For many types of engineered products, the sooner you can discover and solve design and manufacturability problems, the better. Nowhere is this more true than in plastic injection molding. High-quality molded parts require a perfect synergy between part design, mold design, and molding processes.
That’s why New Berlin Plastics conducts a preliminary design for manufacturability (DFM) analysis during the part quoting process. Its purpose is to obtain an overview of the design intent, the application of the part, and the environment in which it will live. Once those points are understood the next step is to identify any obvious issues with the part design, tool design, and material selection.
“During the design phase, making changes is still relatively easy and inexpensive. Once you’ve cut steel to build a tool, it becomes much more expensive to correct part design and tooling flaws,” explains NBP engineering director Dan Manning.
Getting involved early in the product development process also helps NBP engineers to remove unnecessary cost from the molding process and results in consistently higher-quality parts. “Identifying these problems early helps us get customers’ parts into production faster and helps to ensure that production runs smoothly. Ultimately, we want to deliver the best quality parts at the most reasonable cost to them,” Manning adds.
The NBP engineering team uses a checklist to identify the most common problem areas. Here are some of the most common issues they encounter in part and tool designs during these preliminary DFMs:
Complex part designs: Because of the way a part is designed it may require slides, or side actions, to enable the tool steel to clear the part so it can be ejected from the mold. Essentially, the core and the cavity become “trapped” by features of the part, making it hard to extract without damaging it. Often, slight modifications to the part design can significantly decrease the cost and complexity of its tooling by eliminating the need for some mold features.
Walls too thick: With metal parts, thicker walls mean stronger parts. In plastic injection molded parts, walls that are too thick can cause a myriad of problems. First, they take longer to cool. This means the cycle time to produce that part, one of the biggest contributors to piece price, is longer. Thicker parts also require more material, another big component of a part’s cost.
In many cases, thick walls can be replaced by ribs, which provide the strength required but use much less material and a shorter cycle time. These changes can have a big impact on reducing part cost.
Inconsistent wall thicknesses: Inconsistent wall thicknesses can cause inherent stresses on parts as thinner walls cool faster than thicker ones, leading to an inferior bond as the resin cools and sets.For best results, parts should be designed with a nominal wall thickness – within 10-15% of the same wall thickness throughout the part.
Inadequate draft: When plastic is injected into a mold cavity, a vacuum forms between the molten material and the walls of the tool. Parts must be designed with adequate draft so this suction can be easily broken and the part can be ejected from the mold without damage.
If a part has inadequate draft, it may stretch as it’s pulled from the mold cavity, potentially causing mechanical or chemical damage to the parts. Scuffing and other cosmetic damage may also occur. Textured parts require an even greater amount of draft. This type of problem can easily be solved by ensuring you’re adding appropriate draft in the part design.
Tight tolerances: Sometimes, parts are designed with metal casting or machining in mind. Often, their drawings include tight tolerances for certain features, which can’t be accommodated with the proposed part design or material. In situations like these, the OEM engineer needs to optimize the part design for injection molding and apply tolerances that are more appropriate to the manufacturing method and material selected.
Sharp corners: Sharp corners in designs tend to cause stress risers in the molded parts. They can also create conditions within the tool where these corners don’t completely fill with resin, which can cause defects in the finished parts.
Examples of successful DFMs
A preliminary DFM analysis can often uncover major opportunities for cost savings and improved part quality. Here are two examples:
Outboard engine manufacturer: A marine engine OEM was designing a decorative latch for the hood of one of its outboard engines. The part used a metal insert to provide weight in order to prevent unintentional closing of the hood due to the boat swaying. Because this part would be exposed to salt water, a salt spray test and stainless steel weight was specified by the customer. There was the question of how to attach the weight to the plastic part. The options explored by the OEM drove significant cost, and once everything was added up the OEM wasn’t sure the project was commercially viable.
As part of its DFM process, New Berlin Plastics recommended using a two-stage molding approach to fully encapsulate the insert in plastic. This allowed the OEM to move to a sintered metal insert, rather than stainless steel, as well as avoid the need for a salt spray test and the labor to adhere the insert. This redesign helped the OEM to meet its cost targets for the latch and enabled its launch to proceed.
Breast cup sizing: A clothing manufacturer that plans to sell bras direct-to-consumer approached New Berlin Plastics about producing a series of breast measurement cups in 13 sizes, which customers could use to determine their bra size for online ordering.
NBP’s engineers analyzed the most cost-effective way to make them. Producing all 13 of them in a multi-cavity mold would have created significant over-filling problems. In other words, the chambers that filled first would become overfilled as plastic continued to flow into the other chambers, causing flashing. Other chambers farther away from the gate may not fill adequately. The alternative, creating separate tools for each of the 13 parts, was cost prohibitive.
NBP engineers devised an innovative and cost-effective solution: a single tool equipped with a gating system that incorporated sensors and sequential valves. When a chamber fills, the valve closes, cutting off the flow of resin to that chamber while others fill. This solved the overfill problem and ensures that all 13 chambers are filled with the optimal amount of resin. This kept tooling and part costs reasonable and allowed the project to move forward for the OEM.
The bottom line
Conducting a preliminary DFM analysis helps the NBP engineering team to identify part design and tooling problems early, and to recommend solutions that will help it deliver high-quality parts at an affordable cost.
“We believe in providing prospective customers with a lot of value up front,” explains business development manager Karl Held. “The preliminary DFM is a preview of our expertise and the kind of value we can provide if you decide to do business with us.”
Contact us today to learn how we can bring our best thinking to your injection molding challenges.