Adding injection molding capacity

When buying additional molding machines you don?t have to be right in every decision. But, you can?t allow yourself to be wrong all the time. Your decisions must fit within one overarching principle: your overall operation must be profitable.

First, determine whether you really need an additional machine. Suppose that your contracts, machine outputs, and delivery times indicate you need one or more additional machines. Before you spend the money, ask yourself a hard question: Do you need an additional machine short term or long term? If it?s a short-term need, subcontracting may be the best option. If you project a long-term need, it?s time to look into the purchase of one or more additional machines.

Here?s a calculation you need to do when considering a machine purchase: Classify all the machines in your shop into groups. Classify all the molds that will be run in each particular group. Calculate the amount of hours each of the molds will run each year. Don?t forget setups and changeovers. Add all the hours together. Look at your workweek and calculate the number of hours you have available to mold in the year. Multiply those hours by 85%. This will give you a number that allows for downtime, mold or machine maintenance, etc. Divide the number of machines into available machine hours per year. Now divide the total amount of needed production hours into the total amount of available hours. If the percentage is less than 80%, this group is not a candidate for a new machine.

If it is more than 80%, begin putting additional machines into this group until you get a percentage that is at or below 80%. These extra machines represent the ones you?ll have to buy. Don?t be surprised that in micro molding or very large part molding the loads are 30% or less. These machines are under utilized but you need them to keep the overall business picture in place. If your ?overload? seems permanent, go back and recalculate as if you began running your operation 24/7. That automatically puts 40% more time into the equation, but it also removes the normal downtime on weekends that allows you to catch up if you?re behind.

Which Brand?

This is a loaded question. Every injection-machine builder wants their machine to be recommended, at the expense of all the others. An exception could involve a machine of a size or type that is not supplied by a particular builder. Even then, a machine builder would probably try to convince you that you do not really need such-and-such a size or type of machine after all.

Buying a molding machine is a lot like buying a car. The choice is heavily influenced by personal preference. Most builders make reasonably reliable machines. Furthermore, machine prices are competitive. What really matters is that your profitability depends on having a molding machine in good running order. That means having good access to service and parts. Look at your own plant: how many machines do you have? What brands? In all probability, the brand with the strongest presence also has a strong service and parts presence in the area.

The purchase of a new or used machine must be evaluated according to machine cost, the expected return on investment, and what you expect the machine?s performance and useable lifespan to be. A new machine can be ordered to fit your exact needs. A used machine, on the other hand, gives you what someone else specified. Nevertheless, you can usually buy a used molding machine for an embarrassingly low price. Then, send it off to a machine remanufacturer (in the United States, you need not go overseas) to be refurbished for about 1?3 of the cost of a new machine. Depending on what you specify, you can get the machine brought up to its original capabilities when new, or enhanced to levels of current technology, with new controls, for example.

Do some homework before you go out for bids; on a map, draw a 50-mile radius around your plant. Identify every molder in that location and give them a call. They may view you, perhaps accurately, as a competitor, and be reluctant to talk. But it won?t hurt to try. Ask them how many and what brand of machine they have. If possible get them to comment on the ease of hanging a mold and the individual quirks consistent with each brand of machine. Before you acquire additional machinery, look at the space available in the plant. Make sure the area includes space for auxiliary equipment. An often forgotten consideration, especially when located in a rented facility, is the load-bearing ability of the floor. The normal 4- to 6-inch-thick cement floor is usually sufficient for small- and medium-sized production cells. Machines 400 tons and up, along with their heavy molds, may break the floor depending on its construction.

Machine Types

There are three major machine types. The classic hydraulic machine closes the clamp with either a main hydraulic cylinder to achieve the machine?s tonnage; or with a small cylinder that locks the platen to the tie rods and then a series of large pancake cylinders engage, stretching the rods and achieving the clamp tonnage. The reciprocating screw both turns and injects with the use of hydraulic motors and cylinders. Hydromechanical machines have a screw mechanism identical to hydraulic machines. However, the clamp pressure is achieved by stretching a series of levered arms known as toggles.

Electric machines have come of age due to the screw drive and accurate electronic positioners. These machines gain their clamp pressure with a screw drive closing the mold, reaching the appropriate pressure and then locking up. The screw is rotated with an electric drive motor, and the material is injected using a screw drive. If you need hydraulics on an electric machine, bring in a separate hydraulic pump. You will still save considerably on the cost of energy.

Hydraulic and hydromechanical machines use hydraulic pressure generated by pumps. The main drawback of pumps is that they are always on and only used for a small percentage of time. The rest of the time the energy produced through these pumps is converted to noise or heat. With the addition of an AC inverter drive a large utility savings can be enjoyed. This slows down the pumps when not in use and speeds them up when pressure is called for. It is easy to tap into the hydraulic system to actuate hydraulic core mechanisms or external hydraulic ejector systems.

Machine Size

Consider the size of the platen, how much daylight you want between the platens when fully open, the speed of opening and closing, the maximum clamp force, the shot size of the injection unit, and the throughput required of the screw and barrel. Buying a new machine is like ordering dinner a la carte. Within very few restrictions you can buy almost any sized clamp unit with any sized injection unit along with a choice of the type of screw, nozzle, and machine control systems to fit with your needs.

Shot size for all parts should be between 20-80% of the maximum shot capacity. With the fastest possible cycle and the largest possible shot, the extruder?s capacity should exceed this demand by 10-25%. The screw?s L/D ratio determines the material residence time and shot inventory in the barrel. You don?t want a barrel so long that in the normal course of molding material spends so much time in the barrel it degrades even though you are using an optimal cycle. A separate measure is the number of shots in the barrel. This is barrel capacity divided by shot size.

Installation and Startup

Machines, depending on their size, are either transported by truck or rail directly to or near your facility. Sometimes the machine is so large that the carriage is first put into place, then the clamp and injection units are mounted on the carriage. This is not something to do with your own personnel in the name of saving a few thousand dollars.

It is important that the platens are parallel, the injection carriage is completely horizontal, and the floor mounting pads have been laser leveled to the machine within thousandths of an inch. This requires a specialized type of rigger. The crew that installed your cooling tower or material silo does not have the skills or experience to install a molding machine. Call the manufacturer and get a recommendation of a rigging company in your area who has done this before. Even if you have to pay a premium, this is money well spent.

Editor?s note: The author wishes to acknowledge the assistance of Jose Luis Chavez, injection molding plant manager, Sanyo Manufacturing Televisions, Tijuana Mexico; and Brent G. Borgerson, process engineering manager, Matrix Tooling Inc./Matrix Plastic Products, Wood Dale, IL.Bill Tobin is a frequent contributor and consultant who also publishes an independent monthly e-newsletter with tips on injection molding. E-mail [email protected] to request to be put on the circulation list.