Tek-Tip: Tools & Techniques for Cutting Rubber Molds

Some rubber molds require one or multiple cuts in order to remove the original model and subsequent castings.

 

Poured Blanket Mold with Mold Shell

 

 

 

Cutting_RubberMold

 

 

Here are some tips regarding the tools and cutting techniques that we use here at Polytek:
 
 
Tools that we use: 

A tool like your kitchen paring knife will not work well for cutting rubber molds. At Polytek, we most often use the following:

  • Mold Key Knife
  • Scalpel
  • Razor Blade (one-sided for safety)
  • X-Acto Knife

 

Cutting Rubber Molds - Tools
[in order from left to right: mold key knife, scalpel, razor, X-Acto knife]


 
 
 General notes about these tools:

 

  • Mold Key Knives automatically create a tongue-and-groove pattern.
  • Scalpels, razor blades and X-Acto knives can fit into tighter spaces/corners than mold key knives. Often, a mold key knife is used to create the first cut (this cut should not reach the model) and then a scalpel/razor blade/X-Acto knife is used to make the final cut.
  • Depending on the rubber being cut, blades can degrade rather quickly (i.e., after cutting 2-3 molds), so it’s a good idea to keep multiple on-hand.

 
 
General notes on cutting rubber molds: 

  • It is very difficult to cut mold rubbers that are harder than Shore A45.
  • Silicones are generally easier to cut than polyurethanes.
  • To avoid damaging the model, the first cut should not go completely through to the model. As mentioned in the previous section, the final cut should be made with a scalpel or X-acto knife as these tools are more precise.
  • Avoid making a straight cut in the mold; make a zig-zag/tongue-and-groove pattern for better alignment when casting.

 

 

ZigZag_CutinMold

 

 

Planning ahead:

  • If you are planning on cutting a mold, it is a good idea to create a thicker section of rubber in the area that you intend to cut (~0.75″). An example is shown on the far right below. You can also see this technique in a video tutorial on our YouTube Channel.

 

 

Rubber Flange for Cutting-01

 

 

For brush-on blanket molds (like the mold pictured above), we make these thick pieces of rubber by casting the rubber in strips (we have a silicone mold that we made specifically for this purpose) and then applying them to the mold:

 

 

Casting Rubber Strips-01

 

 

  • When selecting a location for the cut, choose an area where seam lines will not be as noticeable if they appear on the casting (e.g., avoid the face).
  • Repetitive opening and closing of the mold can cause tears over time. Embedding Tietex® Fabric in the area where the cut ends can improve strength. The red arrow below indicates an area where TieTex would be beneficial.

 

TieTex next to cut-01


 

 

TieTex Fabric for Cut Molds

 

[^TieTex Fabric]

 

 

Are you working on a mold making project and have a question about technique?

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