Relationship Between Injection Mold Dimensions and Product Dimensions & Adjustment Methods
2026-04-15 15:35:12
Injection Mold
In injection molding production, mold dimensions are the decisive factor for product dimensional accuracy. The two are not simply equal but have a strict logical relationship affected by material shrinkage, mold structure, processing accuracy and molding parameters. Many dimensional defects such as oversized or undersized parts, warpage and mismatched assembly are caused by poor matching between mold dimensions and product dimensions. This article explains the core relationship between mold size and part size and provides practical adjustment methods for on‑site debugging and mold modification.
Core Relationship Between Mold and Product DimensionsThe fundamental reason for the difference between mold size and product size is the shrinkage of plastic during cooling. After molten plastic is injected into the cavity, it undergoes volume shrinkage as temperature decreases. Therefore, mold dimensions must be enlarged in advance to compensate for this shrinkage. The basic formula is: mold cavity dimension = product target dimension × (1 + shrinkage rate). Shrinkage rate varies greatly among different materials. General plastics such as PP and PE have a shrinkage rate of 1.5%–3.5%. Engineering plastics such as PA and PBT are 0.5%–2.0%. Glass fiber reinforced materials significantly reduce shrinkage to 0.2%–1.0%.
Actual shrinkage is also affected by wall thickness, injection pressure, holding pressure, mold temperature and cooling time. The cavity determines the outer dimensions of the product, and an oversized cavity leads to an oversized product. The core determines inner dimensions such as holes and grooves, and an oversized core results in a smaller inner size. Gate and runner systems affect filling pressure and holding effects, which in turn influence shrinkage and stability.
Common Causes of Dimensional Deviation
Dimensional deviation usually comes from several aspects. Incorrect shrinkage rate setting is one of the most common reasons. If the shrinkage value used in mold design is inconsistent with the actual shrinkage of the material, the product size will inevitably deviate. Low mold processing accuracy is another key factor; errors in milling, grinding and assembly will directly be reflected on the product. Improper injection parameters, such as insufficient holding pressure, unstable mold temperature or uneven cooling, will also cause size changes.
In addition, long‑term production will cause mold wear or deformation, especially in molds used for glass fiber reinforced materials. The guide pillars and bushings gradually wear, leading to increased matching clearance and reduced positioning accuracy. Changes in material batches or production environment will also affect the final product size. Therefore, dimensional stability requires comprehensive control from design, processing, production to maintenance.
Process Adjustment MethodsProcess adjustment is the preferred method for small dimensional deviations, usually within 0.05mm, because it requires no mold modification and is low in cost. Increasing mold temperature will increase shrinkage and reduce product size, while lowering mold temperature will reduce shrinkage and make parts larger. Holding pressure plays a vital role in size control. Insufficient holding pressure causes excessive shrinkage and small size. Increasing holding pressure and holding time can effectively compensate for shrinkage.
Appropriate injection speed and temperature ensure uniform filling and stable shrinkage. Excessively high material temperature will increase shrinkage, while excessively low temperature will lead to insufficient filling. Through reasonable parameter matching, most minor dimensional problems can be improved without modifying the mold. This method is especially suitable for trial production and small‑batch debugging.
Mold Dimension Adjustment Methods
When the dimensional deviation is large and stable, usually more than 0.05mm, mold modification must be carried out. If the product is oversized, the cavity needs to be ground and polished to reduce its size. If the product is undersized, methods such as welding, electroplating or adding inserts can be used to increase the cavity size. For local dimensional deviation, only the corresponding core or cavity area needs to be modified.
Hole size problems are usually solved by adjusting the diameter or position of the core pins. During modification, each processing amount should be controlled within a small range to avoid excessive correction. After each adjustment, trial molding and measurement are required until the size meets the requirements. Precision parts often require multiple corrections to achieve ideal accuracy.
ConclusionThe relationship between injection mold dimensions and product dimensions follows the principle of shrinkage compensation. Stable dimensional control requires correct shrinkage setting, high‑precision mold manufacturing, reasonable process parameters and regular maintenance. By following the sequence of “process first, mold later”, manufacturers can effectively solve dimensional problems, improve qualification rate and ensure stable mass production.
