Check the Weather Report!
- Category: Tech Tips
When polyurethanes don't perform as expected, companies typically look for explanations such as technician error, mixing machine issues or supplier/raw materials problems. A common culprit is often overlooked...the weather. Uncontrolled shipping, storing and casting environments can cause polyurethane resins and hardeners to form crystals, altering handling characteristics. Eliminating these issues is frequently as easy as implementing better control over shop temperature and humidity.
Effects on Cure. Shop temperature should be maintained at 70°F to 77°F to ensure polyurethane system stability and consistency. The catalysts and reactive-crosslinkers used to formulate polyurethane materials generate heat when mixed. If the ambient temperature in the shop during mixing and pouring varies greatly from the controlled laboratory temperature of around 72°F, the polyurethane's gel time will be different than that stated on the technical data sheet. If the temperature is greater than 77°F, the gel time and work life of the mixed system will be reduced and linear shrinkage may increase. Similarly, a lower than "normal" room temperature will increase the gel time and lengthen curing and demolding schedules.
Effects on Shipping. When temperatures hover around 60°F, both resin and hardener system components may begin to freeze. If resin freezes, small white shards or crystals will form or the material will take on a "milky" or cloudy appearance. Hardener subjected to cold temperatures may become more viscous and form "waxy" chunks.
Crystallized resin and hardener usually can be returned to their original, usable state with the application of heat following several easy steps:
1. Loosen the caps on the resin and hardener containers to relieve pressure that can cause cans to swell and, in some cases, rupture as the material is warmed. DO NOT remove covers completely. Open containers can be contaminated by airborne debris and moisture.
2. Place containers in an oven temperature heated to between 125°F to 150°F.
3. Inspect the material every 30 minutes. Over-heating system components can cause irreversible damage. If crystals remain in the resin or the hardener is still viscous, agitate the cans and return them to the oven. Material MUST BE agitated to avoid localized heating. In most cases, resin and hardener will return to their normal, very clear state. Depending on the volume of product in the container, the reversion process may take up to 16 hours.
NOTE: In rare cases, crystallized or partially solidified materials will have dimerized, or undergone a molecular change, that prevents reclamation.
When polyurethane is exposed to water, the byproduct is CO2. When a polyurethane resin is moisture contaminated or a mixed system is cast in a very humid environment, eradicating the trapped air bubbles is very difficult. In some cases, vacuum and/or pressure can be used to remove air but it’s always easier to prevent the problem in the first place.
Air Conditioner. During summer months, when temperatures and humidity are high, it’s important to use an air conditioner in the areas where polyurethanes will be handled and poured. A relative humidity below 50% is the ideal environment for casting materials. A dehumidifier may also be helpful.
Replace Wood and Paper Products. Exposing polyurethanes to moisture-retaining surfaces can cause bubble formation. For example, paper mixing containers and wooden mixing sticks should be avoided in favor of plastic buckets and paddles. Similarly, silicone, epoxy or aluminum tools are preferable to wood molds.
Dry Nitrogen. When containers are opened to remove resin or hardener, clean the edges with a rag or paper towel to remove residual material. Then, replace the lids and reseal securely. If desired, spray a layer of dry nitrogen into the can before closing. Because the gas is heavier than water vapor, it will settle over the polyurethane surface to prevent contamination. NOTE: Innovative Polymers fluorinates its one-gallon jugs to help protect against moisture migration through the plastic.
Desiccant. When meter/mix equipment is used or material is dispensed from steel drums, a desiccant cartridge should be utilized. The cartridge is designed to dry the air being pulled through it, eliminating the harmful effects of water molecules.
Liquid Protection. Molecular sieves that absorb moisture and block water molecules from reacting are also incorporated in many Innovative hardener formulations. NOTE: These fillers will settle out of solution, forming a thin white layer at the bottom of the hardener container. Shaking the can will resuspend the sieves.
Quality Control Ensures Reliable Products
- Category: Tech Tips
At Innovative, product quality and consistency is a top priority. As various products are manufactured, our quality control (QC) team conducts a variety of tests on both simple, uncured materials as well as mixed and cured polymers. Our technicians utilize laboratory equipment that is frequently inspected, calibrated and certified as accurate. The QC results are then recorded and retained as needed should any questions about product performance arise. Our goal: to provide customers with polyurethanes that exhibit outstanding batch-to-batch consistency and meet the specifications as stated on technical data sheets.
We begin our QC process by drawing samples of the initial product mixture and testing such properties as:
- Gel time -- time to solidification
- Viscosity -- material liquidity
- Specific Gravity or Density -- used to calculate how much material will be required to fill a mold.
Additional tests are conducted based on the material being manufactured. For example, the percentage of NCO in a prepolymer is verified to ensure that the product will cure and perform as expected when the customer molds a part.
Quality tests on cured product samples include hardness and shrinkage; both properties are critical considerations in selecting polyurethanes for a specific project. NOTE: The laboratory measurement of shrinkage must be supplemented by the customer before beginning a new casting project because shrink can be affected by release agents, mold configuration, casting thickness, and ambient temperature.
A number of other performance properties are verified according to the class of product to ensure that the expected in-service capabilities will be achieved.
- Elastomers: Mechanical properties such as tensile strength/elongation, tear strength and flexural modulus are measured
- Clear Polyurethanes: Must be visually examined for clarity
- High Performance Systems: Advanced testing includes Notched Izod impact strength and heat deflection temperature
Customer-specific quality tests may also be conducted, such as the compression strength of polyurethane foams.
Ensuring Fully Cured, High Quality Parts
- Category: Tech Tips
Just as pilots perform a pre-flight check to verify that all systems are working properly, it is often helpful for molders of plastic parts to review material handling methods. While many of the steps involved in mixing, pouring and curing polyurethanes are regarded as standard procedure to experienced technicians - improper methodologies are a most frequent cause of calls to our technical hotline!
Correct Materials Choice
- Category: Tech Tips
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. Innovative Polymers’ 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?”
Silicone Glove Molding
- Category: Tech Tips
The low-pressure rotocasting process today is seeing a resurgence in popularity fostered, in part, by the development of improved process controls and easier-to-handle plastics. Moldmaking methods have improved as well. Specifically, the combination of high-quality silicone rubbers with rigid polyurethane systems is yielding glove molds that facilitate both casting and demolding of one-piece hollow parts with severe undercuts, fine surface detail and/or threads and handles.