80 Questions Moldmakers Should Ask Themselves … and Their ...

As a consultant, I continually see mold and molding issues that could easily have been avoided. The most common problem has little to do with cutting steel. The problem is failing to consider, obtain and verify all the pertinent information up front—prior to the mold’s construction.

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It’s the responsibility of the moldmaker to ensure the mold will produce dimensionally, functionally, and aesthetically acceptable parts for the life of the program. There is no way to achieve that goal without first determining what the dimensions, function, aesthetics, and lifespan requirements are from the customer.

Most clients are not well versed in our industry, but all too often think they are experts. Still, they prefer to work with knowledgeable vendors—someone who is looking out for their best interest. It is the moldmaker’s responsibility to educate the customer, which also serves to protect the moldmaker. For example, no matter how insistent a customer may be, the moldmaker should never agree to build a seven-cavity hot-runner family mold for parts of various sizes and wall thicknesses in a high-shrink semi-crystalline material.

Don’t laugh. This is a real example, and the project is 11 months late and will soon be converted into four molds. The expensive hot-runner system is now a boat anchor. If that wasn’t bad enough, the moldmaker agreed to amortize the mold into the piece price, with no down payment. Ultimately, if the molder can’t produce acceptable parts, the finger is going to get pointed at the moldmaker.

This tooling checklist is focused on communicating all the pertinent information about the mold from and to the customer.

With all this in mind, I compiled a list of questions that should be asked both internally and to the customer to help avoid any delays or oversights that could affect getting paid for the current mold, as well as any future tooling orders. I am not going to expand on why each question should be asked. If you are an experienced moldmaker, you already know why. None of the questions below are about the details of the mold design, such as the type of interlocks, plate thicknesses, eyebolt holes, etc. The list is focused on communicating all the pertinent information from and to the customer.

If the molder can’t produce acceptable parts, the finger is going to get pointed at the moldmaker.

Initial Inquiry Questions

 1.  How did you hear about us?

 2.  What is the end use of the product (medical, automotive, consumer, etc.)?

 3.  Are there any mating parts?

 4.  Do you need any part-design assistance?

 5.  What are the expected annual production quantities?

 6.  What is the expected lifespan of the product?

 7.  Do you have a CAD file of the part? If so, what is the file format?

 8.  Do you have or need a 3D-printed or machined prototype?

 9.  Do you have a 2D dimensioned drawing of the part?

 10.  What is the material type?

 11.  Will the parts be made in more than one type of material or more than one color?

 12.  Do you have any mold-design specifications that need to be adhered to?

 13.  Can we get your molder involved in future discussions?

 14.  Is the number of cavities desired based on the mold cost, part cost, or production requirements?

 15.  If the mold is to be a MUD or other quick-change insert type, is a specific frame size required? If yes, what is the frame size or number?

 16.  Do you know if you want a cold runner, hot bushing, hot runner, valve gate, etc.?

 17.  Are there any interchangeability requirements, such as various versions of the part or engravings?

 18.  Are there any inserts that will be overmolded?

 19.  Are there any secondary operations that might affect the mold design?

 20.  What are the surface finish requirements for the cavity?

 21.  What are the surface finish requirements for the core?

 22.  How many samples will you need initially?

 23.  Are any inspection or testing criteria required for the parts?

 24,  Are there any mold qualification requirements?

 25.  Is there anything else we should know?

 26.  How many other moldmakers are quoting this project?

 27.  Are any of these moldmakers offshore?

 28.  When are you expecting to place the order for the mold?

 29.  How critical is the delivery date?

 30.  If you gave the potential client a ballpark mold cost, what was their reaction?

Internal Review of the Supplied 3D Model

 31.  Did the model translate well?

 32.  Should the part design be modified to reduce the mold cost or increase its longevity?

 33.  Can the model be modified in-house?

 34.  Are any actions required (cams, lifters, two-stage ejection, unscrewing, etc.)?

 35.  Are there any holes or features requiring shutoffs?

 36.  Does the part have uniform wall thickness?

 37.  Is additional coring needed?

 38.  Is there sufficient draft to prevent sticking or scuffing in the cavity?

 39.  Is there sufficient draft to prevent sticking or scuffing on the core?

 40.  Are there any undercuts?

 41.  Are there radii on both internal and external corners?

 42.  Should any radii be removed to eliminate cavity impression in both halves of the mold?

 43.  Are there any ejection concerns?

 44.  Are there any gating concerns?

 45.  Are there any filling concerns?

 46.  Should a flow analysis be considered?

 47.  Are there any cooling concerns?

 48.  Should high-conductivity materials, or conformal cooling be considered?
 

Internal Review of the Supplied 2D Drawing

 49.  Does the 2D drawing have everything defined? Everything includes

        a.  Dimensions, 

        b.  Tolerances,

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        c.   GD&T requirements (straightness, flatness, concentricity, etc.),

        d.   Surface finish or texture,

        e.   Notes.

 50.  What is the date and revision level of the drawing?

 51.  Are the critical dimensions identified and toleranced?

 52.  Are there any dimensions or tolerances that are not obtainable for tooling or molding?

 53.  Has the material type and grade been specified?

 54.  Does the material contain any abrasive fillers, such as glass?

 55.  Does the material contain any flame retardants or other additives?

 56.  Do any of the notes refer to documents that were not provided?

 57.  Should a prototype tool be considered?

The more steps we take up front to minimize risks, through the collaboration of the concerned parties, the more competitive and profitable we will be.

Part Design Review with Customer—Prior to Mold Design

(In addition to any concerns or questions from above.)

 58.  What is the controlling document—a 3D model or a 2D drawing?

 59.  Are you familiar with the inherent issues of family molds?

 60.  Are runner shutoffs required?

 61.  Is the intended gate location acceptable?

 62.  Can a gate recess be added?

 63.  Can ejector-pin bosses be added on deep ribs?

 64.  Are samples and 2D drawings of the mating parts available?

 65.  Are alternate material types or grades acceptable?

 66.  Are you familiar with anisotropic shrinkage?

 67.  Who is specifying the shrinkage factor?

 68.  Who is supplying the material and colorant?

 69.  Are the intended molding machine’s specifications available?

 70.  Is the molder planning on using a robot or picker?

 71.  Is it acceptable if the engraving is raised on the part?

 72.  Is a recycle symbol, date clock, revision symbol, or cycle counter desirable?

 73.  Do you have artwork for any engraving?

 74.  Are the font type and size specified?

Mold-Design Review with Customer

(In addition to any concerns or questions from above.)

 75.  Is the cavity number location, size, and depth acceptable?

 76.  Are the ejector pin locations acceptable?

 77.  Are the witness lines from any actions acceptable?

 78.  Should some areas be left steel safe and adjusted after sampling?

 79.  Should any spare parts be made?
 

Molding Considerations

If you think molding issues have no effect on the part or mold design, think again. For example, sink and knit lines will affect the gate location. Splay and jetting will affect the gate type, size and location. Flash will affect shutoff considerations, steel types and heat treatment. Sink, warpage, and material type will affect both the part and mold design. Burn marks and shorts will affect the vent types and locations. Color values will affect the part’s wall thickness and mold’s surface finish.

 80.  What are the limitations on each of the following?

        •  Color (L, a, b, Delta E, gloss),
        •  Gate vestige,
        •  Sink,
        •  Warp,
        •  Knit or flow lines,
        •  Flash,
        •  Shorts,
        •  Splay,
        •  Burn marks,
        •  Black specks,
        •  Grease or dirt,
        •  Other.

 

Our business is full of risks. The more steps we take up front to minimize these risks through the collaboration of the concerned parties, the more competitive and profitable we will be.
 

1. Avoid Non-Uniform Wall Thickness for Molded Parts

When designing a part for injection molding, having the wall thickness come as close to uniform as possible will make for a stronger part. Otherwise, for example, thinner areas will tend to sink and warp. If you need to design thinner walls in places, they should be no less than 40-60% the width of adjacent walls. And watch out—some materials have quite specific thickness requirements to keep parts strong. Knowing material specifications can save you a lot of pain later.

2. Transition Gradually From Thick to Thin Areas in Molding

No one likes weak, saggy parts, so here’s something to think about that will help you avoid them. After ejection from a mold, parts have to cool. Thin areas will inevitably cool faster than thick ones. That temperature differential can create weak areas, which lead to sink and warp. The solution is to avoid sharp differences in wall thickness, something we just touched on. Instead, create gradual transitions between these areas. You and your parts will be happy you did.

3. Be Cautious with C-Shaped Molded Parts

No matter how you turn it, C-shaped parts are inherently weak, unless supported. Poorly designed molded parts have a tendency to warp, especially when using glass-filled materials. Fiber-reinforced materials are used for their added strength properties, also for heat resistance. Fibers in the plastic counteract shrinkage effects due to molecular orientation whether in amorphous or semi-crystalline materials. But fibers don’t expand or contract as temperature changes, so fiber-filled materials typically experience reduced shrinkage in the direction of their orientation.

Because the fibers cause non-uniform shrink within the molded part, supported C-shaped parts can warp even more than an unfilled material.

4. Consider Carefully Your CAD File Format

Sometimes we get CAD files that were translated from .STL files. While .STL files are acceptable for 3D printing, they create problems with molded parts. That’s because they reveal the part’s surface as a series of triangles, rather than the true curves you would find on real parts.

The bottom line is that we will be unable to quote these designs and send them back requesting changes, which costs you time.

Instead, output your designs as STEP files using CAD software such as SOLIDWORKS, Inventor, Pro-E, Catia, or any number of others and define thicknesses clearly, then you’ll be good to go.

5. Use Radii Wisely for Injection Molding

Building radii into a part can be a hit and miss proposition. Only use them in necessary areas, for example, to eliminate sharp edges that an end user may touch, or in critical functional areas such as a lead-in to an assembly.

Some radii are used on inside corners of critical features to make more robust geometry. Inside radii can also help on material flow patterns. In general, proper placement of corner radii can create stronger molded parts.

6. Be Careful with Parting Lines and Radii for Molding

Parting lines always make for interesting design and manufacturing challenges. Typically, we want them to be as unnoticeable as possible, and deciding where to locate them is an exercise filled with aesthetic and mechanical considerations. But watch out. If you try putting a fillet or radius around a parting line, you might end up with undesirable minor undercuts in your mold and maybe even flash.

7. Eliminate Undercuts in Molded Parts if Possible

Undercuts are features that make it difficult to eject a part from its mold. Sometimes these are created using techniques such as side-action cams or pick-out inserts, but these add manufacturing time due to molding costs and complexity. If it’s possible to eliminate undercuts, it will definitely speed up manufacturing time.

8. Determine if Cosmetic Finishes are Necessary

Cosmetic finishes are possibly the most overlooked piece of the design puzzle. If you don’t need them, don’t order them. You can always go back and add cosmetic finishes to future parts, if necessary. It’s yet another way to save yourself some time and expense. That said, if you do have any questions, don’t hesitate to call our applications engineers to discuss specific finish grades. Also, don’t forget that all-over finishes take longer to, ahem, finish, than targeting specific areas.

9. Treat Each Part as Separate Quote

Assemblies represent a collection of parts, and sometimes you only want one of those parts quoted and manufactured. Even if you want pricing on every element contained in an assembly, treat each part as a separate quote. It makes it easier for us to figure out what you want quoted, and doing that will help speed up the quoting process.

General Injection Molding Rules that Totally Rule

Beyond those issues to avoid or carefully consider, here are some best practices items to keep in mind. We couldn’t go through all this information without listing a few things you should do.

Shutoff angles
Maintaining a 3-degree shutoff between mold components is critical for long-lasting, robust molds.

Text/Engraving
To get raised (embossed) text in your part, use engraving in your mold design. The text you mill into the mold will become raised text on your part. Milling the text into your mold is a fast process, leading to faster turnaround times. Also maintain at least a 0.020 in. (0.508mm) stroke width to achieve clean text.

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