Shrinkage Compensation Calculation and Key Points of Plastic Mold Dimension Design for Sleeve Parts
Sleeve plastic parts are mostly thin-wall cylindrical structures. The shrinkage in radial direction differs obviously from axial direction, with radial shrinkage larger than axial shrinkage. Cavity dimension design of plastic molds must carry out accurate compensation combined with material shrinkage rate. Meanwhile, dimension fluctuation caused by demolding, cooling and assembly should be considered to guarantee stable dimension of finished sleeves.
Basic Calculation Formula for Shrinkage Compensation of Sleeve Parts
The core formula for mold cavity dimension calculation: Mold dimension = Nominal part dimension + Nominal part dimension × Material shrinkage rate - Tolerance correction of plastic part. For cylindrical sleeves, inner diameter and outer diameter should be calculated separately instead of applying a single shrinkage value directly. For outer diameter cavity based on outer circle size of part: Cavity diameter = Sleeve outer diameter × (1+S). For core inner diameter based on inner hole size of part: Core diameter = Sleeve inner diameter × (1+S). S stands for molding shrinkage rate. The radial shrinkage rate of thin-wall sleeves is higher than nominal value in material datasheet, so small margin should be reserved during design. Axial length compensation formula: Mold length = Sleeve axial dimension × (1+Sₗ). Axial shrinkage Sₗ is generally smaller than radial shrinkage Sᵣ. Axial shrinkage parameter cannot be directly used for radial dimension.
Distinguish the Difference between Radial and Axial Shrinkage
Sleeve belongs to rotary thin-wall part. In cooling stage, free shrinkage space in radial direction is limited and wrapping force is larger, so radial shrinkage value is higher. For ordinary PP and PE soft sleeves, radial shrinkage is obviously greater than axial shrinkage. For PA and PBT glass-filled materials, the difference between radial and axial shrinkage decreases, but anisotropy still exists. Do not apply uniform shrinkage rate to all dimensions during design. Outer circle, inner hole, wall thickness and length should adopt shrinkage parameters separately. If sleeve wall thickness is relatively large, shrinkage of thick wall section rises, and compensation fine-tuning should be made separately for thick wall segments to prevent small inner hole and large outer diameter of finished parts.
Tolerance Allocation and Correction on Molds for Plastic Part Tolerance
Sleeves usually have assembly matching requirement with strict tolerance of inner and outer circle dimensions. Mold machining tolerance takes 1/3 ~1/4 of finished plastic part tolerance. Mold manufacturing tolerance cannot be fully assigned to single side. Bidirectional dimensions adopt symmetric allocation. For benchmark dimension with fitting requirement, control mold dimension in the middle to upper limit of plastic part tolerance band preferentially, to counteract shrinkage variation caused by mold temperature fluctuation during production. For non-matching free dimensions, mold machining tolerance can be relaxed moderately to reduce mold processing cost. For sleeves requiring interference fit, reserve margin for shrinkage fluctuation to avoid assembly jamming or excessive gap due to out-of-tolerance dimension during mass production.
Key Points for Cavity and Core Dimension Design
Molding core of sleeve mold is the key component to control inner hole dimension. The core is prone to thermal expansion. Secondary correction by adding thermal expansion amount should be carried out during design. For multi-cavity sleeve molds, uniform machining benchmark must be adopted for all cavities and cores to ensure dimension consistency of each cavity. Thin and long cores of thin-wall sleeves will deflect under injection pressure, leading to uneven wall thickness of sleeves and indirectly changing shrinkage result. Add reinforcing structure at core root to reduce compression deformation. Try to avoid parting line on matching cylindrical surface of sleeve inner and outer circle, to reduce impact of flash on roundness. Roundness deviation causes assembly failure.
Influence of Cooling Structure on Shrinkage Compensation
Uneven cooling changes local shrinkage amount and directly destroys pre-calculated compensation effect. Adopt surrounding cooling channels preferentially for cylindrical area of sleeves to guarantee uniform temperature of cavity and core. Design straight or spiral cooling structure inside cores to reduce temperature difference between inner and outer mold. Higher mold temperature leads to larger overall shrinkage of plastic parts; lower mold temperature reduces shrinkage amount. Select shrinkage rate according to mass production mold temperature during compensation design instead of only using standard sample parameter at room temperature. Keep cooling channels close to molding surface to reduce dimension drift caused by temperature difference and stabilize shrinkage rate.
Influence of Demolding Structure on Dimension
Sleeve demolding mostly uses stripper plate or sleeve ejector pin. Uneven ejection force causes elliptical deformation and indirectly changes measured inner and outer diameter. Select large-area stripper plate to disperse ejection force and reduce demolding deformation. Demolding draft angle should balance dimension and demolding resistance. Excessive draft angle causes obvious dimension difference at both ends of sleeves; insufficient draft angle leads to demolding scratch and part deformation. For long sleeves, add auxiliary venting in the middle to reduce vacuum adsorption inside cavity and dimension variation caused by tensile force during demolding.
Material and Process Margin Reservation
Different batches of the same raw material and proportion of regrind material will change actual shrinkage rate. Select shrinkage range instead of single fixed value during mold design. Reserve grinding allowance for cavity and material removal trimming space for core within processing allowable range. When shrinkage is too large, polish outer circle of core conveniently to adjust dimension. Measure sample dimension after mold trial when samples are fully cooled to room temperature, calculate actual molding shrinkage rate reversely, compare with previous calculated value, and trim cavity or core as required.
Mold Trial Check and Dimension Verification
Measure inner and outer diameter and length after samples cool fully to room temperature after mold machining and trial run. Do not test high-temperature samples directly. Test samples of each cavity separately for multi-cavity molds, count dimension fluctuation range, calculate actual shrinkage value and compare with theoretical calculation result. Judge whether mold modification is needed according to measured data, fine-tune injection molding parameters synchronously to match mold compensation amount and stabilize mass production dimension.
