Injection molding is a highly repeatable, highly scalable process. But there are a few best practices that ensure tolerance, form, fit, and function.

In a recent webinar on Designing for Manufacturability without Compromising the Time-to-Market, John Sidorwicz Vice President of inside sales and Customer Service Xcentric Mold & Engineering, and Petro Tsarehradsky, chief marketing officer at Xcentric Mold & Engineering discussed 10 important best practices.

1. Material choice. Producing high quality, consistent plastic injected modeled parts relies heavily on the chosen material. One source to explore is WhichMatWeb.com. There you can browse materials by name, type, performance, and characteristics. It’s a free site. Another option is go directly to material manufacturers.

Consider mixing materials. With today’s injection molding processes, you have the option to mix materials or include additives. Additives can be a way to enhance materials to meet your needs. One additive to consider is glass. Another is carbon fiber, which will add strength and static dissipation.

2. Appropriate wall thickness. Next to your material selection, maintaining a uniform wall thickness throughout your part is critical.

Wall thickness will often determine mechanical performance, cosmetic appearance, ease of molding the part, and the cost of the part.

Achieving an optimal wall thickness is a balance between strength and weight. A 10% increase in wall thickness provides about a 33% increase in stiffness with most materials. But avoid changing wall thicknesses within a design, going from thick to thin or from thin to thick over sections.

This will be mentioned later, but ribs and curves can provide strength to a part without resorting to increased wall thickness.

3. Draft. Draft is the angle designed into your part that aids part removal from the mold. Include draft in a design to prevent sticking an ejector pin push into your parts, especially on a cavity or show surface.

Draft, annuals, and tapers all ease a part out of mold during ejection. Less draft will sometimes damage parts during ejection, a condition known as drag, especially at the parting line.

A minimum recommended draft angle is a half a degree. A preferred draft angle is one and a half to two degrees, especially if you’re going to be adding textures.

4. Runners and gates should be designed and incorporated into a mold to ensure the consistent flow of material to fill the mold at the right pressure. A gate is the connection between the runner system and the molded part. The location and size of the gate is integral to the molding process.
5. Appropriate tolerances. A part with too tight of a tolerance can lead to loss of performance or even part failure. A team skilled on advanced manufacturing technologies can advise on the best ways to safely and effectively reach exact specifications.

Many factors come into play with tolerance, including materials, part complexity, tooling, and of course the injection molding process. A tight tolerance part for injection molding is plus or minus two thousandth of an inch. Starting with a good part design will ensure your tight tolerances.

If you have a tight tolerance chart, look at using low shrink rate materials such as ABS and polycarbonate materials, which have a low shrink rate. Thicker walls produce different shrink rates depending on the material, making repeatability difficult.

Higher shrink rate materials are going to be materials like nylons, unfilled nylons, polyethylenes, and polypropylenes.

Avoid tight tolerance areas around the alignment of mold halves, the parting line, or in moving mold components such as slides. If you have two parts coming together, such as in a housing, you may want to do a beauty reveal, where you can get away with hiding the mismatch.

6. Ribs. Ribs are often used for structural reinforcement. They allow for greater strength and stiffness in parts without the need to increase wall thickness.

As a general rule, design ribs as approximately 60% of your nominal wall, or whatever the wall that the rib is joining, to minimize risk of sink. Glossy materials require a thinner rib, typically about 40% of the wall thickness. Thin ribs may be more difficult to fill in, especially once you start adding draft to them.

A proper rib design involves five parameters:

–Thickness, this affects cooling rate and degree of shrinkage, which may cause warp.

–The height of the ribs should not exceed three times the rib base thickness.

–Location. Ribs added to uncritical areas can actually reduce impact resistance.

–Quantity. It’s easier to add ribs than move them, so they should be used sparingly and as needed.

— Moldability. Thin ribs can be difficult to fill. A good check is to request a mold flow and a manufacturability analysis through your injection molding provider.

7. Bosses are used for locating, mounting, and assembly. Wall thickness and height are the biggest factors to consider. Wall thickness around a boss design feature should be 60% of the nominal thickness, similar to ribbing.

If a nominal part thickness is greater than one-eighth, the boss wall thickness should be around 40% the nominal wall. The height of the boss will also have a role. As a general rule, the boss height should be no more than two and a half times the diameter of the hole on the boss.

8. Reduce undercuts. An undercut is any indentation or protrusion that prohibits the ejection of a part from a mold. Most commonly, it’s called an undercut, internal, external, or inaction. Undercuts typically increase mold complexity and can lead to higher mold construction costs. Use creativity and design them in a way where actions won’t be needed.
9. Corners and transitions tie into wall thickness. Any sharp corners can cause molded in stress, just from the resin flow.

Minimize the stress by using rounded corners, adding rads on inside corners, outside corners, and trying to maintain a consistent wall thickness.

10. Thick and thin transitions. Design these as smoothly as possible. Try to avoid steps; use a ramp to improve the flow of material through your part.

Follow these best practices in the following order to ensure an easy, cost effective molded part: materials, wall thickness, tolerances, draft, ribs, bosses, undercuts and corners, and transitions.

Leslie Langnau
llangnau@wtwhmedia.com