Description:

Low gloss can be defined as a dulling of the product finish, usually caused by insufficient force to push the molten plastic against the steel surface of the cavity.

Possible Causes & Remedies:

Machine Cause

◇ INADEQUATE INJECTION PRESSURE

Remedies: Increasing the injection pressure will force the plastic against the steel of the mold cavity and duplicate the gloss of the finish on that steel.

◇ INADEQUATE RESIDENCE TIME

Remedies: Optimize the residence time by making sure the mold is sized to the proper machine. Also, optimize the cycle time to ensure the material residence time is adequate to properly melt the plastic.

◇ LOW BARREL TEMPERATURES

Remedies: Increase the barrel temperature to that recommended by the material supplier. Adjust as needed to eliminate the flow lines. And, remember to keep the profile set so the material is heated from the rear towards the front of the barrel.

◇ LOW NOZZLE TEMPERATURE

Remedies:  Increase the nozzle temperature. As a rule-of-thumb the nozzle temperature should be set at 10 degrees F higher than the setting for the front zone of the barrel. This helps compensate for heat loss due to metal-to-metal contact between the nozzle and the sprue bushing, and keeps the material hot enough to pack the mold, eliminating low gloss.

◇ INADEQUATE CYCLE TIME

Remedies: Increase the cycle time. The easiest change to make is to add time to the cooling portion of the cycle. That is when the plastic is absorbing the most heat in the barrel. Increase barrel temperatures 10 degrees F at a time, allowing 10 cycles between changes to re-stabilize the process.

Mould Cause

◇ LOW MOLD TEMPERATURE

Remedies: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.

◇ SMALL GATES AND/OR RUNNERS

Remedies: Examine the gates and runners to determine if any burrs or other obstructions exist. If possible, perform a computer analysis to determine the proper sizing and location of gates and runners. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.

◇ INADEQUATE VENTING

Remedies: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. The viscosity of the plastic being molded determines the depth of the vent. Stiff materials can utilize deeper vents but fluid materials require thinner vents. In either case, the concept is to remove air from the mold as fast as possible with as deep a gate as the material viscosity will allow. At least 30% of the parting line perimeter should be vented, but additional vents can be selectively placed for any area where localized low gloss occurs.

◇ IMPROPER GATE LOCATION

Remedies: Relocate or redesign the gate so that the molten plastic is directed against a metal obstruction instead of across a flat surface. This will cause the material to disperse and continue to flow instead of slowing down.

◇ POOR POLISHING OF CAVITY SURFACES

Remedies: Prepare the molding surface finish to the requirements of the molded product. There are industry standards available that describe the degree of gloss required for specific finishes. These should be utilized to ensure consistency of finish on all molded products, and the finish should be specified on the product drawing as well as on the mold design.

Material Cause

◇ IMPROPER FLOW RATE

Remedies: Utilize a material that has the stiffest flow possible without causing non-fill. Contact the material supplier for help in deciding which flow rate should be used for a specific application.

◇ MOISTURE

Remedies: Dry the material before processing. All materials require drying, even if they are not hygroscopic. After drying, the material must be used within two hours or moisture may return.

Operator Cause

◇ INCONSISTENT PROCESS CYCLE

Remedies: If possible, operate the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles. 

(Source: plastictroubleshooter.com)