Fw: Another Mission Impossible

From: B. J. Arnold-Feret (ppsltd@airmail.net)
Date: Wed Jun 06 2001 - 02:11:01 EEST

The functionality and the requirements make this a tough order to fill. Unfortunately, getting parts to test that will have the same characteristics of an injection molded ABS part is not going to be done unless the part is indeed injection molded. The reasons for this include:
1. Material packing affects the relative strength of the teeth and can impact the functionality. I.E., a prototype injection molded part may not pack the material into the mold at the same pressures that a standard injection molded part, and thus characteristics may be compromised. Many prototype injection molding operations use low pressure machines, and prototype tools where packing and pressure is not the same as the production part and thus will not mimic the performance of the production part. Another factor on the molded parts is the cycle time and injection pressures will affect the part's fill, shrinkage, etc
2. Gating and mold flow can affect the filling of the part and of the fine features, and thus impact the functionality.
3. Machining the parts from standard ABS or nylon stock may not allow for the same material use as the injection molded part will have. There are lots of nylon and ABS grades, and each one is different. Bar or sheet stock is typically not the same material as an injection molded piece would be.
4. On the teeth, (though I don't what exactly what the part looks like...) if they have any force being applied to them, machining may actually cause notch sensitivity if corner radius is not observed and extreme care is not taken on the finish out.
5. Plus, the strength of the teeth will be different depending on the draft and radius that the mold is cut with. Different mold makers use different radius and draft angles as a routine unless the call-outs note the angles and radius. Thus a machined part may not reflect the real thing.
There are a ton (every pun intended) of reasons that getting a prototype ABS or nylon part with fine detail that is going to be tested for functionality for the usefulness of an injection molded part may run into trouble. I have sympathy for you. Doing the customer education is always challenging, and experience has shown that "cheap molds" coupled with intense requirements often lead down a path of trouble.
Call me if you have more questions.
B. J. Arnold-Feret

-----Original Message-----
From: Ron Clemons <rclemons@harvest-tech.com>
To: RP Mailing List <rp-ml@bart.lpt.fi>
Cc: Michael Mayes <mmayes@harvest-tech.com>
Date: Tuesday, June 05, 2001 12:15 PM
Subject: Another Mission Impossible

    I'm sure every bureau gets their share of these, but.....I have a customer who needs some "choice plastic" protos, with very small detail for functional testing, and of course they don't want to spend a lot of money! Therefore, tooling/molding and fine-detail machining come to mind first. The standard RP methods have been ruled out because the teeth need to be fully filled and the strength requirements exceed the capabilities of the RP stuff. The customer is having low cost tooling done overseas, but wants some parts beforehand for testing. Any realistic realistic suggestions are welcomed.
    "Ring Part"
    * outer dims: 1.30"D x .180"H
    * teeth: on top of ring, .046" base, .034" from base to apex
    * ribs: .29" base x .009" apex
    * material: nylon 6
    "Tubular Part"
    * outer dims: 1.135"D x 4.913L
    * has threads on one end on the OD
    * material ABS
    Ron Clemons
    Harvest Technologies
    "Where Innovation Takes Shape"

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