Injection Molding I - Lecture Review

Practice flashcards covering polymer science, injection molding machine zones, process cycles, and manufacturing quality parameters.

Manufacturing

The process of transforming raw materials into shaped parts.

Plastic (Etymology)

From the Greek word "plastikos", which means to form or mold.

Polymer

A macromolecule composed of poly (many) + mer (structural unit); for example, poly[ethylene] has the unit $-[C_{2}H_{4}]n-$.

Thermoset

A type of plastic characterized by a cross-linked 3D network structure.

Thermoplastic

A classification of plastic materials that includes amorphous and crystalline types.

TEFLON

A polymer application known for its no-sticking properties.

SPANDEX

A polymer application known for its stretching capabilities.

$$T_{g}$$

The glass transition temperature, marking the transition between glassy and rubbery states.

$$T_{m}$$

The melting temperature of a material.

Heating Bands (Zones)

Temperature control components on the cylinder consisting of a Rear zone, a Center zone ($10^{\circ}F-20^{\circ}F$ hotter), and a Front zone ($10^{\circ}F-20^{\circ}F$ hotter).

Mold Closing

The process of closing a mold first with low pressure, and then with high pressure just before completion.

Mold Filling

The step where the nozzle touches the locate ring and molten plastic is injected into the mold.

Packing and Holding

The stage where pressure is applied to compensate for shrinkage and complete the filling until gate closure.

Ejector pins

Components used to remove the final part from the mold after it has opened.

Relative Thermal Diffusivity

In injection molding heat transfer, the property relation is characterized as $\alpha_{tool} > \alpha_{polymer}$.

Boundary Condition of 1st Kind

Applied to injection molding because the tool is often maintained at a constant temperature.

Initial Injection Pressure

The pressure applied to the molten plastic, typically ranging from $1,000\,psi$ to $5,000\,psi$ inside the mold.

Hold Pressure Rule of Thumb

Hold pressure (packing) is typically set to $150\%$ of the initial injection pressure.

Clamp Force

The total force required to keep a mold closed, calculated as $\text{Projected Area} \times \text{Injection Pressure}$ ($A \times P$).

Clamp Tonnage Rule of Thumb

A standard of $4$ to $5\,\text{tons/in}^{2}$ used for most plastics.

Shortshot

A defect occurring at the lower limits of pressure ($P_{min}$) and temperature ($T_{min}$) in the process window.

Flash

A defect caused by excessive pressure ($P_{max}$) in the injection molding process window.

Shear Thinning

A property of polymer viscosity where it decreases as the shear rate increases.

Frozen surface layers (skin)

Solidified layers that form against the mold walls during non-isothermal flow while the core remains hot.