Buyers with experience sourcing rigid plastic injection molded parts are often surprised the first time they see a liquid silicone rubber (LSR) part's actual finished dimensions relative to the mold cavity. The shrinkage is noticeably larger than what they're used to from thermoplastics — and more importantly, it behaves differently, which has direct implications for how a mold needs to be designed and validated before it's considered ready for precision production.
Why LSR Shrinks More Than Rigid Plastics
The underlying cause is fundamentally different from thermoplastic shrinkage, which is primarily driven by cooling and crystallization behavior.
- LSR shrinkage is driven by the curing (crosslinking) reaction itself, not primarily by cooling. As the polymer network forms during cure, the material's actual volume changes as part of the chemical reaction — a mechanism that doesn't have a direct equivalent in most thermoplastic processing.
- Post-cure and post-demolding shrinkage continues over time, as discussed in the context of dimensional stability more broadly — a portion of total shrinkage isn't complete the moment the part leaves the mold, which is different from most rigid thermoplastics where the vast majority of shrinkage occurs during in-mold cooling.
- Shrinkage direction and magnitude can be less uniform than in many rigid plastics, particularly in parts with varying wall thickness, because cure completeness and rate can vary somewhat across the part depending on section thickness and how close each area is to the mold surface.
The practical result is that LSR shrinkage rates are generally higher, and somewhat less predictable without proper validation, than shrinkage rates buyers may be accustomed to from rigid plastic parts of similar geometry.
Why This Makes Mold Design More Demanding, Not Just the Shrinkage Number Different
A higher shrinkage rate alone wouldn't be a major design challenge if it were perfectly consistent and uniform — a mold designer could simply scale the cavity up by a fixed percentage and be done. The real design challenge comes from the factors that make LSR shrinkage vary:
- Wall thickness variation within a single part can produce different local shrinkage rates, meaning a single uniform scaling factor applied across an entire cavity may not produce accurate dimensions in every region of a part with mixed thick and thin sections.
- Specific compound formulation affects shrinkage behavior, meaning shrinkage data from a similar-looking part in a different silicone formulation isn't a reliable substitute for validation with the actual compound being used in production.
- Cure profile and post-cure process influence how much shrinkage occurs before versus after demolding, which affects how much of the total shrinkage a mold needs to compensate for directly versus what stabilizes afterward.
How Mold Designers Typically Determine Compensation Allowance
Rather than relying on a single generic shrinkage percentage, a properly engineered approach usually involves:
- Starting from material-specific shrinkage data for the exact compound being used, ideally validated by the material supplier or through direct trial molding, rather than a general "LSR shrinkage range" figure that doesn't account for formulation differences.
- Accounting for wall thickness variation within the part — thicker sections and thinner sections may need different local compensation rather than a single uniform scaling factor across the entire cavity, particularly for parts with significant thickness variation.
- Building in a trial-and-adjust qualification step, where an initial cavity is cut slightly conservative, first-article parts are measured against target dimensions after appropriate stabilization time, and the cavity is adjusted based on actual measured shrinkage before finalizing critical dimensions — rather than assuming the first-pass calculation will be precisely correct.
- Measuring parts after appropriate post-cure and stabilization time, not immediately after demolding, since measuring too early can lead to a mold being "corrected" based on a dimension that hasn't finished stabilizing yet, resulting in overcorrection.
What This Means for Buyers Setting Tolerance Expectations
- Tolerance specifications should reflect LSR's shrinkage behavior, not be carried over directly from a rigid plastic version of a similar part, since applying plastic-appropriate tolerances to an LSR part can create dimensions that are difficult or impossible to hold reliably in production.
- First article inspection timing matters. As covered in mold acceptance practices more broadly, dimensional measurements taken immediately after molding may not reflect final stabilized dimensions — this is especially relevant for shrinkage validation specifically, since the entire purpose of that measurement is to confirm the mold's compensation was calculated correctly.
- Design changes require re-validation. A wall thickness change, even a modest one, can shift local shrinkage behavior enough to require re-checking compensation in that region, rather than assuming the original compensation calculation still applies.
Questions to Ask a Mold Supplier
- What shrinkage data is being used for this specific compound, and is it based on supplier-published values, prior production experience with this exact formulation, or dedicated trial data for this part?
- How is compensation being adjusted for wall thickness variation within the part, if applicable?
- At what point after molding are first-article dimensions measured, and does that account for post-cure and stabilization time?
- If first-article dimensions show shrinkage different from the initial calculation, what is the process for adjusting the cavity, and how many iteration cycles should be expected before final tolerance is achieved?
The Practical Takeaway
Higher and less uniform shrinkage isn't a flaw in liquid silicone as a material — it's a predictable consequence of how the material cures, and a well-designed mold accounts for it directly rather than treating it as an afterthought to correct during production. For buyers evaluating a mold supplier, asking specifically how shrinkage compensation was calculated and validated for their exact part and compound is a much more useful question than simply asking whether the supplier "has experience with silicone."
