5 Must-Have Features in a Plastic Mold Manufacturing

Plastic injection moulding is perfect for high volume production of plastic parts and components for a wide range of industries. This is largely due to the fact that once in production, it is an extremely efficient, cost effective and quick method of making parts. In addition, a wide range of materials can be used for injection moulding with different plastics providing particular properties, making the process extremely versatile for producing high quality parts with minimal finishing required and low waste.

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To optimise cycle time and part quality, there are 5 main considerations when designing for injection moulding:

1. Wall thickness
To ensure your part isn’t at risk of warping and cracking as the plastic cools down, it’s important to maintain a continual and even flow of molten material around the mould tool. To achieve this wall thickness should be kept as uniform as possible.

By having an even wall thickness your components will be stronger and at less risk of having unsightly sink marks or air pockets (voids) that can be caused by walls being too thick.

Having consistent wall thickness also means that the material will fill the mould cavity much easier which improves cycle times. Different materials have different recommended wall thicknesses which is why it’s always best to seek the advice of your plastic injection mould company when designing for injection moulded parts and deciding on which materials to use.

Wall thickness can also have an impact on costs and productivity. A thin wall obviously uses less material, weighs less and cools down much quicker so cycle times are reduced and more parts can be produced over time. These combined factors can deliver substantial cost savings particularly for high volume production of your components.

Producing moulded parts with differing wall thicknesses is possible but it’s important that any changes in thickness are very subtle and your moulded part may need support structures such as ribs. Need advice? Talk to one of our design for injection moulding experts.

2. Material choice
Material choice doesn’t just have an impact on the appearance or strength of your finished part. Other reasons to consider material type include:

• Flow rates
• Drying times
• Special features you are including in your part design
• Whether post-production machining will be required
• Hygiene regulations your component needs to meet
• Cost

There are many points you need to take into account when choosing which polymer to use so it’s a discussion you should be having at an early stage in the design for injection moulding process.

3. Round edges
Designing in round edges, rather than sharp corners, ensures that your chosen material will flow smoothly around your mould to produce a part that is strong enough to withstand warping. Sharp corners will restrict material flow resulting in stress during the plastic injection mould process and weak spots in your finished parts.

Our injection mould design experts recommend a minimum interior radius of 0.5 times the wall thickness and a minimum exterior radius of 1.5 times the wall thickness to provide a high-quality part that can easily be extracted from the mould during production.

The importance of avoiding sharp, 90 degree, corners shouldn’t be underestimated as they are one of the key contributors to part failure. However, if it’s not possible to use purely round edges in your design, an experienced injection moulder like OGM will be able to advise you on the best options to produce a component that meets all of your requirements.

4. Ribs
Ribs introduce additional strength to plastic injection moulded parts, offering greater rigidity without adding thickness. They are used to support the walls or replace thicker walls to prevent issues such as warping, sink marks and air pockets also known as voids.

Rib thickness should be 40-60% of the thickness of the wall they are supporting or they can also be placed away from the walls in completely separate areas of the mould. By using short ribs it is easier to fill the mould with the molten plastic and the part can also be more easily removed from the mould once it has cooled.

If ribs are needed to maintain the integrity of your part and make the manufacturing process as simple as possible, your injection mould designer will be able to advise:

• how many ribs you will need
• how thick and high they should be
• how far apart they should be positioned (if multiple ribs are needed)

5. Draft
Draft is our fifth consideration and there’s a reason this is the one we are finishing with. Draft is important at the end of the injection mould process when your part is ejected from the mould tool. As the molten plastic starts to cool and harden it shrinks into the walls of the tool. Without draft, considerable force would be needed to extract the hardening part. This could cause drag or punch marks on your components or damage the mould itself.

The amount of draft, and the angle it is applied at, depends on:

• the size of the part
• whether it has a textured finish
• the size and shape of the mould
• the material used

By calculating the correct draft required, your design for injection moulding partner will be able to improve the quality of your part as well as reduce the production cycle times.

The different variables and parameters above are reasons why design for manufacturing for the injection mould process is so important. 

Originally published on fastradius.com on February 18,

For more information, please visit Plastic Mold Manufacturing.

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.

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).

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.

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.

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