DESIGN WITH RUBBER IN MIND

BRUCE: Come take a look at this, Benny.

BENNY: You mean that prototype insert molded rubber part you’re holding, Bruce? I can see from here that it’s going to be expensive to mold.

BRUCE: Yep. It was originally an assembly of plastic, metal, and rubber components, and it looks like the designer had some good ideas, but…

BENNY: OK if I just “but-in” here? The parting line locations, tolerances, and surface finish are the same as would be applied to a molded plastic part, and even though there are many similarities between plastic and rubber, there are some big differences.

BRUCE: And it’s possible to take advantage of some of those differences when designing molded rubber parts. For instance, this part is going to need a retaining flange, when it could easily have been designed with an undercut.

LET IT RUN: Allowing some rubber to run to the
end of the inserts makes tool shut off easier.

STAY AWAY FROM THE EDGE: Don’t try to
ake rubber to the edge, and allow no rubber
on periphery.

BENNY: I was just going to say that, Bruce. Since a plastic part is ejected after it solidifies, undercuts are generally not allowed unless they’re small, and this part needs a big undercut. With rubber, undercuts are more easily accommodated because a rubber part is flexible and can be pulled out of the tool. That flexibility opens up all kinds of possibilities.

BRUCE: You have to remember that the size of an undercut is based upon how much the rubber can move without tearing — although unlike plastics, the part will be hot, which means reduced tear strength. Generally speaking, the width of the undercut, which is the dimension of the overhang normal to the opening direction of the tool, should not be more than two times the length of the undercut feature along the pull direction.

BENNY: The key here being “normal to the opening direction of the tool.” It’s a consideration when you’re laying out the mold.

BRUCE: And here’s something else, Benny. Look at the flash on the end of this molded-in insert. There should have been some consideration given to the way the tool “shuts off” against the insert.The shut-off is how you keep material from flowing around or over the face of inserts such as the bolt compression limiters in torque couplers.You don’t want flash over the end of this type of insert because it would end up between the bolt head and the insert, making proper assembly impossible.

BENNY: Looks like some secondary hand finishing is going to be necessary on this one. That’s going to add to the cost.

BRUCE: Sure, and it could also slow up production. In order to prevent this, it’s a good idea to have a small thickness, 0.5 mm or more, of rubber extending up around and to the end of the insert as I show in the figure. This allows the tool to shut off squarely on the end of the insert, which is much easier than trying to control a shut-off on the OD of the insert.

BENNY: That’s because of insert tolerances, right? Go ahead and tell me I’m brilliant.

BRUCE: Sure, Benny, you’re brilliant. Now, with a molded-in sheet metal plate, such as is often seen in gaskets, you should give some thought to the shut-off at the periphery of the plate. If there is rubber on both sides of the plate, it’s best to allow the rubber to flow around the edge at least 0.4 mm thick, which prevents a difficult shut-off due to plate tolerances and makes trimming flash much easier. It also prevents problems with bonding at the edge of the plate. Now, if rubber cannot be on the edge of the plate or is on one side only, stop the rubber 1.0–1.5 mm back from the edge. Again, this leaves a clean shut-off on the top of the plate.

BENNY: Great minds think alike, Bruce. I was just going to add that with a design like that, flash could occur on the surface toward the edge of the part and would need to be trimmed away. In general, you should design the part and mold to make flash removal unnecessary.

BRUCE: Right again, Benny. And that reminds me: whose turn is it to buy coffee today?

BENNY: I’m sure it’s my turn again, so why don’t we talk about slits in molded rubber parts instead? Slits are common in wire grommets and duckbill check valves, and they can be molded in or cut in after molding as a secondary operation. Of course, for items like check valves where a good seal is desired, the slit must usually be done in a secondary operation, but for grommets, the slits can often be molded in to minimize cost.

BRUCE: But there’s a limit to how thin a slit you can mold. Generally, the thinnest slit that can be molded in is 0.75 mm because there’s a limit to the strength of the thin piece of metal required in the tool. If you try to go thinner than this, the pressure of the rubber as it’s forced into the mold can deflect or even break the thin metal. Now how about that coffee?

BENNY: OK, sure, just as soon as we talk about something that can make or break a molded rubber part design: tolerances and finish requirements. Rubber parts shouldn’t be dimensioned to the same tolerances as are applied to rigid products. It’s not enough to specify a highly precise mold, even if your budget allows it,because even small variations in temperature, cure time, and compound composition can make a significant difference in shrinkage.

BRUCE: We see that often, don’t we, Benny? While mold tolerances and mold registration are fixed, they become increasingly expensive as tolerances are reduced. In addition, conditions encountered during shipping, handling, and storage can cause changes in a flexible part’s dimensions. That’s why we prefer to specify parts to RMA A2 Precision tolerances unless they can be looser — as with very flexible parts such as bellows, where RMA A3 commercial tolerances are more appropriate.

BENNY: Guess it all means that with rubber, you have to be more flexible in your thinking. Flexible, get it, Bruce?

BRUCE: Yeah, I get it, and you make a good point, Benny. And speaking of flexible, here’s something else to keep in mind: the flash call-out in the tolerance note .T .80mm, meaning 0.80 mm of flash extension allowed, is the best place to start for a general note as this much flash can normally be hand trimmed. If a tighter tolerance is required in certain areas, it’s best to apply the tighter tolerance only in those areas. And don’t ask for T0 because it’s never possible unless there is no parting line. If necessary, specify the tool parting location if very tight control is required.

BENNY: And we’re not finished until we talk about finish. In the RMA note, we usually specify F2 for standard mold finish. Standard microinch surface finish call-outs are not used as the finish is controlled by the mold finish. This is analogous to SPI plastic mold-finish call-outs. Since rubber and plastic components are generally not machined parts, finish is normally specified for the machined mold components.

BRUCE: Hey, it looks like we’re, uh, finished, Benny. Now, how about that cup of coffee?

If you have questions about Pro/E or any other design engineering topic, please email Benny and Bruce at benny&bruce@rlhudson.com.

Pro/E® and Pro/ENGINEER® are registered trademarks of Parametric Technology Corporation (PTC).

Solutions 4Q08

DESIGN WITH RUBBER IN MIND