Top 5 Common Problems in TPE Injection Molding: Flow Marks, Demolding Issues, Weld Lines, Flush, and Silver streaks - Causes and Solutions [Supervised by a Material Manufacturer]
This article explains the causes and countermeasures for common problems such as flow marks, demolding issues, weld lines,flushs, and silver streaks that occur during injection molding of thermoplastic elastomers (TPEs). Improvement methods starting from the material itself are explained in detail under the supervision of the technical department of a material manufacturer.
Table of Contents [hide]
- 1.Thermoplastic elastomers (TPEs) and challenges during molding
- 1.1.Overview of Defects and Improvement Approaches
- 1.2.What are flow marks? Causes and countermeasures
- 1.3.Why do mold release problems occur? Causes and countermeasures
- 1.4.What are weld lines? Causes and countermeasures
- 1.5.Causes and countermeasures for burrs (flush)
- 1.6.Causes and countermeasures for silver streaks silver streaks
- 2.Frequently Asked Questions (FAQ)
- 3.Our technical support
Thermoplastic elastomers (TPEs) and challenges during molding
Thermoplastic elastomers (TPEs) are materials that combine the flexibility and elasticity of rubber with the moldability of thermoplastics. Because they can be processed using common injection molding and extrusion molding machines, they are used in a variety of product designs to improve productivity, simplify processes, reduce weight, and ensure recyclability.
On the other hand, injection molding of TPEcan present different molding challenges than those for general-purpose plastics due to the material's flexibility, flow properties, and solidification behavior. This section introduces typical molding defects and points for improvement from the perspective of molding conditions and material selection.
Summary of bugs and improvement approaches
Defect | Typical factors | Molding conditions and mold configurations | Solutions based on materials |
Flow marks | Lack of flowability, unstable melt flow front | Adjust resin temperature, injection speed, and mold temperature. | A material designed for high flowability and stable melt flow front |
Demolding issues | Flexibility, insufficient cooling, mold surface condition | Temperature reduction, extended cooling, surface treatment | Materials Designed for Easy Release from mold |
Weldline | Gas generation, temperature and injection velocity condition, Insufficient Vnnding | Optimization of temperature and metering conditions; addition of vents | Materials designed to minimize gas generation and improve weld line appearance |
Flush | Excessive injection pressure, excessive flowability, insufficient solidification, mold gap | Review of pressure, temperature, and holding pressure; mold inspection. | Material designe to balancce floiwability and flush suppression |
Silver Streaks | Moisture absorption of materials, moisture condensation, volatile components | Pre-drying, management of storage and transport conditions | Check the drying conditions for each material. |
Actual optimal conditions vary depending on the type and grade of material, product shape, mold structure, molding machine, etc. For recommended molding conditions for each product, please refer to the product catalog or contact our customer service.
What are FLOW MARKS? Causes and countermeasures
Occurrence phenomenon
This phenomenon involves the appearance of record-like or striped flow marks on the surface of molded products. In products with high aesthetic requirements, this dedects can compromise appearance quality.
Main causes
· The material lacks fluidity.
· The flow front’sResin Fountain Flow))is unstable
· The product shape, gate design, molding conditions, and material properties are not compatible.
Common improvement methods
· If insufficient fluidity is the primary cause, increase the resin temperature, injection speed, or mold temperature within an appropriate range.
· If the main cause is instability at the flow front, adjust the injection speed in stages.
· If excessive flowability is causing unstable flow, consider lowering the resin temperature.
Why do DeMolding Issues occur? Causes and solutions
Occurrence phenomenon
This refers to phenomena such as molded parts being difficult to remove from the mold, deforming during ejection, or sticking to the mold surface.
Main causes
· TPEis more flexible than typical rigid plastics, and depending on the product shape, the release resistance can be high.
· The resin temperature or mold temperature is high.
· The cooling time is short, and the molded product does not solidify sufficiently.
· It is affected by the condition of the mold surface. In particular, mirror-like surfaces have a large contact area, which can make mold release difficult.
Common improvement methods
· Lower the resin temperature and mold temperature within an appropriate range.
· Extend the cooling time to ensure the rigidity of the molded product during removal.
· The actual contact area with the resin is reduced by processes such as texturing and blasting the mold surface.
· Check the mold and product design, including draft angles, undercuts, and protrusion positions.
What is a WELD LINE? Causes and solutions
Occurrence phenomenon
This phenomenon occurs when separate flow fronts merge, resulting in a visible line and potentially reduced mechanical strength.
Main causes
· The resin temperature drops at the flow front junction, resulting in insufficient bonding.
· Gas generated by the materials and molding conditions accumulates where the flow fronts merge.
· The resin temperature is too high, increasing the amount of volatile components and decomposition gases.
· The injection speed is too fast, and the gas cannot be expelled in time, or the gas is drawn in.
· The resin temperature rises due to shear heating during metering.
Common improvement methods
· Optimize the resin temperature, and lower it if overheating is suspected.
· Increase the mold temperature to suppress resin temperature drop where the flow fronts merge.
· Gas vents (installing or adding vent channels to release air from inside the moldand ensuring an venting path)
· Adjust the injection speed and balance it with gas emissions.
· Reduce the screw rotation speed and apply moderate bacl pressure to stabilize the metering process.
Causes and solutions for burrs (flushing)
Occurrence phenomenon
This phenomenon occurs when resin leaks from the parting line, sliding parts, or around the ejector pins of the mold, resulting in a thin, film-like excess.
Main causes
· The injection pressure is too high or holding pressure is high, exceeding the clamping force or the mold's sealing capacity.
· The material is too fluid, or the resin temperature is too high.
· High pressure is applied whenthe TPE has not fully solidified.
· Gaps are created due to mold wear, deformation, and the inclusion of foreign objects.
· Near the gate, the localized pressure increasing due to high flow resistance.
Common improvement methods
· Review the upper limits for injection pressure and injection speed, and adjust the V/P switchover position and holding pressure conditions.
· Lower the resin temperature within an appropriate range.
· Check the clamping force and the condition of the mold.
· Check the design factors related to flow resistance, such as gates, runners, and product wall thickness.
Causes and solutions for SILEVER STRIKES
Occurrence phenomenon
This phenomenon involves the appearance of silvery-white streaks on the surface of molded products. It can occur when moisture or volatile components vaporize during the flow process and are stretched across the surface.
Main causes
· The material has absorbed moisture during storage.
· When materials are moved from a low-temperature environment, moisture condensation occurs due to the temperature difference.
· The materials, colorants, and additives contain moisture or volatile components.
· Gas is being generated due to overheating or resin stagnation in the barrel.
Common improvement methods
· Perform pre-molding drying according to the recommended conditions for each material.
· Review the management of ingredients after opening, storage locations, and temperature differences during transportation.
· By utilizing hopper dryers and other equipment, re-moisture absorption after the product is put into the molding machine can be suppressed.
· Check cylinder temperature, residence time, and purging conditions.
Drying conditions for TPE before molding
TPO,TPS, andTPV(partially cross-linked types) generally do not require drying.
The following are standard drying conditions for our materials. It is important to check the recommended conditions of each manufacturer. Also, the required drying time and other conditions will vary depending on the storage conditions, so please use these as a guideline.
Examples of materials | Drying temperature guidelines | Estimated drying time |
TPV(Fully Crosslinked Type) | 80～100℃ | 2-3 hours |
TPC | 70～110℃ | 2-5 hours |
Frequently Asked Questions (FAQ)
Q1.What is the most common defect in TPE injection molding?
While it varies depending on the product shape and application, products with high appearance requirements tend to have more flow marks and weld lines,while flash and mold release issues are more common with softer materials.
Q2.What is the difference between flow marks and weld lines?
Flow marks are striped marks caused by unstable flow front, while weld lines are linear defects that occur where two or more branched flows merge.
Q3.Can TPE be molded without drying?
This varies depending on the material and grade.For hygroscopic materials such asTPV(fully cross-linked type) andTPC, pre-molding drying is often recommended, so you should check the recommended conditions in each product catalog.
Q4.What should I do if I don't know the cause of the molding defect?
It is crucial to examine the external appearance, location, and filling behavior to identify whether the defect is caused by the molding conditions, mold, or material. Our company supports improvement proposals by conducting root cause analysis from the perspectives of materials, molds, and molding conditions.
Q5. How can we minimize Flush inTPE while also avoiding insufficient filling?
First, review the molding conditions, such as injection pressure, holding pressure, and resin temperature. Then, select a material that can prevent insufficient filling while minimizing flush.
Our technical support
Molding defects occur due to the interplay of not only material properties, but also product design, mold design, molding machine, and molding conditions. At our company, we confirm our customers' applications and required characteristics, and support them in resolving issues from both a material selection and molding condition perspective.
· Material selection based on application and required characteristics
· Technical consultation regarding the optimization of molding conditions.
· Analysis of the causes of molding defects and proposals for improvement.
· Prototype evaluation and grade proposal
