Automotive Tools

Magnetic Base

A Molder's Guide to Magnetic Quick Mold Change Systems

Having established a need to improve mold changeover time, when deciding upon the clamping process, the molder is confronted by three technology choices.

• Manual-Mechanical - typically for smaller presses. Involves the mold being modified to accept some kind of clamping ring or spigot. Specially designed plates are mounted to both platens which through the use of a lever allow the user to locate and clamp the mold securely to the machine.

• Hydraulic/Pneumatic-Mechanical - For all sizes of press. Takes existing clamp technology (toe-clamps) and automates the process without the need for Operator intervention. For best results, Mold bases should be the same size to avoid the need to reposition the clamps.

• Magnetic - Typically for presses greater than 50 ton. Magnetic Plates are fixed to the machine platens and electronically switched to clamp the mold.

Over the past 15 years, the Molding industry has seen the "magnetic" option become the primary choice on the following provisos:

• Where the application is Thermoplastic Injection Molding and operating mold base temperatures rarely exceed 250 deg. F.
• Where the daylight of the press allows for the thickness of (both) magnets.
• Where the molds vary in size.

The first two points above address the magnet's primary weaknesses: Magnets are susceptible to high temperature, and although they have become thinner over the years, loss in machine daylight is unavoidable.

However, magnets have become much more affordable and when considering installation cost that includes the absence of mold modification, they are the most cost competitive option.

Other advantages over their more conventional competitors include:

• Less Maintenance. There are no moving parts, electrical power is only required for the switching process, which depending on the size can take as little as 0.5-3 seconds.
• Greater Accuracy. Clamp force is uniformly distributed over the entire mold base surface, just as the mold was (ground) manufactured in the first place.

Having made the decision to buy a magnetic system, the biggest hurdle for the molder is choosing the right product and supplier.

Despite the fact that magnets have been around since the beginning of time, it is an unknown science to most - including Engineers! This makes the customer vulnerable to distortion and exaggeration of the facts.

The following questions and answers are designed to provide a brief "check list" for the first-time buyer:

What happens when there is an electrical power failure?

All manufacturers utilize permanent-electro magnetic technology (some say "electro-permanent"). This technology invented in the UK in 1963, involves internal permanent magnets which when activated by an electrical pulse remain permanently magnetized. They cannot lose power unless the cycle is reversed. Some rare-earth magnet materials can be affected by excessively high temperatures, check with the manufacturer/supplier for their recommended maximum operating temperature.

Is Magnetic Quick Mold Change safe?

Yes - providing sensible operator guidelines are observed. Operation of the magnet is undertaken when the guards are closed, there is no reason why any Operator should be within the vicinity of the mold unless it is clamped or attached to the crane/handling device. Ask the Manufacturer for the Operation sequence and how the magnet is interlocked with the Press.

Does the Magnetic field affect the Press, Ejectors, Controls, Personnel etc.?

If the pole design is properly determined, the mold base will absorb most of the magnetic field. Additional (stray) fields are localized and kept relatively close to the magnet surface and will not affect controls. It is unlikely that personnel with pacemakers will be affected by the magnetic field, but as a precautionary measure, all manufacturers will insist that personnel with pacemakers be kept away from the immediate area.

Is it possible for the mold to break away from the magnet?

Yes - The magnet like any clamp has a maximum clamp force, if the press can generate a force greater than the clamp force, the mold will move. Under normal operating conditions, a magnet has more than adequate clamping force and should never let go of the mold. Mold security, however becomes vulnerable in the following instances:

1. Over-packing/Mold Lock: In the event that the mold seizes up and the opening force of the machine exceeds the magnetic clamp force, then at some point one side will let go. The mold will remain attached to the other side and the Operator will need to intervene to rectify the condition.
2. Excessive nozzle force: Some machines apply a large nozzle force applied to the fixed side of the mold. If the mold is small, it may be that this force is greater than the magnet force. Ask the manufacturer/supplier for the likely clamp force applied to the smallest mold and the nozzle force can be reduced to accommodate.
3. Ejector/Knock-Out Over-stroke: The worst condition for a magnet which (generally) can only happen at the time of set-up. Over-stroked Ejector Rods applied at production speed provides a "dynamic" force which subject to size and condition of the mold base may cause the moving side to release from the magnet.

Responsible manufacturers/suppliers should be in a position to provide data for clamp forces at the min and max mold sizes. In addition, through training and technical manuals they should be able to provide comprehensive operator guidelines to minimize accidents.

What are the advantages/disadvantages of different pole designs?

Specialists in magnetic products are more than capable of making claims that cannot be substantiated by the inquirer. Try not to be distracted from the most important questions:

• What is the maximum clamp force generated by the magnet?
• What is the maximum clamp force generated by the magnet on the minimum mold size and what is the recommended minimum mold size?
• What is the minimum recommended mold base thickness to absorb all the magnetic flux?

The fact is that regardless of whether the poles are square, round or rectangular, and or whether a supplier is offering larger poles versus another, the answers to the above questions is all that is required to make an informed decision.

All reputable manufacturers should provide this information and it should be made available on a simple layout drawing for the Molder to appreciate.

What is the thickness of the magnetic platen?

Magnetic platen thickness x 2 will determine loss in daylight.

Does the magnet have resin between the poles and is the resin subject to temperature?

Some manufacturers employ the use of resin in the non-magnetic areas between the poles to keep cost down. In the early days, resin would expand and contract with temperature. If the resin "grows" above the pole surface, then the molder will need to sand it away, failure to do so may cause resin cracking and reduce holding force.

More recently, manufacturers have designed the resin to reside below the surface to allow for natural expansion some have gone with an all-metal surface.

What is the maximum operating temperature of the magnet?

The grade and type of the raw magnet material determines temperature susceptibility; some companies offer special magnets for hotter applications where the internal coils are also specially adapted. All manufacturers will provide maximum operating temperature conditions for their product.

What safety interlocks does the QMC system employ?

All magnets should have a direct link to the IMM Emergency Stop circuit. In the event of a failure of any kind, the press needs to be stopped in the shortest possible time.

• Magnets need to be interlocked to ensure that the mold is in contact before activating
• Magnets need to be interlocked to ensure that the machine guards are closed before activating/deactivating.
• Magnets need to be interlocked to ensure that if a mold moves during production is can shut down the machine very quickly.
• Ideally, Magnets need to be interlocked making it impossible for the operator to deactivate whilst the machine is in "production". Some manufacturers may employ the additional use of an enabling key switch.

All magnets will have some kind of monitoring sensor(s): Proximity sensors or flux sensors are relatively common.

What additional preparation will be needed before production can commence?

Check the recommended system voltage - most manufacturers will require a separate isolated supply.

Check maximum current and power required, ask for recommended fusing.

Since IMM Platens are (still) designed around conventional clamping, fixing holes in accordance with SPI/Euromap are limited to the outer areas. Some magnet manufacturers may require additional holes to be placed nearer the center for fixing the magnet securely and uniformly to the machine platen.

What training is provided?

As with most businesses, the buyer/initiator of the magnetic system is not usually responsible for its operation. Whilst the operation of the magnet is quite simple, it can be a little daunting to see a heavy mold clamped invisibly. The Operator will have questions and concerns that should be addressed and as with many technologies there are some basic do's and don'ts that should be followed along with some basic maintenance tips.

Does the magnet require specialist installation assistance?

In most cases the installation of a Magnetic Quick Mold Change System is relatively straight forward. Subject to the product's specific requirements, knowledge of the IMM and where to access various electrical interlocks will be required.

The planning process associated with the implementation of a Magnetic Quick Mold Change System cannot be under-emphasized. By asking the right questions and insisting upon proper training can only maximize the success of the installation and experience.

About the Author

DocMagnet Inc. designs and supplies Magnetic Workholding systems. DocMagnet Quick Mold Change systems are designed around the machine platens to ensure maximum clamp force. They are not limited by pole design as with most other manufacturers.

Why are some material magnetic and others not?

Why are some materials (iron, steel, heamatite) magnetic and others (like copper or aluminium) not? If it's about electron spin and alignment, why aren't all electrical conductors also magnetic or attracted to magnets even if they're not actually magnetic themselves, afterall they generate EM fields when a current is induced in them? Are there any magnetic or magnetisable non-metals? Why are only ferrous (iron based?) metals affected?

It's all about unpaired electrons. The transition metals add electrons to the second shell in, so there is the possibility of them being unpaired. Hence Fe Co and Ni. The other ferromagnets are the rare earths, which likewise are filling the second rather than the outer shell.

Why don't people use that fabulous resource, Wikipedia, rather than post questions here?

Bulls Eye Dart Pen with Magnetic Base