Cast Urethane Soft Tooling | Basics of Urethane Casting | SD3D Printing
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Cast Urethane Soft Tooling

By January 20, 2019Casting

Silicone molds and the cast urethane soft tooling process are very useful for creating duplicates of an existing part. This method is typically employed when a small to medium quantity of parts is needed (10-100). This is due to the fact that the tooling and piece price are more cost-effective for lower quantities when compared to hard tooling options. Silicone molds are usually used for prototype, bridge to production applications. Most silicone molds are good for making about 25-50 shots per cavity depending on the part complexity. Silicone molds are injected with material that are manually gravity-fed using a tube. Depending on the brand and type of urethane material, it takes about 1-24 hours for the part(s) to cure. Once the parts contained in the mold cure, the molds are manually opened. Lastly any other necessary finishing processes are then carried out by hand.

Silicon molds and soft tooling are a bridge between prototyping and mass production.

Prior to the invention of cast urethane soft tooling, there were few cost-effective options for the production of small to medium runs for bridge applications and initial market releases. In as much as injection molding is an amazing solution for large scale production of an uncountable number of products, it doesn’t provide an economical solution for small volume production needs or highly customized products.

cast urethane soft tooling

Providing answers to this gap in production options involves lowering the cost of tooling and speeding up production while using materials with corresponding functions and properties as those of the molded products. A combination of 3D printing and cast urethane soft tooling has demonstrated to be one of the best alternatives to hard tooling for low volume production. This includes medical device end use applications, consumer products and many others. Contact us today to discuss your project with an expert. We will let you know whether soft tooling vs. 3D printing will be right for your application.

The advantages of silicone molds over hard tooling methods are low cost, speed, versatility, and accuracy.

Costs for the stereolithography (SL) or 3D printed master and the molding materials are usually measured in hundreds of dollars. This is significantly more cost effective for small production runs over hard tooling which would cost thousands of dollars. Once the molds have been built, replicas can be cast in just a couple of days as opposed to weeks or  months. The adaptable nature of cast urethane soft tooling also allows for the engineer to test different casting materials without having to change the CAD model.

How to build soft tools for cast urethane molding: 

A mold container (typically acrylic or MDF) is constructed first. The original part is then placed within. A false or sacrificial feature (shown by the yellow soil in this image) can be included so that the original part won’t be completely covered in the cured silicone mold. Before the upper half of the mold is poured, the clay soil is removed. By doing this, the two mold splits can be parted after they have been cured.

 

The liquid silicone is transferred into the vessel, completely covering and encasing the original part. After a few hours, the silicone cures (positive pressure and heat may be able to speed up this process). Next, the hardened urethane mold is detached from the retaining box.

 

The next process is to remove the original enclosed part without destroying it or the newly made soft tool. The cured silicone will need to be carefully cut in the necessary places in order to be able to remove the original model safely. The resultant flexible mold can then be twisted or bent open to remove the original part. If the part is one that is complicated, then it may have to be partitioned. 

 

After reassembling, the empty space where the original stereolithography part was initially located is casted with the chosen material, most times a two-component urethane or epoxy. The filled material is then cured, and the new replica cast is taken away from the soft tool. Based on the material used, we can create a numerous quantity of duplicates from one mold.  A close up look at the picture shows “sprues” on the cast part. These are results of holes that were added to the mold to relieve bubble formation in the finished part. The sprues are detached during post-process finishing.

Cast Urethane Soft Tooling Built From 3D Printed Master Patterns

The most economical, swift turn master patterns are manufactured by using 3D printing. Several additive manufacturing techniques are used for mater patterns. Tradeoffs between these methods include varying feature detail, surface finish, and production speed:

  • MJF and PolyJet 3D printing – PolyJet master patterns are naturally built with 16-42 micron layer thickness (0.001” to 0.002”) with minimum feature sizes running around 0.012”. This is one of the fastest and highest resolution additive manufacturing methods, but also one of the most costly.
  • Stereolithography (SLA) – SLA layer thickness ranges from 25-75 micron (0.001” to 0.003”) with details as fine as 0.012” making use of High Definition processes explicitly for master pattern creation. This is the middle range option that provides a good balance between cost, resolution and production speed.
  • Fused Deposition Modeling (FDM) – FDM layer thickness used for cast urethane molding are typically around 100 micron or 0.004” with minimum feature sizes running around 0.03”. FDM is by far the lowest cost method used for creating master patterns, but it is also the slowest and provides the lowest resolution.

It is important to note that MJF, PolyJet and SLA master patterns must undertake post-process finishing before proceeding to tool creation. FDM can typically be taken to cast urethane soft tooling creation immediately after production or with minimal post processing effort.

Did you know you can now source your cast urethane soft tooling directly from our instant quoting tool? Just drag and drop your 3D file and select one of the casting resins as your material choice.