7 Effective Plastic Molding Tips

1.           Uniformity Is Something to Consider When Designing Your Parts

Injection molding refers to forcing resin in liquid form into the 2 separate halves of a mold. Any changes or restrictions in the thickness of your parts will affect the flow and this may have other adverse effects later on.

The best option would be to ensure that the thickness stays constant and around 2.0mm to 3.0mm. Avoid designing custom moulding with less than 1.0mm or larger than 4.0mm wall thickness since it only creates extra headaches.

2.         Your Parts Should Have Drafting

You may have or may not have heard about drafting when it comes to injection molded parts. Drafting, or the addition of a draft angle, allows your parts to be released from the injection mold. With draft angles, you should use at least 1° on untextured molds and at least 3° if working with a textured mold surface.

Such minimums will allow your parts to easily release from the mold without the need to pry them out. Your customer might insist on a design that demands a tight mating area. In such a situation, try localizing the zero-draft area as close to the mating portion as opposed to a complete surface.

3.         A Radius Should Be Added Wherever Possible

Sharp corners on an injection molded part tend to be difficult to completely form. Air becomes trapped in the sharp corners, so your best course of action would be to design them out. A radius usually lends itself to a draft angle to ease transitions and make sure that your part can be easily removed from the injection mold.

4.         Resin Flow Should Always Be Designed to Flow from Thick to Thin Sections

Designing in injection molded parts sometimes requires thicker sections for strength and structure. Molten resin flowing through the injection mold loses both temperature and pressure.

If a mold flows from a thin tight section to a larger wall thickness, it will have trouble completing far away from the gate once the resin is introduced in the mold. Gates need to be located at thicker sections of the part’s design and resin should ideally flow to the thinner sections last.

5.         Decide Which Molding Defects Are Acceptable

Injection molding results in defects. Two halves of the injection mold have to come together and there’s a chance of a parting line at the interface. On the mold’s thicker sections, there could be sinks that are caused by bosses, which are designed into the surface’s backside.

Strengthening ribs in the part, which are specifically designed to add structure can also add the potential for visual defects. While it is possible to limit some of the defects with advanced molding conditions, you can’t always negate everything.

You can choose a surface finish that reduces the chances of it becoming a quality problem. You can also consider moving features to locations on the part that will not be a problem even if they end up leaving a mark. Decide what defects are and aren’t acceptable, then design around their limitations.

6.         Strengthening Rib Sizes Should Be Reduced as Much as Possible

Strengthening ribs play an important role, but having too large of a feature can end up creating more problems. There are 3 key design considerations for each rib: the rib height, the thickness of the base, and the overall thickness.

The rib base should ideally be designed at 60 percent or less of the wall thickness to reduce the chance of a sink mark on the surface. The rib height needs to be as short as possible to reduce the chance of it getting struck in the injection mold.

The standard when it comes to rib height is keeping it less than 3x the part thickness. If you have a part 1.0mm thick, the rib should be a maximum of 3.0mm in height. The overall thickness will end up being smaller than the thickness of the base that’s directly related to the draft angle designed in. More is not necessarily better when it comes to strengthening ribs, which is why they should be used sparingly.

7.         Undercuts in the Tooling Should Be Avoided

Undercuts in the injection molding tools are when the opening and closing of the tool doesn’t allow for the formation of a feature. In such a case, it would be better to use a lifter and slide to form the feature as opposed to complicated shapes. Simpler is always better in molding. The lifter and slide can form complex shapes that still allow the part to be removed from the injection mold.