Making Yellow Windows Appear Clear in Polyurethane

By Bob Hatch

Related links: Polyurethane | TPU | ASTM D746

A rigid TPU and color additive package make all the difference in an application requiring clarity.

This month’s topic came to me as I was moving my office. I noticed a large, clear part in my archives file and decided it represented a good story that I could write about and bring some important facts to light.

The molded part, basically a window, was one of several similar parts I have worked on made of rigid polyurethane produced and marketed by one of the national manufacturers. Typical applications are for sight glasses or windows in storage tanks where it’s important to able to see the product being stored; material requirements include chemical resistance to the liquid in the tank and clarity so the color of the storage tank contents can be determined.

Often the material of choice for applications like this is acrylic or polycarbonate, but these are attacked by many chemicals. In this case, the molder made the correct choice for an alternate material: a rigid polyurethane. Rigid TPU is difficult to dry, easy to scratch, and hard to mold; but other than these weak points, it’s chemically resistant and is capable of being molded into a fairly clear, rigid part. The chemical resistance is why it’s picked as a good material for parts that function as sight glasses or windows in many industrial applications.

I have seen brackets molded out of rigid TPU where the part was designed with a 90° angle separating two sections of the part. The interesting aspect of this material is that the 90° angle doesn’t need a rib designed into the part to prevent it from collapsing or warping during postmold cooling; it isn’t compromised in any way, thanks to the material’s low shrinkage feature and its high rigidity numbers.

Except for these helpful features, the design process with rigid TPU is the same as if you were designing for PC—same shrinkage, wall thickness, gate locations, gate depth, gate width and land dimensions, and venting requirements. The only difference between processing PC and rigid TPU that I have noticed is the extra attention you need to put into drying the latter. Check with the manufacturer for recommended drying temperatures and length of drying time, but I usually recommend 200-220°F for at least 12 hours and have found these numbers to work well for desiccant-type dryers. (Some of the newer dryers might do a better job with this material.)

Cleaning up the gate and runners

Let’s review the gate. The customer’s complaint was that the natural color was too yellow. He also mentioned unusual flow marks at or near the part’s center, mostly straight out from the single gate location and in a curved pattern 90° to the material flow. I was sure I could solve the flow mark issue through my normal runner system and gate review.

The sprue O-diameter measured 0.225 inch and fed into the half-round, 0.190-inch-deep main runner. The main runner fed into a chisel-style or modified fan gate, which is not good. This type of gate design causes a fairly severe flow restriction and creates high shear. The gate was 0.080 inch deep and 0.540 inch wide where it attached to the part. The gate land—measured from the runner to the part—was 0.150 inch. These dimensions were not even close to what they should have been. Based on the part’s wall thickness of 0.140 inch, the gate depth should have been 90% of the wall thickness being gated into, or 0.126 inch. The gate needed to be twice as wide as it was deep, so 0.252 inch. Finally, the land is always half the depth but never exceeds 0.030 inch.

A yellowish tint and flow marks in the center of the part landed this window on the Troubleshooter’s desk. (Scratches on the easily marred rigid TPU are the cause of the other marks.)

I also noticed a potential shear point where the gate attached to the runner. This shear point could be corrected by recutting the gate so that it attached to the runner at its middle, not at the side. This would eliminate the shear point and correct the restriction to material flow.

On to the runner: The half-round runner needed to be cut to a full-round diameter of 0.250 inch for rigid TPU. Plus, a half-round runner is not recommended for feeding an edge gate.

The sprue O-diameter needed to be opened up to 0.312 inch by buying a new sprue bushing with this dimension or using a wire EDM to open up the old sprue bushing. In the second option, the toolmaker needs to be careful to maintain the draft angle of the original sprue bushing design; typically I find this draft angle to be 0.0175 inch per inch of sprue length. The molding machine nozzle should have been a full taper design with an orifice diameter of 0.290 inch.

These changes to the sprue, runner, gate, nozzle style, and nozzle orifice sizing would ensure that the material would not degrade as it passed from the barrel through the runner system, and then into the mold cavity. They would also no doubt correct the curved flow lines on the part.

Color correction

The next problem was the yellowish appearance of the material—its natural color. The corrective action was to add a small amount of color to the rigid TPU, either by having the material manufacturer do it or by getting a color concentrate that would need to be mixed in with the natural material prior to being dried.

I know that PC producers add a blue color to their material to give it a water-clear look, and the same goes for polystyrenes, acrylics, and SAN. However, based on information I received from Clariant in Minneapolis, MN, I found they recommend a violet color be used with rigid TPU to make it water clear. The customer used the recommended Clariant color package and the yellowish tint disappeared. The result was a nice water-clear appearance.

It was a decent day’s work and a good story about another kind of molding problem that many molders face from time to time.

October, 2007 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.