Every new product starts with a concept. A fresh idea to make a current product better, or to fill a void in the marketplace. Plastic injection molding is an excellent manufacturing process to create cost effective, but profitable products in a timely manner. If you have a fresh product idea, but don’t know what to look for in development, these tips may help you understand the small details that need to be discussed with your molding partner.
Tip #1: Start with the Function and Finish
Most consumers see a finished product and just have the end in sight. The first step on the road to development is identifying what the product does, how it operates, where it will be used, and what it should look like. Each of these items will help hone in on the key design features need for the product to surpass customer expectations and look good doing it.
What environment will your product be used in? Many designs often overlook how and where the end-user will use the product. If only a fraction of the environments and usages are initially identified, you can expect future failures when the customer finally uses the product to their full intended use. Usually this drives costly redesigns to ensure durability in all scenarios. It happens more than you think. Identifying all environments and usages upfront will help determine the potential materials needed for the perfect product.
Once you know how your customers will really use your product, you can start to select the optimum plastic resin for cost control, durability, and overall performance. There are thousands of resins available for plastic injection molding. Many engineered resins will withstand extreme temperatures, corrosive environments, electrification, have more recycling options, and even more characteristics. The key will be to discuss what your product needs with your designer and molding partner, then select the optimum resin available.
Lastly, before your designer really gets started on the fine details, you need to select the optimum surface finish. 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 choices for surface finish. Steel can be polished for a smoother surface finish, which 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
- Smooth finish prepared for painting or secondary graphics
- Etched with a custom logo
- Blasted for a rough but uniform texture
- Gloss, matte, or satin polish
- Mirror or lens finish
A surface finish or texture can also be used strategically to hide parting lines, surface defects, and allow gases to escape the injection mold during molding operations. The surface finish needs to be selected very early in the design process to allow for proper design in the mold.
Tip #2: Design Your Parts to be Uniform
Injection molding uses pressure to force molten resin into the injection mold. Your designer should focus on eliminating or reducing restrictions or changes in wall thickness that can affect the flow of the resin. These restrictions and thickness changes can also have other negative effects with your product. The best design option is to keep the wall thickness constant, and between 2.0 – 3.0mm. Avoid wall thickness less than 1.0mm or larger than 4.0mm as it creates extra headaches.
Tip #3: Design Resin Flow from Thick to Thin Sections
Plastic injection molding may require thicker wall thickness to offer more strength and durability in strategic areas. As molten resin flows through the injection mold it loses temperature and pressure. Resin that flows from thin sections of the mold to thicker sections will have difficulty completely filling far away from the injection gate. One design improvement can be made to position the injection gate in the thicker sections of the mold.
Tip #4: Incorporate Draft Angles to the Design
Draft angles allow your parts to be released from the injection mold. A draft angle of at least 1° on an untextured mold should be designed in, and a minimum of 3° should be design in with a textured mold surface. Incorporating these minimum angles will allow your parts to release from the mold without requiring extra effort to remove them. If your customer’s design intent requires a tight mating area, the design should attempt to localize the zero-draft area as close to the mating portion rather than a complete surface.
Tip #5: Incorporate Radii Everywhere Possible
Draft angles aren’t the only problem area that can make your new design unable to be removed from the injection mold without force. A radius incorporated ensures the part will form completely without air becoming trapped in the mold. Draft angles and radii should be designed in tandem to work together.
Tip #6: Keep Strengthening Feature Size to a Minimum
Adding strength to an injection molded part typically involves adding rib features. Size can equate to strength but adding too large of a feature can also bring extra problems. Each rib should be designed at the base to equal 60% or less of the wall thickness to reduce the chances a sink mark will form on the surface. The rib should also be designed as short as possible to prevent it sticking in the mold after the part is formed. The standard for rib height is to keep it less than 3x the part thickness. The overall thickness should also be smaller than the base thickness, which is directly related to the draft angle designed in.
Tip #7: Avoid the Temptation to Add Undercuts in the Tooling
An undercut in the injection molding tool is when the opening and closing of the tool doesn’t allow a specific feature to be formed. Avoid the temptation to add an undercut to form the shape, and instead design a lifter and slide to form the feature. The lifter and slide may add a little extra cost to the mold, but it will be worth the investment to allow the part to be removed from the injection mold without extra force.
Tip #8: Make the Tough Decision on Which Molding Defects Will Be Acceptable
One simple fact of injection molding is that the process will produce defects. It may be a parting line where the two halves of the mold meet, there may be sinks in the surface, or there may be other visual defects left after molding. Some of these defects can be reduced with advanced molding conditions, but not every defect can be avoided. Your design can incorporate a strategic surface finish to limit the detection of a defect, or you may be able to move features to less noticeable areas. Make the tough decision on which defects will be acceptable, and then design around their limitations.
Tip #9: Design for Manufacturing and Include Error Proofing
Plastic injection molding is used for many industries, and most parts molded are included in a larger assembly. If you’re designing a large assembly, incorporate alignment datums to ensure the part will be assembled the same way every single time without any errors. If your goal is a world-class manufacturing result, your design should be aligned with manufacturing to reduce the error potential.
Tip #10: Use Rapid Prototyping to Identify Early Problems
Rapid prototyping can be used for more than just samples of the product. They offer a chance to identify improvements or early problems with the design, the manufacturability, and any secondary processes required. There are currently many options available to benefit the molding industries, and your designer should be able to use one of these options to create early samples.
These Options Are Available for Rapid Prototyping:
- 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.
Bringing products to a global market is a complex process. The global economy is fast-paced, but we have designed many products to be a leader in the global economy. We can offer advice on the best surface finish to exceed your design expectations, and also offer advantages beyond function and 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. SEA-LECT Plastics is a turn-key supplier that can handle the intricate details. We are a leader in plastic injection molding with options for assembly and logistics. We can aid with resin selection on a new product, offer turn-key assembly options, and program management to see the complete development cycle through with success. Give us a call at (425) 339-0288 or email us at email@example.com. At SEA-LECT Plastics, we specialize in military product applications, outdoor adventure gear, musical instruments, supporting the medical parts manufacturing and all kinds of consumer product industries. Our goal is to make your project efficient and cost-effective to manufacture, assemble, and ship no matter how complicated your concept is.
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.