March 2010 Design Tip

Woe With the Flow (and How to Avoid It)

We’ve written often about issues—sink or gas bubbles—that can be caused by overly thick walls in plastic parts, but thin walls can be problematic too. Obviously a feature must be thick enough to serve its purpose and handle expected stresses, but it must also be thick enough to allow proper resin flow during moulding.

Identifying features that are too thick is relatively easy; too thin is another matter. Resin flow is affected by many factors including resin characteristics, gate location, wall thickness, and other aspects of part geometry. Any of these can slow resin flow, resulting in incomplete mould filling or structurally weak knit lines. For this reason it is important to anticipate the flow of resin into and through a mould. This can be difficult in complex designs, which is why Protomold developed ProtoFlow®, a sophisticated program that models resin flow needed to produce a customer’s 3D CAD design. Its analysis is offered free with Protomold’s ProtoQuote® online quotes. Developed using cutting-edge algorithms and run on Proto Labs’ massive compute cluster, ProtoFlow considers geometry, wall thickness, and resin characteristics in simulating resin flow through a gate and into a mould.

We can view mould filling as a race in which the goal is to get resin to every part of the mould before it hardens. In running that race you have several factors working in your favor. One is the ability, within limits, to choose the location of injection gates. Another is the ability to apply pressure of up to 15,000 psi to move resin through the mould. At the same time there are several factors working against you. First is the fact that resin cools, thickens, and eventually hardens as it travels through the mould. Also, whenever resin flows meet—either coming from multiple gates or after flowing around an obstacle—they can form structurally weak knit lines if the flows have cooled too much to fully meld where they meet.

To help anticipate and head off problems, Protomold offers free, computerized flow analysis. A ProtoFlow analysis is added to any ProtoQuote in which there appear to be potential flow issues. The ProtoFlow analysis is a 3D animated simulation that shows the temperature and fill pressure of your chosen resin as it flows through the mould (see Figure 1). If you do not specify a resin, the system assumes ABS as the default.

Figure 1

Figure 1

The ProtoFlow animation includes two simulations. The temperature analysis shows cooling of resin as the mould is filled which can help predict potential warp in the finished part. The pressure analysis uses both color and isobars to show pressure at various points in the resin flow during mould filling. Greens and blues indicate low pressures, which are not likely to cause problems; golds and yellows, along with “crowded” isobars, indicate areas of high pressure, which can cause weak knit lines, increased flash, “short shots,” and gas trapping.

Where problems are indicated, the first step a designer can take to reduce pressure is to increase wall thickness in the problem areas. In some cases, increasing thickness by as little as 15 percent can resolve the problem. In other instances, thickness may have to be increased by 100 percent or more, but this can be done incrementally allowing you to address potential fill issues without increasing wall thickness more than is necessary. In addition to thickening walls, you can also consider specifying a resin with a higher flow rate. Protomold customer service engineers can help you determine how best to address any of these issues.

A “successful” ProtoFlow analysis can significantly reduce the likelihood of flow problems, but it does not guarantee that there will be no issues in producing parts from your model. Also, while a ProtoFlow analysis is not automatically generated with every ProtoQuote, you can request a free ProtoFlow analysis with any ProtoQuote by contacting customer service at +44 (0) 1952 607447.

For basic information on minimum wall thickness for various resins, go to our recommended wall thickness information. While ProtoFlow helps ensure that wall thickness is appropriate from a mouldability standpoint, only you can ensure that your design is suitable for its intended purpose.