequence of steps for forming the pre-form in which the compression of the thermoplastic material is commenced prior to the completion of the introduction of the material into the closed space. The process may be used in the manufacture of containers, e.g., for the packaging of margarine, from polyethylene or polypropylene.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for the manufacture of plastic 
containers, for example, containers for margarine, and especially to an 
improved method for the thermoforming of such containers from polyethylene 
or polypropylene. 
2. Description of the Prior Art 
Methods for the manufacture of plastic containers are known in the art. In 
particular, in U.S. Pat. No. 3,602,946, issued Sept. 7, 1971, G. E. 
Curetti, et al. describe a method which involves the formation of a 
diaphragm of plastic material in a transfer ring with the periphery of the 
diaphragm being retained in an internal annular groove of the ring. The 
ring containing the diaphragm is applied to a moulding station where the 
diaphragm is moulded into a container. The container is then severed from 
the plastic material retained in the groove, the annulus of plastic 
material so formed being discharged to waste or re-used. 
In U.S. Pat. No. 4,005,164, issued Jan. 25, 1977, B. Proctor describes a 
method for the manufacture of plastic containers with integral rims in 
which plastic material is compressed into an annular groove in a transfer 
ring. The material in the annular groove forms the integral rim of the 
container on moulding. In the process described by Proctor an annulus of 
plastic material is not formed. 
The method and apparatus described by Proctor have had significant 
commercial success with a number of thermoplastic materials, for example, 
polystyrene. However, containers manufactured from polyethylene or 
polypropylene have been found to be capable of substantial improvement, 
especially with regard to the characteristics of those containers, e.g., 
aesthetics and uniformity of the thickness of the walls of the containers. 
A method for the manufacture of plastic containers from polyethylene or 
polypropylene so as to obtain containers of improved quality has now been 
found. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides an improvement in a method for 
the manufacture of plastic containers comprising: 
A. forming a pre-form of thermoplastic material in a transfer means, 
B. applying the transfer means carrying the preform to a mould having a 
cavity corresponding to the shape of the container, 
C. forming the pre-form into a container in the mould cavity, and 
D. separating the container from the cavity, the improvement comprising (a) 
forming the pre-form by introducting into a closed space between opposite 
sides of the transfer means a predetermined amount of a thermoplastic 
material selected from the group consisting of polyethylene and 
polypropylene, the material being introduced through a passage of at least 
0.50 cm diameter in one side of the transfer means, and the closed space 
being defined by surfaces having a surface roughness in the range of 
50-125 .mu.m rms, and (b) mechanically compressing the thermoplastic 
material within the closed space by applying pressure from the other side 
of the transfer means prior to completion of the introduction of the 
material into the closed space, and completing the compression after the 
passage is closed. 
As used herein, the expression "rms" means root means square and is used 
with reference to the measurement of the roughness of the surface of the 
closed space described herein on the basis of the average deviation from 
the mean position of the surface. 
The present invention relates to a method for the manufacture of plastic 
containers that is generally referred to in the trade as thermoforming. 
The thermoforming process to which the present invention relates consists 
of three basic processes performed on a horizontal indexing table. The 
three processes are normally performed simultaneously on different 
pre-forms of thermoplastic material. A particular example of such a 
thermoforming process is described in detail in the aforementioned patent 
of B. Proctor. 
In general terms, a reciprocating extruder meters a predetermined amount of 
molten thermoplastic material into a closed space in a compression mould. 
The molten thermoplastic material is then compressed to form a pre-form. 
The pre-form is applied to a forming station where a plug-assist is 
normally used to stretch the pre-form, the stretched pre-form then being 
conformed to the shape of a mould by means of air and vacuum. The 
container so obtained is then transferred to an ejection station where the 
container is removed. The cycle is then repeated.

Referring to FIG. 1, a compression mould, generally indicated by 1, is 
comprised of upper transfer ring 2, lower transfer ring 3, transfer plate 
4, compression ring 5, compression piston 6, injection plate 7 and conduit 
8. Transfer plate 4 is formed with a recess having an internal lip 9 which 
can support upper transfer ring 2 and lower transfer ring 3. Retaining 
groove 10 is formed between upper transfer ring 2 and lower transfer ring 
3, groove 10 forming the edge of closed space 11. Closed space 11 is 
primarily formed by the lower surface of injection plate 7 and the upper 
surface of compression piston 6. 
Conduit 8 is equipped with a passage 12 which, in FIG. 1, is in fluid-flow 
communication with inlet 13. Conduit 8 is positioned in recess 14 which is 
centrally located in injection plate 7. Inlet 13 is located in recess 14 
and communicates with closed space 11. Inlet 13 is of a similar but 
slightly larger diameter to passage 12, but increases in diameter as it 
merges with closed space 11. 
The upper surface of compression piston 6 conforms to the lower surface of 
injection plate 7, the upper surface of compression pistons 6 having a 
projection 15 corresponding to inlet 13. 
In FIG. 2, thermoplastic material 16 is shown in conduit 8, passage 12, 
inlet 13 and part of the closed space. The closed space, designated 17 in 
FIG. 2, is of smaller volume, even including that part of thermoplastic 
material 16 that is within the closed space, than closed space 11 of FIG. 
1 as a result of compression piston 6 being closer to injection plate 7 in 
FIG. 2 than in FIG. 1. 
In FIG. 3, conduit 8 is located in recess 14 so that passage 12 is not in 
fluid-flow communication with inlet 13. Compression piston 6 and injection 
plate 7 are spaced apart so that the thermoplastic material forms pre-form 
18. In the embodiment shown, the thickness of pre-form 18 is the same as 
the thickness of retaining groove 10. 
Although not shown herein, the apparatus shown in FIGS. 1-3, is used in 
conjunction with means for forming a container from pre-form 18 and means 
to eject the container so formed from the apparatus. Such means are shown 
and discussed in the aforementioned patent of B. Proctor, the disclosure 
of which is incorporated herein by reference. 
DETAILED DESCRIPTION OF THE INVENTION 
In operation, compression piston 6 is initially in the spaced-apart 
relationship with injection plate 7 that is shown in FIG. 1. Conduit 8 is 
located so that passage 12 is in fluid-flow relationship with inlet 13. A 
predetermined amount of molten thermoplastic material 16 is injected from 
conduit 8 through passage 12 and inlet 13 into closed space 11. At the 
same time as the thermoplastic material 16 is injected through passage 12, 
compression piston 6 is moved towards injection plate 7. The injection of 
thermoplastic material 16 and the movement of compression piston 16 is 
coordinated so that at all times during the formation of pre-form 18 shown 
in FIG. 3, the thermoplastic 16 is in a state of flow. In particular, the 
front of thermoplastic material continuously advances through closed space 
17. When the required amount of thermoplastic material 16 has been 
injected into closed space 17, conduit 8 is moved out of fluid-flow 
communication with inlet 13. Compression piston 6 continues to advance 
towards injection plate 7 until closed space 17, including retaining 
groove 10, is completely filled, thereby forming pre-form 18. Compression 
piston 6 moves until it is at a predetermined distance from injection 
plate 7. 
Subsequently, using apparatus not shown, pre-form 18 is applied to a 
moulding station using upper transfer ring 2 and lower transfer ring 3 so 
that a moulded container may be formed. The moulded container is then 
transferred, using upper transfer ring 2 and lower transfer ring 3, to an 
ejection station where the container is ejected. 
The thermoplastic material of the present invention may be polypropylene or 
polyethylene. The polyethylene may be a homopolymer of ethylene or a 
copolymer of ethylene and a higher .alpha.-olefin, especially a higher 
.alpha.-olefin having 4-8 carbon atoms, e.g., butene-1, hexene-1 or 
octene-1. Such polymers must have the physical characteristics necessary 
to permit the thermoforming process to be carried out. Preferably, the 
polyethylene is a homopolymer or copolymer of ethylene having a density, 
as measured by the procedure of ASTM D-1505, in the range 0.940-0.970 
g/cm.sup.3 and a melt index, as measured by the procedure of ASTM D-1238 
(condition E), in the range 0.4-1.0 and especially in the range 0.4-0.8. 
It is important that passage 12 have a diameter of at least 0.5 cm in order 
to facilitate a uniform flow of thermoplastic material through passage 12 
and inlet 13 into the closed space. Moreover, inlet 13 should be reamed 
out, as shown in the drawings, so as to not restrict the flow of material 
into the closed space. If the flow is not uniform, melt fracture of the 
polymer may occur, thereby resulting in the manufacture of containers that 
tend to exhibit so-called stress lines, as well as variations in the 
thickness of the walls thereof and which are capable of significant 
improvement aesthetically. Melt fracture is a phenomenon known in the art 
relating to non-uniform flow through an orifice as a result of high shear 
stresses in the polymer. The resultant polymer extrudate has a rough 
exterior surface which manifests itself as visible stress patterns in the 
containers manufactured therefrom. 
The entire surface of the closed space that comes into contact with polymer 
must be a uniformly roughened surface. In particular, the surface must 
have a surface roughness in the range 50-125 .mu.m rms, especially 80-100 
.mu.m rms. A surface roughness in the range 85-95 .mu.m rms is preferred. 
Such a surface may be obtained by wet sandblasting a surface of the closed 
space having a surface roughness of less than 50 .mu.m rms. 
An important feature of the method of the present invention is that the 
thermoplastic material is injected into closed space 17 at the same time 
as the compression piston 6 compresses closed space 17. Such simultaneous 
injection and compression is carried out in a manner that ensures that the 
front edge of polymer, i.e., the polymer/air interface, is continuously 
advancing. As is exemplified hereinafter, if the front is permitted to 
become stationary at any time, the resultant container is capable of 
improvement, especially aesthetically. A line of differing optical 
properties, a so-called haze line, and variations in thickness tend to 
occur. 
The method of the present invention may be used in the manufacture of 
thin-walled plastic containers, for example, containers for margarine, 
tubs, cups, flower pots, and the like, from polypropylene or polyethylene. 
The present invention is illustrated by the following examples: 
EXAMPLE I 
Apparatus for the thermoforming of containers and of the type generally 
described hereinbefore as being of the prior art, especially that of the 
patent of B. Proctor, was obtained. The apparatus was a MONAFORMER.RTM. 
140/30 obtained from the Hayssen Manufacturing Company of Sheboygan, Wis., 
U.S.A., and was adapted for the manufacture of containers of the type used 
for the packaging of margarine. 
The apparatus was tested using polystyrene (DOW.RTM. type t8526) and was 
shown to produce 0.25 liter and 0.5 liter containers of good quality. The 
apparatus appeared to be relatively insensitive to changes in operating 
conditions, containers of good quality being thermoformable from 
polystyrene over a wide range of operating conditions. However, subsequent 
attempts to produce 0.5 liter containers of good quality from 
polypropylene and polyethylene were unsuccessful. 
In order to thermoform polypropylene and polyethylene, the following 
modifications were made: 
Modification A--the diameter of the passage was increased from 0.295 cm to 
0.625 cm and the inlet was reamed out so that the angle of the surface of 
the inlet to the axis of the passage was increased from 41.degree. to 
55.degree.. 
Modification B--the surfaces that form the closed space were wet 
sandblasted to a surface roughness of 85-95 .mu.m rms, the roughness prior 
to wet sandblasting being ca. 10-15 .mu.m rms. 
Modification C--the sequence of steps for forming the pre-form was altered 
so that the mechanical compression of the thermoplastic material commenced 
prior to the completion of the introduction of material into the closed 
space instead of after completion of the introduction of material. As a 
result, the thermoplastic material was in a state of flow at all times 
between the commencement of introduction of material into the closed space 
and the completion of the forming of the pre-form. 
The apparatus and process modified according to A, B or C above was tested 
in the manufacture of 0.5 liter containers using a number of polyethylenes 
as the thermoplastic material. Further details of the tests and the 
results obtained are given in Table I. 
TABLE I 
______________________________________ 
Poly- 
mer 
Melt 
Temp- 
Poly- Modifications** 
erature 
Run mer* A B C (.degree.C.) 
Comments 
______________________________________ 
1 1 yes yes yes 272 Good quality 
container, 
uniform high 
gloss finish. 
2 1 no yes no 272 Container 
showed a 
haze line. 
3 2 yes yes yes 272 Good quality 
container, 
uniform matte finish. 
4 2 yes no no 272 Container 
showed stress marks 
and was not of 
uniform thickness. 
5 3 yes yes no 204 Melt fracture 
at inlet, 
container had 
haze line. 
6 4 yes yes yes 272 Good quality 
container. 
7 4 yes no no 272 Container 
showed a 
haze line. 
______________________________________ 
**runs in which all three modifications had been carried out are of this 
invention. Other runs are included for comparison. 
*Polymer 1--ethylene/butene1 copolymer of a density of 0.956, a melt inde 
of 1.0 and a stress exponent of 1.30. 
Polymer 2--ethylene homopolymer of a density of 0.960, a melt index of 0. 
and a stress exponent of 1.8 (SCLAIR.RTM. 19A). 
Polymer 3--ethylene homopolymer of a density of 0.960, a melt index of 7. 
and a stress exponent of 1.26 (SCLAIR.RTM. 2908UV). 
Polymer 4--Polymer 1 plus 1% TiO.sub.2. 
EXAMPLE II 
The procedure of Example I was repeated using polypropylene as the 
thermoplastic material. Details of the tests and the results obtained are 
given in Table II. 
TABLE II 
______________________________________ 
Poly- 
mer 
Melt 
Temp- 
Poly- Modifications** 
erature 
Run mer* A B C (.degree.C.) 
Comments 
______________________________________ 
8 5 no no no 272 Fair container, 
showed streaks 
and stresses. 
9 6 yes no no 204 Good container, 
some evidence 
of stresses. 
10 7 yes no no 210 Good container 
but haze line. 
11 7 yes yes yes 272 Good quality 
container. 
______________________________________ 
**runs in which all three modifications had been carried out are of this 
invention. Other runs are included for comparison. 
*Polymer 5--AMOCO.RTM. polypropylene 506104. 
Polymer 6--AMOCO.RTM. polypropylene 8104. 
Polymer 7--PROFAX.RTM. 7523 polypropylene.