The three major issues to consider when making a prototyping decision are good, fast and cheap. A very strong, if not emphatic, tilt toward "good" is strongly recommended for any molding job that involves complex tooling, demanding requirements, maximum tool efficiency or repeatable tolerances.
The highest quality route to a prototype, yet also the most expensive, is construction of a single cavity insert for a production tool that produces prototype parts that have exactly the same properties as a real production part. "The biggest consideration in making a prototype is that you want to do it exactly the way you would do it in production," comments Greg Warkoski, process technology manager for Solvay Advanced Polymers.
If you're going to edge gate the part in the prototype tool, you should also edge gate the part in the production tool. If you're using a three-plate tool in the production tool, then you should do the same for the prototype tool. Gate type and location affects fiber orientation, packing pressure, fill time, or other variables that change the part's mechanical characteristics. "Either know that and accept it up front and be willing to make modifications on your production tool, or do it right the first time," says Warkoski.
One way to reduce costs is to design and build a "production-capable prototype" core and cavity set that bridges the prototype to production phase.
"If you are contemplating construction of an eight-cavity tool, you can build a single cavity for the prototype process, validate the process, and then make the other seven core and cavity sets," says Warkoski. It's the most expensive prototyping option, but production of an actual core and cavity set in production materials is the only way to make sure the prototype part has the same mechanical properties as the production part.
One caution: if you go the single core and cavity route, make sure fill time is unchanged when you go to a multi-cavity tool. If a single cavity operating alone fills in two seconds, then the screw injection velocity on the injection molding machine would need to be four times higher to ensure similar fill rates in a four-cavity tool.
There are other important nuances to consider as well.
It's a common practice to use hand-loaded inserts to avoid use of expensive side action in prototype tools. These six tips can save you plenty of headaches:
Idiot-proof the way inserts fit into the tool. Otherwise, they may be loaded backwards or even upside down. Murphy's Law applies. If you're using more than one insert, number them and make sure they can't be interchanged.
Allow for thermal expansion of the inserts if the tool is running hot. Otherwise the inserts will stick in the tool or not fit into their pockets.
Make sure the insert fits snugly, however. Nothing is worse than a stray insert damaging the tool.
Don't close the mold quickly and then slow it down as you would in production. This could dislodge the insert.
Design insert removal as part of the process. Build a fixture that facilitates removal of the inserts, which will be hot after the platens part.
Calculate how much time passes as inserts are removed and loaded. Check if that time exceeds the material supplier's recommendations for material residence time. Some materials, particularly the newer bioabsorbable medical compounds, are very thermally sensitive and can substantially degrade in the machine's barrel or manifold system if left too long.
Consider thermal issues
Follow basic rules when making a production-capable prototype tool.
Thermal management of the tool itself is also a concern. For most short-run prototyping, a single, straight through water channel will suffice. Use oil for tools running hotter than the boiling point of water. "Also make sure you draft the tool as you would in the production tool and provide for sufficient part ejection," says Warkoski. "It's not fun to pull the tool after only one shot that gets stuck." For more information on proper drafting, see Part IV in this series: Avoid Molding Pitfalls at (http;//rbi.ims.ca/4922-536) There are two special considerations when analyzing the parts after ejection. First don't measure parts until several hours after they cool. QC can wait a day. Secondly, if parts warp, don't shrink fixture them unless you plan on the same procedure during volume production. "If a part is exposed to elevated temperature (a temperature above the mold temperature), it will stress relieve anyway," comments Warkoski. If your design permits, a less expensive and faster alternative is use of a service bureau that has automated the mold quoting, mold design, and mold machining process using 3D CAD data and basic rules based on a variety of considerations. Each step of the mold build process is based on your answer to the previous question. Problems or issues in the design are immediately pointed out. Size limitations One of the constraints may be size. One service bureau, Protomold of Minneapolis, lists these size requirements:
Maximum part outline is approximately 7.5 inches by 14 inches. There is, however, an overall limitation of 75 square inches of projected mold area.
Maximum part volume is about 15.75 cubic inches.
Maximum part depth is 3 inches if the parting line can pass through the middle of the part or 1.5 inches if the parting line must be at one edge.
Protomold can support simple undercuts-up to four side actions per mold, and they all must be at the parting line. Side pulls must be perpendicular to the primary (A-B sides) pull direction. There are also some limitations created by the CNC machining process, such as inability to make sharp corners. These types of prototype tools are made from aluminum and can be used for limited production quantities in some cases. Potential runs depend greatly on materials and processing conditions. Highly glass-filled compounds run at high pressures can reduce tool life to less than 40 parts. Unfilled resins run at low pressures permit tool life beyond several thousand parts. Tools like this made by service bureaus are generally based on two-plate mold designs. Straight pulls are used for ejection. Steel or aluminum? Go back to your initial analysis of the relative importance of speed, quality or cost (see triangle diagram). To access The Design Engineer's Portal for High-Performance Plastics, go to http://rbi.ims.ca/4922-537.
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Good engineering designs are those that work in the real world; bad designs are those that don’t. If we agree to set our egos aside and let the real world be our guide, we can resolve nearly any disagreement.
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