Description:

Sink Mark can be defined as a depression, resembling a dimple or groove, caused by excessive localized shrinking of the material after the part has cooled.

Possible Causes & Remedies:

Machine Cause

◇ BARREL TEMPERATURE TOO HIGH

Remedies: Decreasing the barrel temperature allows the molecules and voids to expand and contract normally and provide consistent shrinkage values. Consistent, uniform shrinkage minimizes the condition that causes sink marks.

◇ INSUFFICIENT INJECTION PRESSURE OR TIME

Remedies: Increase the amount of pressure or the time applied. Upon initial startup the mold should be filled incrementally starting with intentional short shots (if the mold design allows) and progressively increasing pressure shot-by-shot until the mold is filled and packed properly. If injection pressure and time are adequate the shrinkage should be uniform and consistent resulting in parts without sink marks.

◇ INADEQUATE COOLING TIME

Remedies: Increase the cooling time portion of the cycle. It is true that longer cycles make the part more expensive but there is a minimum amount of time required for the resin to form a proper skin. It depends on what material is being molded. A general rule-of-thumb for a part with a wall thickness of 0.100'', the cooling time should be 20 seconds. The overall cycle would then be 25-30 seconds.

◇ INSUFFICIENT CUSHION AND/OR HOLD TIME

Remedies: Maintain a cushion that is somewhere between 1/8'' and 1/4'' at the end of the injection stroke. This provides something against which hold pressure can be applied. The amount of time for holding pressure should be long enough for the gate to freeze, normally 3-4 seconds. 

◇ EXCESSIVE NON-RETURN VALVE CLEARANCE

Remedies: Inspect the non-return valve mechanism and replace worn or damaged components. This wear is normal but is accelerated by molding materials that have reinforcements (such as glass) in them. The valve should be inspected at least every three months.

Mould Cause

◇ MOLD TEMPERATURE TOO HIGH OPPOSITE RIBS

Remedies: Decrease the mold temperature to the point at which the material has the proper flow and packs out the mold without shorting. 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 optimize their size and shape. Do not overlook the sprue bushing as a long sprue may solidify too soon. Use a heated bushing or extended nozzle to minimize sprue length. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.

◇ IMPROPER GATE LOCATION

Remedies: Relocate or redesign the gate so that the molten plastic is directed against an obstruction such as a core pin. This will cause the material to disperse and continue to flow instead of slowing down.

◇ EXCESSIVE THICKNESS AT MATING WALLS

Remedies: Although it is good design practice to maintain all walls at a uniform thickness, in areas where a junction is formed, one of the walls should be between 60% and 70% of the mating wall thickness. This will minimize the mass at the junction until the shrinkage is equal in all areas and sink marks will not develop.

Material Cause

◇ IMPROPER FLOW RATE

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

Operator Cause

◇ INCONSISTENT PROCESS CYCLE

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

(Source: plastictroubleshooter.com)