• Troubleshooting guide for molding

• Chiller sizing guidelines

Troubleshooting Guide for Molding

Typically the Technician, Engineer and Operator are looking to get the parts to spec quickly, because of scheduling
restraints. This standard approach does not let you analyze the defect, the mold and the machine.

A common sense approach plus the published guides for the material being used are essential. In all cases follow the
KISS rule " Keep It Simple". Molding machines are extremely complex devices and they seem to be getting more
complex. This is all the more reason to suspect your Molding machine First. Once you have decided that the
machine is not the cause of the problem look at the mold. If the mold checks out and there is no problem, check the
material. If the material checks out to be correct including being properly dried, it is time to look at your molding
parameters.

Try not to make more than One set of changes at a time, this will help isolate the problem and add less confusion.

1. Back pressure - use the published spec for the material. Make your changes in 10 PSI. increments.
2. Barrel Temperatures - Set your profile according to the material manufacturers spec. As you make
changes be sure to let the temperatures stabilize, the larger the machine the longer it will take. If you are
not sure how fast your barrel heats up, use the figure of 30 to 45 seconds per °F. If you need to cool the
barrel down use a figure of 1 minute per °F. Remember changes here will affect fill time and pressure.
3. Clamp Tonnage - Make sure the machine has sufficient clamping force for the parts being molded. The
absolute minimum acceptable tonnage is, the injection pressure times the surface area of part (Square
inches) times 1.15 divided by 2,000. Remember this is a low figure.
4. Cycle Time - These vary dramatically depending on material and mold design. Start with the
recommendation form the material supplier and try to keep your adjustments to under 10% per adjustment.
Remember to give enough time for the adjustments to show up approximately 10 to 15 machine cycles.
5. Drying of Material - Materials must by dry. Use the manufacturers spec and if anything go with more
drying time never less, be more careful with using higher temperatures because you can cause discoloring,
degrading and other adverse effects depending on material.
6. Gating - Incorrect gating can cause knitting lines, jetting, burning and surface blemishes. Look at the
material suppliers spec and talk to your mold maker.
7. Hold Time - Make sure that the part has sufficient time to cure before the pressure is released from the
part. This is often called the curing or packing time.
8. Injection Pressure - Look at the material supplier spec and start from there. Make your changes in
increments of 10%, be sure to allow sufficient time between changes, most machines will stabilize in 5 to
10 machine cycles.
 9. Injection Speed - Again start from the material suppliers spec and make your adjustments in 10%
increments. Allow sufficient time for the changes to stabilize in the machine usually 5 to 10 machine
cycles.
10. Melt Temperature - This should be measured using an accurate Pyrometer and sensor. Purge an air shot
onto a piece of cardboard and measure the internal temperature of the plastic. If this test is performed using
an Infrared Instrument it must be done with no time loss because the surface of the plastic cools quickly.
The other method, is to use a special needle that is inserted into the nozzle, to measure the actual melt
temperature in the barrel.
11. Mold Temperature - Start with the material manufacturers recommendation and go from there. Unless
your mold is of an unusual design it will heat and cool slowly. As you make changes in 10° increments,
you should allow a full minute per°. The part will always tend to stay on the hotter side of the mold and a
hotter mold will tend to produce a shinier part with greater strength.
12. Nozzle Temperature - Start with the manufacturers spec and make your changes in increments of 2° to
5°. Most nozzles heat up pretty quickly, so you should allow 20 to 30 seconds per°.

Chiller sizing guidelines

BTU/hr =Gallons/hr x 8.33 x temperature differential in °F

One Ton of cooling is 12,000 BTU's/hr.

If your flow is in gallons/minute (gallons per minute) multiply by 60 to get gallons/hr.

When you have your result add 15% to 25% for safety, wear and tear, and some future expansion.

Some rough examples are:

Injection molding 30 lbs. per hour of high density Polyethylene requires 1 ton of cooling.

Injection molding 35 lbs. per hour of low density Polyethylene/Polypropylene requires 1 ton of cooling.

Injection molding 40 lbs. per hour of Nylon requires 1 ton of cooling.

Injection molding 50 lbs. per hour of Polystyrene/ABS requires 1 ton of cooling.

Injection molding 65 lbs. per hour of PVC/Polycarbonate requires 1 ton of cooling.

Injection molding 70 lbs. per hour of PET requires 1 ton of cooling.

Extrusion of 50 lbs. per hour of Polyethylene/Polypropylene requires 1 ton of cooling.

Extrusion of 75 lbs. per hour of Polystyrene requires 1 ton of cooling.

Extrusion of 80 lbs. per hour of PVC requires 1 ton of cooling.

Blowmolding 35 lbs. per hour of Polyolefins requires one ton of cooling.

Air compressors require 0.2 Ton per horsepower.

Vacuum pumps require 0.1 Ton per horsepower.

Hydraulic cooling requires 0.1 Ton per horsepower.

Hot runner molds require 0.1 Ton per 10 kilowatts.

Water pumps require 0.2 Tons per horsepower.

Feed throats on 400 Ton machines or larger require 1 Ton.

Feed throats on machines under 400 Tons require 0.5 Tons.

We will be adding to this area regularly so please check back. If you have particular questions or comments please
E-mail us at technical support.

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