Knit lines, also called weld lines, form where two or more flow fronts of molten plastic meet during mold filling. They often occur around holes or other features that divide the flow, or in parts with multiple gates, and can leave a visible line on the surface.
Flow lines are wavy patterns or streaks that follow the material’s path through the mold. They are often associated with variations in flow and cooling during filling.
Splay appears as silvery streaks on the surface. Common causes include moisture in the resin, trapped air, or gases generated during molding.
Gloss variation occurs when an area appears duller or shinier than the surrounding surface, even when the color is consistent.
These defects require evaluation before repair. A knit line, for example, can also involve reduced strength. Per-Fix® corrects cosmetic appearance; it does not restore structural strength or correct an underlying physical molding problem.
Per-Fix® is designed for automotive interior parts and other molded parts where cosmetic appearance defects need correction in low-gloss applications. A dashboard or trim component, for example, may meet its functional requirements yet be rejected because of a visible surface blemish.
The appropriate repair depends on the resin, the type and severity of the defect, and the required finish. Matching the coating to those needs helps restore a consistent appearance while maintaining the part’s low-gloss character.
Not sure whether your part is a fit? Tell us what you are seeing, and we will help you evaluate the options.
The long-term answer to a recurring molding defect is in the process. Depending on the cause, that may mean improving resin drying, adjusting temperatures or injection speeds, or reviewing gate design and venting. Those changes matter, but they do not recover the parts already molded.
Per-Fix® gives molders another option for parts that are suitable for cosmetic repair. Its micro-thin clear coating optically normalizes the affected surface, reducing the visible contrast between the blemish and the surrounding area while preserving the underlying molded color. Selecting the appropriate gloss level helps the repair blend with the existing finish. Pigmented versions provide additional hiding power when needed.
The coating can be applied at the mold station or in a secondary repair area. Rapid drying helps keep handling delays short: Chem-Pak’s aerosol application guide lists typical handling in 2 minutes, packaging in 10 minutes, and full cure in 24 hours at 70°F (21°C). Follow the selected product’s instructions and confirm drying times under your actual application conditions.
The resin is the starting point for product selection. A coating that adheres well to polypropylene may not perform the same way on nylon, and solvent-sensitive plastics require suitable formulations that use different coating resins and solvent systems. Per-Fix® is offered in five specialized clear-coating series, with Black available for selected resin families:
These recommendations provide a starting point. Confirm suitability for the actual resin grade, surface condition, and application requirements on representative parts.
Clear is the starting point for many light to moderate defects, including knit lines, flow lines, and localized gloss variation. It improves visual uniformity while retaining the part’s underlying color.
Black adds coverage for more pronounced blemishes on black parts, including splay and color variation. It is a practical choice when the standard black finish meets the part’s appearance requirements and a custom color match is unnecessary.
Color Match™ provides additional hiding power with a custom color and gloss match. It is available for PP, TPO, PVC/vinyl, and limited ABS applications, including black parts that require a specific shade or finish.
The Per-Fix® range includes triple matte (AAA), double matte (AA), matte (A), eggshell (B), and satin (C) finishes. Available gloss levels vary by product series. Color Match is formulated to the part’s specified color and gloss.
Use the following workflow with the selected product’s technical data sheet, label, and aerosol application guide.
1. Confirm the part and coating are suitable. Verify that the part meets its functional requirements, identify the resin, and select the appropriate coating and gloss level.
2. Test representative parts. Confirm compatibility, adhesion, color, and gloss before treating a production lot.
3. Apply according to the instructions. Prepare and shake the can, clear the valve as directed, and use a steady sweeping motion. The aerosol guide recommends a spray distance of 8–12 inches and a single coat of two to three passes to bring the surface to a just-wet condition.
4. Allow drying and inspect the result. Observe the recommended handling and cure times. Confirm that the repaired part meets the required appearance and adhesion criteria before release.
Will the repair last?
Per-Fix® is designed to provide a durable, permanent cosmetic repair when the appropriate system is selected and properly applied. Confirm performance against the part’s required adhesion and service conditions.
Can I use Per-Fix® on exterior parts?
Standard Per-Fix® Clear and Black are intended for interior parts. Color Match is designed primarily for interior use, with potential for limited exterior applications. Exterior applications require customer testing and approval under the intended exposure and service conditions. Chem-Pak does not perform exterior testing.
Is a flaw repair coating a substitute for fixing the molding process?
Per-Fix® is typically used as an interim solution, allowing production to continue while operations or maintenance teams address the underlying quality issue. However, conditions such as excessive mold wear may require repairs that cannot be completed quickly, while production demands may prevent taking the mold out of service. In these circumstances, Per-Fix® has also been used as a longer-term production solution to address visual defects on otherwise acceptable parts.
I am not sure which resin my part is made from. Where do I start?
Share the part information, any available resin specifications, and the defect you are seeing. We can help you narrow the selection. Identify the substrate before choosing a coating; whether the part is black or another color does not establish compatibility.
Browse the full Per-Fix® line by resin, with technical data and safety data sheets available on the product pages. For help with a specific part, contact our team with the resin, the appearance defect, and the required finish.
Randy Duane is President and CEO of Chem-Pak, Inc. He has spent his career in aerosol and liquid manufacturing, bringing decades of hands-on experience to product development and customer applications.
