Mold Building Basics | Mold Classifications | Material Types | Design Standards

Knowledge

Design Standards

On finished parts, a small rough spot will appear at the gate, a small line will occur at the parting line, and a round mark will occur where each ejector pin is located. Keep these features in mind and consider how they will impact your part.

A well-designed injection-molded part has common traits. The design of a part can make a difference in how well the part molds and accomplishes your goals. Some of the most frequently considered features include:

• Wall thickness
• Rounding corners
• Angled walls

Wall Thickness

When looking at wall thickness, there are two considerations. The walls should be as thin as possible, and all walls should be uniform in thickness.

In general, the thinner the wall, the higher the cost savings. Thinner walls mean parts cool faster, which means that cycle times are shorter, resulting in more parts per hour. Further, thin parts weigh less, using less plastic per part. On average, the wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch). Thin-wall injection molding can produce walls as thin as .05mm (.020 inch).

Molding Defects Caused by Varying or Oversized Walls

Warping

Parts with walls of uniform thickness allow the mold cavity to fill more easily because the molten plastic does not have to be forced through varying restrictions as it fills.

If the walls are not uniform, the thin section cools first. Then as the thick section cools and shrinks, it builds stresses near the boundary area between the two. Because the thin section has already hardened, it doesn’t yield. As the thick section yields, it leads to warping or twisting of the part, which, if severe enough, can cause cracks.

Voids and Shrinkage

Walls that aren’t uniform in thickness can cause shrinkage problems. Examples might include ribs, bosses, or any other projection of the nominal wall. Since thicker walls solidify slowly, the area they are attached to at the nominal wall will shrink as the projection shrinks. This can result in a sunken area in the nominal wall. To reduce shrinkage, rib thickness should be between 50 and 60 percent of the walls they are attached to.

POSSIBLE SOLUTIONS TO VARYING OR OVERSIZED WALLS

Transitions

If design limitations make it impossible to have uniform wall thickness, consider making the change gradual by using a “slope.”

Coring

Coring is a method of removing plastic from the thick area, which helps to keep wall sections uniform. This eliminates the problem altogether.

Ribs

To strengthen parts, instead of using thicker walls, use additional structures such as ribs. Rib thickness should be less than wall thickness to minimize sinking effects. The recommended rib thickness should not exceed 60 percent of the nominal thickness. Plus, the rib should be attached with corner radii as generously as possible.

Recommended Wall Thickness By Resin

Resin Type	Wall Thickness
Acrylic	.025-.150
Polypropylene	.025-.150
Acetal	.030-.120
Polystyrene	.035-.150
Polycarbonate	.040-.150
Acylonitrile Butadiene Styrene	.045-.140

Rounding Corners

Round corners and edges wherever possible. Sharp corners greatly increase stress concentration, which can lead to part failure. Sharp corners often come about in non-obvious places, such as a boss attached to a surface or a strengthening rib.

Angled Walls

In a straight pull mold, it’s important to angle the walls to create draft on the part. Draft creates a taper on the walls, allowing the part to be removed from the mold with less possibility of cosmetic defects such as drag marks. For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) – especially for textured walls and walls higher than 0.25".