Low-Volume Plastic Part Production Techniques

21 Dec 09

ManufactureLink fields all manner of enquiries from industry and the general public seeking either a supplier to manufacture their components or assemblies, or looking for ideas and solutions to their manufacturing challenges. One of the more common types of enquiries we field is for the low volume manufacturing of plastic components.

The enquiries typically come in 2 forms. In the first form the buyer knows they need only low volume manufacturing and directly ask who can make just a few of the components they need. The second form is the buyer who comes to us seeking an injection mouding company or someone who can make moulding tools. One of the first questions we always ask is, "How many components do you need to manufacture?" Often the answer is, "I need about 100 parts."

So it becomes immediately apparent that the buyer in fact needs a supplier who uses techniques to manufacture plastic parts in low volumes and at low cost. Typically the buyer has not heard of rapid prototyping techniques or considered other methods to manufacture low volume plastic components which is understandable considering many of these buyers are not from an industrial background. Most of the plastic components they see around their home are injection moulded so it is a natural conclusion they draw that to make a plastic component, you need to mould it.

So just what are the options available for the manufacture of low-cost, low-volume plastic components and assemblies without needing to use moulding techniques? In this discussion we are talking about fully functional components to be used as intended, not just representations for presentations or to check form, fit and function.

Options for low volume plastic manufacturing include:

Machining and plastic fabrication.
3D printing (SLA, SLS, FDM etc).
Pour moulding
Dip Moulding
Thermoforming
Silicon tooling or bridge tooling.

We will discuss how each of these techniques work and how they can be applied in turn. But the first step to determining the best method is to consider the part geometry, material properties required and preferred material.

For example, if a parts has very complex geometry, with undercuts and internal detail it can't be moulded or machined. So 3D printing is the natural choice. But the part needs to perform like a moulded part with similar strength so perhaps laser sintering would be the final choice if the materials available are suitable.

Or for example, the geometry may be simple but the material needs to be able to withstand high temperatures, be mechanically strong and with low friction properties. In this case the required materials may be readily available in bar form and simple machining and plastic welding techniques may be most suitable.

So to make the right choice, get to know the different techniques and how they can be applied. Also get to know the availability of various materials and the forms they are available in. The following short descriptions are a good starting point.

Machining and Fabrication

There are a wide variety of engineering plastics available in the form of bars, flats, rods, sheets and blocks. It is almost staggering the array of properties available from these very advanced materials. You can find an engineering plastic made for almost any application available right off the shelf. The trick is to be able to turn the raw materials into your finished components.

Most engineering materials are easily machined using standard CNC machining techniques. Part geometry is not just limited to prismatic shapes but can easily include 3D, contoured geometry. Components can include threads and metal inserts. Some 'undercut' part geometry is possible depending on the cutter access to the area in question. Most plastics are easy to cut and can be machined quite fast. But it is well worth keeping in mind that many engineering plastics change shape slightly after machining and maintaining tight tolerances can be difficult. Experienced plastic machinists know this and know how to machine accurately. Otherwise, selecting alternative materials can help.

Most thermoplastics can be readily welded using simple plastic welding equipment. Weldable plastics make the process of joining pieces together quite simple. For engineering plastics that are not weldable, gluing and/or fastening techniques are readily available for joining. Heat staking is another fast method of plastic joining that can be used.

Generally, machining and fabrication techniques can manufacture low volume plastic components and structures as easily as making metal parts. Even for very large components, if low volumes are required then consider plastic fabrication ahead of rotational moulding for example. Plastic fabrication can also be combined with some of the other techniques below to make low volume, complex assemblies. To manufacture components using machining and fabrication techniques, you simply need to discuss your requirements with a plastic fabricator.

Click here to locate suppliers with plastic fabrication expertise.
Click here to locate CNC machininsts who machine prototype components.

3D Printing

3D printing is one of the most common methods for producing low volume plastic components. 3D printing technology has advanced so far as to be considered a genuine 'rapid manufacturing' technique and can eliminate the need for tooling altogether. 3D printing or 'direct digital manufacturing' produced plastic components of any geometry, directly from your 3D CAD data. The 3D model is 'sliced up' into layers and the machine builds the part layer by layer by accurately melting or curing thin layers of new material on each layer. Designers have complete freedom as to part geometry and designs can even include internal moving parts.

The main considerations for using 3D printing is the material properties required. Some 3D printing methods such as Stereolithography (SLA), Objet or Polyjet printing produce very accurate, low-cost parts with excellent surface finish, but the strength of the final component can be too low for some applications. Components with better than 99% of the strength of a moulded part are possible using Selective Laser Sintering (SLS), but the range of materials available is limited and the surface finish may not be suitable. Fused Deposition Modeling is one of the lowest cost techniques available with good part strength, but range of materials are limited and the surface finish can be poor compared to other techniques.

Size can also be a major limitation when considering using 3D printing. The component being produced needs to be able to physically fit within the printing envelope of the chosen machine. If the component size exceeds the available machine capacity, then you will need to either print your components in sections and join them, or use another manufacturing technique altogether.

But if you are designing your components using 3D modeling techniques, 3D printing is almost always your first port of call. To choose the right technique for your components, locate a rapid prototyping company and discuss in detail the properties you need your plastic component to have and they will be able to recommend the right method.

Click here to locate rapid prototyping companies who have 3D printing capability.

Pour Moulding

Pour moulding is a very useful technique that is often forgotten about due to the large amount or promotion for 3D printing techniques. But pour moulding is still an extremely useful technique for producing a few plastic components with excellent surface finish. Providing you can make a suitable mould, pour moulding can also be accomplished by almost anyone. Pour moulding materials are readily available.

Pour moulding put very simply is the process of pouring a liquid resin into a mould, allowing it to set and then removing the completed part from the mould. As a general rule of thumb, components need to be designed with the same rules that apply to moulded plastic parts such as having draft on the walls and avoiding undercuts so the component can be pulled form the mould. Although if internal details and undercuts are required, moulds can be made from silicon and other materials which allow the mould to be pulled or broken away.

As with most low volume plastic component manufacturing techniques, the materials available are a prime consideration as there is a limited range or them. Urethane is the most common material used in pour moulding and this technique is often referred to as 'urethane casting'. But the urethanes available do come in a range of hardnesses.

Making the mould is the only tricky part of using this technique. Techniques include hand making a sample part from clay or other materials and then casting a mould around the component using silicon. The silicon is cut down the 'split line' to give a 2 part mould.

Pour casting moulds can be CNC routed from wood or plastic, CNC machined from metals or even produced using 3D printing or thermoforming techniques described elsewhere in this article.

Click here to find a pour moulding expert.

Dip Moulding

Dip moulding is another technique for producing low cost plastic parts that is often forgotten. Dip moulding entails making a mould that is continually dipped into a liquid polymer to build up the component to the required thickness. Dip moulding is a very easy and low-cost way to produce plastic components that have a shell-like geometry. Some examples of products commonly dip moulded are plastic caps, covers, casings, housings etc. After the required thickness is built up, the mould is removed from the inside of the component. In most cases the liquid polymer used is fairly soft which makes it easy to remove from the mould. Harder plastics are possible but the normal rules of requiring draft on the surfaces apply.

Accuracy of dip moulding, control of outer shapes and dimensions and range of materials are low so these are the main considerations. But dip moulding is another technique that can be accomplished by almost anyone.

Click here to locate a supplier with dip moulding capability.

Thermoforming

Thermoforming or vacuum forming is an excellent technique for low volume manufacturing of plastic components. In thermoforming, a sheet of plastic or plastic film is heated and placed over a mould to shape it then cooled. The excess is trimmed away leaving a completed part. Often a vacuum is applied to the lower half of the mould to enable forming of tight contours or geometry.

Thermoforming can be used to make anything from very small components right up to large plastic panels. For low-volume parts, the moulds themselves are cheap to produce can can be made from wood, plaster, body filler or other materials depending on the application. Simple thermoforming can be accomplished at home using you oven for heating, a simple mould and a vacuum cleaner for suction but the size of parts is limited to the size of your oven and the detail in the mould is limited to your available methods of production.

Professional thermoformers can produce a mould quite simply and at low-cost using CNC routing, CNC machining or 3D printing techniques. Moulding is fast, accurate and low-cost.

Choice of materials is limited to thermoplastics available in thin sheet form. The other main limitation is the part geometry. By the nature of the technique, parts need to be uniform thickness and the thickness needs to be within the sheet plastics available. The component geometry must be able to be pulled off the mould and some part detail can cause a thinning of the wall thickness. Specialist thermoforming companies are expert at producing quality parts using this technique.

Thermoformed parts can be joined in 2 halves to produce a complete component having hollow internal detail making this a very versatile process. Thermofoming is also a useful technique for producing pour moulding moulds or as an input to fabrication techniques.

Click here to locate manufacturers with thermoforming or vacuum forming expertise.

Soft Tooling or Bridge Tooling

There are times when nothing but a moulding tool will do for the production of your components even though they are low volume. Often this is because the material you have to use is not available with the other techniques, or the surface finish required cannot be produced using other techniques. But having to go to a full moulding tool needn't mean spending a large amount of money on a full-fledged injection moulding tool. Soft tooling or bridge tooling is the answer.

Soft tooling or silicon tooling is the process of making a moulding tool from silicon. The technique typically involves first producing a 'pattern' or actual part by one of the techniques discussed above complete with sprues and risers. The part is placed in a moulding box and the silicon poured in liquid form around the pattern. After curing the soft mould is sliced through at the split line and the pattern removed resulting in a mould with 2 halves. This mould can be used to produce anything from 1 to a few hundred parts (depending on part geometry) typically using pour moulding or casting.

Bridge tooling is given its name because it is designed to be a low cost tooling technique to bridge the gap between producing prototypes of early production and full production. Bridge tools are produced using a variety of techniques including CNC machining of aluminium moulds, casting of urethane moulds, 3D printing of moulds and inserts, investment casting of moulds and even incremental forming of thin metal moulds. These moulds are suitable for anything from 200 to several thousand parts. The parts can be injection moulded meaning the full array of materials are available to choose from.

While any part geometry that can be produced by conventional injection moulding can be produced by a bridge tool, it is worth considering your component design to lower the costs of your bridge tool. If your part has sharp, external corners this translates to sharp, internal corners in the mould which may need to be EDM machined. If these corners can be radiused it may then be possible to produce the tool completely by direct high speed machining which will lower the cost substantially.

If a moulding tool is absolutely required but your volumes are low, talk to a rapid protyping company or a company with experience in bridge tooling. They can work out a solution and you may be pleasantly surprised how cheap your tool can be produced.

Click here to locate companies who specialise in soft tooling.

Click here to locate companies who specialise in bridge tooling.

This is a somewhat brief summary of the most common options for production of low-cost plastic components. In general if you have anything from 1 to a couple of thousand parts to produce, talk to a specialist rapid prototyping company first. They do more than produce prototypes. They are also expert in low-volume production and getting your designs to market faster at lower cost. Once your market is established, you can then look for a higher volume solution for long term production such as full injection moulding.

Click here to locate rapid prototyping companies.

Or contact ManufactureLink as your first port of call and we will assess your requirements and point you in the right direction which is part of the free services we provide to buyers from anywhere around the world.

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