It seems almost every industry has embraced plastic injection molding. If you have flown on a commercial airplane, you’ve seen and touched many plastic injection molded parts. Trains? They have them. Cars and trucks are full of injection molded parts too. Most home appliances and smart devices such as laptops and cell phones are packed with plastic injection molded parts. They are everywhere. If you have a new idea for a product, plastic injection molding has probably entered your mind. If you’ve never designed a plastic part destined for injection molding, it requires managing little details. We’ve covered 10 great tips that will help you get the most from designing your injection molded parts.
Tip 1: Choose a Surface Finish to Meet Your Function
The surface finish on your parts can have many different functions. The best course of action is to decide what you need from your parts. Is it physical? Does it also need to have visual appeal? Once you know what function you need, you can then start narrowing your focus on a particular surface finish. Another decision pertaining to the finish is tied to what class of mold you need based on production volume and what material the mold will be made from. Steel molds are harder than aluminum molds and offer more options for surface finish. Steel can be polished for a smoother surface finish, which can be a better option if your product requires painting or another secondary operation to enhance your product.
Some of the finish options available include:
- Gloss, matte, or satin polish
- Leathery texture
- Mirror or lens finish
- Geometric or patterned shapes
- Etched with a designed logo
- Prepared for painting or secondary graphics
- Blasted for a rough uniform texture
Surface finish has more function than just physical attributes and visual appeal. Strategic texture and finishes can blend and hide parting lines and make undercuts possible. Finishes can also allow trapped gasses to escape the injection mold, which improves the molding process efficiency.
Tip 2: Keep Consistent Uniformity
Liquid resin doesn’t want to change direction as it flows through an injection mold. There are a few design do’s and don’ts that can help with consistent uniformity. Keep your part thickness between 2.0 and 3.0mm if possible, and reduce the thickness changes where possible in the mold. Don’t design a part that is thicker than 4.0mm or thinner than 1.0mm. these extremes can cause many problems with injection molding.
Tip #3: Add Draft Angles to Your Design
Your parts must be able to eject from the injection mold. Texturing or surface finish can add a level of difficulty to parts being ejected from the mold. Adding a draft angle (or drafting in general) allows your parts to be ejected from the mold without requiring excessive force or special tools. A general rule of thumb is to use at least 1° on an untextured mold and a minimum of 3° with a textured mold surface. These minimums will allow your parts to release from the mold without having to pry them out. If the design requires a tight mating area, localize the low-draft area as close to the mating portion rather than a complete surface.
Tip #4: Eliminate Sharp Corners and Edges
Sharp corners and edges can trap air in the injection molding process. They are difficult to form and can also lead to transition issues with draft angles. Every design should eliminate sharp edges and corners to ensure parts can be fully molded.
Tip #5: Resin Should Flow from Thick to Thin Sections
As mentioned in tip 2, uniformity is always recommended, but that is not always feasible in every injection molded part design. Thickness may need to vary for structure and strength characteristics. In the cases that part wall thickness must change, the resin should flow from the thicker section into the thinner sections. The gates should be placed at the thickest sections to ensure the part can be fully formed in the injection mold.
Tip #6: Accept That Molding Defects Are Going to Happen
Injection molding is capable of producing excellent parts, but it’s not a perfect process. The mold has two halves that squeeze together to form a part out of molten resin. There could be a parting line at that junction surface. Thicker sections of the molded part could show sink marks or strengthening ribs could contribute to visual defects. Some of the defects can be limited with molding settings, but that doesn’t guarantee every defect is preventable. Accept that molding defects will occur and decide which can be acceptable in specific locations on the part. Adjust your design accordingly based on the limitations.
Tip #7: Reduce Strengthening Rib Sizes or Eliminate Their Use
Strengthening ribs can add durability to your designed part, but they can also add molding problems too. There are three primary features that your design should consider if strengthen ribs are to be used: the base thickness, the rib height, and the overall thickness. The rib base should be designed at 60% or less of the wall thickness to reduce the chance of sink marks on the adjacent surface. The rib height should be reduced to the minimum height allowable. This will reduce the chance it will become stuck in the injection mold or it may not fill 100% in the mold either. The design standard for rib height is to keep it less than 3x the part thickness. If your part is 1.0mm thick, the rib should be 3.0mm or less in height. The overall thickness should be smaller than the base thickness, which is directly related to the draft angle designed in.
Tip #8: Eliminate Undercuts in the Injection Mold
An undercut in the injection molding tool is when the opening and closing of the tool doesn’t allow a feature to be formed. If there is a design feature that causes an undercut, a lifter and slide should be added to form difficult shapes. The lifter and slide can form difficult shapes and still allow the part to be removed from the injection mold. Injection molding designs should be kept as simple as possible to prevent future problems.
Tip #9: Utilize Design for Manufacturing (DFM) and Error Proofing
Injection molding can create amazing and complex assemblies, and with complexity comes errors. If your parts are part of a larger assembly, utilize Design for Manufacturing (DFM) and error proofing concepts to make the parts assemble easier and with less errors. Use a datum scheme to ensure the part will be assembled the same way every time. Feature 1 should always be used first, feature 2 is used second, then all extra features, etc. World-class designs should eliminate a high percentage of error potential and allow manufacturing teams to assemble products quickly and efficiently.
Tip #10: Use Rapid Prototyping to Detect Problems
Rapid prototyping has revolutionized the early stages of injection mold and parts design. In the past the design was digitally verified, then a mold made to start creating physical parts. On occasion that would show a problem with the design that had to be corrected in the injection mold. Currently a sample can be made via rapid prototype before the first attempt at making an injection mold. Rapid prototyping can also be used for revisions or iterations with the design, and also to create low-cost quick samples that can be used for manufacturing and secondary process verification. There are many options currently in use in the molding industries, and your designer should be able to use one of these options to create early samples.
What Options Are Available with Rapid Prototyping Samples?
- Metal 3D Printing– Perfect for generating complex samples with low weight and high strength
- Stereolithography (SLA) – good for low volume production parts with a quality finish and increased strength
- Selective Laser Sintering (SLS) – SLS is for plastic and metal prototypes with intricate internal designs
- Selective Laser Melting (SLM) –SLM is the preferred option for parts requiring high strength, high durability, and a complex intricate design
- Digital Light Processing –It has a good tolerance to the design and has a good surface finish.
- CNC Machining– An ideal choice for plastic or metal without costly tooling. It holds tighter tolerances and has a better surface
- Fused Deposition Modeling (FDM) – FDM includes a low cost, ease of use, and using multiple plastic types and colors in one prototype
- Rapid Injection Molding – A rapid mold can be used for a small amount of parts for low cost
- Binder Jetting –A big advantage in making multiple parts at one time with a lower cost
- Laminated Object Manufacturing –A good option using thin laminates laid layer by layer for paper, plastic, or metal. The designs should not be complex, but LOM is low cost compared to others.
If you’re interested in learning more about each rapid prototype option, check out our article with more in-depth information on each.
SEA-LECT Plastics has decades of experience designing, molding, and assembling plastic injection molded products. We aim to exceed our customer expectations. If you have a new project that needs guidance, we can offer support to get your project off the ground. Surface finishes should be appealing, but also offer the function your design needs. We can help select the perfect resin for your new project.
For more information on SEA-LECT’s design and production opportunities, call (425) 339-0288 or email us at mattp@sealectplastics.mystagingwebsite.com.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.