Choosing the Correct Casting Resin

With so many cast polyurethane systems available in the marketplace today, selecting the best material for a new prototype or part can be a most challenging process. RAMPFS’ experienced technical service team specializes in helping customers evaluate and choose materials. To provide effective input, however, we must have specific project information including:

• Part performance properties
• Processing and handling criteria
• Special requirements

It is by working in partnership with customers that we can most accurately answer the often asked question, “Which material will meet my needs?”

The first step is evaluating the part to be molded.

Part Performance

The mechanical stresses, required dimensional tolerances, surface finish, and service environment/temperatures in which the part will be used are keys to polyurethane selection.

Our technical representatives frequently receive difficult to answer questions…

Question 1. “I’m looking at a part specification that requires a tensile modulus of 250,000 psi. What do you have?”
Why is this hard to answer? Because every polyurethane is formulated with a series of physical characteristics. When a single characteristic is deemed “most important”, as is tensile modulus in our example, this should be regarded as the starting point. Then, additional characteristics can be considered to select a material that best fits the application. RAMPF suggests that customers do as much research as possible to determine all physical part parameters before calling our hot-line (1-800-340-4563).

Question 2. “Do you have a polyurethane that replicates ABS?”
Unlike the previous query that was too narrow in scope, this question is a bit too broad. It is important to understand that cast polyurethane systems (thermosets) cannot attain all of the physical properties of injection molded thermoplastics. While thermoset polyurethanes can provide similar performance and can meet individual property needs, the full combination of properties available in an ABS material cannot be attained in a polyurethane system. If a customer calls asking for ABS, we typically ask which specific property is most important. For example, if the part must be ultra-durable and able to withstand impact, our Tech Team would probably recommend a polyurethane that offers high impact resistance. However, this same material might not meet the flexural modulus indicated on the engineering specification.

Also, customers must keep in mind that all thermoplastics (ABS, polyethylene, polypropylene, etc.) have many grades. While one ABS plastic may have a very high flex modulus of 800,000 psi (very stiff), another may have a flex modulus of 275,000 psi (relatively stiff). Again, direct information about performance requirements for the cast part is critical before we can make a recommendation.

Question 3. Can you color the polyurethane I selected a specific color?
It is important to provide the Technical Representative with details about color requirements at the beginning of the conversation. Some materials are, for example, naturally dark in color and cannot, therefore, be pigmented to attain light color shades. If a comprehensive discussion has already taken place and a specific material selected before color is considered, the whole recommendation process will have to start again from the beginning.

Question 4. I don’t have many details on required part performance but I do know it will be used outdoors. What do you recommend?
In the absence of information about required physical properties, it is at least important to evaluate anticipated service conditions. For example, if a fixture is being molded for an outdoor planter, the selected polyurethane must be able to withstand exposure to temperature extremes, humidity and rain, and the destructive effects of ultra-violet rays from the sun. RAMPF InnoClearTM water clear polyurethanes are UV-resistant and may be the answer to this type of application.

Question 5. Are you willing to develop a system to meet our specifications?
Our chemists custom formulate polyurethanes at no additional cost to the customer to ensure that the material we supply is a “best fit” for the application. Using the right thermoset system from the beginning of a program can mean the difference between success and serious losses! When a new material is required, however, adequate time must be allowed for needed formulating work.

Processing/handling

Among the factors to consider in selecting the best polyurethane for a project is its work life. The gel time of the material must be long enough to provide for complete mold filling before components begin to cure. Remember that shop temperature affects work life; higher than 77oF temperatures will decrease gel time while lower than 77oF can increase gel time and affect the cure cycle.

Molding process is also important. If a part will be vacuum cast, for example, a polyurethane like those in the RAMPF RapidVacTM line should be selected. These products feature low viscosities with rapid gel times and perform well with the equipment used in vacuum casting. Specific parts may require degassing or pressure casting. Or, if being processed via meter/mix equipment, the polyurethane may be subjected to higher pressures and temperatures than when being hand cast in a silicone rubber mold.

Special Situations

Some projects may require a special handling. A common example is new designs/parts that are covered by secrecy restrictions.  RAMPF is very willing to sign a non-disclosure agreement to keep all discussions confidential. We are also happy to participate in conference calls with the end customer to review particularly complex projects and critical materials properties to help select the right polyurethane.

We look forward to working with you on your next project! Customer satisfaction is our number one priority!