Page 448 - Injection Molding Advanced Troubleshooting Guide

448 44 Stringers

44.3.2.3„ Mold: Lack of Cooling
Molds should be designed with cooling around the tip locations. Without adequate
cooling, the plastic in the tip will not reach a freeze point and will either drool or
string. Evaluate cooling designs prior to mold build to ensure that the drop area is
being cooled.

44.3.2.4„ Mold: Valve Gates

If a hot runner system has valve gates it is critical that the valve pins reach a full
forward position to ensure that the gate is closed. If the pin is not traveling all the
way forward the heat at the tip will tend to create stringing problems.
Verify if the valve pins appear to be traveling forward. If a valve gate is having
stringing defects it is time to take the mold to the tool room to determine why the
pins are not reaching a full forward position. The cylinder may have worn seals or
if the pin is adjustable it may be out of adjustment. Also determine if the pressure
(pneumatic or hydraulic) is adequate for the pin to actuate completely.

44.3.2.5„ Mold: Hot Tip Orifice
The larger a hot tip orifice is the more difficult it will be to cool. When there is a
large mass of plastic at the hot tip the plastic acts as an insulator and can restrict
how well the plastic cools, which in turn can lead to stringing problems. There
have been cases where going with a smaller tip orifice led to a reduction in string-
ing problems.

44.3.3„ Stringers Troubleshooting Machine Issues

A few basic issues on the machine can lead to stringers including:
ƒ Nozzle heater setup
ƒ Machine performance
ƒ Nozzle tip

44.3.3.1„ Machine: Nozzle Heater Setup
A key part of injection molding is maintaining accurate and repeatable tempera-
tures of the melt delivery system. An often-underappreciated part of the machine
is the nozzle and various adapters required for the nozzle (see Figure 44.4).
Evaluate how well the nozzle is covered with heater bands and thermocouples. If
the nozzle is long and only has a thermocouple located in the wrench flats of the
nozzle the balance of heat from end to end on the nozzle can be problematic. If
using long nozzles and adapters make sure that they have separate thermocouple