Thermal Stability

The first purpose of the injection unit for molding a crystalline material is to deliver to the mold the necessary amount of a homogeneous melt ( with no unmelt and no degraded material ). The rules of construction of the injection unit are then dependent on the molding material requirements in term of thermal behavior and heat needed. The first point to take into account for a crystalline material is the thermal stability at melt temperature, to avoid degradation. Then, screw, nozzle, backflow valve, adaptor should be designed to provide efficient melting of crystalline material and delivery of molten polymer to the mold.

Melting behavior is different between plastics are different. The amorphous polymer starts softening just after Tg and presents a continuous change in viscosity. This gives a very large temperature range to operate ( but a large variation of viscosity with temperature). In contrast, the crystalline polymer stays solid up to the melting point and suddenly melts to the liquid phase at a high temperature. This limits the processing range of temperature between unmelt and thermal degradation.

The second factor is the time the material stays at that temperature. For all polymers, the molecules can withstand a certain temperature before degradation can start. Obviously, the acceptable time limit becomes shorter when the temperature is higher. Degradation will result in the generation of gases which cause bubbles in the melt, splays on parts, mold deposit, yellow and brown marks on the parts.

The average residence time (or Hold-Up Time, HUT) in the injection unit is linked to the amount of polymer in the cylinder, the shot weight and the cycle time and can be calculated with the following:

HUT= (Weight of Resin in Cylinder) x Cycle Time / Shot Weight

or

HUT= (Maximum Screw Stroke x 2) x Cycle Time / Current Screw Stroke

Note: Effective screw stroke = distance the screw travels during rotation only

Machine Start-up

There are two basic modes of machine operation.

1- Setup:  This includes manual operations. In this mode, every press action requires manual pushing of an appropriate button to bring about the desired action.

2-  Semi-Auto or Full-Auto: In automatic operation timers, relays, and limit switches are electrically coordinated to produce the proper sequence of operations, so that each cycle is repeated from shot to shot, and the end result is a finished product with consistent characteristics.

No buttons should be actuated if the machine is in a shut-down condition. In that case, the first move should be to open the water lines to all connections, such as the heat exchanger, the hopper throat, and any other component requiring water coolant. Next one should make sure that all pump suction valves are open.

One shoud check the setup record, which indicated not only the set of parameters for the job but also the accessoroes needed.

Having assembled all the items needed for mold mounting and setup, one can proceed to manipulate the machine.

Before any model is selected, power has to be available in the control

Injection System Specifications

Injection Capacity (Theoretical): the maximum calculated the swept volume (or trapped volume in a plunger unit) is cubic inches that can be displaced by a single stroke of the injection plunger or screw, assuming no leakage and excluding the use of a rotating screw to displace additional volume.

Thermoset Injection Capacity: Injection capacity can be measured in cubic inches of swept volume, but as there is non-return valve on the thermoset screws, this figure cannot be used to convert to true shot weight because some material flows back over the screw during injection. The amount of backflow is dependent on variables in both the machine and molding material.

Plasticizing Capacity: the maximum quantity of a specified plastic material that can be raised to a uniform and moldable temperature in a unit of time (Pound/hr).

Recovery Rate: the volume or weight of a specified moldable material discharged from the screw per unit of time, when operating at 50% of injection capacity as determined by SPI test procedure ( cubic inches/sec).

Injection Pressure (PSI): the maximum theoretical pressure of the injection plunger or screw against the material expressed in psi.

Maximum Injection Rate: the maximum calculated rate of displacement of the injection plunger or screw, expressed in cubic inches per second.

Minimum Injection Rate: the minimum calculated rate of displacement of the injection plunger or screw, expressed in cubic inches per second.