7 Tips for Perfect Plastic Parts
Life in plastic, it’s fantastic!
Whether you’re old enough to recognise that line (or if it’s one you’d rather forget), the fact is we’re surrounded by plastic. Toys, appliances, phones, furniture, cars, medical products, kitchenware – plastic is everywhere! Why? Probably because it’s versatile, flexible, durable and cost-effective. So, understanding plastics and plastic moulding techniques is very important for anyone involved in product development.
Without a doubt, one of the most common processes for manufacturing plastic parts is injection moulding. During this process, liquid plastic is injected under pressure into a mould tool. The tool generally consists of a core and cavity side, allowing it to open so the manufactured parts can be ejected. It’s a great way to produce cheap, high-volume plastic parts.
Before we begin, it’s important to understand the basic process, so check out the video below.
Design Tips
Bad design can be extremely costly, so let’s take a look at some of the main things to consider when developing your product.
Gate position and weld line
Have you ever looked underneath a plastic bottle and noticed the small protrusion at the centre of its base? Well, that’s the gate mark and it’s the point at which the molten plastic enters the mould cavity. There are a number of gate types, and the type and position should be carefully considered for a couple of important reasons. Firstly, the gate should be located to allow effective material flow throughout the cavity. For larger parts, it’s not uncommon to have multiple gates. Secondly, the gate point is generally visible on the finished part, so locating it on a visible or cosmetic face isn’t a good idea. The resulting gate mark may also protrude slightly, so it’s is often desirable to specify a ‘sub-flush’ gate. And finally, it’s important to consider weld lines when selecting a gate position. Weld lines occur when two or more material fronts converge and join up. This often produces a witness mark, the position of which depends on the gate location and the way the material flows after entering the mould. Your injection moulder should conduct a mould flow analysis to determine where any weld lines will occur and the most appropriate gate position.
Wall thickness
Parts should have a uniform and consistent wall thickness where possible. Any changes in wall thickness should be graded with fillets to smooth out the transition. Abrupt changes in thickness should be avoided as they can result in cosmetic issues and potentially weakened parts. A wall thickness of between 1 - 4mm is generally recommended for most common plastics.
Ribs and bosses
Most plastic parts require screw bosses, snap arms or other features that allow assembly of the finished components. And due to the wall thickness limitations mentioned above, many parts (particularly larger ones) require stiffening ribs to add strength and rigidity. When designing ribs, bosses and other protruding features, it’s important to consider their thickness. Adding features that are too thick can result in ugly and unsightly sink marks on the part’s external faces. This happens when the part cools and it looks terrible, particularly when it occurs on a visible or cosmetic surface. As a general rule-of-thumb, the thickness of these features should be no more than 2/3 of the part’s wall thickness.
Draft
Pick up any plastic part and take a close look at it. If you look hard enough, you may notice that the ‘vertical’ faces aren’t quite vertical at all. They’re all slightly angled. This is known as draft and it’s very important as it allows the moulded part to be ejected from the tool without becoming stuck. A minimum of 0.5° draft is usually required, but it’s preferable to use at least 1° if possible. Draft is vital for any injection moulded part, regardless of size. All parts shrink slightly during cooling and, without draft, the part may become stuck to the core side of the tool, causing big problems during manufacture.
Material selection and colour
A wide range of plastics are available, so consider your application carefully. Things to consider include material cost, durability, dimensional stability, UV resistance, heat resistance, flammability, chemical resistance and sustainability.
There are a couple of options for coloured parts; master-batch and pre-coloured plastics. A huge range of pre-coloured materials are available, but for more specific colour matching, master-batch may be required. Master-batch material is usually white and colour is added to it during the moulding process. This allows tailored colours and more flexibility but can result in colour inconsistencies if it’s not well controlled.
Finishes
A variety of finishes are available for plastic parts, ranging from high-gloss and optical-grade polished surfaces to textured finishes. The finish of your part is determined by the finish that’s been applied to the injection mould tooling. Textured finishes may be applied to the tool either through a spark-erosion process or a chemical etch, and they are usually specified using a VDI or Mold-tech number. Most manufacturers can supply finish swatches to help with your selection and it’s important to specify surface finishes on your drawing. Where no specific finish is required, a general polished finish is usually applied.
Cooling and jigging
In order to harden the molten material, parts are cooled within the mould, however, some further cooling may occur after they are ejected. For most parts this doesn’t present an issue, but some deformation may occur in parts that have thin, unsupported areas. In these instances, cooling jigs may be required to support the parts while they cool to room temperature. Ask your manufacturer for advice if you have any concerns, as they should be able to help.
Over to You
We hope you’ve found this topic helpful. If you’d like any more information or have a particular project you’d like to discuss then we’d love to hear from you. Please feel free to get in touch with us at enquiries@designvisionid.com. Alternatively, you share this post on social media by using one of the sharing buttons below.