Hardened Tooling Steel Selection and Injection Shrinkage Tolerances
Verify tool steel composition and heat treatment phase stability independently to ensure injection shrinkage predictions match finished part tolerances.
ASSAB S136
Meaning
High chromium martensitic stainless steel provides the primary material base for manufacturing plastic injection molds that require superior corrosion resistance and optical clarity in the finished product. Within industrial fabrication circles, assab s136 behaves as a specialized pre-hardened grade that resists the corrosive effects of chemically aggressive plastics such as PVC and acetic polymers. The material specifications for this steel restrict its use to applications where thermal fatigue is not the primary mode of failure but where dimensional stability during heat treatment is a requirement.
Manufacturers utilize this specific grade for medical components, optical lenses, and electronic housings because the clean microstructure allows for a high degree of mirror finishing. Compliance with the relevant production standards for assab s136 involves adhering to controlled cooling rates during the tempering phase to avoid the precipitation of carbides that would diminish its stainless properties. In the context of Chinese toolmaking facilities, the steel typically originates from certified suppliers who can provide mill certificates proving the chemical homogeneity of the batch.
Precise control over sulfur levels during refining ensures that the mold surfaces remain free from pitting during extended production runs in high humidity environments. The usage limits for assab s136 define it as a non-structural material intended exclusively for tooling that operates under five hundred degrees Celsius.
Refining Process
Production of high purity alloys requires an electro-slag remelting stage to filter out non-metallic inclusions that would otherwise compromise the structural integrity of the mold surface. Because assab s136 must sustain a mirror polish, the elimination of oxygen and hydrogen through vacuum degassing becomes a mandatory part of the manufacturing sequence. The initial casting of the steel creates a base ingot that then undergoes multi-directional forging to refine the grain structure.
Large grain structures in the finished product result in uneven cooling and potential cracking during the later machining stages. After forging, the steel moves through a controlled annealing process that reduces internal stresses and prepares the metal for CNC milling operations. If the cooling rate during this phase is too fast, the material becomes too hard to machine with standard carbide cutters.
Specialized foundries monitor the chemical composition of the melt in real time to verify that chromium remains between twelve and fourteen percent by weight.
Polishing Characteristic
Achieving a standard SPI A-1 surface requires the absence of pinpoint porosity that characterizes lower quality martensitic alternatives. The clean response of assab s136 during the diamond paste buffing stage results from the uniform distribution of chromium throughout the metallic matrix. If the steel contains high levels of carbon segregations, the finishing process reveals unsightly orange peel textures that render the mold unsuitable for lens production.
Skilled toolmakers utilize a sequence of progressively finer abrasive stones before switching to manual cotton wheel sets. The duration of the manual polishing stage depends heavily on the initial heat treatment results and the absence of overheating during rough grinding. High pressure mold surfaces for clear acrylic parts depend on this steel to avoid refractive defects that smaller micro-cracks would otherwise introduce into the light path.
Operational Limit
Exposure to temperatures exceeding six hundred degrees Celsius causes the hardness of the matrix to decline rapidly as the martensite transforms back into softer ferrite. In environments where the mold face encounters high velocity glass fibers, the wear rate of assab s136 increases dramatically compared to harder tool steels like D2 or CPM grades. Moisture and humidity in Southern China factories demand the corrosion protection that this specific steel offers over conventional P20 categories.
If the cooling channels within the mold block are not treated with protective inhibitors, mineral scale builds up more slowly in this grade due to its lower reactivity. The financial advantage of using assab s136 diminishes when the production run exceeds one million cycles without regular surface maintenance. For long-term viability, the operator must avoid using high pressure steam for cleaning, which can lead to stress corrosion cracking over several years of use.
Internal mold pressure must stay below the calculated yield point of the steel to prevent permanent deformation of the cavity walls during the injection phase.