What is Compression Molding? - an Overview

Compression Molding

Compression Molding, also known as Silicone Compression Molding, Rubber Compression Moulding and Compression Forming, which is the process of placing a given amount of prepare & uncured solid silicone rubber material into a mold cavity, then close the mold, applying both heat and pressure to the solid silicone and allowing it to fill the cavity. When the cavity is filled, excess pre-form material spills out into overflow grooves, when the time is up, the mold will open, then demolded the  silicone rubber compression molded parts by hand. Silicone Compression moulding has been used to mold silicone rubber parts since the beginning of the rubber manufacturing industry. It is one of the oldest methods still used to mold silicone rubber parts.

How Does Compression Molding Work?

To complete compression molding process, there are steps as follows:

• Make mold tools - prepare steel material, complete the product & mold design

• Prepare raw materials - Selected the type of material you want to use and calculate the proper amount of material. If too much material, excess material will seep out of the cavity and result in big flash which would be difficult to remove.

• Set up the machine parameters - after the mold is well-done, can get ready for the sampling to ensure the part dimension and surface is what we required, need to set up the machine parameters and heat up the mold.

• Placing prepare raw material - Place the prepared raw material in the center of the bottom mold.

• Compress Molding part - Close the mold tool, apply both heat and pressure, and wait for the part to form.

• Demould - Remove the molded silicone rubber products from the mold manually.

• Tear trimming & Quality Inspection - Resin flash around the edges/ holes must be manually cut off or removed.

Notes:

There are some ways to remove the waste edge or flash from a molded rubber product:

• Tear trimming manually

• Cryogenic processing

• Tumbling

• Precision grinding (for the molded silicone rubber parts, like silicone sleeve with big parting line, we can minimize the parting line via this way)

Compression Moulding Pros

• Compression molding is cost economic than rubber transfer molding and LSR injection molding, and the compression molding process is quite simple when compared to LSR injection molding. (It is a low-cost process and can be used for both low & high volume production.)

• Short set-up time, easy to change material’s hardness & colors quickly.

• Compression Molding is the only way to produce silicone products at Marble Colors.

• The tolerance can be controlled at 0.02-0.1mm by using good steel material, good located pins, and good mold-making equipment.

• Compression Moulding has more material options: Compression molding is suitable for many kinds of silicone rubber materials, as parts can remain pliable after the curing process. The most common materials include common silicone, food grade silicone, medical grade silicone, natural rubber, EPDM , Neoprene Rubber, Nitrile Rubber, etc.

Compression Molding Cons

• Because of the mold structure & compression molding way, it is hard to achieve non-flash effect by located pins & mold-making equipment.

• Material Wasted due to the flash

• Hard to achieve automation unless the product structure is quite simple & production volume is large. When flash (the extra resin that escapes between two parts) occurs, it must be manually trimmed, thus slowing down production efficiency. Additionally, we can not re-use the excess material, which creates more waste.

• Higher Human Capital Investments: As previously mentioned, flash is removed manually on compression molds, which generates higher labor costs.

• Slower Cycle Time: The time needed to start and repeat the sequence of operations is between 1-6 minutes for compression molds; it is a much slower cycle time than injection molding.

Compression Molding Materials

Compression molding is suitable for flexible materials, as parts can remain pliable after the curing process. The material choice is based on the nature of the material handled, temperature, pressure, and frequency of operation. Commonly used materials include silicone, natural rubber, EPDM, NBR (Nitrile Rubber) , CR (Neoprene Rubber), FKM (Fluororubber).

• Silicone: it includes common silicone, food grade silicone, medical grade silicone, this kind of silicone rubber can only be completely vulcanized in high temperature no matter which kinds of vulcanizing agent is used in order to get the ideal physical performance. Solid silicone rubber can be made into all kinds of silicone products after vulcanization, which generally have good & stable physical properties. Solid silicone rubber can reach food grade standard by FDA approval after two-times vulcanization process. Therefore, solid silicone rubber can be applied in both industry and foodstuff.

• Natural Rubber (NR): has excellent abrasion resistance, tear resistance & water resistance; but its ozone resistance, heat resistance and mineral oil resistance are bad.

• Neoprene (CR): CR has good heat, weather, ozone resistance, good chemical properties, good flame retardance and tackiness.

• Nitrile Rubber (NBR): performs excellently at oil resistance and liquid fuel resistance and has good gas barrier properties.

• FKM : have good resistance of oil and strong acid | alkali |oxidant, but its low temperature properties and elasticity are poor.

• FVMQ: has good high-low temperature resistance, oil & solvent resistance, oil resistance.

• EPDM: performs excellently in heat, weather, ozone resistance, and has excellent electrical properties.