Tecnoextr S.r.l. - Guarnizioni Industriali Estrusi e stampati in elastomeri

The moulding processing of technical articles in rubber requires essentially a press, which may be designed to open its workplanes with a vertical or horizontal linear motion of translation, a mould and a serie of successive operations to implement an added value to the product and to grant the respect of several requisites.
The process of moulding may be brought back to three different technologies:
• Compression moulding
• Transfert moulding
• Injection moulding

Compression moulding may be considered as the most traditional method to get technical moulded rubber parts.
It is still used because of its versatility and of the cheaper cost of the dedicated moulds and requires an horizontal workplanes press.
Slabs of uncured rubber in a semi-finished shape are positioned inside the opened and thermically conditioned mould.
Closing the workplanes, the material, which has been carefully weighted before so to slightly overfill the whole cavities of the mould, flows inside the mould and vulcanize itself due to high temperature and pressure.
After a certain period, which may vary depending upon the material being used and the volume of the part, the mould is opened and the parts released. Transfert moulding is an evolution of compression moulding and it has been developed to consent rubber to enter

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inside a mould in a closed position, to get such those parts whose shape prevent use of a traditional compression moulding.

During this process, to be run for an obvious reason on board of the same kind of press used for compression moulding, while the mould is closed a raw rubber slab is positioned inside a chamber, or more, on the upper plate of the mould. This chamber communicates with the cavities inside the mould thanks to capillary runners.
While the press workplanes close, the material is pushed through the runners and injected inside the mould cavities.
Injection moulding, a well balanced mixture obtained adding to the traditional technology an hydraulic unit for the breakdown, batching and injection of rubber, has brought the necessary streamline which has made possible the production of massive series of rubber parts, granting the repeatibility of the process.

 

The press for injection moulding may open horizontally or vertically, because it is no more necessary to position the slabs inside the mould exploiting the gravity force.
The material is batched ed injected inside the mould while it is closed and its cavities are reached and overfilled thanks to runners machined on the surface of the mould plates or sometimes thanks to injection before the mould has been completely closed.
This process shows as mentioned before the highest repeatibilty and reliability, guarantees high and constant quality standards and makes the moulding cycle possible of being automated, parts releasing included.

 

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Tecnoextr s.r.l. uses all of the three moulding techniques mentioned here above, being at its own disposal a complete press yard, reliable and constantly updated, so, depending upon the required article, our technical department may suggest the most favourable and suitable process.
Depending upon the kind of process being used, the lay out of the mould changes consequently.
Anyhow it is mostly the article shape to suggesting and sometimes to imposing the moulding technique and the best solution one may adopt for the lay out of the mould.
Being obvious that simple geometries may be moulded with any technique, in this case the best solution consists of the cheaper solution possible or it depends even from the available kind of press.
The mould lay out changes if choosing a compression or an injection moulding.
It results intuitive that the cavities inside the mould for compression moulding are differently positioned than in a mould for injection.
In a mould for compression moulding the designer fills as much as possible its surface with cavities, which are usually positioned according longitudinal axis, while in an injection mould instead cavities are positoned to be reached simultaneously and as soon as possible by the rubber flow, so at the end of the runners machined on the mould surface.
As a first conclusion it is allowed to affirm that it is possible to get more cavities in a compression mould than in an injection one with identical dimensions and so that it is possible in theory to get more parts per unit time.

A correct balance between the two techniques is reached while taking in consideration that the single press cycle is usually faster for an injection press, even because the only manual operation in this case is the releasing of parts from the mould, sometimes it is automated too, and that rubber slabs for compression have to be carefully weighted one by one. Process costs are often balanced, sooften a solution is chosen evaluating several
other variabilities.

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The list of these variabilities may be wide, for instance due to the presence of cores it may be necessary to inject rubber while the mould is closed, as it is made for the manifacturing of bellows or moulded hoses.
For other reasons, for instance the moulding of grommets or parts with strong undercuts, the article shape in itself requires a long time to release the parts, so it is unuseless to run a fast injection cycle if it impossible to exploit it and it is better to increase as much as possible the number of cavities in a compression mould.
Sometimes, due to the intrinsic chemical structure of rubber materials, compression moulding is preferred because the preformances of a part moulded this way may be better than by injection.
During injection rubber has a low viscosity and it flows rapidly inside the mould runners.
Sometimes under these conditions the polymer chains may break or it is furthermore possible the material to scorch too rapidly generating an unperfect vulcanization.

Talvolta, per motivi legati alla struttura stessa dei materiali polimerici e della gomma in particolare, lo stampaggio a compressione è preferibile in quanto le caratteristiche meccaniche del pezzo stampato possono essere migliori di un omologo prodotto ad iniezione.
Durante lo stampaggio ad iniezione, infatti, il fuso viene iniettato a temperatura e pressione elevata ed a conseguente bassa viscosità nei canali ricavati nello stampo. In queste condizioni e con determinati presupposti, nei polimeri possono spezzarsi le catene di cui sono composti oppure può essere indotta la vulcanizzazione in punti ed in tempi non desiderati e può prodursi quindi una struttura imperfetta nel vulcanizzato, conseguenza del flusso troppo spinto che l’ha generata.

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Last but not least, but it is still underscored that one has to consider the highest possible number of variabilities and cases, a decision must take in account the cost of the moulds and it is undeniable that an injection mould is more expensive and that it needs more than a compression mould an accurate tuning and set up to get the required results.
All of these considerations and the solution of these kind of technical problems are property of our technical department, skilled personnel whom our customers ask for their know how to understand choices been made for the manufacturing of moulded articles.
As previously discussed, a mould lay out, so the amount of cavities, the number of plates and their relative motion is closely related to the geometry of the part to be manifactured.
Due to this reason, even if at a first sight it may seem of a secondary importance, it is fundamental to knowing the function of the moulded part and to being informed as much as possible about what is has to be in contact with the part and about forces, pressure, service temperature, life expectation and media.
One of the first analysis to be made by the mould designer concerns the positioning of the mould parting lines, which subdivide the mould into different plates and components, i.e. cores. Close to these lines are machined the cutters, necessary volumes which receive the overfill of rubber from the cavities.

The rubber flash connected to the parting lines may be detrimental to the part functionality and it has to be removed by deflashing and this is why it is better to know in advance which are the sealing surfaces of the part.
If the mould designer ignores the positioning of the sealing areas, it is possible to get a wrong mould design or, on the contrary, absolutely unuseless or expensive solutions chosen for safety to avoiding flash as much as possible.

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The extreme difference between the coefficient of linear thermal expansion of a carbon steel used for moulds and of a rubber material, more or less ten times more for rubbers, makes visible especially while releasing parts a characteristic phenomenon known as shrinkage.
It is observed that the moulded part is smaller than the cavities, so it hangs over cores and somehow releases itself from hollows.
This phenomenon, which varies depending upon the hardness and the material being used, influences the mould design because the dimensions of the cavities have to be increased of a certain percentage.
Furthermore a mould designed for a certain material and hardness usually cannot be used for others.
After the moulding process, if required the parts may be subjected to further treatments.
First of all, depending upon the polymer being used, a postcuring in an hot air oven is required to completing vulcanization, to releasing cure agents completely and to increasing so the performances of the parts.
At the end of this phase a further shrinkage is noticed, even if usually of a minor entity than what it is shown after moulding.
The phase of deflashing may take place before or after postcuring and by means of several different methods: handmade, if the cutters of the mould are able to finish accurately the surfaces, using nitrogen to get a fragile flash, but only if on the part thin sections are missing, by traditional tumbling, widely used for rounded geometries like o-rings, by die cut, when certain causes do not allow the other techniques.
The last but most important operation to be done is the visual inspection of the parts.

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Control plans requires the moulded parts to be continuously checked during manifacturing, so they are inspected after being released from the mould to correct the soonest incoming defects and later on they are sorted by skilled personnel, according the quality standard and quality level required.
An abstract from a control plan actually applied by Tecnoextr s.r.l. and according ISO 2859 is shown here below.
Depending upon the quality level required, sorting is made starting from a sample amount of parts until the whole production batch, always according clear instructions.
Tecnoextr s.r.l. manufactures technical articles in rubber using the widest material range offered by the market, of a certified quality and constantly checked since its consignment to compounding and manifacturing, by means of continuous laboratory tests.
To what it may concern the available materials and their own characteristics and application go to the introduction of this catalogue.
Material properties shown are anyhow related to standard compounds and so it is suggested for specific cases or further information to get in contact with our technical department.
Specific international standard norms, as per extruded parts, must be applied to get the correct dimensional tolerances for mouldings.
Tecnoextr s.r.l. applies and suggests the use of dimensional tolerances, with the exception of o-rings whose norm is different, according UNI ISO 3302-1 class M2.

 

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