Injection Molding for Prototyping and Low-Volume Production: It’s More Affordable Than You Think
Ian Wright posted on July 11, 2016 |
On-demand manufacturing makes injection molding a viable production process.
Injection molding press. (Image courtesy of Proto Labs.)

Injection molding press. (Image courtesy of Proto Labs.)

When it comes to comparing production processes, there’s a longstanding assumption that injection molding is a high-cost method reserved for high-volume production runs. 

However, as on-demand manufacturing becomes more common, that assumption is becoming increasingly less accurate.

For low-volume production runs or even prototyping, injection molding represents a flexible and reliable option. If you’re considering using injection molding for your next prototype or short run production, there are several points to consider.

Injection Molding Materials

Compared to 3D printing, injection molding offers engineers a variety of material options.

“We offer anything from polypropylene, to santoprene, to glass-filled nylon and LCP,” said Tony Holtz, technical specialist at Proto Labs. “You can get several materials with varying levels of additives and fillers such as glass, mineral and colorants in addition to some very high heat- and creep-resistant resins such as Ultem and PEEK.”

Proto Labs preferred mold material is aluminum due to its ability to be manufactured fast and cost-effectively. But on occasion it uses steel for its tooling material due to molding constraints such as mold temperature. Materials such as PEEK or Ultem require higher molding temperatures than most thermoplastics, so steel is better suited for high-temp plastics. Aluminum molds are typically able to produce well over 10,000 parts at Proto Labs and are maintained in-house throughout the life of the project.

“Aluminum is a great heat dissipater,” said Holtz. “It draws heat away from the part quickly and disperses it into the atmosphere. We maintain consistent heating temperatures in our molds, without the same heating and cooling lines that take a lot of time and money with traditional injection molds. This does slightly increase our cycle times compared to traditional molds, however, depending on part size and complexity.”

Aluminum mold for an optical liquid silicone rubber part. (Image courtesy of Proto Labs.)
Aluminum mold for an optical liquid silicone rubber part. (Image courtesy of Proto Labs.)

That being said, Proto Labs also offers a variety of engineering-grade materials , many of the same materials as high-volume molders. “We’re also using liquid silicone rubber as a mold material, which is an elastic material with many benefits over thermoplastic elastomers as well as clear polycarbonate or acrylic,” said Holtz. “The most common resins are probably ABS, polycarbonate acrylic and nylon materials. We have more than a hundred resins that are, and that’s in any combination—so we might have eight nylons available, and then colorants that are added to those as well.”

Single-Cavity vs. Multi-Cavity Molds

A crucial question for any engineer working with injection molding is whether to opt for a single-cavity or multi-cavity mold. The short answer to this question is that the choice of mold type depends on how many parts you need to make. However, there is no simple cut off point beyond which multi-cavity molds are more economically viable than single-cavity molds.

For example, suppose you need 10,000 parts.

“Single-cavity aluminum molds are good for well over 10,000 parts,” said Holtz. “Primarily, a few thousand parts is what people want to be using those for. The same technology can also produce multi-cavity tools, up to around 8 cavities, depending on the size and complexity of parts.”

Injection molded part. (Image courtesy of Proto Labs.)
Injection molded part. (Image courtesy of Proto Labs.)

If you need to produce this many parts all at once, then a multi-cavity mold may be your best bet. But if that’s the total number of parts you need over a span of two years, then it may be better to opt for the less-expensive single-cavity mold.

Another consideration is whether you expect your production needs to ramp up in the future.

Having questions like these explains part of the appeal of on-demand manufacturing. “In our initial quote, we have the ability to show you what your single-cavity pricing is, what your multi-cavity pricing is, as well as the pricing structure for a part for your production run,” said Holtz. “You could easily forecast—say, if you need 100,000 parts—what number of cavities will be most cost beneficial. If you start with that single-cavity tool and realize that you need a multi-cavity mold, we’re going to deduct that initial tooling cost from your second multi-cavity mold, as long as the part geometry stays the same.”

Designing for Moldability

If you’re planning to use injection molding to produce your parts, you’ll need to design them with moldability in mind. But where do you start?

“If customers are looking at injection molding as their end process, the first thing they need is the CAD design. Send us that CAD model now, rather than waiting for weeks of redesigning in-house,” said Holtz.

For example, sharp corners can weaken parts as a result of molded-in stress from resin flow. Similarly, thin edges restrict resin flow and can break during gate trimming. These and other issues can be anticipated before the injection molding process begins through proper mold flow analysis early in the process.

Reducing sharp corners improves part strength. (Image courtesy of Proto Labs.)
Reducing sharp corners improves part strength. (Image courtesy of Proto Labs.)

“We have a proprietary mold flow simulation called ProtoFlow,” Holtz commented. “It’s unique to our method of manufacturing molds. Some customers have their own mold flow analysis, so we’ll compare the two analyses to give them a better understanding of our processes. Ours tends to have a slightly higher pressure of molding—that’s why we use our own simulation.”

“We can also provide an automated manufacturing review, removing cores and undercuts so you can decrease your manufacturing costs,” said Holtz.

Injection Molding in On-Demand Manufacturing

Flexibility is a key advantage of on-demand manufacturing. When it comes to injection molding, that means doing more than just prototyping. “We talk about on-demand manufacturing in terms of bridge tooling, end-of-life planning and supply chain emergencies,” said Holtz. “Depending on when you need it, some inventories never exceed a certain amount, so you can continue using the aluminum tooling.”

Thermoplastic injection-molded part. (Image courtesy of Proto Labs.)
Thermoplastic injection-molded part. (Image courtesy of Proto Labs.)

The traditional model for injection molding had customers own the tooling, but Proto Labs takes a different approach as part of its on-demand philosophy. “Proto Labs owns the tooling, but the customer owns the exclusive right to the parts, the geometry and that mold,” said Holtz. “We don’t discuss customer projects, parts or molds publically without their consent.”

For customers, this means not having to cover tool maintenance, storage and upkeep costs.

“Proto Labs will maintain the tool for the life of the project,” said Holtz. “If the mold wears, breaks or needs to be re-polished, customers typically do not pay for that. We’re going to repair the tool, or even replace the mold for the customer, as long as they continue to order parts from the mold.”

Two particular features of Proto Labs’ rapid injection molding make this possible. First, the use of soft tooling from aluminum allows for modifications that can’t be done easily with steel molds. Second, and more importantly, Proto Labs keeps all its mold manufacturing in-house.

“We have more than 400 CNC machines manufacturing molds and parts, and over 200 injection molding presses globally,” Holtz explained. “As one machine is completing an order, the operator is moving to the next press, setting it up and getting that one started while another operator finishes the previous one.”

This is what allows on-demand manufacturers like Proto Labs the ability to mold parts within days.

“You can get an injection mold starting at $1,500 in 15 business days or less,” said Holtz. “We work directly from a 3D CAD file and build the mold around that. We have proprietary software that generates code for CNC machines. We’re CNC milling, applying EDM and hand-finishing aluminum blocks, which is our actual mold. We don’t incorporate multi-plate molds, and heating and cooling lines like traditional injection molding companies. We’re taking what was once a complex process and simplifying it to produce quality parts as fast as possible.”

For more information, visit the Proto Labs website.



Proto Labs has sponsored this post. 

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