TIPS WITH TONY: Mold Flow Analysis

Last week we discussed wall thickness by resin types, where we learned the importance of uniform wall thickness and provided a guideline on thickness based on your material selection. Another very useful resource in selecting materials is a mold flow simulator, which tests different resins and how they fill using accurate molding pressure.

Melt Flow Index
All materials have a different melt flow index (MFI), but what does this exactly mean? MFI is a measurement taken on how well a material flows at a high temperature through a specified diameter during a 10 minute test. This measurement for MFI is calculated into grams per 10 mins. Typically, higher numbers mean you have a much better flowing material that can fill thin wall geometry easier. But that doesn’t tell the entire story as a higher MFI doesn’t always mean that you won’t encounter any issues on thin part geometries. All materials have varying melting temperatures, so compare MFI between different families of materials such as polyethylenes and polypropylenes, which have about a 40° difference in testing temperatures.

Knowing the Material
Work with Proto Labs’ customer service engineers (CSEs) as soon as you begin quoting your parts and tell them what material you are considering having the parts produced in. Having this information early allows them to properly analyze part geometry for appropriate wall thickness using the MFI of the selected material. Often times, a part that is too large or has features that are too thin for a selected material will require an increase in wall thickness or an alternative material to be chosen.

Simulation
How does a mold manufacturer know the selected material will work? This is where the software takes over. Proto Labs uses a proprietary ProtoFlow® fill analysis program that is truly unique to our molding technique. We have several available materials that can be tested using a resin’s MFI and your CAD geometry.

The ProtoFlow simulation shows the resin fill of a part through a single gate location at the end of the part and the color represents the part filling through to completion.

After your CAD model has been uploaded, gate location and quantity of gates are selected based on your part’s geometry and material. A simulation is then run by our mold designers to review:

  • gate location
  • knit lines
  • incomplete fill
  • balanced fill
  • and most importantly, fill pressure

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TIPS WITH TONY: Wall Thickness by Resin Type

Knowing the material your parts are going to be manufactured in early on in your development process can save you time, money and a lot of frustration. You should work closely with your manufacturer during design, so they can help you identify potential material issues before any parts are actually molded. Because some part geometries inherently work better with certain resins, your manufacturer can help guide you toward the appropriate material options.

Issues that can result from selecting an incorrect material:

  • Warp
  • Sink
  • High fill pressure
  • Poor cosmetic finish
  • Shorting or burning
  • Brittleness

Uniform Wall Thickness
With injection molding, we talk a lot about how uniform wall thickness helps improve mold fill versus thin features that can restrict the material and create a number of the aforementioned issues. Having connected walls that are too thick and too thin can affect how a part cools, thus creating sink and warp. Furthermore, the same issues can arise if your entire part is too thick or thin.

 

Watch rib-to-wall thickness ratios. To prevent sink, rib thickness should be about half of wall thickness.

This is why the appropriate rib-to-wall thickness ratio must be followed. The appropriate thickness for a rib that is extruded from another surface is approximately half the thickness of the adjacent surface. This is the optimal part design to provide strength while at the same time reducing your chances for significant warp or sink. Learn more on uniform wall thickness in plastic parts on our website.

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TIPS WITH TONY: Fine-Tuning Your Additive Resolution

When you’re watching an epic movie filled with sweeping cinematography, you probably want the highest on-screen resolution possible with, say, a Blu-ray disc or high-definition stream. But if your children are watching old Disney movies in the playroom while arguing with each other over Legos, a standard picture from a classic DVD will probably suffice. The point: Don’t overpay for something that isn’t really necessary.

Normal resolution.

Normal Res
The same thought can be applied during 3D printing when you’re prototyping with stereolithography (SL). Proto Labs uses three resolutions that range in cosmetics and functionality. Normal resolution (NR) provides the lowest cost, but lacks fine detail. With NR you get a layer thickness of 0.004 in. with a minimum feature size of 0.010 in. — but that might be all you need in early prototyping.

High resolution.

High Res
If your part requires an elevated level of precision, there’s high resolution (HR). Here, you get a layer thickness (0.002 in.) and minimum feature size (0.004 in.) half of NR. It costs more, but the boosting the part quality may be well worth it depending on your intended application.

Micro resolution.

 

Micro Res
You can even step up to a higher level of precision, which most manufacturers are unable to provide. Micro resolution (MR) — the Blu-ray of additive resolutions, if you will — can provide optimal part detail on the smallest of part features. With MR, you get a layer thickness of 0.001 in. and minimum feature size of 0.002 in. Yes, that is an actual life-sized ant (not an evil oversized ant) atop a microscopic chess board. You can even see the staircase inside the rook!

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TIPS WITH TONY: Machining Versus Molding for Magnesium Parts

In our latest tip, we’re talking about magnesium, and how to use it in product design for metal parts that need to be lightweight yet strong.

Magnesium works well for reducing component weight in place of steel or aluminum as it’s the lightest structural metal currently available. This lends itself well to a range of applications in the automotive, aerospace and electronics industries.

It’s heavily used in vehicle lightweighting to lower fuel consumption, reducing automakers overall carbon footprint. It’s also used to create lighter and thinner electronics or simply whenever a lightweight yet strong backbone material is needed in a part. Be sure to test and review magnesium’s material properties closely as it’s not always the proper substitute for other metal or plastic materials.

Magnesium versus

Steel

Aluminum

Plastic

75% lighter

33% lighter

Greater stiffness

Thinner walls

Similar or greater mechanical properties

Improved strength and wear resistance

Consolidation of parts

Consolidation of parts

Higher temperature

Machines faster

Machines faster

Creep resistant

Reduced tooling costs

Improved corrosion resistance

Fewer supports needed

What manufacturing methods are there for magnesium? Proto Labs offers both CNC machining and injection molding for prototype and low-volume mag parts, though magnesium die casting also used in the industry.

How do I choose the right manufacturing method? Quantity, lead time, size and material properties will greatly impact which manufacturing method to use.

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TIPS WITH TONY: Ribs and Radii

Designing ribs and radii into your injection-molded part is not only important to increase strength and stability — it improves material flow, eliminates thick areas that create issues like sink, and ultimately, enhances the cosmetic appearance of your part.

Ribs
A thick part will have several issues with sink or voids creating cosmetic, functional or molding concerns. The addition of ribs reduces the amount of volume in part thickness while still providing the part with the same overall height. Some believe that eliminating the volume/part thickness can decrease the strength of a part. This is not true — adding ribs can actually improve stability, strength and cosmetics depending on the material selection and part geometry.

Watch rib-to-wall thickness ratios. To prevent sink, the thickness of the rib should be about half of the thickness of the wall.

A major design consideration that is often overlooked is rib-to-wall thickness ratios. If you have a rib feature that is too thick on a wall, you’ll create thick areas that can result in unsightly sink or shadowing on the opposing surface. Avoid this by following a guide of 40 to 60 percent wall thickness for any rib.

 

 

Coring Out
Coring out is a technique where you remove material from a plastic part, leaving distinct walls and ribs that provide enough strength and mating surfaces for other parts in the assembly. It is necessary to make the part moldable and also saves cost and weight. Leave ribs in the right location and size to maintain strength, particularly in bending, and retain surfaces and features that interface with other parts in the assembly.

Ramps and Gussets
Continuing the discussion on strength and resin flow improvement, ramps and gussets are important features that you can build into your design. Sharp corners create high stress points whereas gussets and ramps are stress relievers, working to improve overall part quality.

Radii
Adding radii — edges or vertexes that have been rounded — to your part will improve how resin fills the mold as plastic can flow poorly around sharp corners. When you have sharp corners, resin can create stresses that cause the part to warp or bend, and may provide a location that can break since this is a weaker transition point. Resolve this by adding a generous angle to both the outside and inside corners.

Ideally, you should model radii so the inside and outside radii use the same center resulting in larger radii on the outside curves. This will help you retain a consistent wall thickness throughout your part.

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