Page 255 - Injection Molding Advanced Troubleshooting Guide

246 26 Dimensions

which impacts how well the second-stage pressure is applied through the cavity.
Note also that oversized gates can lead to over-pack conditions that can lead to a
part that is too large near the gate.
Flow analysis software can be used to determine the required fill pressure through
a given gate size, which will help minimize the risk of a process being pressure
limited due to gating. Flow analysis software will also provide a frozen layer plot
that will indicate if the wall stock is still shrinking after the gate has frozen.
Also make sure to account for gate spacing. If the flow distance from a gate is ex-
cessive the part will have uneven distribution of pack pressure through the cavity.
Plastic can only flow and pack at limited distances from a gate; this is influenced
by factors including wall thickness, material viscosity, melt temperature, and fill
velocity. Plan ahead to provide an adequate number of gates to effectively fill and
pack a mold. Total volume of gate area should be considered for the amount of
plastic to fill the mold; often times a wider but thinner gate will help fill a mold and
still allow adequate pressurization of the cavity.
When gating it is important to ensure that adequate pack pressure can be distrib-
uted within the cavity. An area of concern to watch for is thin to thick transitions
in the wall stock. If a thick section of the part is located downstream of a thin sec-
tion the thin wall stock will freeze and limit the ability to pack the thick sections.

26.3.2.2„ Mold: Sprue and Runner Size
Restrictions in the sprue, runner, or hot runner system can lead to pressure drops
that limit the ability to pressurize the cavity. A sprue and runner must be sized to
provide adequate pressurization to the cavity without being oversized, which will
increase cycle time. STOP troubleshooting questions every step of the material
flow path from the end of the cavity all the way back through the nozzle of the
machine.
Be aware that too large sprue and runner sizes can contribute to situations where
a part may be over-packed. Do not assume that larger gates, runners, and sprues
are always a good thing, because they impact cycle time and may lead to cases of
over-packing that can influence dimensions and warp.

26.3.2.3„ Mold: Cooling
Mold temperatures that are not running at the desired point can result in differ-
ences in cooling. If the mold is running cool it may freeze the part more quickly
resulting in a smaller part. A hot mold may allow the part to pack out better result-
ing in reduced shrink but also may impact warp and crystallinity, which could lead
to increased shrink. Verify uniform cooling on the mold surface and evaluate the
temperature versus the recommended mold temperature for the material.