The Low Tolerance Spec

The United Seal & Rubber Co. Blog

Methods for Making a Custom Gasket Design

Posted by Virgil Alonso on Tue, Aug 20, 2013 @ 10:08 AM

Lathe-Cut gaskets

Injection molding is one of mainstream manufacturing forms of making a gasket or seals, but what are the other unsung heroes are not as well known? Most people might not know that several other process exist to create gaskets or seals other than injection molding. These include, but not limited to extrusion, splicing - vulcanizing, lathe cuts and gasket die cutting. This article will walk you through each process and give you the advantages and disadvantages of each one.

 

Extrusion

Extrusion is a process used to create objects of a fixed, cross-sectional profile.   Rubber material is pushed or drawn through a pin & die of the desired cross-section.  The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections and work with materials that are brittle, because the material only encounters compressive and shear stresses.  It also forms finished parts with an excellent surface finish.

It is often produced in continuous lengths (long lengths) or in cut-to-lengths.  Rubber extrusions can be provided in solids, hollow, sponges & include all shapes and sizes.   Extruded options include all type of rubber compounds, conductive, nonconductive, TPR (plastics), sponges, urethanes and offered in many more options…

 

Splicing and Vulcanizing

Splicing involves cutting the extrusion to a desired length.  The cut can be at an angle or a straight joint (butt-joint) splice.  The vulcanizing process involves gluing the ends (proprietary adhesives) and placing them into high heat conducting aluminum die(s) with the ends matched up together.  Next the aluminum tool is placed under a pneumatic press that applies heat and pressure to the rubber to aid in vulcanizing the ends together.  The dies are kept under the presses for a predetermined time limit and allowed to cool down after being removed from the presses.  This process is often not recommended for vacuum, high pressure or dynamic applications.  In some cases, cold bonding of an extrusion is an option.

Other options included under this category include:

  • Serpentine Molding

Fully molded in serpent shape with no splicing joints, can provide larger ID sizes but there are limits.  Tooling and mold cost will be much higher than the dies for splicing and vulcanizing.  This is not a good option for sealing gases, high pressure or dynamic seals.

  • Continuous Molding

Highest cost due to time, scrap rates and tooling.  The performance is similar to fully molded products.  This option can be good for sealing gasses, vacuum and dynamic applications.

 

Lathe Cut/Precision Cut Gaskets or Washers

This process starts with the extruded tube.  The tube is loaded onto a mandrel, wrapped and placed in an autoclave for curing.  After it has been cured, the tube is unwrapped, often placed on another mandrel and goes through a grinding process to achieve the required OD size and finish for the part.  After achieving the correct OD size and finish the tube/mandrel is loaded into a cutter and the operator produces the cut thickness of the part.  The cut thickness often takes the shape of a square cut with flat edges.  However, radial, axial and specialty profiles can also be achieved through this process.

  • Advantages of this type of process are low-to-no-cost tooling: When tooling is involved, it is less expensive than tooling required for their counterparts, the molded rubber product.   Another advantage is tighter tolerances than a traditional die cut gasket can be held.  The funnel shape and/or hourglass shape often associated with die cut gaskets is eliminated due to the mandrel, grinding & cutting processes.  This becomes a factor as the cut thickness of the gasket/washer increases in size.

Disadvantages for this process include: There are size limitations to the extrusion that can be produced.  Generally speaking, with an ID/OD round lathe cut the size limitation would be around 19” ID.  With custom profiles, size limitations have to be addressed on a case by case basis. There are minimums placed on the amount of material that must be produced.  The process itself will require that a minimum amount of material is fed through the extrusion equipment in order to produce the rubber extrusion.   As a general rule of thumb, an extrusion run will produce at a minimum of 50 ft. of material (can be greater).  Depending on the thickness cut, there may be a required minimum footage that is higher than your needs. 

For example, lets say you have a gasket that has a thickness of 1/4".  Lets say the minimum run for the chosen compound is 50 feet of extrusion.  Now there will be some waste, so the end result will not be 50 foot of good material.  For our example, lets say there was 40 feet of good material.  40' x 12" gives us 480" of good material.  480"/.25" = 1,920 pcs.  So for our example you would end up with roughly 1900 pcs. of your desired gasket.  Maybe you only need 500 pcs.??  The benefit is you will end up with a better performing product (no OD hourglass).

Gasket Cutting

In its simplest form, this process involves material(s), some type of die cutting equipment/tools and often includes a steel rule die.  The presses involved have traditionally been hydraulic punch presses of different varieties.  Depending on which type of press is utilize, production of hundreds to a few thousands of gaskets per hour is possible.  Machine that slit materials are part of the die cutting process.  Think of this process as producing a product similar to weather-stripping.

  • Advantages of die cutting process include but not limited to: Low cost tooling that on average is a couple to a few hundred dollars.  Can be water jet cut or laser cut when it makes sense and tooling is avoided.  Prototype parts can be provided in less than 7-10 days when the material is readily available.  A wide variety of materials are available for use, including solid rubber, sponge, foam, non-asbestos, conductive, nonconductive, mil-spec and more…

Disadvantages of die cutting: The tolerance will not be as tight as a lathe-cut gasket (for example).  When the material gets thicker, the gasket’s ID and OD can begin to take the form of a funnel and/or hourglass shape.  How pronounced the funnel or hourglass shape is depends on the material used to produce the gasket(s).

The intent of this blog is to offer the options avaialble with producing seals and gaskets.  While, we skipped around the various molding options for the purpose of this article, do not discount molding as an option.  Please contact us and allow us to offer the solution that is right for you!

 

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Tags: Value Engineering