February 2010 Design Tip

New Decade, New Rules

Just kidding! The fundamental injection moulding design rules for this decade are the same as those for the last decade (and several before that). But we wanted to reiterate them to help keep your projects on track as we stride confidently into the two-thousand-tweens.

When Proto Labs Customer Service Engineering proposed the following list of rules, we asked him to put them in order of importance. “They’re all Number One,” he replied. “Ignore any one of them and it can stop you in your tracks.” For that reason, we present Four #1 Rules for Successful Rapid Injection Moulding:

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

Uniform wall thickness

Uneven wall thickness is an open invitation to a multitude of problems. Depending on gate placement, it can lead to incomplete mould filling if resin has to 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® 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. For an example of this technique, go to a past Design Tip “Put Your Parts on a Diet.”

Appropriate draft

For a moment, let’s think football. Picture a defender sliding in for a tackle . Now imagine the condition of his kit (and maybe his leg as well) as a result of that slide. That’s what can happen to undrafted surfaces when a mould opens and the part is ejected. Proper draft ensures that the part surface and mould 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 mould 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 mould faces meet—that are parallel to the direction of mould or cam opening require draft as well. Detailed information on draft can be found at on our website. If a submitted design needs additional draft, this will be noted in the ProtoQuote.

Undrafted and drafted moulds

Past Design Tips on draft:

Resin characteristics

The characteristics of various resins differ across too many dimensions to discuss in detail here, but we want to remind you of the issues that can affect the moulding 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 mouldability. 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.
Resin characteristics

Basic information on these and other characteristics can be found on our website at Material Selections page.

Manufacturing process

Protomold’s rapid injection moulding process is significantly faster and more affordable than traditional injection moulding while sharing many of the traditional method’s capabilities. However, our process does have a few limitations that users should understand. These 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 under the heading “Protomold Specific Design Guidelines” in the middle of the page at Design Guidelines page.

Of course, if you have questions regarding these or other issues, Proto Labs service representatives are available to help at +44 (0) 1952 607447.