In injection molding, particularly with multi-cavity molds, all inserts must be manufactured and maintained with absolute precision to ensure consistent, high-quality parts. Even minor deviations or surface defects in the inserts can result in rejected components. Molds are subject to significant stresses from the plastic materials being processed during the injection molding cycle. In addition, some plastics can be chemically aggressive or highly abrasive, leading to increased wear on mold components. To safeguard process reliability and part quality, RKT conducts continuous inspection of our molds during production as part of a structured preventive maintenance program. Targeted measures are implemented to minimize wear and protect critical mold areas.
Plastics and Their Impact on the Injection Mold
Influences of Different Plastics on the Mold
Standard plastics such as polypropylene (PP) or polyethylene (PE) are generally easy to process and typically do not require exceptional measures in mold design. However, materials modified with fillers, such as glass fibers, minerals, or other reinforcing additives, required special consideration. Abrasion and mechanical stress increase, making the careful selection of suitable steel grades, hardness levels, and protective coatings essential. RKT processes thermoplastic polymers according to customer specifications and designs molds tailored precisely to the material and application. This ensures long tool life, stable processes, and consistently high product quality.
Potential Challenges for Injection Molds
Flame retardants: Chemically aggressive flame retardants are sometimes added to plastic compounds to reduce flammability, particularly in electrical and electronic components. During processing, these additives can attack the steel surfaces of the mold, accelerating corrosion and wear. Without appropriate material selection and protective measures, this can shorten the service life of critical mold components.
Glass and Other Abrasive fillers: Abrasive fillers such as glass fibers, carbon fibers, glass beads, titanium or other reinforcing materials enhance the mechanical properties of the finished part when higher strength or stiffness is required.
Carbon fibers also provide electrical conductivity. The high flow speeds and pressures during injection cause these fillers to act abrasively on the mold surfaces. The continuous flow of reinforced material across steel inserts leads to increased mechanical wear.
Deposits: A further challenge is deposits on the surfaces of the inserts in the injection mold, which can arise from additives/fillers of some plastic grades. Outgassing, predominantly during the injection process, leads to deposits that must then be removed from the mold, as otherwise, e.g., unwanted marks appear on the surface of the plastic parts. This effect can be reduced by introducing venting channels (vents) at the end of the flow path in the inserts – small channels with a depth of 5 to 15 µm – to discharge possible outgassing.
Steel Grades for Injection Molds – Selecting Based on Plastic Properties
The selection of the appropriate steel grade for an injection mold is determined individually for each project and depends largely on the plastic material used and any fillers it contains. Material properties such as abrasiveness, corrosion potential, processing temperature, and surface requirements all play an important role.
- For mold inserts for commodity plastics such as PP and PE, we typically rely on high-quality tool steels such as Böhler 1.2343 ESR or W403 VMR. 1.2343 ESR is a premium hot-work tool steel known for its toughness at elevated temperatures, excellent hot strength, good machinability, and outstanding polishability. W403 VMR offers comparable properties and, due to its refined microstructure, is particularly well suited for delicate or highly detailed mold geometries.
- For mold base plates, pre-hardened steels such as 1.2312, 1.2085, or 1.2099 are commonly used. 1.2085 and 1.2099 offer enhanced corrosion resistance due to their higher chromium content, combined with good dimensional stability, which is an important factor for long-term precision and durability.
- When processing more demanding materials, such as POM, reinforced polymers including PA, PBT, PC, PPA, PEI, or LCP, higher-performance mold steels are required. In these cases, we use grades such as Böhler M340, M333, or M390, as well as Stavax and Tyrax from Uddeholm. These steels are characterized by high corrosion resistance and are available in various hardness and toughness grades, with excellent polishability. The ultimate selection is always tailored to the your specific application in order to provide optimal tool life, process stability, and part quality
Plastics in Medical Technology
RKT processes a wide range of high-performance thermoplastics tailored to demanding applications and regulatory requirements of the medical industy:
- OC and COP are frequently used in medical and diagnostic applications. These materials offer exceptional transparency, very low optical anisotropy, and excellent biocompatibility, making them ideal for cartridges, microfluidic components, and optical elements.
- PP, PE, and PS are cost-effective and easy to process. They are commonly used for high-volume disposable products where reliability and economic efficiency are key.
- POM, PA, and PPA are engineering thermoplastics selected for components that must withstand high mechanical loads, dimensional stress, or repeated movement.
- ABS, PC/ABS blends, polyamides, and PBT are impact-resistant materials frequently used for housing parts and structural components that require durability combined with good surface quality.
- Engineering plastics reinforced with high glass or carbon fiber content are used when exceptional strength and stiffness are required. These materials often serve as lightweight alternatives to metal components while maintaining high mechanical performance.
How RKT Prevents Material-Related Mold Damage
Design
Molds can be protected from abrasive, chemically aggressive, or otherwise challenging plastics through careful design and engineering. One of the most effective ways to minimize mold wear is through thoughtful construction, particularly in the design of the gating system where the molten plastic enters the mold.
The melt is introduced into the cavity through a small opening, and the design of this area plays a critical role in managing the effects of different plastic properties. One common solution is the use of a hot runner system, which eliminates the sprue and allows for precise control of the melt flow. Hot runners can be equipped with an open design or valve gate technology which offers the added advantage of avoiding a gate vestige entirely.
Various alternative gating systems are also available, depending on the part geometry. A film gate (or flash gate) is a wide channel used for large, flat components, though the excess material must be removed mechanically afterward. Diaphragm gates (also called umbrella gates) are often preferred for radially symmetrical parts, due to their round design and balanced flow characteristics.
By carefully selecting the type, size, and position of the gate, RKT can effectively reduce mold wear to provide consistent part quality, even when processing demanding materials.
Coatings
For certain mold components, special coatings exist depending on requirements:
- Sliding coatings/emergency running properties for moving elements to avoid lubrication with grease: e.g., DLC.
- Reduction of deposits at the end of the flow path on mold inserts or easier cleaning of the coated surfaces: e.g., TiN or CrN.
- Protection of the high-gloss polish on mold inserts against minimal scratches and corrosion: e.g., Caveo, CrN.
Cleaning/Maintenance
Certain plastics, such as POM (Polyoxymethylene), have a tendency to form deposits in molds, making regular cleaning and maintenance essential. The frequency and type of service depend on the plastic grade, part geometry, and production requirements.
With A-Service maintenance, molds are cleaned at regular intervals, as frequently as every eight hours, using methods such as dry ice blasting. This type of cleaning can be performed on the mold while still in the press to maintain optimal mold performance.
B-Service maintenance is scheduled based on the complexity of the mold and the materials being processed. Typically, after approximately 50,000 to 200,000 shots, the mold is removed from the machine, disassembled, and thoroughly cleaned and maintained in the tool shop to ensure continued precision and longevity.
For the medical sector and/or upon customer request, C-Service is performed. This involves a comprehensive assessment of the mold condition, replacement of worn components based on observed wear patterns, and intensive cleaning of all mold parts including all cooling channels. C-Service is typically conducted at fixed intervals, such as annually, depending on the intensity of mold utilization.
Screw Wear
The plastic materials can cause wear not only in the mold but also in the injection unit, particularly the screw and barrel. At RKT, we primarily use highly wear-resistant screw designs to ensure consistent process stability and long service life. Screws and barrels are regularly inspected with specialized measurement equipment, and any components showing signs of wear are promptly replaced to maintain optimal performance.
Do you need a special plastic compound for your product, or do your components need to have specific mechanical or conductive properties? We would be happy to advise you on the selection of plastics and additives. In addition, our in-house toolmaking department will ensure that you receive a precisely fitting tool that can withstand the stresses of injection molding over the long term and meets your product requirements 100%.
Stefan Preis
Head of Project Business
S.Preis@rkt.de