A prospective new customer comes to you with an idea and asks that you design the injection mold for their new injection molded part. After going through a big discussion on the function of the part for their end-user, what resin they may need to ensure quality and durability, and the surface finish they need for visual appeal and function, you may be all set to start the initial design. There are a lot of little things that need to be considered though. Our goal is to ensure you have all the tips you need to design the perfect plastic injection molded part for your customer and your manufacturing team to ensure smooth delivery.
Tip 1: Choose the Right Surface Finish for the Design
The surface finish has many functions beyond visual appeal, but before you start picking out the final surface finish, you should already have decided what class of mold you need based on production volume and what material the mold will be made from. A steel mold will be harder than an aluminum mold and offer more options for surface finish. Steel has the advantage that it can be polished for a smoother surface finish. That can also be advantageous for painting or another secondary turnkey operation to enhance your product.
Some of the finishes available include:
Geometric or patterned shapes
Leathery texture simulating grain
Prepared for painting or secondary graphics
Etched with a logo
Blasted for a rough uniform texture
Gloss, matte, or satin polish
Mirror or lens finish
Product surface finish and texture has multiple advantages including being used to make undercuts and hide parting lines. A strategic texture or finish in those areas can blend or hide the parting lines. Beyond visual appeal, texture can improve grip, offer improved paint adhesion, and can also allow gases to escape the mold during the injection process. The surface finish needs to be decided very early on to allow for proper design in the mold.
Tip #2: Design Your Parts with Uniformity in Mind
Injection molding is the process of forcing liquid resin into the two halves of the mold. Any restrictions or changes in thickness of your parts can affect the flow, which may have other negative effects down the line. Your best option is to keep the thickness constant, and around 2.0 – 3.0mm. Don’t design with less than 1.0mm or larger than 4.0mm wall thickness as it creates extra headaches.
Tip #3: Add Drafting to Your Parts
You may or may not have heard of drafting in relation to injection molded parts. Drafting, or adding a draft angle, allows your parts to be released from the injection mold. With draft angles, 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. Your customer may insist on a design that requires a tight mating area. In that case, try to localize the zero-draft area as close to the mating portion rather than a complete surface.
Tip #4: Add a Radius Everywhere Possible
Sharp corners on any injection molded part are difficult to form completely. Air becomes trapped in the sharp corners, so designing them out is your best course of action. A radius also lends itself to a draft angle for easing transitions and ensuring your part can be removed from the injection mold.
Tip #5: Always Design Resin Flow from Thick to Thin Sections
Sometimes designing injection molded parts requires thicker sections for structure and strength. As molten resin flows through the injection mold it loses pressure and temperature. A mold that flows from thin tight sections to a larger wall thickness will have trouble completely filling far away from the gate when resin enters the mold. Gates should be positioned at the thicker sections of the part design, and the resin should flow to the thinner sections last.
Tip #6: Pick Which Molding Defects Are Acceptable
Injection molding produces defects. There are two halves to the injection mold that must come together and at the interface there is a chance of a parting line. On thicker sections of the mold, there may be sinks caused by bosses designed into the backside of the surface. Strengthening ribs in the part, purposefully designed to add structure may also add the potential for visual defects. While some of these defects can be limited with advanced molding conditions, not everything can be negated. You can choose a surface finish that can reduce the likelihood it will become a quality problem. You may also be able to move features to locations on the part that won’t be a problem if they do leave a mark. Choose what defects will and won’t be acceptable, then design around their limitations.
Tip #7: Reduce Strengthening Rib Sizes as Much as Possible
Strengthening ribs have a purpose, but having too large of a feature can create extra problems. There are three main design considerations for each rib: 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 a sink mark on the surface. The rib height should be as short as possible to reduce the chance it will become stuck in the injection mold. The 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 will be smaller than the base thickness, which is directly related to the draft angle designed in. More isn’t always better for strengthening ribs, so use them sparingly.
Tip #8: Avoid Undercuts in the Tooling
An undercut in the injection molding tool is when the opening and closing of the tool doesn’t allow a feature to be formed. In this case, it will be best to use a lifter and slide to form the feature rather than complicated shapes. In molding, simpler is better. The lifter and slide can form difficult shapes and still allow the part to be removed from the injection mold.
Tip #9: Design for Manufacturing and Error Proofing
Most injection molded parts are designed to be part of a bigger assembly. Sure, there are Legos and other small plastic toys that don’t require any assembly. They are the exception, and not the usual rule. When you design parts for assembly, utilize datums in the design to ensure the part will be assembled the same way every single time. Feature 1 is always first, feature 2 is second, etc. World-class manufacturing requires designs perfected for manufacturing and reducing error potential should be part of every design.
Tip #10: Use Rapid Prototyping to Find Early Problems
Rapid prototyping can be used for improvements with the design, the manufacturability, and with secondary processes. It can also be used to find early design problems that you may not recognize in a 3D model. 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 us, and using multiple plastic types and colors in one prototype
- Rapid Injection Molding – A low cost 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.
It’s your job to design plastic injection molded part or product that goes beyond your customer’s expectations, and can also be manufactured by anyone. Choose a surface finish and texture that will be visually appealing, but also offer advantages for function beyond form. When you need a resource to design your next injection mold, help select a resin with all the parameters needed for a successful product, or design your injection molded parts for manufacturing, we’re here to help. For more information on SEA-LECT’s production opportunities, call (425) 339-0288 or email us at firstname.lastname@example.org.
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.