4 Things To Consider That Affect The Cosmetic Quality Of An Injection Molded Part
Injection molding is the best way to quickly create plastic parts with high levels of accuracy, but there are some challenges inherent to this form of modeling. If you're trying to manufacture a part that is always visible and needs a high level of cosmetic quality, you'll need to make some basic changes to your prototype design before sending it to the manufacturer. Watch out for these four common challenges that affect how a finished piece looks.
Make Text Large Enough
For most visible plastic parts like handle housings or part shields, it's a convenient to add the brand or product's name in the form of raised letters. You can also note patent numbers, safety warnings, and other important details by adding text and symbols to an injection molded piece. Aside from making the text large enough to be legible, you should also make sure that the letters will fill properly inside the mold by
- Sticking to a stroke width of at least 0.02 inches for recessed designs
- Leaving a minimum of 0.02 inches between letters, especially if they're raised
- Limiting the depth or height of the lettering to avoiding mold sticking issues.
Even Out the Cross Sections
Warping usually interferes with the fit or use of a part, but sometimes the warping is so minimal that it only makes a part less attractive. This is still a major problem for visible components, so you need to minimize drastic differences in material thickness across the part. If a thin cylinder wall joins with a very thick base section, the resulting cooling differences will leave depressions in the thick surfaces or cause the thinner areas to warp. Aim for the most even thickness across the various surfaces of the injection molded part to avoid visible distortion.
Consider the Cost Trade Off
Before redesigning your part over and over again aiming for absolute cosmetic perfection, consider what small flaws you can put up with in the finished product. Each improvement to the design specifications or mold quality will drive the cost of the part up. If you're trying to release a new product on a budget, aiming for cosmetic perfection may hold you back and limit your release so much that your exposure to new customers is restricted. It's better to balance cosmetic perfection and manufacturing cost, especially in the beginning when first prototyping new parts.
Prepare for Texture Challenges
Finally, don't forget about how surface textures effect the molding process. Since injection molding polymers shrink slightly as they cool, molds are usually designed with a little extra space known as the degree of draft. This slight angle between a part's surface and the mold wall it touches allows it to slide out cleanly instead of sticking or ending up with a marred surface.
Textures increase the amount of draft needed inside the mold to keep the plastic part from sticking to the frame as it shrinks and hardens. Only an experienced plastics designer can determine exactly how much draft is right for each textured pattern added to a part because extra draft is only necessary on surfaces running parallel with the mold opening. Different textures also work better with specific polymers, but the right plastic for the function of a part may not provide the right cosmetic characteristics. Trying to get a fine grain pattern on a plastic known for not holding a lot of detail will only end in frustration and sub-par results.
Consider having your designs run through an injection molding simulator before paying for the first prototype to go through manufacturing. Simulation helps you determine how curves and ejection ports affect the final look of the piece by leaving behind drag or press marks. A simulation can help you fix a lot of small cosmetic issues before spending the money for mold setup and production.
For more information on injection molding, check out a company like Accurate Products Inc.