Avoiding Common Cosmetic Defects in Molded Parts

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Avoiding Common Cosmetic Defects in Molded Parts | Plastics Technology

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The global plastics industry has been navigating through what is arguably the most volatile period in decades. Unprecedented amounts of new production capacity are scheduled to start in North America, Europe, and China in the near term and compete for demand during a period of economic challenges. How will trade flows shift? Will this lead to regional cost disparities and rationalization? Energy transition and sustainability targets continue transforming the plastics market and increasing the competitive landscape. As the market evolves, what impact will new technology, policy, regulation, the growing role of chemicals versus fuel and other factors have on industry restructuring and business models? At GPS 2024, leading global experts will come together to discuss pivotal impacts and initiatives shaping the plastics industry. Join us and participants from across the globe to gain the latest insight and deep analysis as you connect with your peers and industry professionals. This year’s conference will explore the theme Disruptive Global Dynamics Reshaping Plastics and include a full day workshop focused on the Global Plastics Business and Plastics Transition to Circularity, 1.5 days of expert content and numerous networking functions.

Every three years, leaders from almost every major industry gather at NPE to advance their businesses through innovations in plastics. The largest plastics trade show in the Americas, NPE offers six technology zones, keynote speakers, workshops and opportunities to build partnerships.

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The Society Plastics Engineers (SPE) Extrusion Division and the SPE Eastern New England Section will co-host the Screw Design Conference-Topcon on June 19-20, 2024 @ UMass Lowell in Lowell, MA. This highly technical program will focus upon screw design principles for single and twin screw extruders with wide ranging topics relating to screw designs for feeding, melting, mixing, venting and pumping plastics products and parts. Areas of focus will include screw designs for melt temperature and gel management, gel minimization, bioplastics, recycled materials and foaming. In addition to the technical sessions, a tour of the UMass Lowel Plastics Processing Laboratories will be integrated into Day 2 of the event. This program is not just for screw designers, but to help anyone responsible for any type of extrusion operation to evaluate existing extrusion equipment; and also to prepare for future projects. Price to attend: Less than $1000! Registrations will be accepted in early 2024. Call for papers – To be considered to give a presentation, please submit a talk title and abstract on or before December 15 to: Technical Chair: Eldridge M. Mount III, e-mail emmount@msn.com Corporate sponsorships - A limited # of corporate sponsorships (15) are available on a 1st come basis. Included is a 6’ tabletop display (must fit on table), denotation in all promotional activities, and 1 no charge registration. To become a sponsor contact: Charlie Martin, Leistritz Extrusion, e-mail cmartin@leistritz-extrusion.com, cell 973-650 3137 General information: A reception on Day 1 and a tabletop display area will allow the attendees to meet and discuss state-of-the-art screw technologies with industry experts. The SPE Extrusion Division will issue a “Screw Design Certificate” to all participants who have attended the program. Students are encouraged to attend and will receive a discounted rate. For additional information contact: Program Chair: Karen Xiao, Macro Engineering, KXiao@macroeng.com

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Consistent quality is paramount within the production of pipe and tubing applications. Additionally, significant material savings can be obtained by tightly controlling product dimensions with the correct process equipment. In this webinar, Conair will cover gravimetric control of an extruder and production line speed to ensure optimal quality and cost savings are achieved in your product run. A detailed discussion of the upstream material handling system includes: blending resins upstream of the feed throat, detection of the extruder rate at the throat, control of the extruder rpm and control of the product itself in feet per min — all accomplished with a simple recipe configuration which includes product weight per length desired and production line speed.

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Fast facts SPE International Polyolefins Conference 2024: The Preeminent Polymer Conference in the World dedicated to Polyolefins since 1975 The conference will in-person and virtual The powerful software platform for the conference will allow access to all papers on-demand, even several months after the conference, access to the program, access to virtual exhibitor/sponsor booths, and easy communication with speakers and other participants In-person participants can download a mobile or web app for the conference to access all the features of the software platform for the conference We anticipate over 900 people from around the globe to participate in-person or virtually The improved layout of the exhibit floor offers easy access and flow for the 60+ companies that we anticipate to exhibit Over 20 sponsors expected 150+ Technical papers, Sunday afternoon Tutorial Student Poster Competition 2 Networking Socials Meeting rooms available for rent to meet with customers and suppliers Exhibits from Monday through Wednesday until noon The Conference is organized by the SPE South Texas Section, the SPE Polymer Modifiers and Additives Division, the Thermoplastic Materials and Foams Division, the Engineering Properties and Structures Division, the Building and Infrastructure Division, and the Flexible Packaging Division.

Identify familiar flaws and then work to eliminate them with these mold and part design tips, and process considerations.

Today’s consumers expect a certain level of finish in the products they purchase. Increasingly, style has become an important and deciding factor when comparing products. So, it’s important to understand how a part is manufactured to ensure that even basic design issues don’t become bigger problems for consumers or affect part functionality.

Injection molding comes with a specialized set of design guidelines, and design engineers who understand the best practices will increase their chances of catching these issues early and developing structurally sound and cosmetically appealing parts and products.

There are a number of common issues that we see in injection-molded parts at Protolabs. Here are some ideas on how to avoid them in the first place.

Draft can be as simple as adding a degree or two to a wall to avoid right angles. Photo Credit: Protolabs

Injection-molded part aesthetics start with draft. Sufficient draft is an important part of any mold design, even with quick-turn tooling. Vertical walls, meaning those part surfaces parallel to the direction of mold operation, should have a minimum draft angle of a 1/2 degree, though 2 degrees is even better. Heavily textured surfaces may require 5 degrees or more. Without proper draft, part ejection becomes difficult if not impossible, and unattractive drag or scrape lines may occur.

As its name implies, sink appears as a dimple or shallow depression on the surface of a molded part. It’s caused by thicker than normal cross sections, non-uniform part design or improper gate placement — the doorway through which hot plastic first enters the mold cavity. Polypropylene and acetal are very susceptible to sink, while fiber- and glass-filled materials are less prone to sink. We recommend assigning a recommended wall thickness to each material and advise that the minimum wall thickness be no less than 40 to 60% of a workpiece’s thickest section. Material flow within the mold should travel from thick to thin whenever possible, which might mean reorienting the mold cavity or placing the gate in an area originally reserved for a cosmetic surface.

Meeting the recommended wall thickness for a material will help avoid sink. Photo Credit: Protolabs

If you design a part with walls that are too thin for the target material, it’s likely to curl up like a potato chip. Warp, as this phenomenon is known, is easily avoided by following the same rules used with sink, namely staying within the general wall-thickness guidelines. Ironically, the glass-filled materials that work well with sink-prone parts are more susceptible to warp. That’s because, as the part cools, the glass fibers tend to line up in the same direction, creating internal stresses. You can improve your parts with internal support structures (such as gussets) to support thin walls or ribbing of large, flat surfaces.

The type of uniform wall thickness displayed here is vital in preventing part distortion or warp. Photo Credit: Protolabs

Trimming of parts is often necessary to avoid flash, depending on the material. Photo Credit: Protolabs

Look closely at a rubber O-ring and you’ll see a thin line of material at its outermost periphery. That’s a parting line, the seam where the two halves of the mold come together. If you’re working with free-flowing materials such as Santoprene or unfilled nylon, a small amount of flash can sometimes ooze into the seam and typically requires trimming once the part has cooled for improved aesthetics. On a donut shape such as this, there’s little choice about the parting line location, but many orthogonal parts have sharp corners, which make a clean, crisp junction that’s a perfect place for the mold to separate. Flash or no, you should expect a parting line on most molded products. Digital manufacturers like Protolabs are able to identify the parting line location during the quoting process and may suggest ways of modifying the part geometry to avoid one.

Adding and mixing colorants consistently can prevent swirling. Photo Credit: Protolabs

From honey beige to cornflower blue, manufacturers stock a variety of colorants. These are mixed with natural resin pellets just before the molding run and are usually quite close to the target color. Still, the final product color may vary due to the polymer being used; texture and polish of the tool; and swirling during the mixing process. If you want an identical color match on your parts, it would be best to purchase color-matched pre-compounded resin from an external vendor.

Worried about those fine lines that look like hairline cracks in your injection-molded part? Don’t be. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. Commonly seen at the edge of a hole or other cored feature, as a rule, knit lines are purely cosmetic but may create a physical failure point if present in an area of the part that receives substantial stress, such as the head of a screw. In this case, designing a strengthening boss feature around the hole is a good precaution, or just skip the hole entirely and drill it afterward.

Knit lines have the potential to create structural issues for parts in the areas where they’re located. Photo Credit: Protolabs

If you select a PM-F0 noncosmetic finish on a tool, the finished part will likely show small, circular, end-mill marks and tool transition lines. If you need a surface finish that’s more cosmetically appealing, it’s generally a simple, if more expensive, matter to manually polish the tool. A PM-F1 finish removes most tool marks, while an SPI-A2 will be smoother than a fresh jar of peanut butter.

Texturing via bead blasting is another option that generally leaves a uniform matte finish (except in thicker areas) around knit lines and in darker materials. Bear in mind that deep slots and cavities are difficult to reach for polishing and texturing, and that fine finishes may affect turnaround time because of the additional effort needed for polishing. Suppliers sometimes offer multiple surface finish options to choose from (like pad printing and laser engraving) as well as finishing options (like mold texturing and part marking).

A secondary operation like bead blasting can ensure a uniform finish. Photo Credit: Protolabs

Gate vestige is that small ugly spot at one end of the part left by removal of the gate after molding, usually with a side cutter or razor knife. It’s an unavoidable fact of injection molding. For the most part, the only thing that can be done to avoid it is to orient the part in the mold such that cosmetic surfaces are unaffected. For example, when molding a Statue of Liberty replica, the gate should be placed on the soles of Lady Liberty’s feet. When submitting a design to an outsourced manufacturer like Protolabs, always be sure to speak to an applications engineer to ensure surfaces that require a vestige-free appearance can be accommodated. There may be options to change a gate style depending on the material and part geometry. It is much easier to do this during the review stages rather than after the mold design stages have begun.

There are several other problems that can crop up with injection molding, some of which can be tied back to wall thicknesses that exceed general recommendations:

A draft adjustment or a new finish option will help prevent drag marks. Photo Credit: Protolabs

Thankfully, most of these issues can be resolved through slight modifications to part design and/or selecting a different material. Difficult part geometries often require fine-tuning of the molding temperature, injection speed, hold times or all three. Material selection also plays a big part with cosmetics. Two examples of this are polypropylene and HDPE, which tend to sink more than polybutylene or acetal, but flow better into small part details. It is possible to test different materials using the same mold, but unfortunately shrink factors may prevent parts from having dimensions that match the CAD. In some cases, after testing multiple materials, a new mold may be required for further testing or production parts.

About the Author: Rob Young is the senior global product leader for injection molding at Protolabs. Young has nearly a decade of experience in digital manufacturing and specializes in on-demand production and supply chain optimization for Industry 4.0. He came to Protolabs after working with early stage startups at the Ina Mae Rude Entrepreneur Center and Center for Innovation Tech Incubator in Grand Forks, North Dakota. Young holds a B.B.A. in Entrepreneurship from the University of North Dakota. Contact: 877-479-3680; rob.young@protolabs.com; protolabs.com.

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Avoiding Common Cosmetic Defects in Molded Parts | Plastics Technology