GLOSS (low)

 

 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.

Machine

Inadequate Injection Pressure

Explanation: If too little injection pressure is used, the molten plastic material will tend to cool off and solidify before the mold cavity is packed. If no packing is achieved, the material will not be forced against the mold cavity hard enough to replicate the finish. The less dense material will appear dull and have very little gloss.

Solution: 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

Explanation: Residence time is the amount of time that the plastic material spends being exposed to heating conditions in the injection barrel. The required time depends upon how much heat the material must absorb to be processed properly. Inadequate residence time results in under heated material. This will cause the material to be stiff when injected and it will not flow enough to fill the cavity before solidifying. The flow patterns will be imprinted on the surface of the molded part because they were not forced out in time.

Solution: 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

Explanation: Low barrel temperatures have the same effect as short residence time. The plastic material does not become fluid enough to fill the mold before solidifying and the material cannot be forced against the steel of the mold cavity. Therefore, the steel finish will not be replicated on the plastic and a dull part will result.

Solution: 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

Explanation: As material is transported through the heating barrel, it is gradually brought up to the ideal processing temperature by absorbing heat from the heating bands and frictional heat, which is created by the shearing action of the rotating screw within the barrel. In the last heating zone, the material is exposed to is the nozzle. By the time the material gets to the nozzle, it should already be at ideal molding temperature and only a small amount of heat needs to be applied at this point to keep the resin flowing. If the nozzle is not hot enough, however, the material will begin to cool off too quickly as it leaves the barrel and the flow front will not be forced against the cavity steel to squeeze out the flow lines.

Solution: 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

Explanation: If the overall cycle time is too short there is a good possibility that the material in the barrel cannot absorb enough heat before it is injected into the mold. This will cause premature solidification and low gloss may appear because the plastic was not packed out enough (before solidifying) to push against the steel cavity of the mold and replicate the steel finish.

Solution: 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.

MOLD

Low Mold Temperature

Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to pack against the mold steel before they solidify. This results in a dense part with high gloss. If the mold is too cold, the molecules solidify before they are packed out and low gloss may result.

Solution: 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

Explanation: Gates and/or runners that are too small will cause excessive restriction to the flow of the molten plastic. Many plastics will then begin to solidify before they fill the cavity. The result is an unpacked condition of the molecules and the material will not have a chance to be pressed against the cavity steel. Low gloss will result.

Solution: 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

Explanation: If there is not enough venting in the mold, the material will push into the areas that are not vented and will not compress against the mold steel because trapped gases are in the way. The material will actually solidify before packing can be achieved. This early solidification will result in a dull surface finish.

Solution: 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

Explanation: With some plastics, if material is injected directly across a flat cavity surface, it tends to slow down quickly as a result of frictional drag and cools off before the cavity is properly filled and packed. When this happens, the molded part surface has low gloss because the material has not been forced against the cavity steel before solidification.

Solution: 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

Explanation: A properly molded product will duplicate the finish that is present on the molding surfaces of the mold in which it was formed. If that finish was not properly prepared (normally by hand polishing), the molded part will not have the high gloss that is normally desired.

Solution: 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

Improper Flow Rate

Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that has a stiffer consistency. It is better to use the stiffest flow possible because it improves physical properties of the molded part. However, the stiff material will require higher injection pressures, which may blow the mold open and cause loss of pressure, which results in low surface gloss. If an easy flow material is used, the physical properties will not be as great. In addition, the material will maintain pressure and create proper gloss.

Solution: 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

Explanation: If moisture is present in the material it will turn to steam during the transition through the heating barrel, forming voids on the surface of the molded part. Groups of voided areas will gather to form a dull area because the underlying plastic cannot replicate the steel surface of the cavity.

Solution: 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

Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, some of the material may not absorb enough heat to travel far enough to pack the cavity and low gloss will occur because the material cannot replicate the steel surface.

Solution: 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. 

HOME  DEFECTS  RESOURCES  BOOKSTORE  SHOWCASE  SEMINARS  CONTACT US
Copyright Texas Plastic Technologies
Worldwide rights reserved