Efficient production of rubber medical stoppers by injection molding.

Rubber stoppers are used for the sealing of containers for pharmaceuticals and blood. These stoppers are termed piercing stoppers since the container's contents are used by piercing the seal with a needle after removal of the metallic protective cap. A hemispherical impression makes it easier for the needle or other device to be inserted in the middle of the stopper and in addition reduces the force required for piercing.

The articles are all constructed in a similar manner and are characterized by the maximum diameter at the head. The common sizes lie between 13 and 30 mm. In this region the stopper is later covered with a cap. The cap with stopper is pressed on to the vessel, which has been previously filled, in such a way that a seal is ensured.

To prevent infections, an absolutely clean surface must be ensured. Even the smallest fissures, burrs or fluff can be a potential health hazard. The surface areas, which come into contact with the substance in the vessel, may not have any furrows. Visible mold separation marks are not permissible in this part of the article. In addition, to avoid disruptions in the course of automatic filling, tight tolerances must be maintained.

Since pharmaceutical stoppers are normally single use articles, there is a continual demand for them. An idea of the size of this demand is given by the fact that a patient in a hospital for intensive medical treatment will use between 15 and 30 stoppers per day.

Rubber stoppers must be gas tight, elastic and temperature-resistant. Good resistance to the usual sterilizing agents is a further requirement. The most important requirement in respect to the material are the purity and uniformity of the elastomer. Butyl rubber is commonly used due to its low permeability to gases and vapors and its good resistance to aging. Both chlorinated as well as brominated butyl are used. In addition to the basic polymer, various auxiliaries and fillers are used. Since each individual component of an elastomer must be completely inert to the pharmaceutical to be sealed, the number of components to be employed should be reduced to a minimum. The use of additional processing aids increases the proportion of products capable of being exuded in the course of vulcanization, and such aids should only be used where necessary and then only in small quantities.

The method of manufacture has an influence on both the quality and cost of an article. In addition to the cost-effectiveness of a particular manufacturing route, special product requirements such as freedom from burring and the precise maintenance of dimensional tolerances must be taken into account when selecting the production route for pharmaceutical stoppers.

Processes currently employed are:

* Compression molding of the pad - characteristics: large output per hour; fair part quality.

* Injection molding by the ITM process - characteristics: high part quality; however, it is very expensive due to the high mold cost and the limited number of cavities.

* Injection molding of a pad - characteristics: good combination of high quality and high output particularly with two deck molds.

In the following sections, injection molding with the use of a pad will be described in more detail for the production of pharmaceutical stoppers.

The objective of the project planning phase is selection of the machines and molds required to produce the articles to be manufactured. In the case of the machine configuration, the special process requirements for the processing of butyl rubber and the needed output figures planned must be taken into account.

As a first step, the design of the mold cavities is defined. This gives the information necessary to determine the number of cavities which can be accommodated by a particular pad and mold. The arrangement of the cavities and the pad size are dependent on the flow behavior (viscosity) of the compound. This determines the injection pressure needed from the injection unit in order to fill the pads and cavities.

The internal mold pressure required for the particular compound limits the maximum pad projected area, since the total force within the cavities must be less than the clamping force. From this the maximum pad size can be calculated.

The minimum distance between the cavities depends on the punching process, the part contour and size, and whether cavity inserts are used in the injection mold or not.

Once the distance between the articles has been determined, the size and number of the pads is laid down in accordance with the clamping force of the machine. Fundamentally the diameter of the pad should not be too great in order to ensure that variations in pad thickness are small and that the flow paths are short. Good results have been obtained with pad diameters of up to 300 mm.

The size and number of the machines required for each production step are decided on the basis of the number of stoppers to be produced. All the machine requires for the fabrication of the stoppers should be arranged in a manner that takes into account the sequence of fabrication. The objective here is to achieve an unhindered flow of materials/articles with paths which are as short as possible. When designing the factory, consideration should be given to the cleanliness classes required for the individual rooms. In addition to the production rooms, rooms or areas must be provided for the power supply and other utilities equipment, laboratory, warehouse and maintenance equipment.

Injection molding machines of the type that are used for all common elastomers are suitable for the production of rubber infusion, injection and plunger stoppers.

When selecting a machine, one must differentiate between whether the pads will be removed manually or automatically. For manual removal, a vertical injection molding machine is advantageous since the platen can be accessed on three sides. With automated pad removal with the aid of a portal-type handling device, a horizontal machine is well suited. The size of the machine and its clamping force are determined mainly by the number of stoppers to be manufactured on it. Machines with a clamping force from 250 to 600 tons make it possible to have an economically large number of cavities in the mold in combination with the production of articles with a high level of dimensional stability. An injection unit which is capable of plasticizing the volume of material to be injected within the relatively short cure times should be selected. In order to be able to inject the material into the relatively thin pads, the injection unit should be capable of generating a pressure of at least 2,000 bar.

The high process stability and control required for the fabrication of pharmaceutical stoppers requires a control system which monitors all process and machine parameters and stores these to provide batch documentation.

If the demand for a particular article is large enough, double deck molds should be considered in order to achieve the most economical production. With such molds, the for the pads is injected through a cold runner in the center of the mold. The use of cold runner technology reduces the proportion of waste in the sprue and at the same time shortens cure times.

To successfully mold butyl rubber, particular attention must be paid during mold design to maintaining homogeneous temperature throughout the mold and in achieving optimum air evacuation. When being molded, butyl rubber tends to form air inclusions in the cavities. This tendency can only be eliminated by having multiple adequately dimensioned vacuum connections to each pad in the mold.

Even small differences in temperature have a great influence on the cure time of articles, as well as on the hardness of the finished product. A change in temperature of 10 [degrees] C can change the cure time by up to 50%. Uniformly high part quality can be achieved through the use of several independent heating zones, each with its own temperature monitoring and regulating system.

Punching tools are used for separating the pharmaceutical stoppers produced in the molded pad. The punching tool is loaded with the complete pad and the individual stoppers are punched out. The stoppers that have been punched out fall down into a container for further treatment. The punched-out pad is removed as waste. Pads with a diameter of up to 300 mm can be punched out in one operation. For pads of greater diameter, it is recommended that punching is carried out in a number of steps.

Eccentric presses are well suited for the punching process. It is important that the punching machine has an adequate level of parallelism and is equipped with a vacuum device. A soap solution is used as a lubricant during punching.

When constructing the punching tools, care must be taken that they match exactly to the injection mold. An offset in the region of just 1/10 mm between the position of a stopper on the pad and the position of its punch on the punching tool can lead to difficulties in loading the pads.

Punching tools for the punching of elastomeric articles must be constructed in accordance with the highest level of precision. In order to ensure that no burr is formed, the cutting gap between punch and bottom die may be no more than a few [micro]m.

The designing of the punching tool also includes the defining of the diameters of the holes in the bottom die for the positioning of the pads. Here two factors must be taken into account. On the one hand, the diameter of the hole in the bottom die must be larger than/equal to the maximum diameter of the article, while on the other hand, the prescribed maximum dimension of the article may not be exceeded.

If the compound is changed or if parts with a different contour have to be produced, it is often not possible to determine the processability, and thus the final design of the mold, without first carrying out full scale tests. For conducting these tests and for getting samples for customer approval, it is recommended that a test mold be built.

The objective with test molds is to be able to produce a limited quantity of samples at low cost under production conditions and to determine all the parameters that will arise in subsequent high volume production.

If a mold with pad and cavity insert plates is available, tests can be carried out under production conditions by just changing one insert plate.

If the objective is merely the production of samples with a new contour or using a new compound, and it is not necessary to determine molding parameters for production, the very simplest test molds fabricated without cavity inserts can be employed. If a test mold is built, attention should be paid to the fact that the flow relationships determined by pad size and thickness are comparable with those of the production mold to be built later. The ability to efficiently produce the sample articles can be determined from these trials.

In order to keep the amount of waste as low as possible, efforts are always made to keep the pad thickness to a minimum and to maximize the pad diameter. However, as pad thickness is reduced and pad diameter increased, the tendency to scorch during the injection process increases and becomes a limiting factor. To reduce scorching, the mixture should be of low viscosity and be formulated with low scorching in mind.

To achieve the prescribed article dimensions, the mold must be manufactured taking into account the shrinkage factor. The shrinkage arises mainly through the change in temperature which takes place as the demolded article cools. In addition to the magnitude of this temperature difference, the amount of the shrinkage also depends on the chemical constituents of the compound.

When bromobutyl rubber is being converted, the shrinkage factor lies between 1.5 and 2.5%. To determine shrinkage precisely for the fabrication of injection molds and punching tools for volume production, samples produced by injection molding must be measured. If it is a matter of rotationally symmetrical articles and if the pad is also round, orientation of the shrinkage in the X/Y directions does not have to be considered. To achieve a reliable result, elastomeric articles should be measured optically without a testing force being applied (i.e., without contact).

A change to the components of a compound or to the method of manufacturing changes the process parameters and they must be dete rmined again.

To minimize the time required before production, the parameters relevant for fabrication should be determined as far as possible on the test mold. The results obtained from preliminary results with the test mold can be reliably applied to the production mold when the article contour, pad size and pad thickness of the production mold and of the test mold are identical.

Important parameters are the maximum processing temperatures and injection speeds which should be selected to minimize cure time without the mixture scorch. The internal mold pressure necessary must be determined in order to determine the clamping force of the injection molding machine required to overcome the mold-opening force produced by the internal mold pressure and the given projected area of the mold.

Various auxiliaries, such as mold release agents and talcum to prevent the uncured rubber strips from sticking together, are required for the production of rubber stoppers The auxiliaries to be used in production should be determined and used during the testing phase. As a result of their chemical composition, many mold releases are not suitable for medical use.

To make sure that there will be no interaction between the auxiliaries used for the fabrication of the stoppers and the substances with which they will come into contact, the auxiliaries used must be approved before they are put into use for production.

Deposits are formed on the pad and cavity surfaces of a mold during production. The extent to which this occurs depends both on the properties of the compound and on the processing parameters. In order to ensure that the quality of the finished product remains uniform, the molds must be cleaned at preset intervals. The length of time which can be permitted between one cleaning process and the next can also be determined from tests with the test mold.

For the initial start-up, the injection molding machine must be equipped with all the necessary accessories such as cold runner temperature regulating units, cure time optimization systems, etc. An essential precondition for the checking of the machine functions and the optimization of the process parameters is that the initial start-up is carried out with the production mold installed and with the proper compound.

To enable the different parts of the mold to be heated homogeneously, modern injection molding machines have a number of heating zones for each heating platen and a self-optimizing system for the temperature controllers. The self-optimization system has to be matched to the mold in the course of the initial start-up.

When all the measures for a stable production cycle have been taken, production should be carried out for a number of cycles without interruption.

After the plant has been successfully started up, the final values for the process parameters are determined starting from the values obtained in the preliminary tests. For optimization, it is essential that the systems work in a continuous process without disruptions.

The objective of process optimization is that of realizing a high output of articles (short cure time) in combination with the certainty that the required quality will be maintained. Here the mutual interactions between the different parameters must be taken into account.

Once the optimum production parameters for the production of an article have been determined, they should be stored in the machine control system and safeguarded against being improperly changed by the operator. The most important items of information for the operating of the process and the evaluating of the article quality should be put together in a sequence plan and put up at the work place or on the machine.

The justification for automation is reduction of production costs while maintaining quality. Counterbalancing the advantage of reduced direct labor is the higher capital costs of automation and often reductions in flexibility. Consequently, automation should be considered when similar products have to be produced in large batches.

In the molding of rubber stoppers, injection molding machines offer the best preconditions for automation. The injection molding process can be set up in such a way that the operator does not have to intervene in the production process. Potential for automation lies in the removal of the pads and in the application of the mold release. The pads can be removed from the injection mold with a handling device mounted on the machine. The pad is gripped at a number of points and demolded with a peeling movement. The mold release for the next cycle is then applied from spray nozzles mounted on the handling device.

Acknowledgements

"Water-based SPMRAs: Improved productivity, quality and environmentally safe" is based on a paper given at the October, 1998 International Rubber Conference.

"Designing EP(D)M structure to achieve exceptional processibility and product attributes" is based on a paper given at the September, 1999 Rubber Division meeting.

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