Design engineers can realize important gains by working collaboratively with the companies (often at the tail end of a supply chain) that design and manufacture the tooling that critically influences the economics and quality of a plastic part.
"Some people want to build molds before they even know what the critical elements are," says Bill Muldoon, president of NyproMold of Clinton, Mass. "That's a big mistake. It's important to meet together upfront and
establish the design criteria -- how we are going to define success."
Fairly often, original equipment manufacturers (OEMs) run into problems with part quality and cost because they failed to consider mold design issues.
"Every day I am faced with things like parts that have core-shift issues," says Vince Lomax, vice president of Tech Mold in Tempe, Ariz. "Had they looked at designing the part a little differently initially, they wouldn't have had the problem. Once they have approval and begin making parts, making a print change is almost like an act of Congress."
The top center area of the mold insert shows a section made with conformal cooling channels via a laser sintering process.
Core shift in injection molding refers to an uneven distribution of the polymer meltflow around a core during the filling and packing phases. Deflection of the core by uneven meltflow causes wall thickness variation of the part. Lomax recommends consulting with the production mold maker prior to the design freeze.
"If the customer had participated in a mold design review, you could have put a ring down where the gate is located," says Lomax. "And then the polymer would have been given the opportunity to flow to the path of least resistance, which is the thicker wall section. After that area is filled, the polymer melt would go up the core consistently and evenly, and you would alleviate or even eliminate the core-shift problem."
It's an example of a significant issue because the OEM typically gives the molder a very specific core-shift specification for a part with a certain length versus diameter requirement. If the polymer melt races up one side faster than the other, the core is pushed over, moving the part out of spec. Part quality suffers and reject rates rise, severely reducing the productivity of the tool.
Four simple rules
Lomax's point is one that reflects his more than 40 years in mold engineering. Design engineers, however, can benefit greatly from a few basic rules.
"In my 27 years of plastic design experience, 95 percent of the problems in molds created by part design issues can be solved by following four simple rules," says Wayne Phillips, senior design engineer at Phillips Plastics in Hudson, Wisc. "When you're designing a model in CAD, you're essentially designing a tool, so you need to take into account the factors that allow the tool to operate properly."
It often happens in a preliminary mold design meeting that a seasoned mold designer will propose changes to the part design that will improve its production and reduce the cost to produce it. Sometimes this will stimulate a request to detail the proposed changes. Unfortunately in a second meeting when the details of these proposed changes are reviewed the decision will often be to keep the part as originally designed. Then after the mold is in production and there are issues with molding the part, a decision to follow proposed changes is then mandated.
You can take a donkey to the pond, but you cannot force the donkey to drink.
Total agreement that it is important for the design engineer to see the parts being produced. Seeing first hand voids, sinks, and knit lines helps future desigsn. Even if the mold maker can make a mold to produce parts, it is not always guaranteed that a processor can produce quality parts from that mold. A good mold shop will no-quote a design rather than enter into a losing project.
Everything described as “Design Rules” in this article is important, but its only an introduction to part design. As design engineers gain experience, they gain a wealth of knowledge that makes them so much more valuable to their employers. An inter-related skill of part-designer and mold-cavity designer emerges. This skill is often learned, but not often taught. This article is a start in the right direction. When we were struggling through it, we called in “concurrent design”.
Flash-back 25+ years. When I designed my first plastic part as a CAD designer in the early 80’s I knew NOTHING about tool design. Constant wall thickness? Draft? Gate location-? (what’s a Gate?).This is basic stuff, not taught to CAD jocks at the time. Not to mention the “zillion” issues (knit lines, glass content, shrink, fill rates) pointed out by my associates at Phillips in Wisconsin. When that first horrid little part design was reviewed by an experienced tool designer, egos were bruised, but greater knowledge ensued.
The interrelationship of part design with mold design should be a corporate mandate. Any person who designs a CAD database ought to be the same person who sits in the tool design consult at the mold-maker’s office. The concessions can be discussed on paper, before they’re locked in steel. Later, that same Jr. part designer needs to be assigned to review the tool design, to gain understanding from the mold-maker’s perspective. Subsequently, the same person should visit the tool shop, see the tool being built, see it assembled and finally; sampled at press-side. There is no greater education in manufacturing than to take part design geometry the full course; from an awkward CAD concept to the smell of molten Polycarbonate in the pressroom: the proving ground.
These are great tools for plastic part designers. Keeping the wall thickness as uniform as possible definitely helps in the final part quality. Varaiations in wall thickness especially thick to thin to thick again open up the possibility of quality issues like sinks and voids the end of fill.
In multi-cavity molds, it is always best to designate which cavity the part originated from on the part. It is not safe to assume that all cavities produce the exact same part due to flow, pressure, temperature variation, etc...
Phillips' four simple rules gives engineers a lot to go on. Also, Doug's point about making tooling issues and mold design requirements part of the early development process is spot on and can help teams avoid a lot of the rework and time delays resulting from having to make after-the-fact design changes to accommodate tooling. A lot of the CAD/CAE vendors are beginning to address this issue with new modules and capabilities as part of their broader design tool suites.
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