Rubber molded products refer to a diverse group of vulcanized rubber
products which are manufactured in a mold and the desired size and shape is
obtained. Molded rubber products include diaphragms, vibration isolation
devices, air springs, bushings, all kinds of pads, boots, wiper blades,
chassis bumpers, fascia, conveyor wheels, grommets, and more. They are used
in a wide range of services.
Compounded rubber is transferred, injected or
simply put into a heated molds, and cured to obtain required size and shape
Factors to consider for molded rubber
Molded rubber needs to maintain certain tolerance standards to produce the
finished products. There are many factors which affect tolerances in the
manufacturing of molded solid rubber products. These factors are peculiar to
the rubber industry and are given below:
Types of Rubber Molded Goods
This is the difference between corresponding linear dimensions of the
mold and of the molded part. All rubber materials display some amount of
shrinkage after molding when the part cools. The mold designer and the
compounder must measure the amount of shrinkage and which is
incorporated into the size of the mold cavity. Even though the mold is
built to anticipate shrinkage, there is always an inherent difference
which must be covered by adequate dimensional tolerance. Complex shapes
in the molded rubber goods may restrict the lineal shrinkage in one
direction and increase it in another. The rubber manufacturer always
aims at minimizing these variables, but they cannot be eliminated
- Mold Design
Molds can be designed and built to varying degrees of precision at
different cost. With any type of mold, the mold builder must have some
tolerance. That is why, each cavity has some variance from the others.
Dimensional tolerances on the rubber product must include allowances for
this fact. Most molded rubber goods are made in two plate molds and the
complicated ones require three or more plates.
- Trim and Finish
The main purpose of trimming and finishing operations is to remove
rubber material, like flash, which is not part of the finished product.
This is possible often without affecting important dimensions, but
insome cases, some material is removed from the part itself.
Most insert materials like metal, fabric, plastic, etc. have their own
standard tolerances. When designing inserts for molding to rubber, other
factors like fit in the mold cavities, location of the inserts with
respect to other dimensions, proper hold spacing to match with mold
pins, the room temperature must be considered.
Since rubber is a flexible material affected by temperature, distortion
can occur when the part is removed from the mold or when it is packed
for shipment. This distortion makes it difficult to measure the parts
accurately. However, some distortion can be minimized or removed by
storing the part as unstressed as possible for 24 hours at room
- Environmental Storage Conditions
- Temperature: Rubber changes in dimension with
changes in temperature. It is necessary to specify a temperature at
which the parts are to be measured and the time required to
stabilize that part at that particular temperature.
- Humidity: There are few rubber materials which
absorb moisture. Hence the dimensions of the products are affected
by the amount of moisture in it. This can be minimized by
stabilizing the product in an area of controlled temperature and
humidity for not less than 24 hours.
- Dimension Terminology
- Fixed Dimensions are not affected by flash thickness variation.
- Closure Dimensions are affected by flash thickness variation.