8 Design Rules for Injection Moulded Products

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Injection moulding is a versatile process and can be applied to almost any product. Although injection moulding is the industry standard for fabricating parts for products, it is not without its holdups. There are a few basic limitations to be taken into account. Here&#;s eight rules to follow when designing your product to ensure quality and durability:

1. Maximum wall thickness. The wall thickness of your part is directly proportionate to both the total materials needed to make the part and the cooling time required. By reducing the maximum thickness of the wall of your part, you reduce both these factors, resulting in lower cycle time, thus lower production costs. If the wall of your part is too thick or is inconsistent, problems can be caused involving sinkage and warpage, resulting in rejects and costly redesigns. Ensure your wall thickness is matched to the capabilities of the machine.
2. Corners. They can be a problem in a mould and will not always come out flush. It is almost impossible to force plastic into a perfect corner, and the result will look messy and amateurish, not to mention the strength of the part could be compromised. Round all corners where possible to enhance aesthetics and durability.
3. Applying a draft. A draft is a tiny angle &#; usually one or two degrees &#; applied to the mould on the face perpendicular to the parting line. This will allow for easy removal of the piece from the mould. Not including a draft in your design will mean the automatic ejection system of the injection moulding machine will not operate.
4. Ribs. Ribs are structural elements for your part, used for overall stability control. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.
5. Bosses. Bosses are hollow, cylindrical protrusions usually included in a design for accepting screws or other mating components of your deign. Ensuring these are secured by either attaching them to a wall or adding ribs will mean the bosses will remain straight and accept the part it was designed for without a problem.
6. External undercuts. A protrusion or depression in the outside of your mold &#; the cavity half &#; can create problems when trying to separate parts from the mold. Adjust your parting line to accommodate this.
7. Internal undercuts or overcuts. Similar to external undercuts, these protrusions or depressions are on the inside of your mold &#; on the core half. Adjust your parting line to accommodate this.
8. Threads. If your mould contains a thread, always arrange it perpendicular to the parting line. This will ensure that the fragile thread is not damaged. It is better, if possible, to not include a thread at all in your design. Simplifying your design will lower the chance of something going wrong.

Injection moulding design ensures a quality product and the countless possibilities far outstrip the limitations. Designing for a quality injection moulded product is the essence of the design process, and these limitations are the guidelines for creating a versatile end product.

Originally published on fastradius.com on February 18,

Design for manufacturability (DFM) is the general practice of designing parts so that they are also efficient to produce. While specific best practices vary by manufacturing technology, the ultimate goal of DFM in general is to optimize part design so as to minimize the manufacturing costs &#; without sacrificing on performance or function. DFM also helps you identify potential issues or defects early and avoid disruptive re-designs down the line, which is why assessing possible manufacturing methods is crucial during the initial design and prototyping phases.

Intentional, method-focused design is especially important when attempting to produce parts with complex geometries or intricate features. And while there are many viable manufacturing methods for producing parts with complex geometries, injection molding is among the most common.

DFM is especially important for injection-molded parts, as the hard tooling and molds used to create injection-molded parts introduce a number of variables that may impact design &#; including mold temperature, material temperature, and air pressure. What&#;s more, injection molds are expensive and time-consuming to tool, and the process typically only becomes cost effective when producing parts in high volumes, so consistency and repeatability are critical when designing parts with complex geometries or intricate features.

Here are 5 key tips for how to design plastic injection molded parts with complex features.

1. Take advantage of sliding shutoffs for clips and snap fits

Clips and snap fits are two forms of fastening mechanisms that can be incorporated directly into the injection mold design &#; a few common examples being tool set lids and electronics housings. Both operate similarly: on one side of the mechanism, a flexible tab of material catches on a slot or pocket in the mating part, thereby securing the two.

Sliding or telescoping shutoffs are components machined into one side of the mold that extend into the other half, sliding into place when the mold is closed. This prevents material from flowing into particular areas, which makes it possible to easily incorporate features like hooks and holes (including long through-holes) without the need for expensive side actions, bumpoffs, inserts, and other features that increase the cost and complexity of the mold design.

Sliding shutoffs can be designed to have the same tab and slot to match the part&#;s clips and snap fits, creating features that fit together securely and retain enough flexibility to pull apart without breaking. Shutoffs can reduce mold design and operating costs and also generally be used as a workaround for undercuts and recessed features.

In general, both the part and mold should have a minimum of 3 degrees of draft to prevent metal from rubbing against metal, which can create flash and damage the shutoff.

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2. Choose the right material for living hinges

Living hinges, another flexible lid feature, are an excellent way to attach the two halves of injection molded plastic components (think of the lids on the individual containers of a weekly vitamin dispenser, for instance).

While material consideration is always a critical consideration in design and product development, it should be your primary concern when designing living hinges. Polypropylene, for instance, is better-suited for this feature than polycarbonate (which can be an excellent material for clips and snap fits). Depending on the range of motion that&#;s expected of the lid, you may need to incorporate a radius at the hinge&#;s midpoint to allow the two parts to close more easily.

3. Keep an eye on wall thickness

Wall thickness should remain uniform whenever possible, as variations in thickness can introduce serious complications. Parts with non-consistent wall thickness are at risk of warping (caused by different sections of the part cooling at different rates, which creates internal stress that bends the part permanently).

 Variations in wall thickness, like this example, can lead to sink marks, voids, and warping.

Furthermore, if the walls of a part are overly thick or thin, further issues may arise. For example, thin walls and poorly designed support ribs can impede flowability, causing short shorts (or incomplete mold fills). On the other hand, parts with thick walls and poorly designed ribs are prone to developing sink marks, or impressions on the surface of the part caused by the interior resin cooling faster than the exterior material. If you see signs of either flaw, it might be time to reexamine your mold design.

4. Add draft and reduce the height of tall features

Tall features like bosses, ribs, and standoffs may require you to incorporate greater draft angles (generally up to 3°) to ensure the part leaves the mold without drag lines or other ejection issues. Bosses and tall features allow for threaded inserts and additional part strength, but they increase the risk of developing sink marks.

Furthermore, increasing the height of ribs and other features likewise increases the depth of the mold, increasing the need for longer end mills, more venting, and slower cutting rates during the machining process. One way to work around this is to support bosses with peripheral vertical ribs, which have thinner walls, reducing the chance of sinks.

1.5-2 degrees of draft is typically a safe minimum for most parts.

Angled bosses and other features increase the complexity of production, as the axis of the boss no longer aligns with the parting line or the line of pull &#; which all but necessitates that an insert will need to be manually loaded into the mold before each shot.

5. Be strategic about text and logos

Text (such a product or company name) or logos are commonly added to injection molded products. The good news is that small font sizes are actually fairly easy to achieve through injection molding &#; so long as you follow a few key guidelines.

First, text should be a sans-serif font and the shortest stroke length (the crossbar of a T or a A, for example) must be at least 0.020&#; in length. Raised text is easier to read and to produce than text sunk into a part&#;s surface. Unless the text is inordinately large, it should not be more than 0.015&#; tall.

Finally, unless you&#;re working with flexible materials like silicone rubber or thermoplastic elastomer (TPE), text should face the direction of pull if possible &#; otherwise, manually loaded inserts or side actions might be necessary in order to ensure smooth ejection.

Start refining your injection molding design today

Complex geometry and a high degree of feature complexity aren&#;t the end of the world for injection-molded parts. By paying attention to key design factors like mold design, material selection, boss orientation, and text style and size, you&#;ll be able to improve your part&#;s manufacturability (and therefore cost-effectiveness) and quality at the same time.

Of course, partnering with an experienced manufacturer is another surefire way to streamline the design and production phases of product development. SyBridge brings decades of engineering and design experience to the table, and we work diligently alongside every customer to ensure that your parts are not only made well &#; but that they&#;re made in the most efficient and cost-effective way possible. Contact us today to find out how we can make your designs a reality.

Are you interested in learning more about Bakelite moulding for industrial applications? Contact us today to secure an expert consultation!