Rubber Molding is a molding process in which useful rubber products from elastomers or uncured rubber products are produced. An uncured rubber or elastomer is a material that has the tendency to return to its original shape when high amount of pressure or distortion is applied on the product. Elastomer can be obtained from natural resources providing that resource highly controlled chemical reactions. They are synthetic in nature. Rubber is the best product for the processes in which the materials need to expand and then contract to its normal state.
In Rubber molding process, the uncured rubber is inserted in the rubber molding metal cavity in order to produce the molded rubber for use. They are exposed to high amount of heat followed by various chemical reactions. There is no specific method to create molded rubbers, most of the companies or industries use heat and pressure for production of molded rubbers. Four common steps in which the whole Rubber Molding process can be comprised is briefly described in the sections below
- Injection Molding
- Compression Molding
- Transfer Molding
In injection molding process, the rubbers are placed under heat and with more pressure per square inch of cavity surface so that they could be molded into any shape as required. This process was mainly used for plastic molding in mid 1960’s but with proper experiments and trials plastic molding method was modified to injection molding; it is different from plastic molding because in plastic molding the material is cooled under less pressure.
In this process, the strips are made of the bulk in which the material is mixed with having a dimension of 1.25” wide and 3.75” in length. These strips having the mentioned dimensions are then inserted into screws that fill barrel having the required amount of rubber material from before.
- There is no need to put any effort or labor in the pre-form as the final product gets highly affected due to little bit error in shape and weight.
- As there is no pre-form in this process the need of operator is also vanished to place the pre-form into the cavity like compression and transfer molding.
- The process of pre-heating the material by the injection screws before pressing it into cavity decreases the viscosity and increases the material flow into cavity.
- Total time of the whole process is reduced.
- Flash less tooling.
- Minimum amount of material is wasted and this makes it a economical process.
Another process of Rubber Molding is compression molding. In this method, the rubber compound or raw material is provided with the shape of the end product. This is known as pre-form stage. In this, the pre-form is inserted in the cavity in a manner that the cavity is fully intact. Once both the cavity and pre-form are intact, then the required amount of heat and pressure is provided to the compound to fill the cavity. The excess pre-form compound gets spilled out after the proper filling of the cavity. After this, the rubber is usually de-molded by using hands which leaves behind the molded compound.
- For medium precision, it is an economical process
- Cavity count is maximum
- Cost effective tooling
Transfer molding is like compression molding but there are slight variations. In this process, the secondary raw material is also formed in the pre-form shape and then these pre-forms are placed upon the ‘pot’ located in between the top plate and the plunger. After proper placement, the mold is then closed and the material is compressed by plunger. It is then transferred through spurs to the cavity hole.
- This process requires fewer and simpler pre-form.
- Minimal cost of tools
- From medium to high precision component it is most economical process
- Excess mold doesn’t get spilled out like compression molding, and in this way it has more control of dimensional tolerance.
In this process of Rubber Molding, the removal of excess mold or flash takes place. Various methods are used depending upon the quantity, tolerance level, part size etc:-
- Precision Grinding
- Cryogenic Processing
- Manual Tear Trimming.
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