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It is one of those things that I find hard to believe, yet I keep hearing it again and again from various sources: poor part design is the number-one problem facing molders today. I heard it again just last month when I was attending an Executive Training seminar at RJG in Traverse City, MI.

Clare Goldsberry

October 7, 2013

3 Min Read
Poor part design is problem No. 1 for molders

An impromptu survey of attendees shows that when it comes to quality issues, 30% of the attendees have problems with short shots; 27% have flash problems; 16% get dimensional variation, which typically is poor mold design, said Gary Chastain, of RJG; and 9% get splay troubles and another 9% get burns (generally a product of poor venting).
“There are so many ways to fix something, but we often ignore the root cause,” Chastain said.  And many times the root cause is poor part design which results in a poor mold design. “And at the root of all this is lack of good processing knowledge,” he added. “You have to tie part design to processing in order to get a good mold design – one that can provide optimal cycles, productivity and quality parts.”
It would seem to be a no-brainer but from what I hear, many in the industry just don’t get it. Recently, one mold engineer wrote to me to say, “The mold design portion of the industry has not kept pace with the change from traditional moldmaking to mold manufacturing. Today’s large moldmakers still tend to design by the ‘one man, one design’ philosophy. I find that many moldmakers want to have as many mold designers as possible, while on the manufacturing side they may only have one or two top mold makers and groups of machinists.
“The better way on the design side is to have one top concept person who develops the parting line and basic concept, and support this person with various levels of CAD designers. In many cases these persons could have little – if no – mold/molding experience.”
The second major problem that this moldmaker sees is that moldmakers believe they can’t start the design until they have a perfect part model because they will have to throw too much away when the final part model is available. This causes much of the delay in getting projects started quickly. “The fact is that companies – even if they have to discard part of the design including the cavity and core blocks – would still have determined enough of the design to gain their customer approval and would be able to order and start construction while the design is being finalized,” he wrote.
Finally, couple those two problems with the fact that “way too many mold designers do not even truly understand the molding industry’s wants and needs,” and you have even more delays in getting mold construction started in a timely manner because of repeated design changes before the molder is ready to sign off on the final design concept.
Ultimately that leads to an extremely long and costly part/mold/processing validation that often includes core/cavity revisions and other changes to the mold. The part, mold, and processing validations can often consume several weeks – if not months – of time and, according to come mold makers, costs that can exceed 50% of the cost of the mold. Is it any wonder that more and more mold manufacturers have added molding for mold/process validation and engineering? It’s become a huge part of the total mold manufacturing process.
Chastain told attendees at the RJG seminar much the same thing, providing some good advice:

1)    You need processing speeds and pressures that fit the mold design.
2)    Runner design is critical to optimum molding. “I see too many poor runner designs today,” he said.
3)    Poor part design is a real problem in the industry, and leads to poor mold design.

“Moldmakers need good processing knowledge which equals good part design.” Chastain said. “Tie part and mold design to processing.”

About the Author(s)

Clare Goldsberry

Until she retired in September 2021, Clare Goldsberry reported on the plastics industry for more than 30 years. In addition to the 10,000+ articles she has written, by her own estimation, she is the author of several books, including The Business of Injection Molding: How to succeed as a custom molder and Purchasing Injection Molds: A buyers guide. Goldsberry is a member of the Plastics Pioneers Association. She reflected on her long career in "Time to Say Good-Bye."

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