With a general understanding of plastic polymers we turn our attention to the molding machines. There are two distinct types of molding machines under common use in the industry 1) compression and 2) injection.

Compression molding machines use a delivery process that typically delivers the dry granules to the mold without the influence of preliminary heat or pressure. Injection molding in its simplest terms, incorporates a screw fill feed that passes the dry granules through a heated collar and a compression stage allowing the material to be delivered in a melted state and forced into mold cavities.

The injection machine is the work horse of the industry when volume production is the issue. Through the production of large orders these machines tirelessly mold plastic polymers in a consistent quality manner. Injection machines are made internationally and a company can have a machine that was designed and built in Germany, China or the United States. From a production point of view it is more about the handling of the machine than the machine itself. If the machine is well designed and built it can come from almost anywhere, we are after all in the midst of a global economy. Injection machines can be electric or hydraulic. This variation relates to the clamping drive mechanism and the drive screw operation.

InjectionCompression

To the left we see a typical injection molding machine rated at about 35 tons. To the right is an injection molding machine rated at about 25 tons that allows parts to be inserted in the plastic. The compression is a measure of rating these machines, but their functionality in production create the greatest difference. The size of a machine is delineated by its holding pressure measured in tons, the size of the platen (how big a mold it can accommodate). Larger injection pressure and larger parts can require larger tonnage. The shot size is also a limiting factor on these machines. And we also have the price tag for the bigger machines.

Bigger is not always better, the larger the machine the larger the bore of the barrel and the shot size. Too big a machine can cause feed problems for modest and smaller size parts causing the polymer to fall into the Non-Newtonian range. What we want is not the biggest machine, we want the right machine.

There are debates on the level of control, whether electric or hydraulic provides more consistent control. Hydraulic seems to be the choice for absolute control. Electric machines cost slightly less to run and can run a little less in price. Whether your plastic manufacturer uses hydraulic or electric machines may have little interest to you the buyer. Most plastic polymer parts can be produced on either machine with no noticeable difference when the operators and plant engineers know their job.

In the drawing below we see a simple layout of a injection molding machine. It is comprised of 3 major sections/units 1) Injection, where the material is feed to the mold, 2) Mold Assembly, where the material in molded in to its final form and 3) the Clamping Unit, where pressure is applied to hold the mold shut, during the injection phase.

Raw plastic polymer pellets specifically selected for your part’s functional requirements are placed in the hopper to the right in the drawing below. The machine hopper is feed by a larger system that pulls materials from a local cache of materials brought to the plant floor or through hose feeds from the adjoining warehouse. Smaller runs may be supported by manually loading bags of material throughout the production cycle.

Basic Injection processStarting the drive motor allows material to travel along the feed screw through the barrel. The barrel is shrouded in heaters and the heat can be controlled by range allowing the polymer to gradually transition from its Glassy to Melt state. Friction, direct heat and compression all three influence the melting of the polymer. Knowing the design limits for the polymer the operator will set the heaters to attain the optimal temperature defined by the supplier.

Because your part is new to the plant the mold (tool) you provide has been inspected and if not in good shape cleaned. simple things like dirt particles, scratches, oil or rust can detract from the finished part. The core mold is clamped on to the machine with one half bolted to the movable platen and the other bolted to the stationary platen. Alignment of these parts is critical to avoid damage to the mold and to ensure proper part alignment.

Depending on the size and complexity of a mold sometimes weighing into the thousands of pounds (one cubic foot of steel 12″ x 12″ x 12″, weighs 489.02 pounds)and with a myriad of parts more complex molds can take days to properly install on a machine. Especially, if there are heaters, cooling lines or injectors to connect and adjust. Dropping a mold is never a good thing especially when they weigh so much, but scratching the surface or even touching the surface of some is just as bad. Real care needs to be taken when handling a mold at this stage to protect your investment, the mold, and their investment the machine.

Where it is simple enough to turn the feed screw on and the heaters to a temperature recommended by the supplier, it is never that easy. The bore of the barrel on machines vary and the cavity within the screw (feed depth and metering depth) can also vary. Feed depth is the larger cavity in the feed screw at the hopper end of the screw. Meter depth is the smaller cavity in the feed screw at the end of the feed screw near the stationary platen.

The screw is segmented into three functions: 1) the feed picks up the polymer from the hopper and begins the melting process softening the polymer, 2) the transisition zone where the center of the shaft expands in size shrinking the feed cavity and compressing the polymers (this step helps eliminate air pockets) and 3) metering is where the space between the Flight’s (those spirals surrounding the shaft) becomes minimal in size and consistent providing an accurate area for gauging the shot (amount of plastic granules needed to fill the mold one time).

From the screw the polymer enters the mold assembly typically through a check valve that prevents blow back into the screw during the injection phase (lower pressure systems may not require a check value relying on the compression fit of the nozzle to the mold alone). From the nozzle (a reduction point from screw that interfaces with the mold) it enters the mold, sprue cavity(plastic initially entering the mold prior to the actual part or runners), the runner cavity (if present, is channels cut in the mold allowing plastic to flow to more than one part)and the part cavity. Channels are cut into the mold too small for the polymer particles, but large enough to allow air to escape. without these air channels a mold could never fill completely. These air vents are so small they are barely visible to the unassisted human eye, little more than scratches on the surface of the mold. Most people deal in sixteenths of an inch thinking a thirty-second is small. Plastics manufactures persist in measurements to thousands of an inch and beyond.

The two halves of the mold are held together by the clamping unit while the material is being injected into the mold until the material has solidified enough to be ejected. At that time the the mold cavities are opened (some molds are made of numerous parts and must be opened in a specific sequence). Injector pins or other devices help release the part from the mold dropping them onto a conveyor for further production steps. Simpler parts may only require cleaning and inspection.

It all seems simple enough until a part under-fills a cavity or flashes (blows material beyond the mold cavity edges). Blowing beyond the mold edges is an important issue since it pries the mold apart by a fraction of an inch shifting the size of the product. Don’t forget we still have density issues as well, if we do not watch the Newtonian and Non-Newtonian ranges.

Wait a minute we are starting to sound like we know a little something about the business. We are a long ways from the talent and background to make plastic parts, but we can now appreciate the conversations of the plastics manufacture we have teamed with and we can certainly appreciate at least a few of the intricacies of the business.

When a part comes out of a mold assuming there is more than one part, it will have a runner. That is a cavity built into the mold to allow molten polymer to feed from one part cavity to the next. The sprue is a stem or nub that feeds the runner. These are released from most parts when ejected from the mold but depending on the mold and ejection process may require manual intervention.

Proper design of molds helps place joint lines and points where the sprue or runner detach less visible. But we also need to remember the expansion rate of the polymer (based on the type) to determine the cavity size based on whether the channel is in-line with or perpendicular to the feed of the material.

Final polishing may be required to remove all traces of molding.

In this article we have looked at the work horse of the industry injection molding. These machines are where volume production is done, but not all plastic manufacturing can be done on these machines. There is more to learn and more to share with you.