In a previous post, we outlined the advantages of compression molding, which include low cost, little waste of material and suitability for manufacturing large parts. In today's post, we delve into the compression molding process.
Compression molding, one of the oldest techniques used for processing polymers, is the act of using compression and heat to shape the raw material (in our case uncured rubber compounds) by means of a mold.
The process involves four stages:
1. A mold comprised of high-strength metal is designed and machined to the exact dimensions needed to produce a finished product fitting exact specifications.
The mold generally consists of two parts: a top plate and the cavity. The top plate defines one side of the part while the cavity defines the other. The mold is installed in a press and heated in such a way as to control the temperature through the entirety of the process.
2. The uncured rubber compound is pre-formed to a controlled weight and shape that resembles the contours of the end product. The pre-forms provide a sufficient amount of surplus material to ensure the cavity is completely filled. Any excess material is channeled into overflow grooves and discarded at the end of the molding process.
3. The operator places the pre-formed material into a mold, which is then closed with intense top force so that the material is compressed into contact with the mold areas. The combination of heat and pressure causes the rubber to flow and fill out the entire mold cavity, thus forming the finished part.
Compression temperatures can range from 140 to 200-degrees centigrade. Mold pressures vary from 20 to 700 bars, and curing times can alternate from one minute for thin parts to more than one hour for very thick rubber parts.
Depending upon the type of rubber utilized, and the particular part being manufactured, an exact cycle time is established to reach an optimum level of cure.
4. Once cured, pressure is released, and the custom molded rubber product is ejected from the mold ready for use.
Four primary factors contribute to the successful compression molding process:
- Amount of material used
- Heating time and technique
- Force applied to the mold
- Cooling time and technique