Plastics Molding & Manufacturing/Inject System

The injection unit is responsible for both heating and injecting the material into the mold. The first part of this unit is the hopper, a large container into which the raw plastic is poured. The hopper has an open bottom, which allows the material to feed into the barrel.

The barrel contains the mechanism for heating and injecting the material into the mold. This mechanism is usually a ram injector or a reciprocating screw. A ram injector forces the material forward through a heated section with a ram or plunger that is usually hydraulically powered.

Today, the more common technique is the use of a reciprocating screw. A reciprocating screw moves the material forward by both rotating and sliding axially, being powered by either a hydraulic or electric motor. The material enters the grooves of the screw from the hopper and is advanced towards the mold as the screw rotates. While it is advanced, the material is melted by pressure, friction, and additional heaters that surround the reciprocating screw.

The molten plastic is then injected very quickly into the mold through the nozzle at the end of the barrel by the buildup of pressure and the forward action of the screw. This increasing pressure allows the material to be packed and forcibly held in the mold. Once the material has solidified inside the mold, the screw can retract and fill with more material for the next shot.

Injection Unit Barrel Sizing

The injection units should be ideally designed such as the barrel can hold at least 2 cycles of injected materials. Meaning, for each injection cycle should emptied at least half (50%) of the barrel capacity. However the 50% is taken as the references due to the barrel sizes should not be designed lesser than 20% or more than 80% of the barrel capacity for each injection molding shots (which is known as 20-80% thumb rules in plastic industry).

For examples, if the injection shot weight is 60g . Therefore the ideal barrel sizes should hold at least 120g. But however , the range of barrel sizes can be as little as 75g (80%) or as much as 300g (20%). The purposes of this barrel sizing rule thumb is to ensure that the material did not degrade due to staying inside the heated barrels for too long (long residence time). 20% rule thumb is for materials that are able to withstand the heat ( harder to degrade ) meanwhile 80% rule thumb is for material that are very heat sensitive (degrade in heat easily)

Components inside the injection molding

Injection System

Hopper

This is where raw plastic pellets are stored before they are introduced to the barrels. The hopper has tapered sides to drop pellets using force of gravity into the barrel. The base of the hopper should have a magnet.The purpose of the magnet is to attract metal contaminants that may have been in the raw plastic. These metal contaminants comes from the blades of a plastic granulator used to produce regrind. Metal particles can ruin the sleeve of the injection barrel or the surface of the screw.

Barrel

Barrel is usually manufactured in the form of a long, round tube and is made of an inexpensive grade of steel. The inside of the tube is lined, usually with a thin sleeve of high-quality hard tool steel that can withstand the abrasive nature of the injection process. Normally, the sleeve has a high chromium content.

Heating Bands/Elements

Heating bands are strapped to outside of the barrels. The bands are electrically activated and are placed along the entire length of the barrel with minimal space between them. Each zone contains three or more heating bands and each zone is individually controlled by an electrical unit located in the control panel of the machine. Each temperature control unit is fed temperature information by a thermocouple in a hole in the wall of the heating barrel in the area of the zone it is controlling. The control unit then decides whether more heat is required and, if so, energizes the heater bands in that zone. When the selected temperature is reached, the thermocouple informs the control unit, which stops sending electricity to the heater bands until the temperature drops again, at which point the cycle repeats. Minimum and maximum temperature limits are set on the control unit and used by the unit to determine whether the heater bands should be energized or de-energized.

Injection Screws

Injection screws are the screw-like shape that function primarily to pull the pellets thru the barrel to the nozzles.This allows screws functions as mixer to homogenize the molten plastic. The screw also generates heating friction to raise the temperature of the plastic. The friction is created because there is just a slight clearance between the surface of the screw flights and the inside wall of the barrel. As thepellets is brought forward along the screw flights, the plastic is squeezed tighter and tighter generating heat due to frictions.

Nozzles

Typical Needle Type Nozzles for Injection Moulding

The nozzle of the machine is a two-piece, tube-shaped component that bolts to the end of the injection barrel.The nozzle needle/cap has an internal taper that matches that of the screw tip. There is a tapered hole through the nozzle tip itself. The radius on the face of the nozzle tip fits up against a matching radius in the sprue bushing of the injection mold. Note that there is a heater band on the nozzle tip. This is called the nozzle heater and it is controlled much like the other heater bands on the injection barrel.

There are some nozzle designs that incorporate shutoff devices in the form of needles, springs, sliding balls, or combinations of these. The picture on the right shows needle type nozzles. Their purpose is shut off the flow of plastic for those materials that are not highly viscous, such as nylon, and that tend to drool from standard nozzles.