diff --git a/src/main/java/com/ffii/core/utils/PrinterLockManager.kt b/src/main/java/com/ffii/core/utils/PrinterLockManager.kt
new file mode 100644
index 0000000..cf2fc8f
--- /dev/null
+++ b/src/main/java/com/ffii/core/utils/PrinterLockManager.kt
@@ -0,0 +1,26 @@
+package com.ffii.core.utils
+
+import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.locks.ReentrantLock
+
+/**
+ * Single-server printer lock manager.
+ *
+ * Keyed by "ip:port" so all print jobs targeting the same printer are serialized,
+ * preventing interleaving outputs when multiple users print concurrently.
+ */
+object PrinterLockManager {
+    private val locks = ConcurrentHashMap<String, ReentrantLock>()
+
+    fun <T> withLock(printerIp: String, printerPort: Int, block: () -> T): T {
+        val key = "${printerIp.trim()}:$printerPort"
+        val lock = locks.computeIfAbsent(key) { ReentrantLock(true) } // fair lock
+        lock.lock()
+        try {
+            return block()
+        } finally {
+            lock.unlock()
+        }
+    }
+}
+
diff --git a/src/main/java/com/ffii/core/utils/ZebraPrinterUtil.kt b/src/main/java/com/ffii/core/utils/ZebraPrinterUtil.kt
index fbefae4..7b9ac36 100644
--- a/src/main/java/com/ffii/core/utils/ZebraPrinterUtil.kt
+++ b/src/main/java/com/ffii/core/utils/ZebraPrinterUtil.kt
@@ -7,6 +7,7 @@ import java.awt.Graphics2D
 import java.awt.image.BufferedImage
 import java.io.File
 import java.io.OutputStream
+import java.net.InetSocketAddress
 import java.net.Socket
 import java.util.*
 
@@ -42,39 +43,51 @@ open class ZebraPrinterUtil {
                 throw IllegalArgumentException("Error: File not found or not readable at path: ${pdfFile.absolutePath}")
             }
             val renderDpi = dpi ?: 203
+            val safeIp = printerIp.trim()
+            if (safeIp.isEmpty()) throw IllegalArgumentException("Error: Printer IP is blank")
+
             try {
-                // 1. Load the PDF document
-                PDDocument.load(pdfFile).use { document ->
-                    val renderer = PDFRenderer(document)
-                    val totalPages = document.numberOfPages
-
-                    println("DEBUG: PDF has $totalPages pages")
-
-                    // Print each page for the specified quantity
-                    repeat(printQty ?: 1) { copyIndex ->
-                        println("DEBUG: Printing copy ${copyIndex + 1} of ${printQty ?: 1}")
-
-                        // Iterate through all pages in the PDF
-                        for (pageIndex in 0 until totalPages) {
-                            println("DEBUG: Processing page ${pageIndex + 1} of $totalPages")
-
-                            // 2. Render each page of the PDF as a monochrome image
-                            val image = renderer.renderImage(pageIndex, renderDpi / 72f, ImageType.BINARY)
-
-                            // 3. Convert the image to a ZPL format string
-                            val zplCommand = convertImageToZpl(image, printDirection)
-
-                            // 4. Send each page as a separate print job
-                            Socket(printerIp, printerPort).use { socket ->
-                                val os: OutputStream = socket.getOutputStream()
-                                val printData = zplCommand.toByteArray()
-                                os.write(printData)
-                                os.flush()
-                                println("DEBUG: Page ${pageIndex + 1} sent to printer")
+                // Serialize all jobs per printer to prevent interleaving.
+                PrinterLockManager.withLock(safeIp, printerPort) {
+                    // 1. Load the PDF document
+                    PDDocument.load(pdfFile).use { document ->
+                        val renderer = PDFRenderer(document)
+                        val totalPages = document.numberOfPages
+
+                        println("DEBUG: PDF has $totalPages pages")
+
+                        // 2. Open ONE connection and stream the whole job.
+                        Socket().use { socket ->
+                            socket.tcpNoDelay = true
+                            socket.connect(InetSocketAddress(safeIp, printerPort), 5_000)
+
+                            val os: OutputStream = socket.getOutputStream()
+                            val copies = printQty ?: 1
+
+                            // Print each page for the specified quantity
+                            repeat(copies.coerceAtLeast(1)) { copyIndex ->
+                                println("DEBUG: Printing copy ${copyIndex + 1} of ${copies.coerceAtLeast(1)}")
+
+                                // Iterate through all pages in the PDF
+                                for (pageIndex in 0 until totalPages) {
+                                    println("DEBUG: Processing page ${pageIndex + 1} of $totalPages")
+
+                                    // Render each page of the PDF as a monochrome image
+                                    val image = renderer.renderImage(pageIndex, renderDpi / 72f, ImageType.BINARY)
+
+                                    // Convert the image to a ZPL format string (contains ^XA ... ^XZ)
+                                    val zplCommand = convertImageToZpl(image, printDirection)
+
+                                    // Send to printer (same socket for whole job)
+                                    os.write(zplCommand.toByteArray(Charsets.US_ASCII))
+
+                                    // Small delay between pages to ensure printer can process each page
+                                    Thread.sleep(100)
+                                }
                             }
 
-                            // Small delay between pages to ensure printer can process each page
-                            Thread.sleep(100)
+                            os.flush()
+                            println("DEBUG: Print job sent to printer ($safeIp:$printerPort)")
                         }
                     }
                 }