Polyurethanes and Soft Tooling

Interview conducted by G.P. ThomasApr 4 2014

AZoM talks to Mitch Johnson, PhD, Senior Technical Director of General Plastics Manufacturing Co, about the many applications and benefits of polyurethane, especially for soft tooling.

Could you please give a brief overview of General Plastics and how it is involved in the polyurethane industry?

General Plastics has been in business for over 80 years and we’ve been located in Tacoma, WA the entire time. We manufacture both rigid and flexible foams, including molded foam parts and machined parts. We’ve been manufacturing polyurethane foam for a number of decades, really since the mid- fifties.

Could you give an introduction to polyurethane and the different types that you work with?

Polyurethanes are a family of incredibly versatile industrial products and most people use these products every day. To make polyurethane foam, you can mix an isocyanate and, in this case a formulated polyol, so that they polymerize at room temperature to form a cellular plastic . It’s really a process that requires very little energy and can utilize renewable resources.

Polyurethanes are used a lot in flexible foams, but coatings are also a huge application. They can be very flexible, very tough materials and the formulation capabilities for urethanes are really endless. There’s so many combinations you can put together to achieve different properties.

We manufacture a lot of rigid foam and we’re considered a high density rigid foam manufacturer. We do make some three and four PCF (pounds per cubic foot) foam, but we do a lot in the 10 to 40 PCF range. In industry, that’s considered high density urethane foam.

Which industries typically use polyurethane?

The biggest industry for rigid polyurethane foam is insulation, including spray foams for insulation of walls, refrigerators, coolers, as well as automotive applications.

We are in more of a niche industry which caters to tooling and core materials. For example making sandwich panels, where there is a layer of fiberglass or carbon fiber, then a layer of foam, and then a layer of the same carbon fiber or fiberglass on the opposite side. That material will be much, much stronger and stiffer for the weight and lighter in weight than a solid sheet of all carbon or glass fiber. As I touched on before, one of our largest markets is the aerospace industry, where strength and light weight are the keys to success.

Staying on that topic, what are the benefits are of using polyurethane in those applications, compared to some traditional materials?

Traditionally, rigid polyurethane foam can be considered a synthetic wood. Wood is a very strong material, but it carries a lot of moisture with it and there can be a lot of variation in density, strength, resin content and possibility of decay over time.

The rigid polyurethane foam that we make is closed celled, so it doesn’t absorb moisture, does not support microbial or fungal growth and will last forever if protected from UV light. A urethane foam core will maintain its properties over the life of the part, if properly designed.

How does General Plastics generally manufacture these foams and how are you unique in that area?

Of course it’s all proprietary information and we have special manufacturing processes, but I think we want to communicate that we are a vertically integrated company. We bring in basic feedstocks, and formulate all of our own formulations.

We are also able to customize things on a batch basis - that’s really one of our strengths, being able to develop and manufacture small, short runs of bun stock or sheet stock products for customers. We have a very fast turnaround with types of projects.

Could you highlight what tooling board is and what tooling boards you particularly make?

Tools in general are basically molds or shapes that you can make composite or thermoform parts from. Our polyurethane tools are a low cost, very fast way to make those types of tools.

Traditionally, those tools have been made out of wood or metal, but with urethane you’re able to get a much more dimensionally stable material than with wood, and obtain much tighter tolerances over time, because wood of course is very sensitive to environmental conditions such as heat and moisture.

You can machine these very complex shapes from polyurethane, and our tooling boards are able to hold very sharp edges and hold tolerances. There’s also very low residual stress on our materials - they don’t warp and move around on you.

Most urethanes are designed to be used at fairly low temperatures such as room temperatures typical for ‘wet lay-up applications’, or low temperature curing Prepregs. So it’s been in the last couple of years that we’ve been looking at foamed polymers for much higher temperature processes. This allows the use of a much wider variety of composite matrix resins and curing requirements.

Could you go into a little bit more detail perhaps about why you would use a soft tool over a metallic tool?

The biggest driver is project cost. Machining or shaping aluminum tools is quite expensive time consuming and it’s also a challenge to do a very large aluminum tool because you always have seams to bring together and then you have a potential for vacuum leaks.

Also, for example, if you’re only going to make 1-4 parts off of it, using a metallic material like INVAR would be very expensive and time consuming. So that’s where soft tooling comes in, because you can machine it very quickly, seal it and pull a part off if it. This makes it a much lower cost process in terms of raw materials compared to using aluminum or metallic tooling in general. However, foam tools can usually not hold extremely tight tolerances that an INVAR tool can maintain, there are always trade-offs. Metallic tools traditionally used for larger runs of parts due to durability and repeatability.

Why are the curing times crucial for success?

Polyurethane foams for tooling typically range from around 20 PCF up to 50 PCF, and they are much better insulators than aluminum. Aluminum of course conducts heat (about 4000 greater than 20 pcf PU foam), and so you get cure from both sides of your composite and you can cool it much more quickly as well.

Urethane foams are good thermal insulators, so the part only sees heat from the side exposed to the hot air . So it takes longer to cure out the part, because you usually have to ramp your temperature slower and dwell longer to achieve proper cure of the composite part.

When looking at the ‘cool down’ aspect, the foam is hotter on the outer surface than the inside. This causes the foam to expand more on the outer surface of the foam. When the oven or autoclave begin to cool, the outer shell of the foam begins to faster than the warmer foam below the surface. This is due to low thermal conductivity of the foam. If you cool it too fast, the outer surface can crack requiring repair.

Engineers selecting to use foam as a tooling substrate need to be account for the thermal expansion, and the ability to move heat in and out of the foam. We typically recommend requesting a small test block of foam, which we run some quick test on first to validate the cure cycle that you want to use This is the best way to validate calculations.

Looking into the future, are there any advancements on the horizon in terms of tooling or polyurethane?

A couple of years ago we started selling a product called FR-4700, and that material is made of a high temperature resin that can handle the high temperature curing cycles required by advanced composite parts. There are very few other products on the market that are as low cost, rapidly machinable and is easy to work with.

With this product, composite tooling can be made from a monolithic foam master using BMI matrix resins.

So down the road, what we’re looking at doing is reducing that thermal expansion and increasing the thermal conductivity so the tool is more true to the desired final shape, so there is less compensation to be calculated. This increase in thermal conductivity will also shorten cycle times. That’s where General Plastics is heading in the future - low cost, soft tooling that has more metallic-like properties.

To summarize, that the ability to adjust the foam properties to the desired outcome of the process is one of our strengths. That’s something that we want to encourage customers to always request. If they need a process that doesn’t quite fit any current offerings, we can have a discussion about what they really need their process to do and work with them to develop this.

About Mitchell Johnson

Mitch Johnson, PhD, is Senior Technical Director of General Plastics Manufacturing Co. and holds over 15 patents. In this role, he leads new product development and drives continuous improvement of the company’s advanced materials and processes. Responsible for the company’s R&D group, Mitch oversees a team of chemists and technicians, and works with customers to create products that fulfil specific requirements and applications. Mitch joined General Plastics in 2008 from 3M Company.

As a product developer, he created numerous coatings, protective materials and foams. He earned his doctorate from the University of Utah, conducting his thesis work in the field of organic chemistry, and pursued post-doctoral studies at Los Alamos National Laboratory, where he studied organometallic lanthanide chemistry.