Toolmakers – Mould Accessories & Supporting Services

In some cases, injection moulding tools need to meet particular requirements, so can be manufactured by toolmaking companies with specialised knowledge and equipment. Some examples are given below:

Fast Cycling & Packaging Moulds

Fast cycling tools are characterised by four main criteria:

• They usually have multiple identical cavities, as they need to produce very high volumes of one item. This also means that they can have very complex hot runner systems.
• The products being produced are usually classed as ‘thin-walled’, having wall sections below 1.0mm thickness, meaning that tool cores and cavities have to be manufactured to very tight tolerances.
• They are required to cycle very quickly, sometimes operating at sub-5-second levels. They need a construction capable of coping with the stresses involved with 24/7 fast-cycling production, as well as to minimise ‘deflection’. This term refers to the potential movement of the core within the cavity as plastic material is injected under high pressure. All though this movement is small, very thin wall sections can make this an issue, for example resulting in a ‘weak spot’.
• These tools can have very extensive and complex water circuits, as plastic has to be cooled extremely quickly and evenly. Failure to do so will either result in longer cycle times, or products that distort during or after removal from the tool.

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Multi-Material & Twin Shot Moulds

Specialist tools are required when two or more material grades or colours are combined into a single product. This requires them to have more than one material feed system, the position of each material entry point matching the design of the moulding machine.

The most common variation is for twin shot (2K) moulding, where the injection moulding machine has a primary horizontal plasticising unit that typically injects the highest volume of plastic into a central sprue bush on the back of the tool. A smaller secondary unit injects a smaller amount of material via a second sprue bush served by an injection unit that is 90 degrees to the primary unit, or sometimes mounted above the primary unit (piggy-back unit).

There are different methods used for injecting each material in sequence:

• Rotary table. One half of the mould tool is rotated to allow each plastic to be injected in turn. This is usually accomplished using a machine mounted rotary table, the tool sometimes requiring specialist unions for the connection of water or oil circuits.
• Transfer method. The tool has two sets of cavities, with a robot transferring mouldings between stations.
• Mechanical cores. In this method, there is one set of cavities with more than one entry point for molten material. A moving core is used to ‘blank-off’ part of the cavity during the first stage of the process. The core is then retracted to allow the second material to be added.

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Medical Moulds

Medical moulds are required to match the requirements of the environment in which they are intended to operate. Most commonly, this involves an injection moulding machine within a cleanroom environment.

As well as typically being manufactured using grades of steel developed specifically for medical device production, care is taken to minimise product contamination. Examples of this would be the use of lubricants that are formulated for this type of application, or tooling designs that minimise the chance of contact with lubricants.

Companies that specialise in the manufacture of medical moulds are also involved with the tooling validation process. As well as the usual tool trials and product verification through measurement, validation also ensures compliance to standards developed specifically for medical device production. In some cases, validation takes place on the premises of the toolmaker, using dedicated moulding machines that are sometimes themselves operating within a cleanroom environment.

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Thermoset & Compression Moulds

Thermosetting materials have a tendency to be particularly abrasive, so tooling is produced using hardened tool steels that may also have a protective coating applied, e.g. chrome.

Tools are also required to be heated to higher temperatures than those used for typical thermoplastic grades. The use of water channels is not possible, so either oil is used (as oil can be heated to much higher temperatures than water), or electrical heating elements are embedded within the walls of the mould.

Thermoset materials also tend to have a very low viscosity in their molten state. For this reason, particular care is taken when machining mating surfaces to minimise the potential for ‘flashing out’.

Compression moulding is usually performed using simple tools that are fitted to vertical presses rather than injection moulding machines. They are manually filled or ‘dosed’ with thermosetting materials, so don’t require a material feed system. They do, however, need to be resistant to abrasive fillers and be able to operate at high temperatures.

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Micro Moulds

As a rule of thumb, micro moulds should be capable of producing mouldings that weigh less than one gram. They are designed to run on specialist injection moulding equipment that has the required accuracy.

Despite the small nature of the mould cavities involved, components can be relatively complex in design, e.g. small gears and cogs. For this reason, companies that specialise in the manufacture of micro moulds have to use CNC equipment that is capable of working on a small scale and with high levels of accuracy and resolution.

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Large Volume Moulds

Large plastic items such as some automotive components (bumpers for example), larger drainage products, wheelie bins and storage crates require much larger than typical mould tools. These weigh in the order of ten tonnes and above and are therefore difficult to both handle and manufacture.

Companies that specialise in larger mould tools are equipped with heavy-duty lifting equipment and CNC machinery with very extensive working envelopes.

Visit here to view the leading companies that can provide you with comparative quotes for Large Volume Injection Moulds.

More often called extrusion dies, these are used to convert the rod-like output of plastic from an extruder into the required shape. This can involve a lot of stress on the material as it is distributed through the flow channel of the die, so there are several design considerations

The plastic profile design. Sharp corners, very large hollow areas and large variations in wall thickness can make it difficult to produce a consistent profile. Wall thicknesses are probably most critical, as the material will flow faster through thicker sections than thinner ones.

Design of the Extrusion Die. This should ensure that:

• There are no dead spots in the flow channel; flow needs to be steady across the whole cross-section of the die
• There is a steady increase in velocity along the flow channel.
• There are small approach angles and no abrupt changes in flow channel geometry.
• At the output end of the die, there should be a parallel section called the ‘Land’. This has the final shape of the profile and needs to be long enough to provide a stable condition during cooling. The required Land length will depend on several factors such as the material viscosity and processing temperature.

Correct Profile Dimensions. It should be noted that the shape and the size of the finished extruded product may be dimensionally different from that of the die flow channel. This is because:

• The profile can initially expand as it exits the die, partly due to the plastic’s elastic properties.
• Depending on the material grade, the profile will then shrink during cooling. The extent of this shrinking can be calculated using known parameters for each type of polymer.

As vacuum forming and thermoforming are relatively simple production methods, the mould tools are also simple in design. In essence, they are a male shape over which a hot plastic sheet is draped. In the case of vacuum forming, a negative pressure is generated below the sheet, drawing it downwards on to the mould. This can be assisted by having numerous small holes through the mould that aid in the extraction of air.

Materials used depend largely on the length of production run (for reduced wear) and cost limitations. The cheapest moulds are often made of wood, followed by composite moulds made from resin. For a better finish and prolonged lifespan, cast or machined aluminium tools are the norms.

Rotational Moulding tools are hollow and relatively simple, typically comprising of two halves that are clamped together once the powder has been loaded. If products are difficult to extract, then sometimes three-piece tools are required. Joints need to be tight to avoid leakage.

The majority of small to medium products are produced using cast aluminium tools, while sheet steel fabrication is preferred for large products.

Design considerations include:

• Draft angles to aid part removal
• Avoidance of sharp corners, as this can prevent even material distribution
• Suitable mounting provision for attachment to the roto-moulding arm
• Venting if gasses are likely to accumulate during the moulding process.

Most types of tooling (not usually vacuum forming) benefit from having water circuits that surround the core and cavity of the tool. These circuits can carry either heated or chilled water, depending on the application. It isn’t uncommon for one half of an injection moulding tool to be heated and the other cooled.

It is fairly common practice for the tools to have numerous water circuits, i.e. there is an entry point for water and an exit point that transfers the water back to the heating or cooling device.

General rules to follow are:

• As water is distributed from a water chiller or water heater (TCU) and split down via a manifold block that then distributes water to each circuit, the pipework should reduce in size. This is so that some water circuits aren’t starved of sufficient flow. Variations in flow can be compensated for by using a water flow regulator to distribute water to the tool. A water flow regulator is a manifold with clear site glasses that allow the flow to each circuit to be monitored and adjusted.
• If a tool has a lot of circuits, then it is common practice to ‘loop’ some circuits. This is where the exit point of one circuit is connected to the entry point of an adjacent circuit. The issue with this is that the water temperature will differ at each entry point because the heat has been removed or added to the metal surrounding the initial circuit.
  If looping is unavoidable, try and use shorter circuits in combination, or those where a variation in temperature will be less critical to either cycle time or part sizing. For example, if one area of a component is cooled more slowly, it will shrink at a slower rate, possibly causing distortion.
• Where a tool has both hot and cold circuits, use blue connecting hoses for cold and red for hot circuits. It is also good practice to mark the entry and exit points of circuits, particularly if they are to be looped.

Water Fittings

To speed up the process of piping up a tool, it is common practice to use quick release water fittings. These consist of a male and female half that lock and seal when engaged. They are released via an outer moveable ring (sleeve) that slides up and down the larger diameter female component.

You should be aware that several manufacturers produce these fittings, each having a unique design that is not interchangeable.

The male half is fixed into the tool using a male thread (the tool has the matching female thread) that is usually BSP in form. (1/4” and 1/8” BSP are the most common).

The female half is typically terminated by a hose-tail, a pipe with outer raised ridges that is pushed into the correct size I.D. rubber hose. This connection is made secure by using an adjustable hose-clip.

Variations of the male quick release half include those with or without a shut-off valve that forms a seal when the female half is uncoupled. The valve is designed to minimise water leakage and also prevents air from entering the tool during storage, thus minimising corrosion. The drawback of this feature is that water flow is impaired in comparison to flow-through units.

Female couplings are typically available with either butyl or Viton seals, the latter being better suited to higher temperature applications.

If you aren't familiar with SMED, this stands for Single Minute Exchange of Die and centres around reducing downtime. A good example within our industry would be the regular tool change of an injection moulding machine.

As well as improve machine efficiencies, SMED can also make it practical to supply on a J.I.T. basis, therefore offering the potential to win additional business. Below are some effective and relatively lower-cost options for you to consider.

Quick Clamping systems

Easy to use clamping systems such as the ones supplied by PlastikCity Partner company Lenzkes are a good starting point. The clamps come with platen bolts as well as T-nuts, the latter being ideal if your machine has slotted platens. The bolts and T-nuts are supplied according to Euromap standards for the platens fitted to particular sizes of machine.

You can also speed up change over times by trying to standardise backing plate sizes for tools that run in a particular machine size.

There are numerous more expensive quick clamping systems such as Magnetic platens, hydraulic clamps and bayonet clamping.

Magnetic platens affix to the front of the existing machine platens. They use electromagnetic force to secure tooling and release it as required. The technology used means that a power cut will not release the tool.

Hydraulic clamps use hydraulic pressure to hold the tool in place. They are typically used with very large tooling and form part of an automated tool-change system that uses roller systems to move tools into position. The tools are then hydraulically clamped, there usually being backup systems to prevent accidental tool release. These automated systems also tend to have systems for automatically connecting oil and water circuits.

Scheduling of tools

Planning of the sequence of jobs you run should also be a major consideration. There is no point investing in expensive automatic tool changing systems, but then run two alternating tools with polymers that have vastly different processing parameters. Five minutes to change the tool, but a good hour for a material change and to get the barrel to the right temperature!

It isn't however always practical to plan your tools to run in a particular order, so purging agents should be a consideration, especially as some can also be used to clean a tools runner system. A selection of proprietary brands can be found in the PlastikCity Purging Agent category.

Reducing setting up time for tools

With many companies now operating machines with the ability to store numerous tool settings, a good proportion of tool setting time is often taken up by the 'piping up' of waterways.

Hard piping is a cheap and easy solution that saves having to hunt around for the right hoses and fittings. It is also a much tidier solution and prevents miss-connections of hot and cold circuits and the overuse of 'looped circuits' that can 'starve' a tool of sufficiently cool water.

A simple form of hard piping can be achieved quickly and cheaply by using 90-degree hose-tails, flexible hose and tool mounted manifold blocks. The manifolds are typically mounted in colour-coded pairs, with one pair on each half of the tool. If there are water fittings on the underside of a tool, then small feet can provide clearance.

If large quick release fittings are used to connect to the manifolds once the tool is in situ, water connections can be completed in seconds. You can also pre-heat a tool using a spare water/oil heater.

You can ensure the right connections are made by using different types of connectors, e.g. D.M.E. units from the water heater to hot tool circuit, Staubli for chiller circuit etc. All the necessary components can be sourced from Mould Shop Consumable specialists.

Having tools stored near a machine, and raw material to hand (preferably already dried if required!) are also key points to consider.

These items are frequently used in conjunction if a standard injection moulding machine needs to be adapted for multi-material moulding.

Rotary Platens are permanently fixed to the front face of the machines fixed platen. The rotary platen and the half of the mould tool mounted to it are rotated to bring cavities within the tool into contact with different injection units. This allows different grades or colours to be added and permanently combined within a single production cycle.

The platens have rotary ‘unions’ that allow cooling water and hydraulic circuits to be connected with the tool.

Secondary Injection Units allow one or more additional polymer grades/colours (in addition to that provided by the moulding machine’s own injection unit) to be incorporated into a moulded product.

If a particularly small amount of plastic is involved, then the secondary unit may be compact enough to be directly mounted onto the mould tool. Due to their size, these units are frequently powered by a core pulling circuit provided by the moulding machine.

Larger secondary units are more commonly mounted directly onto the moulding machine (for example, vertically positioned on top of the fixed platen) or to the floor at 90 degrees to the machine. Larger units can be electrically powered or may have their own independent hydraulic power pack.

As secondary units have to be synchronised within the moulding cycle, they need to be either directly controlled by the moulding machine’s own electronic control system, or receive a timing signal from the machine via an interface. In the latter case, the secondary unit will need to have its own independent control system.

Visit here to contact the leading UK companies that can provide you with advice and quotes for your rotary platens & secondary injection unit requirements.

Basic Mould Tool Construction

Mould tools are constructed from a series of mould plates and spacers. Back-plates (bolsters) are used to clamp each half of the tool to the moulding machine's platens. These tend to be slightly larger to allow easier clamping. The tool is built up using additional mould plates into which the core and cavity are machined.

Risers (spacers) are used to create gaps between plates, for example, to accommodate a moving ejector plate mechanism. Guides (dowels) are frequently used on one half of the tool, these mating with female bushes in the other half of the tool to ensure correct alignment and increased strength during the closing process.

The rear face of the fixed half of the tool also contains a sprue bush, this being accessible via a central hole in the injection moulding machine's fixed platen. The sprue bush is a round plate with a radiused depression containing a central hole that runs to the mould cavity. The nozzle of the injection moulding machine is pushed, using a set amount of pressure, against the bush to create a leak-free seal. This relies on the nozzle having a matching radius and orifice size, to allow the plastic to be injected into the mould tool without material loss.

Tool mould plates are available in a variety of metal types and grades. Aluminium plates are typically used for less demanding prototyping or low volume production, especially if polymers don't contain abrasive fillers. Pre-hardened steel plates provide a shorter lead-time and are suitable for medium to high volume work. In contrast, tool steel grades have to be hardened after they are fully machined, but provide the ultimate level of durability.

Specialist components

Tools can contain specialist items, some common examples of which are given below:

HYDRAULIC CORES & LOCKING CYLINDERS. Where more complex plastic parts are being moulded, additional mechanical components such as moving side cores are sometimes required. These move in and out of the mould's cavity to create features such as holes in the side of a moulding. This movement can be achieved in two ways:

Using the action of the tool opening and closing to actuate slides/cams that generate the lateral movement. The advantage of this solution is that providing linkages aren't damaged; there are no timing issues that could result in the tool closing when a moving core is not retracted.

Using hydraulic cylinders to actuate the core movement. (Generally called hydraulic core pulling). This negates the need for complex mechanisms within the tool, but an external hydraulic circuit is needed. This is most commonly provided and controlled by the injection moulding machine. In the case of all-electric machines, a separate servo-powered hydraulic power-pack is controlled by the moulding machine. Additional safety measures are sometimes recommended, e.g., a photo-cell that can detect that the moving core is fully retracted before mould closing commences.

LOCKING CYLINDERS are designed to hold far higher forces than conventional cylinders, without the need to maintain hydraulic pressure.

COLLAPSIBLE CORES are specialist items that can create a void within a plastic component that is larger than would typically be possible. This is because once the moulding has been formed, the core both retracts and reduces its external measurements at the same time.

DATE STAMPS. Many plastic components have to be fully traceable for many years. One way to achieve this is through a round insert placed within the cavity of the tool, that carries a date mark. The year and month of production are shown using a system of adjustable rings and pointers.

EJECTOR PINS. In their simplest form, ejector pins are hardened steel dowels that slide through holes in the tool to assist the removal of moulded components. They can be supplied with hardened sleeves that act as a bearing and support structure.

As well as round pins, flattened 'blade' like pins are sometimes used, particularly if pressure is to be applied to the edge/wall of a component. Sleeve ejectors (where the outer sleeve moves and the centre is static) are also used to apply pressure over a wider surface area, or for example when a component is a hollow item such as a tube.

HOT RUNNER SYSTEMS. There is a dedicated section for 'Hot Runner Systems & Controllers'.

Prototype mould tools are used during the design and development stage of a project. They are the preferred option when prototype items need to be produced in the same material as the final mass-produced product, e.g. for testing purposes. They can be of much simpler construction than full production tools, for example, lacking water circuits or ejector systems.

Because only short production runs are needed, prototype tools can be made from softer materials that aren't hardened in any way. Aluminium tools are common, with naturally harder grades used if production runs are to be longer. Aluminium is good for dissipating heat (so no water circuits needed) and is quick and easy to machine.

If production runs need to be longer, you can add water circuits and ejector pins. Sometimes, harder tooling grades can allow the production of several thousand parts, although abrasive materials can shorten this significantly. This type of tooling is sometimes called bridge tooling, as it bridges the gap between prototype tooling and full-production tooling.

3D Printed tools are also a good option for very short production runs. They can be produced quickly, and modifications can be easily implemented; you just modify the CAD model and print another tool. These tools are really printed impressions (cavities) that are inserted into metal bolsters. The assembled unit is then mounted in a conventional moulding machine.