Analysis of four main reasons for unstable dimensions in injection molded parts: materials, equipment, molds, and processes.

Using the same mold and the same parameters, production was successful in the morning, but the dimensions started to drift in the afternoon; within the same batch, some cavities were acceptable, while others remained consistently too small; the product’s dimensions were normal immediately after demolding, but changed again after 24 hours. When encountering these problems, the initial on-site adjustments are often made to the holding pressure, material temperature, or cooling time. After a round of parameter changes, the dimensions temporarily return to tolerance, but then fluctuate again after a period of time.

The main reason is that the size of injection molded parts is not determined by a single parameter, but rather by the combined effects of material volume changes, equipment output repeatability, mold pressure and temperature limits, and the entire molding cycle. Focusing solely on the final size without distinguishing between fluctuation patterns easily turns system problems into repeated machine adjustments.

This article summarizes the experience of Elimold’s injection molding and mold engineering team regarding various reasons for dimensional instability in injection molded parts. Through this article, you can gain a deeper understanding of our analysis and conclusions regarding the four main causes: materials, equipment, molds, and processes .

First determine whether the dimensions of the injection molded part are off or it is loose.

in injection molded parts are not the same issue. If the product is consistently larger or smaller than expected, but with minimal fluctuations, it usually indicates a dimensional center offset, which may be related to mold size, actual material shrinkage, or process center settings. If the average size is generally appropriate, but varies significantly between mold runs, it indicates excessive process fluctuations, and the material condition, equipment repeatability, and process stability should be carefully examined.

In actual analysis, at least four types of manifestations must be distinguished:

Modular fluctuation	The dimensions of adjacent modules vary randomly;
Time drift	Gradual deviation during startup, shift change, or continuous production;
Cavity differences	In multi-cavity molds, some cavities are consistently too large or too small.
Later changes	It passed the demolding test, but changes occurred after placement, humidification, or heat treatment.

Before analyzing materials and processes, measurement issues must be ruled out. Unstable product temperature, inconsistent measurement times, different reference positioning, excessive clamping force, and insufficient repeatability of measuring tools can all create false fluctuations. ISO 294-4 classifies shrinkage into molding shrinkage and post-molding shrinkage.

1. Molding shrinkage is the difference between the mold cavity dimensions and the dimensions of the sample measured for the first time under specified conditions, excluding the effects of moisture absorption.

• Post-forming shrinkage reflects dimensional changes that continue to occur after the initial measurement and may include factors such as further crystallization, stress relaxation, and moisture absorption.

It should be noted that this standard is mainly used for standard sample testing. Actual products should still be tested according to drawings, customer specifications or enterprise standards, with uniform measurement time and condition adjustment conditions.

Injection molding materials: Shrinkage rate is not a constant.

The molding shrinkage rate in the material data sheet is a reference range obtained under specific sample, mold, and process conditions, and cannot be directly regarded as a fixed coefficient for product design and mold modification. The specific volume of polymers changes with temperature and pressure. After the melt enters the mold cavity, its density changes continuously during pressure holding, cooling, and solidification, eventually resulting in shrinkage. Even when using the same material, the actual dimensions may differ depending on the pressure, temperature, orientation, and crystallization process.

Material batch and flowability changes

Variations in resin grade, molecular weight distribution, filler content, lubricant, and color masterbatch ratios can all affect melt viscosity, mold filling pressure, and pressure holding transfer. If there is an overall dimensional shift after a batch change, the batch number, melt flow properties, and batching records should be checked first.

Moisture and dryness

Insufficient drying of hygroscopic materials such as PA, PET, PBT, and PC can lead to hydrolysis during high-temperature plasticization, causing changes in molecular weight and viscosity. Some materials continue to absorb moisture after molding. For example, PA products typically experience dimensional increase after absorbing moisture. The moisture content of raw materials, drying temperature, drying time, and exposure time after opening should be controlled according to the material and type of drying equipment. When using a dehumidifying drying system, the dew point of the drying air and the actual airflow should also be monitored.

Crystallization and Orientation

The dimensions of semi-crystalline materials such as PP, PA, POM, and PBT are also affected by their degree of crystallinity. Changes in mold temperature, cooling rate, and wall thickness can all alter the crystalline state. While glass fiber reinforcement typically reduces overall shrinkage, it increases directional differences, causing different shrinkage in the flow direction compared to the direction perpendicular to the flow.

Recycled materials and thermal degradation

Unstable recycled material ratios, repeated processing times, and the source of pulverized material can alter melt viscosity, fiber length, and additive content. Excessively high barrel temperatures or prolonged residence times can also cause material degradation.

Injection molding equipment: Stable setpoints do not equal stable output.

The display on a stable injection molding machine shows the control target, but what truly determines the size is the actual amount of melt output, injection speed, switching position, holding pressure, and melt state of the machine in each mold.

Repeatability of anti-reverse mechanism

Wear or unstable sealing of the check ring can cause melt backflow during injection and holding pressure stages. This typically manifests as simultaneous fluctuations in buffer volume, product weight, and dimensions. If the residual material position fluctuates wildly, increasing the holding pressure alone should not be sufficient to compensate.

Plasticization and metering stability

Screw and barrel wear, as well as unstable back pressure and screw speed, can all affect metering repeatability and melt uniformity. If the amount of material used in a single mold is too small relative to the barrel capacity, it can easily lead to excessive melt residence time and thermal degradation; if the amount of material used is too close to the machine’s maximum injection capacity, it may result in insufficient plasticizing time, unstable metering recovery, or insufficient pressure margin.

Temperature control and power system drift

When the barrel heating coil, thermocouple, mold temperature controller, hydraulic oil temperature, and servo system drift, the panel settings may not change, but the actual temperature and pressure responses will differ. Therefore, it is not enough to just look at the set parameters; the following should also be continuously monitored: actual injection time; V/P switching position and switching pressure; peak injection pressure; buffer volume; and plasticizing time.

Injection mold and product structure: The cavity is not naturally uniform.

The mold determines how the melt flows, shrinks, and cools, while the product structure determines the sensitivity of each area to shrinkage and deformation.

Premature freezing of the gate

If the gate is too small, the flow path is too long, or the local temperature is too low, the gate will freeze prematurely, preventing the holding pressure from being transmitted to the mold cavity. Extending the holding time is only effective if the gate can still transmit pressure. Continuing to hold pressure after the gate has frozen will only prolong the cycle and will not allow for further shrinkage compensation.

Imbalance between runner and cavity

In multi-cavity molds, differences in runner length, gate size, venting capacity, and hot runner temperature will lead to variations in filling time and cavity pressure for each cavity. If a fixed cavity consistently exhibits abnormalities, the corresponding runner, gate, cooling water system, and core should be inspected, rather than compensating for localized defects using overall machine parameters.

Uneven cooling

Improper cooling water circuit layout, scale buildup, insufficient flow, or excessive temperature differences in the circuit can all cause localized mold temperature variations. For dimensional stability, mold temperature uniformity is generally more important than average mold temperature.

Product structure and mold rigidity

Abrupt changes in wall thickness, excessively thick reinforcing ribs and pillars, asymmetrical structures, and designs with thin, elongated walls can amplify differences in cooling, orientation, and shrinkage. Insufficient core rigidity or mold bulging can cause systematic dimensional shifts; fluctuations in cavity pressure, clamping force, or slider positioning can further manifest as differences between mold cycles.

Injection molding process: The key is not the parameters, but the process window.

The core of injection molding dimension control is to ensure that each mold undergoes a similar filling, holding pressure, and cooling process.

V/P switching determines the starting point of pressure holding.

Switching to the mold too early can lead to insufficient cavity filling, causing excessive reliance on holding pressure for replenishment and potentially resulting in under-holding pressure. Switching to the mold too late can cause pressure spikes, over-holding pressure, flash, and residual stress. Fluctuations in the switching position can directly alter the cavity filling level when entering the holding pressure stage.

Melt temperature affects viscosity and pressure transmission

Increasing temperature generally reduces viscosity and improves flow, but it also prolongs cooling time and alters orientation relaxation and crystallization processes. Conversely, excessively low temperatures can increase pressure loss, leading to insufficient far-end filling or feeding. Therefore, the effect of material temperature on dimensions cannot be simply judged as “the higher the temperature, the greater the effect” or “the lower the temperature, the more stable the effect.”

Holding pressure determines the degree of compensation.

Insufficient holding pressure can easily lead to undersized dimensions, shrinkage, and internal voids; excessive holding pressure can result in overfilling, residual stress, flash, and difficulty in demolding. A reasonable holding pressure profile can reduce shrinkage differences in different regions and improve dimensional accuracy. The holding pressure time can be initially determined through a weight plateau test: gradually increase the holding pressure time until the product weight no longer increases significantly. However, during the test, it is essential to ensure no flash, no backflow prevention leakage, and that other process conditions remain stable. It is important to emphasize that product weight is only a proxy indicator of filler volume and shrinkage status and cannot replace critical dimensional measurements. For dimensions primarily controlled by glass fiber orientation, warpage, or residual stress, dimensional fluctuations may still occur even with stable weight.

Mold temperature and cooling determine the demolding state

For semi-crystalline materials, increasing the mold temperature usually increases the degree of crystallinity, which may increase molding shrinkage, but may also reduce residual stress and subsequent dimensional changes. If cooling is insufficient, the product may be ejected before rigidity is established, making it prone to demolding deformation and subsequent shrinkage.

Quickly pinpoint the cause based on fluctuation characteristics.

Size anomalies should not be guessed based on experience; the range can be narrowed down layer by layer based on the fluctuation pattern.

All cavities drift synchronously.	The key points of inspection include material batch, drying status, material temperature, mold temperature, and equipment thermal balance.
Persistent abnormalities in fixed cavity	The key areas to check are the corresponding runners, gates, vents, cooling water channels, and cores.
Size, weight, and cushioning fluctuate in tandem.	The key areas of inspection include the check ring, metering stability, and pressure holding repeatability.
It passed demolding but changed after being left to stand.	The key areas of inspection include crystallization, moisture absorption, residual stress, and cooling conditions.
Initial fluctuations after startup, recovery after production stabilizes	The key checks are whether the barrel, mold, and power system have reached thermal equilibrium.
Weight is stable, but size still fluctuates.	The key areas of inspection include material orientation, product warpage, mold positioning, and measurement systems.

The recommended analysis sequence is as follows: confirm the measurement system → stratify by time and cavity → correlate weight and process parameters → check materials and equipment → verify cavity pressure and mold temperature distribution → confirm key factors through testing. DOE and other testing methods are only meaningful after measurements are reliable and the equipment and mold conditions are basically stable.

Shift from dimensional inspection to process control

Dimensional stability cannot rely solely on end-of-pipe sampling, nor can it depend on a single, unchanging set of parameters. Companies need to establish a complete control chain covering materials, equipment, molds, processes, and measurement.

• The requirements for material moisture content, drying conditions, recycled material ratio, and batch switching are specified.
• Monitor V/P switching, buffer volume, injection time, plasticizing time, and actual pressure;
• The effective holding time is determined by the weight platform or cavity pressure.
• Monitor cooling water temperature, flow rate, and temperature difference between each circuit;
• Establish dimensional control charts based on the cavity to avoid average values masking individual cavity anomalies;
• Standardize measurement benchmarks, placement time, and condition adjustment requirements;
• Evaluate the process capability after the process stabilizes, and then decide whether to modify the model.

Ultimately, the instability of injection molded parts dimensions is not simply due to inaccurate shrinkage rates, but rather a result of inconsistencies in material condition, machine output, mold boundaries, and process windows. Truly effective improvement does not involve adjusting a single mold to meet specifications, but rather identifying the key variables affecting density, shrinkage, and deformation, ensuring that each mold is formed under repeatable conditions.

Summarize

This article summarizes the four main reasons for unstable dimensions in injection molded parts: materials, equipment, molds, and processes. It provides relevant analysis and conclusions, and you can refer to the solutions analyzed and summarized by our engineers for further processing. If you have precision plastic parts requiring mass injection molding and need related molds making, please contact the Elimold team for assistance.