Non-Uniform Wall Thicknesses Can Create Quality Problems

Posted by Ken Glassen on Jul 22, 2014 2:26:51 PM

Jul 22, 2014 2:26:51 PM

Designing wall thicknesses that are as uniform as possible helps control the shrink rate for that part or product. Shrink rates for different materials vary according to wall thickness of the part. Non-uniform walls can lead to large pressure drops during filling, causing significant differences in shrink rates. In turn, varying shrink rates cause internal stresses within the part, leading to warpage.

For example, thicker areas in the part can act as “runners” within the part that alter the way the plastic fills the mold. The molten plastic prefers to follow the easiest path, so its flow will always favor the thicker wall section first. As a result, molten material may race ahead in some locations, and then “backfill” the remaining space. This can be troublesome, especially if adequate venting has not provided in these areas to allow the escape of any trapped air.

When gating a part, it is important to gate into the thickest section and then flow into thinner areas. This is necessary to properly pack the part out after filling. The flow path of molten material must remain open so the plastic material can continue to flow into the part details during the cooling phase. Gating into a thin wall, or flowing through a thin area to get material to a thicker area, may create flow irregularities. The thinner area may freeze and solidify, preventing the additional material in the pack phase from reaching the thick section of the part. This can cause higher shrinkage due to the underpacked conditions in the thick area, resulting in sink and/or warp in the part.

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Topics: Plastic Part Design

Shrink Rate Has a Big Impact on Quality

Posted by Al Timm on Jul 22, 2014 1:51:32 PM

Jul 22, 2014 1:51:32 PM

Not taking the time to properly determine shrink rate can have a big impact on the quality of the final product—including its geometry, performance, and appearance.

Material suppliers typically provide information on the shrink rate of their materials. This number is usually based on ASTM Standard D955 and a .125-inch thick plaque with a specific gating size and location. Although this is a good place to start, this value is typically not accurate enough for many products, especially critical, highly complex parts.

Wall thickness of the part, mold gate size, and processing conditions such as packing pressure and mold temperature are major factors in determining shrinkage of the part. For example, thinner wall sections cool faster, resulting in less shrinkage. Larger gates will result in longer packing time, also providing less shrinkage.

Shrink rates also vary according to flow direction. After gate locations are selected, it is important to analyze the part to determine the basic direction of flow. On long, narrow parts—gated at the end, for instance—flow will essentially parallel the length of the part. In these cases, the “in-flow” shrink rate is used for this dimension of the part. The “cross-flow” shrinkage is used for dimensions that are perpendicular to the long dimension. For parts with random fill directions, an average of the in-flow and cross-flow directions can be used. For parts with critical dimensions, prototyping is the safest option (the mold can be also left steel-safe so that critical areas can be “tweaked” if needed after a production molding process is established).

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Topics: Injection Molding

Up-Front Collaboration Helps the Environment, Too

Posted by Al Timm on Jun 24, 2014 2:19:30 PM

Jun 24, 2014 2:19:30 PM

There are plenty of great production reasons for getting together with your injection molding team during the earliest design stages to discuss design for manufacturability (DFM). It is also a great way to help the environment.

We all have a stake in doing whatever we can to protect our natural resources, water, and air. That’s why it is important for manufacturers to be proactive in reducing their consumption of natural resources, production of non-recyclable waste, and carbon footprint—which DFM helps us achieve.

Up-front DFM collaboration with your injection molder brings the best minds together to come up with the best design for your product, as well as the best way to manufacture it.

Take, for example, product design. DFM and computer modeling can identify the thinnest part wall thickness and weight that can still achieve the top performance objectives for that part. This is important—not just for reducing material costs and product weight, but also for keeping you from going with design specs that call for more plastic than is actually required. By using only the amount of material you really need, you conserve valuable, petroleum-based resources and reduce waste/scrap that might be sent to the landfill. You will also be spending less energy to produce the part—another plus for the environment (and the budget).

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Topics: Injection Molding

What It Takes to Mold Complex Parts

Posted by Ken Glassen on Jun 24, 2014 11:55:00 AM

Jun 24, 2014 11:55:00 AM

There is no question that manufacturing complex parts takes injection molding to a much higher level. More knowledge, skill, and expertise are required, as well as sophisticated infrastructure, equipment, and environmental controls. Even though they may not always be in demand, these extended capabilities are always available at Kaysun to quickly meet the changing needs of clients, including short production schedules.

It starts with our facility, which is fully air-conditioned to maintain optimum temperature levels—ambient temperatures, and especially humidity, which will affect raw material properties. Maintaining a stable, injection molding climate reduces the environmental impacts that fluctuating temperature can have on operations. Our building is also designed to optimize material flow, from raw materials to finished goods—this improves overall productivity and enables faster/easier mold changes.

Key pieces of equipment in our manufacturing area are high-end Toyo electric presses, which provide better repeatability and more control compared to the more common hydraulic presses. The increased precision and repeatability electric presses provide—for example, a positioning accuracy of ±0.0001 inch—are ideal for complex parts. Other advantages of electric presses include:

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Topics: Injection Molding

Why it's Important to Know Your Tool Maker

Posted by Ken Glassen on Apr 10, 2014 8:32:00 AM

Apr 10, 2014 8:32:00 AM

Success in the injection molding business isn’t just about having the right equipment or the latest technologies. This is a good start, of course—but to completely meet the ever-evolving needs of clients who make complex products under challenging time and cost constraints, injection molders must have top tool makers in their supply chains that embrace the same vision of manufacturing excellence and customer service.

Injection molders and their tool makers must be rock-solid partners that are committed to the same beliefs about how to conduct business. Core to this philosophy is that tool makers must treat the injection molder’s clients as their own.

Over time (and hundreds of projects), a deep trust and understanding develops that establishes a partnership based on consistent performance, shared problem-solving, and trust. It is much like a quarterback and wide receiver—they have worked so closely together, with the same belief system, that they think and respond in the same way to deliver the winning play. The other important result of having a shared vision is that the injection molder and the tool maker help each other improve at what they do—sharing or developing best practices, or expanding capabilities and opportunities, to create a long, profitable relationship—and yes, even friendship.

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Topics: Tooling / Molds

Get the Injection Molding Process Right First

Posted by Al Timm on Apr 8, 2014 11:50:00 AM

Apr 8, 2014 11:50:00 AM

Injection molding is a dynamic, complex process that, simply by the nature of its many variables, requires some testing and some adjustments to get it just right, prior to production. Some manufacturers, however, choose to focus on the specifications of the mold first and then build the process around the finalized mold, thinking this saves time and money. This approach, however, typically results in production problems that slow the whole process down and reduce quality and repeatability. The best approach is developing a consistent, efficient process first, followed by fine-tuning the mold to the process.

The main steps of the process are proper melting of the plastic resin, injection at the correct rate, packing at the proper pressures, cooling with the correct temperature mold surface, and ejecting the part after the proper amount of cooling.

It’s important to get a process worked out that has the largest possible processing window—this gives the engineering team more flexibility and range in designing the mold. If the team strictly processes for dimensions only, the process may be insufficient to create the molding conditions that will yield the most consistent part.

Building the mold with critical dimensions up front is risky because there are subtleties in the process that cannot be fully predicted without testing. For example, injection speed may need to be adjusted to counteract splay created by shear stress, which can be caused by the shape of the part. Injection speed can also influence dimensional results.

The final part will have differences in shrink due to direction of material flow. Final shrink is influenced by many molding parameters. Molding within a range of acceptable parameters, cross-checked with final dimensions, allows the final processing “window” to be established. In general, the larger the processing window is, the lower the risk for problematic start-ups and inconsistent product quality.

The bottom line is that the processing window must be large enough to create a high-quality part that meets performance specifications and looks good.

This is where scientific molding comes into play. Following the principles of scientific molding is typically the best way to factor in the many variables that come into play and determine the best process. For example, with scientific molding, the proper viscosity of the material can be determined by pressure curves. When lot-to-lot material variations occur, the molding process can be adjusted to produce the same pressure curves that were generated during the initial process development—ensuring repeatability and saving time.  Take a look at our Scientific Molding Whitepaper to understand more about this.

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Topics: Injection Molding

Value Stream Mapping Brings Quick Rewards

Posted by Matt Fehrmann on Apr 3, 2014 10:04:55 AM

Apr 3, 2014 10:04:55 AM

In the manufacturing world, “lean” principles reduce inventory and work in process, improve quality, boost productivity, and ultimately lower costs. Lean originated in Japan decades ago and has been readily embraced in manufacturing sectors around the world, especially the automotive industry.

With increasing global competitiveness, lean principles have never been more important for making U.S. companies competitive. One of the most simple, yet effective, lean tools is value stream mapping. It can be implemented within days and can reap impressive results in a short period of time.

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Topics: General Manufacturing

Plastics Made from Carbon Dioxide could Slow down Global Warming

Posted by Matt Fehrmann on Mar 25, 2014 2:03:48 PM

Mar 25, 2014 2:03:48 PM

We all know that carbon dioxide (CO2)—a greenhouse gas emitted into the atmosphere through the burning fossil fuels—is a major culprit in global warming. This gas is also hugely abundant in oil shale deposits, where it is typically burned away by flaring, or sometimes captured and injected via deep wells into porous rock formations.

Companies have tried for years to incorporate CO2 into plastic, but could not make the process cost-effective. Now, however, several plastics manufacturers have found success in using CO2 in plastics—the trick was finding the right catalyst.

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Topics: Plastics / Resins

Injection-Molded Plastic Allows More Creativity than Ever Before

Posted by Al Timm on Feb 25, 2014 11:57:00 AM

Feb 25, 2014 11:57:00 AM

Design teams are always trying to come up with something better—better shapes and designs, better performance, better materials, and lower costs.  They are constantly seeking something that will give them an edge over the competition.

Cost, of course, is a huge factor. After all, there is only so much you can do to reduce costs when you use the same set of materials, designs, and processes. A one-percent gain here, a one-percent reduction there. Lean is good, but it can only go so far.

Being creative is also harder when you are limited by material choices—especially metal.

What if changing from metal components to plastic ones could save you up to 50 percent in operational costs without compromising function or quality—and evenimprove design choices and options?

Right, you say. “You can’t replace metal with plastic—how about that snow shovel I bought last year? Plastic breaks.”  Well, it’s time to change that mindset.

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Specialty Steels Speed up Cooling Times

Posted by Al Elger on Feb 25, 2014 11:00:00 AM

Feb 25, 2014 11:00:00 AM

One of the most important parts of the injection-molding process is cooling—it also takes the longest! In fact, more than 80 percent of the cycle time is related to cooling. You also can’t cut corners with cooling—the cooling rate must be carefully controlled to achieve tight tolerances and no defects.

However, the faster the heat transfer occurs between the part and the mold, the faster the part cools and the sooner it can be ejected from the mold. One way to enable cooling is to make the mold from specialty materials that have higher thermal conductivities compared to standard mold steel. The steel you choose depends on the properties of the selected material (engineered polymers with additives can be more challenging), production and performance expectations, and budget.

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