An Overview of RTV Molds

Injection molding has long been the best method of bulk production, while 3D printing has recently become the best choice for early stage prototyping.

But when and how should you make the switch from single volume production with 3D prints to high volume production? With small run production it can be hard to find the balance between upfront costs, production costs, and quality, but silicone molds can be an effective way to bridge the low-volume chasm.

About Silicone Molds

Silicone molding – most commonly known as room temperature vulcanizing (RTV) molding – offers a great solution for small batch production. The mold material has no problem retaining tiny and detailed features and tolerances similar to those in your 3D printed parts (minimum features 0.025"/0.6mm).

Method

First, a pattern of the item to be manufactured must be produced. You may be starting with an existing item you wish to reproduce, in which case the silicone can be applied directly (assuming the material is suitable).

Wood and metal can be machined to make the pattern, but a pattern can also be made with 3D printing. 3D printing the mold pattern has shown to reduce lead times up to 90% and reduce costs up to 70%, depending on the geometry of your original part.

Once the pattern is ready, the silicone is mixed with curing agent and poured over the pattern; curing can take up to 24 hours and the resulting mold is strong and flexible and then ready to use almost immediately.

Urethanes, a type of thermoset plastic, are the most commonly used casting material and offer a wide range of mechanical, visual and electrical properties; post production work can be reduced to almost nil.

The mold can be used for runs of up to 100 parts, although typically the usage range is between 15-30 units per mold, depending on the casting material.

Costs

The cost for RTV units is broken down into 3 line items:

  1. Pattern cost
  2. Mold cost
  3. Unit cost

Pricing of the pattern varies based on how you produce the geometry. If you are creating a replica model, you can use an off-the-shelf unit, otherwise 3D printing is the primary method for producing patterns, followed by subtractive CNC parts.

After a model has been produced, a mold maker will usually apply a primer and texture for the desired surface finish of the final product.

For the cost of the mold, the main consideration is around how much silicone needs to be used. At around $11/in^3, silicone isn’t cheap!

From there, it’s also important to take into consideration:

  1. Where slides need to be used
  2. How many pieces the mold will be
  3. The lifetime needed out of the mold.

The cost of the individual units is then determined by the geometric volume of the part and the amount of post-processing work required. For simple parts with little material requirements, the unit cost is very low.

After you calculate the cost of all 3 line items mentioned earlier (pattern, mold, and unit cost), it’s best to divide the total by the number of units you can get from the mold.

Here’s an example of what the cost breakdown might look like:

  • Pattern: $320
  • Mold: $900
  • Unit: $70/unit
  • Mold life: 30 sets
  • Per unit cost: ~$110

While $110 per unit might seem expensive, that’s almost 1/3 of the cost to produce the part individually and the lead time can be much faster than cutting an injection molded tool.

How RTV Molds Compare

Here’s a brief look at some of the major pros and cons of RTV molding:

Pros

  • Lead time and upfront costs are far lower than injection molding
  • Production costs are lower than 3D printing
  • Minimal post-production processes
  • Availability of casting material properties is much greater than 3D printed materials – mechanical properties can be improved through choice of material and can simulate production parts

Cons

  • Molds must be reproduced around every 20-100 units
  • Surface contaminants can inhibit the curing process

Comparison with 3D Printing

3D printing is unparalleled when it comes to taking an idea from vision to tangible prototype with next to no lead time. But as volume increases you quickly start to lose money in production costs, even with in-house equipment.

As your part size increases to the capacity of the printer, the costs incurred grow exponentially and to print one at a time will greatly increase manufacturing time.

Comparison with Injection Molding

While nothing (yet) beats injection molding at large scale production, RTV silicone molds do offer some advantages.

The flexibility of the mold and its resistance to stress allows for detailed design features; draft angles and undercuts that could not work in a traditional injection mold can be successfully implemented.

Unfortunately, RTV molds are only a low volume production solution with production runs of sub-500 units. So be sure to carefully analyze when investing in injection molding is most suitable for your business.

When to Use RTV Molds

If your production is at the high (>50) or low (<10) end of the small volume scale, it’s relatively easy to conclude whether 3D printing or casting with a silicone mold is your best option.

Production of 20-30 units is where choosing the best manufacturing method is most difficult.

Small 3D printed parts can be printed in parallel, reducing manufacturing time and operating cost of the machine and thus can be produced more cost effectively than RTV molds, even at higher volumes up to around 40 units.

Larger parts on the other hand, especially those approaching the print bed limit and printed at high resolutions, exponentially increase manufacturing time. So if your part is large and detailed, consider making molds at even lower production volumes, around 10 units.

As you can tell, there are many factors to inform which production method you should use, so be sure to consider the unique features of your product, including size, part geometry, and production volume to find the best solution.

Learn More →

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