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Home / How to Resolve Plastic Molding Part Weight Variations from Pressure Control？

How to Resolve Plastic Molding Part Weight Variations from Pressure Control？

2025/11/17 By le zhan

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At an automotive parts injection molding plant, 10,000 semi-finished sensor housings were left idle due to a 0.5-gram weight deviation in the plastic injection molding process. “We’ve adjusted the pressure knob 50 times today,” their production manager said. “One batch is too light, the next too heavy. We simply can’t guarantee consistent weight.” This situation is not uncommon in plastic molding, with each injection molding machine incurring tens of thousands of dollars in annual losses from resin, labor, and scrap. Even for medical device component injection molding plants, weight tolerances as small as ±0.1 grams pose a greater risk; a single batch of defective parts can result in six-figure losses.

As an engineer at Topstar, I knew the root cause wasn’t an operational error; it was improper pressure control. To address this, Topstar uses low-damping injection units and linear guides in its plastic injection molding machines to achieve sensitive pressure detection and stable control.

What is weight deviation in plastic molding? How does improper pressure control lead to weight deviation?

To solve the weight deviation problem in plastic molding, it’s essential first to understand the definition of weight deviation and why pressure control is its primary driver. Weight deviation is the discrepancy between a part’s quality and its target specifications. Even a deviation of only 1-2% can render the part unusable in applications with strict tolerance requirements. Improper pressure control primarily causes weight deviation through three aspects:

  1. Injection pressure fluctuations: Insufficient pressure leads to inadequate mold filling, while excessive pressure results in overfilling. Even small, uncontrolled fluctuations in pressure within each cycle can cause inconsistent material filling.
  2. Holding pressure mismatch: Holding pressure is used to hold material within the mold during cooling. If the holding pressure is too low, the material will shrink; if it is too high, excessive material will be squeezed into the mold. Improper control leads to fluctuations in holding pressure within each cycle.
  3. Pressure response lag: Traditional injection molding machines adjust pressure too slowly when resin viscosity or mold temperature changes, resulting in a batch of parts with unacceptable weight, which is only addressed after the problem is discovered.

The Impact of Weight Deviation in Plastic Parts

Weight deviation not only causes scrap but also affects every stage of production. First, for customers with strict tolerance requirements, entire batches of products will be rejected if parts exceed weight specifications. One Topstar customer had 3,000 parts rejected due to a 2% weight deviation, resulting in losses of tens of thousands of dollars. Furthermore, while some overweight parts can be reworked, labor and time costs accumulate.

Poor pressure control leading to excessive pressure accelerates wear on the mold cavity and core, shortening mold life by 25% to 30%. Overweight parts waste resin; one customer calculated that they wasted 220 pounds of resin per month, resulting in annual losses of several thousand dollars. Improving pressure control can eliminate 80% of weight deviations. One manufacturer reduced weight-related losses by 92% after switching to Topstar injection molding machines.

The Impact of Weight Deviation in Plastic Parts

Why can’t traditional pressure control solve the weight deviation problem?

Traditional plastic injection molding machines struggle with weight deviations because their pressure control systems are slow to respond, lack precision, or are susceptible to frictional errors. The main problems include: older machines use high-friction injection units, which delay pressure regulation. By the time the machine reacts to a pressure drop, multiple parts with incorrect weights have often already been produced. Uneven movement of the sliding guides also leads to an uneven pressure distribution during injection. Even slight jamming can cause pressure spikes.

Additionally, traditional sensors have a 5-10 millisecond delay, making it impossible to detect and correct pressure changes in real time during plastic molding. Furthermore, operators need to adjust pressure settings based on experience rather than data, leading to over-adjustment and creating new weight problems.

Using Low-Damping Injection Molding Machines and Linear Guides

Topstar solved the weight deviation problem by upgrading its plastic injection molding machines to enable precise, sensitive pressure control. The key lies in two technologies: low-damping injection units and linear guides. First, the low-damping injection unit minimizes friction in the injection mechanism, allowing pressure to be adjusted within 1-2 milliseconds—fast enough to correct deviations during injection. It responds instantly to changes in resin viscosity or temperature, ensuring consistent pressure from the first part to the last. Unlike sliding guides, linear guides allow the injection unit to move smoothly and without friction.

This ensures that pressure is evenly distributed across the mold, eliminating pressure spikes caused by jamming. Topstar equipment uses high-precision pressure sensors that can detect pressure variations as low as 0.1 MPa. The low-damping unit and linear guides elevate pressure control from sufficiently precise to truly precise, which is why Topstar injection molding machines can control weight deviation within ±0.3%.

Linear guide

How to optimize pressure settings for zero weight deviation in plastic molding?

Even with Topstar’s precise control, correct pressure settings remain key to eliminating weight deviation. First, review the material data. Check the resin manufacturer’s recommended injection and holding pressure ranges. For example, PP typically requires an injection pressure of 80–120 MPa, while ABS requires 100–150 MPa.

Then conduct small-batch testing in the plastic molding process: print 20–30 parts at the recommended pressure, then weigh them. If the part is light, increase the injection pressure by 5 MPa; if it is heavy, decrease it by 5 MPa. Simultaneously, fine-tune the holding pressure to 60%–80% of the injection pressure. If the part shrinks, increase the holding pressure by 3–5 MPa; if the part develops flash (becomes heavier), decrease the holding pressure. Then activate Topstar’s automatic compensation function, which adjusts the pressure based on real-time data from the pressure sensor. This eliminates deviations caused by changes in resin or mold temperature. Furthermore, it tracks weight data for each product batch. Topstar’s plastic injection molding machines record pressure and weight data, enabling you to identify trends and proactively adjust settings.

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Achieve Zero Weight Deviation Through Precise Pressure Control

Weight deviations caused by improper pressure control don’t have to be a long-term problem in your plastic molding operations. The solution lies in using a responsive, precise pressure-controlled plastic injection molding machine. Topstar uses low-damping injection units, linear guides, and a sensitive pressure-detection system to control weight deviation to within ±0.3%. By eliminating friction, reducing pressure hysteresis, and enabling automatic adjustment, Topstar’s plastic injection molding machines transform unstable pressure into stable pressure, thereby reducing waste, defects, and costs.

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