When it comes to rapid injection molding, there are four “rules” that are equally important. Ignore any one of them and it can stop you in your tracks. Thus, the four #1 rules for successful rapid injection molding are:

1a) Maintain uniform wall thickness
1b) Maintain appropriate draft
1c) Understand the resins you plan to use
1d) Understand the manufacturing process

Mold resins are available with numerous features and benefits. Check out a manufacturer’s or service bureau’s web site for details, then consult with your chosen vendor to ensure the right choice.

Uniform wall thickness

Uneven wall thickness is an open invitation to a multitude of problems. Depending on gate placement, it can lead to incomplete mold filling if resin must pass through a thin area to reach a thick one. And because resin shrinks as it cools, thick areas may shrink more than thin ones, which can lead to warp in the finished part.

So, if walls are to be identical (or at least similar) in thickness, what should that thickness be? If it is too thin, parts won’t be structurally sound, but if they are too thick they may shrink enough to cause unsightly, potentially risky surface sink.

Also, because dissolved gases are released as resin cools, thick walls can develop bubbles at or below the surface, weakening the part. The ideal thickness of a wall will depend on its function and on the resin used. (Information on acceptable wall thickness for various resins can be found on the Design Guidelines page of the Protomold web site). Our ProtoQuote(r) design analysis can make some guesses about wall thickness issues, but since we don’t require you to select a resin type until you’re ready to order, you need to factor in resin-specific properties when designing your part.

If a feature needs to extend above or below the rest of the part surface, it need not be thicker than the adjacent areas. Instead, it can be designed as a cored-out feature rather than a solid one.

A good rule of thumb in parts design, according to Protomold’s Kevin Crystal is, “When in doubt, draft it.” When you pull a straight-sided plastic part out of a straight-sided mold, drag marks or other damage can occur. To prevent this result, surfaces that are parallel to the line of mold opening may have to be drafted – angled away from the line along which the part will be ejected. The angle causes the part wall and mold wall to move apart during ejection. Your CAD package may not tell you which surfaces should be drafted, but it may indicate which surfaces are drafted (or reverse-drafted). Shallow, smooth-sided features may not require draft, but deep ribs, holes, inside cores, and multiple standing features usually do. Textured surfaces have a particular tendency to stick to mold walls. Typical recommendations are that light texture requires 3 degrees of draft, and heavy texture requires at least five.

Appropriate draft

Let’s use baseball as a metaphor here. Picture a base runner sliding into second base just under the fielder’s throw. Now imagine the condition of his uniform (and maybe his hip as well) as a result of that slide. That’s what can happen to undrafted surfaces when a mold opens and the part is ejected. Proper draft ensures that the part surface and mold surface will draw apart instead of being dragged across one another during ejection. The required degree of draft needed to avoid damage depends on a variety of factors including height, location, and surface texture of the feature.

Draft is almost always required for surfaces that are parallel to the direction of mold opening. In parts with cam-driven side actions, draft is also required for surfaces parallel to the direction of cam action. And shutoffs-surfaces where mold faces meet-that are parallel to the direction of mold or cam opening require draft as well.

Resin characteristics

The characteristics of various resins differ across too many dimensions to discuss in detail, but we want to remind you of the issues that can affect the molding of your part:

• Obviously, mechanical properties such as strength can be an issue; stronger resins may require less material to meet your requirements.
• Shrinkage varies among resins and can definitely affect moldability. This can be of special concern with filled resins, which shrink unevenly depending on the direction of resin flow.
• Viscosity, and the ability to fill small features, also varies among resins.

Manufacturing process

Our rapid injection molding process is fast and affordable, compared with traditional injection molding. However, our process does have a few limitations that you should understand. Other vendors process will also have limitations that you should explore. Ours include:

• maximum part sizes
• specific requirements regarding side actions
• limits on the use of fine detail adjacent to steep walls
• the sharpness of the outside corners of parts
• the need to accommodate ejector pins

Details regarding these requirements can be found at our web site or a representative can answer them.

Proto Labs
www.protomold.com

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